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General Discussions => Drawing Board => Topic started by: erikb1971 on August 20, 2011, 03:47:17 PM

Title: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on August 20, 2011, 03:47:17 PM
I was lucky to be able to buy a set of the PMP EQP1-A and tube gain make up boards here on the forum. For my specific purpose though I would like to ad a sweepable mid, maybe like the mid section on the ez1084 (0,3khz - 10khz + switch for high/low q).  Is there a way of "relatively easy" adding such a thing to the here-for mentioned boards?

Cheers

Erik
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: gemini86 on August 20, 2011, 06:05:05 PM
not without affecting the high and low frequencies... Ian is working on this already, I'm sure he'll come up with something soon, as he's ridiculously good at these things.
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on August 21, 2011, 02:09:53 AM
not without affecting the high and low frequencies... Ian is working on this already, I'm sure he'll come up with something soon, as he's ridiculously good at these things.

Oh that would be the ideal solution... Iam going to work on the other parts of my project and keep two eyes open for Ian's poor man's sweet sweepable mid eq!
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: dirtyhanfri on August 21, 2011, 04:45:35 PM
not without affecting the high and low frequencies... Ian is working on this already, I'm sure he'll come up with something soon, as he's ridiculously good at these things.

That would be sweeeeet
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on August 21, 2011, 04:51:38 PM
yeah! "Go Ian, Go Ian, Go Ian"
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: baadc0de on August 21, 2011, 05:12:41 PM
yeah! "Go Ian, Go Ian, Go Ian"

Is this public?? As in, a mids section addon for the PMPeq??
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on August 21, 2011, 05:21:12 PM
well.. eehhh gemini made it public :-)
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: dirtyhanfri on August 21, 2011, 06:29:00 PM
Would be great, but let´s don´t press Ian.

He makes a great uninterested work, and shares a lot of knowledge.

If he´s working on it, one day he will come with a great Poor Man Sweepable Mid Freq Section. And the all of us will be  :o

Sure.
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: baadc0de on August 21, 2011, 06:32:47 PM
Sorry, sorry, I just got excited about the prospect, since I already have 4 channels of Ian's PM in the pipeline and have just bought a case for it (but haven't decided on a front panel yet). I'd love to add a swept mid section to them :)
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on August 21, 2011, 06:39:35 PM
Of course no pressure! Luckily Ian does not strike me as anyone who would be influenced by pressure.... But silently.. fingers crossed!
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: dirtyhanfri on August 21, 2011, 06:41:49 PM
Mine´s on the bench right now, waiting for some caps, a power transformer and tube sockets, I´ve already got a front pannel, but I wouldn´t doubt in get a 2U rack case and a new front pannel. At all is an affordable project (I´m using a recycled case right now).

A pair of friends asked me to build it for them, would be great too add a mid band, but, hey, it´s great too as it is!
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: gemini86 on August 21, 2011, 07:31:13 PM
yeah! "Go Ian, Go Ian, Go Ian"

Is this public?? As in, a mids section addon for the PMPeq??

...from this thread: http://www.groupdiy.com/index.php?topic=45110.msg569012#msg569012 (http://www.groupdiy.com/index.php?topic=45110.msg569012#msg569012)

Quote from: ruffrecords
Funny you should say that, I have been surveying a range of 3 band EQs with just that in mind. Hi and low are quite straightforward to do with little interaction because the frequencies concerned are far apart, but with a mid range control sitting right between them it is hard to do something simple that does not interact with the Hi or Lo, but I am working on it - watch this, or another, space.

Cheers

Ian


But yes, no pressure on Ian, he's been scratching out schematics for us like crazy lately, maybe somebody can help him with the leg-work.

Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: MatthisD on October 16, 2011, 09:44:52 AM
http://www.jlmaudio.com/passive_mid_cut_fixed.jpg

(Edited: I had thought a 1:2 input transformer would work but maybe a 1:1.5 interstage instead)
What about a Meq5 type filter, taking just the mid-cut section as in the link above and putting it in front of the Poor man's EQ. The loss would be about 7dB. If using the Poor Man's tube gain it has a few dB extra for makeup too but maybe a 10k//20K interstage could be used to get an extra couple of dB.


The extra parts for the mid-cut: 1x transformer, 1x Inductor, 1x pot, 1x 12position switch, 1x bypass switch and 11x capacitors.

Any thoughts? what are the flaws with this idea?

Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on October 16, 2011, 06:15:46 PM

Any thoughts? what are the flaws with this idea?

The good thing about this idea is that it is a cut only - no boost - which makes it an easier task - it is boost that is the tough one.

The flaws are twofold:

1. The poor man's EQ has its pot and cap values tweaked to work with a 10K:10K transformer rather than the 600:600 one used in the original Pultec so you would need to do the same again for the pot, cap and inductor values for a mid cut.

2. The hi and lo cut circuits actually start working at between 5 and 10 times the frequency setting on the switch. Even so, with the highest lo setting and the lowest hi setting together, there is little interaction between the hi and lo circuits. This would not be so with a mid cut and it would be quite possible to overlap the responses of the mid section with parts of either the hi or lo sections. This may or may not be a bad thing - it might produce some interesting curves or it might just sound awful. That said, it is quite common for three band EQ to overlap in this way so I may just be seeing a problem where there is in fact none.

Despite that you got me thinking and I tried a quick simulation of a mid cut addition to the poor man's pultec and it seems to work quite well. It is a simple series resonant circuit connected from the junction of the hi boost and hi cut pots to ground. I worked out some cap values for a Carnhill VTB9050 inductor which seem to give usable frequencies. One difficulty with mid cut circuits is knowing what value of Q to aim for. Manufacturers seem to go for a higher Q for cut than for boost with about 3 being typical for boost and twice that for cut. With that in mind I worked out two sets of values of caps and frequencies with the VTB9050 one for a Q of 7 and the other for a Q of 3.5.

In an attempt not to compromise the hi and lo circuits or to require additional gain boost, this circuit works in a slightly unusual way. The pot is simply in series with the LC which varies not only the attenuation but also the Q - the Q starts out low and reaches its maximum at maximum cut. I added a fixed resistor in series with the pot to limit the cut to about 14dB like the hi and lo controls. One small problem with this simple circuit is with the pot at maximum there is still a small cut of about 0.6dB. To eliminate this you could add a switch in series with the pot as a sort of mid cut on/off switch. I attach a screen capture of the circuit taken directly from LTSpice and annotated with the calculated values.

I have not looked at interaction with the lo and hi controls.

Let me know what you think.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: rmaier on October 16, 2011, 08:26:39 PM
At the risk of stating the obvious, I'd say this sounds like a killer idea!
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on October 17, 2011, 04:13:52 AM
go Ian, go Ian, go Ian!
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: MatthisD on October 17, 2011, 06:33:53 AM

Brilliant, at what amount of attenuation would the Q be at 7 if using the cap values for that Q, at maximum 14dB?

One difficulty with mid cut circuits is knowing what value of Q to aim for. Manufacturers seem to go for a higher Q for cut than for boost with about 3 being typical for boost and twice that for cut. With that in mind I worked out two sets of values of caps and frequencies with the VTB9050 one for a Q of 7 and the other for a Q of 3.5.

In an attempt not to compromise the hi and lo circuits or to require additional gain boost, this circuit works in a slightly unusual way. The pot is simply in series with the LC which varies not only the attenuation but also the Q - the Q starts out low and reaches its maximum at maximum cut.



Any thoughts? what are the flaws with this idea?


1. The poor man's EQ has its pot and cap values tweaked to work with a 10K:10K transformer rather than the 600:600 one used in the original Pultec so you would need to do the same again for the pot, cap and inductor values for a mid cut.

I think I meant this though:

Input:600ohm/600ohm  -  Meq5 Midcut  -  interstage:10k/10k  -  PoorMan's EQ  -  PoorMan's Tube gain stage(or other).

The part I am unsure of is the impedance that the Poor man's EQ will see. If its 5.5k after the Mid cut could you put a 12.5k resistor across the 10k transformer secondary (12.5k parallel with 50k=10k) so to not affect the Poor Man's filter section?
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on October 17, 2011, 10:22:18 AM

I think I meant this though:

Input:600ohm/600ohm  -  Meq5 Midcut  -  interstage:10k/10k  -  PoorMan's EQ  -  PoorMan's Tube gain stage(or other).

The part I am unsure of is the impedance that the Poor man's EQ will see. If its 5.5k after the Mid cut could you put a 12.5k resistor across the 10k transformer secondary (12.5k parallel with 50k=10k) so to not affect the Poor Man's filter section?

OK, now I understand what you were getting at. From looking at the MEQ5 I think it would feed the poor man's 10K transformer OK. You would need extra gain make up to account for the loss in the MEQ.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on October 17, 2011, 05:18:22 PM

Brilliant, at what amount of attenuation would the Q be at 7 if using the cap values for that Q, at maximum 14dB?


The maximum Q is at the maximum cut and  reduces as the cut reduces.

Cheers

Ian

Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on October 18, 2011, 06:42:32 PM
I have been giving this a little more thought. Part of the original idea of the poor man's EQP1A was to get rid of its expensive inductors. If we introduce an inductor now just to do mid cut it rather defeats that object. So I propose we make that inductor work for its money by working as both a mid cut and mid boost inductor. We can achieve this by switching it from the cut position to the boost position. The only downside is that, although the cut and boost frequencies will be the same, the Q (sharpness) of the cut will be much higher than that of the boost. If we set the Q right for the cut it will be too low for the boost. If we set it right for the boost it will be too sharp for the cut. Unfortunately we cannot simply use the same inductor with different capacitor values to get the same frequencies with both boost and cut at the right Q values. Then I had an idea. As the frequencies are the same and we don't need two sets of components, one for boost and one for cut, we can use the existing EQ PCB with a one pole 12 way switch and have a dozen mid frequencies to choose from. With so many frequencies we can space them closer together so if the cut Q is a little high it is not so much a problem as we can home in closer to the required frequency.

So I worked on this some more and concluded probably the best inductor to use is the Carnhill VTB9055 which costs about 20GBP. Unfortunately this has only 5 taps so we can really only get 10 sensible frequencies from it. However, I mentioned in an earlier post that the simple way of doing the cut meant that even with the control at minimum cut there was a small residual cut so we needed an off switch. By leaving a couple of blank positions in the middle of the 12 way switch we could have 2 sets of 5 frequencies with off in the centre.

So, I have ordered a VTB9055 and I'll knock up a prototype and give it a try.

Watch this space.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: MatthisD on October 19, 2011, 11:10:01 AM
great stuff!  I tried to workout some cap values and frequencies with that inductor. But with 10 different frequencies and 5 taps can you keep the Q value consistent from position 1 to 10?

Would these values be right for roughly cut Q of 7:

100nF
362Hz
1.93H
4393 Ohms

68nF
535Hz
1.3H
4372 Ohms

Thanks
Matt
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on October 19, 2011, 08:28:16 PM
great stuff!  I tried to workout some cap values and frequencies with that inductor. But with 10 different frequencies and 5 taps can you keep the Q value consistent from position 1 to 10?

No, you have to compromise on Q variation when you use two different caps on the same tap

Quote
Would these values be right for roughly cut Q of 7:

100nF
362Hz
1.93H
4393 Ohms

68nF
535Hz
1.3H
4372 Ohms

Thanks
Matt

By my calculator the frequency and characteristic impedances (Z) are correct. The series 1K resistor sets the maximum amount of cut to about 14dB. It also determines the maximum Q since Q=Z/R where R in this case is the 1K (plus the pot resistance. So the maximum Q with those values I reckon would be just under 4.4. I think we need to be aiming for a Q about twice that.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: abbey road d enfer on October 20, 2011, 08:36:42 AM
great stuff!  I tried to workout some cap values and frequencies with that inductor. But with 10 different frequencies and 5 taps can you keep the Q value consistent from position 1 to 10?
Some designers support the idea that Q should somewhat increase with frequency. Now, I don't know if it comes from genuine experimentation or just a justification for a workaround.
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on October 20, 2011, 12:49:07 PM
Some designers support the idea that Q should somewhat increase with frequency. Now, I don't know if it comes from genuine experimentation or just a justification for a workaround.

I have not been able to find an original MEQ5 schematic - just the Gyraf one. I notice that for dip frequencies from 2KHz to 7KHz inclusive Gyraf use the same value inductor and just alter the capacitor value. This means the bandwidth stays constant and hence the Q rises as the set frequency rises. It appears to change from a Q of 1.5 at 2KHz to a Q of 6.5 at 7KHz. I have no idea if the original Pultec MEQ5 did this also.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: MatthisD on October 20, 2011, 01:41:37 PM
Here it seems to match the frequency settings as the Gyraf schematic:
http://danalexanderaudio.com/OutboardPic/PultecMeq5.jpg
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: MatthisD on October 20, 2011, 01:56:36 PM
great stuff!  I tried to workout some cap values and frequencies with that inductor. But with 10 different frequencies and 5 taps can you keep the Q value consistent from position 1 to 10?
Some designers support the idea that Q should somewhat increase with frequency. Now, I don't know if it comes from genuine experimentation or just a justification for a workaround.

Which would be consistent in a way but if the Q jumps between a higher, lower then higher again as you go through the frequency positions it could be strange to get used to, but of course much better than having only 5 positions.
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on October 20, 2011, 05:50:22 PM
Here it seems to match the frequency settings as the Gyraf schematic:
http://danalexanderaudio.com/OutboardPic/PultecMeq5.jpg

Yes, the frequencies are identical so that is encouraging. Unfortunately I have been unable to find an MEQ5 manual. Pultec were quite good at showing frequency response curves in their manuals so that might well have shown how the Q actually varies.

Also, our other discussion reagrding the likely value of Q seems to be wrong. I have simulated the Gyraf circuit and also looked at the manual for the UAD powered plug-in of the MEQ5 and both seem to show that the cut Q is around 1.5 which is a lot less than I had assumed (based on what the Helios type 69 EQ does). There are two consequences to this.

1. I need to re-work to L and C values which may mean using a different inductor. Note that minor variations in Q should be quite unnoticeable at such low Q values.

2. If we try to switch this to boost, the Q will be extremely low - perhaps so low as to be unusable.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: noulou on October 21, 2011, 07:35:59 AM
Hey Ian,

Maybe this will help

http://www.kiwistudio.gr/schematics/MEQ5.pdf (http://www.kiwistudio.gr/schematics/MEQ5.pdf)


Nikos
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on October 21, 2011, 09:12:49 AM
Hey Ian,

Maybe this will help

http://www.kiwistudio.gr/schematics/MEQ5.pdf (http://www.kiwistudio.gr/schematics/MEQ5.pdf)


Nikos

Nikkos, that is brilliant! Thanks very much. Looks like the Gyraf is a direct copy of the PUltec in which case the Q does vary across the high frequencies.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: CJ on October 21, 2011, 10:21:04 PM
actually, that schematic was reversed from a scribbled out schemo from greg boblinski up in the frigid northern reaches of canada,

as far as i know it has never been checked against a real meq 5, but it seems to work great, so wtf, over?  :P
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: Michael Tibes on October 22, 2011, 07:51:35 AM
Ian, I've sent you the MEQ-5 manual to ianbellATukfsnDOTorg, unfortunately the pdf is way too large for the schematics collection here. I didn't do the compilation and redrawing, don't know who deserves the credit. Plenty of nice curves there  ;)

Michael

edit: hadn't realized you already got it, so nevermind. I guess the only difference is the redrawn schematic. Anyway, please keep up the good work!
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on October 30, 2011, 07:00:56 PM
Thanks to everyone who sent me info on the various MEQ variants by Pultec and TubeTech. I have fed all this info into a spreadsheet and worked out what the original inductor values should have been and hence the Q factor at each frequency. By looking at the ratio of the inductor values I have been able to choose an inductor that will give a reasonable approximation of the MEQ mid cut when added to the poor man's EQP1A. The inductor is the VTB9042 and the frequencies, inductor and capacitance values required to emulate the MEQ are as follows:

Frequency                       Inductance(mH)                        Capacitance(nF)
200                               1000                                    650
300                               1000                                    280
500                               600                                     170
700                               600                                     85
1000                              300                                     85
1500                              300                                     37.5
2000                              200                                     31.5
3000                              200                                     14
4000                              100                                     15.5
5000                              100                                     10
7000                              100                                     5.1


This circuit gives Q values similar to the original MEQ mid cut. Unfortunately this means that if they are then used  to try to add a mid boost the resultant Q values are very low so it is not really usable as a mid boost.

To add this mid cut to the poor man's EQP1A connect a 47K log pot to the junction of the hi boost and hi cut pots connect a 1K resistor to its slider and connect the other end of the 1K to the switched L and C in series.

At the moment this is a theoretical solution only. I have not built and tested it.


Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: MatthisD on November 01, 2011, 09:06:25 AM
cheers Ian, I'll be giving it a shot!
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: baadc0de on November 01, 2011, 04:50:45 PM
 I hope this doesn't come accross the wrong way, but I'm itching to add a mid boost more than a mid cut to the poorman. I'm willing to live with more makeup gain required, anything you guys think would work?
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 01, 2011, 06:25:38 PM
I hope this doesn't come accross the wrong way, but I'm itching to add a mid boost more than a mid cut to the poorman. I'm willing to live with more makeup gain required, anything you guys think would work?

While I was working out the cut inductor and cap values the other night, it occurred to be that the cut works by means of a series resonant circuit. One way to make a boost is to use the same series resonant circuit across the other arm of the attenuator but because this arm has a much higher resistance the Q is reduced so low as to be unusable.

What we really need is a circuit connected to the same arm of the attenuator as the cut so that the Q remains the same. The way to do this is to use a parallel resonant circuit instead of a series one so I am going to have a look at that next. With a bit of luck and some clever switching we might be able to persuade the same set of L and C to produce either boost or  cut.


Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: baadc0de on November 01, 2011, 08:13:14 PM
Oh very nice... I hope you can pull it off, that would definitely be something! And be a good candidate for the next PCB! :)
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: baadc0de on November 05, 2011, 03:35:14 PM
Oh, are there really going to be 12 positions for the mid band? We had 6 for low and hi I think?
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 05, 2011, 07:37:12 PM
Oh, are there really going to be 12 positions for the mid band? We had 6 for low and hi I think?

12 positions is quite likely. The reason is that for the hi and lo we needed simultaneous boost and cut so we needed a 2 pole switch - one pole for boost and one for cut. With the mid cut/boost we either cut or boost so we only need a one pole switch so we can have 12 frequency settings. Whether it makes sense in EQ terms to have 12 frequency settings is another matter.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 09, 2011, 05:44:41 PM
I have now done some work on a parallel resonant boost version. The table below lists for each frequency the simulated Q for the original cut,  for the series resonant boost (using the same L and C values) and for the parallel resonant boost (again using the same L and C values).



Frequency                       Qcut                        Qseriesboost                     Qparallelboost
200                             1.2                                0.2                              19.0
300                             1.8                                0.3                              12.6
500                             1.8                                0.3                              12.8
700                             2.5                                0.42                              8.9
1000                            1.8                                0.3                              12.7
1500                            2.7                                0.45                              8.4
2000                            2.4                                0.4                               9.5
3000                            3.6                                0.6                               6.3
4000                            2.4                                0.4                               9.3
5000                            3.0                                0.5                               7.5
7000                            4.2                                0.7                               5.4


The Qcut values follow reasonably closely to those of the MEQ5. Unfortunately when the same values of L and C are used in a series resonant boost the resultant Q is very low. When the same values are used ina parallel resonant boost the resultant Q is rather high.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ChrioN on November 09, 2011, 06:27:15 PM
I would gladely support this project with inductors. If we get enough dudes, prices will be very good.
...Or you can just support some else  ;D
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: baadc0de on November 10, 2011, 05:02:44 AM
I have now done some work on a parallel resonant boost version. The table below lists for each frequency the simulated Q for the original cut,  for the series resonant boost (using the same L and C values) and for the parallel resonant boost (again using the same L and C values).
...

What about having a double rotary (either two separate or two deck) with two sets of capacitors and a switch to select which set to use?
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 10, 2011, 07:01:47 PM
I have now done some work on a parallel resonant boost version. The table below lists for each frequency the simulated Q for the original cut,  for the series resonant boost (using the same L and C values) and for the parallel resonant boost (again using the same L and C values).
...

What about having a double rotary (either two separate or two deck) with two sets of capacitors and a switch to select which set to use?

I had thought of using different L and C values for boost and cut and exploiting the fact that we have a 2 pole 6 way switch on the existing EQ PCB to to contain the two sets of capacitors (I am aiming for using the existing PCB rather than designing a special one just for mid cut/boost). Having two switches  is more expensive so we should aim to avoid it if possible. I feel bad enough about using the inductor for mid cut without adding extra switches too ;)

One thing we could do is accept the differences in Q between boost and cut caused by using the same LC values at each frequency. This is what the classic Helios Type 69 EQ does - and its cut Q is much higher than its peaking Q. The ratio between series cut Q and series peak Q is exactly 6:1 in the poor man's Pultec. So we could aim for a nominal Q of 1 for the boost which would give a nominal Q of 6 for cut. The advantage of this approach is that we potentially have 12 frequency settings. The other reason I think this is probably a good way to go is that the MEQ is designed to overlap with its peaking lo and hi EQ and so produce the classic Pultec bump. We don't need to do that at the Lo end because we already create the Pultec bump from simultaneous Lo boost and cut. We cannot do it at the Hi end because we have a shelving Hi EQ rather than a peaking one.

So, I'll warm up the calculator and work out a set of LC values on this basis. That's about as far as I think I want to go with the inductor and I want to return to thinking of other ways of doing mid boost/cut without using an inductor.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 12, 2011, 08:39:40 AM
OK, here's a design for a mid boost/cut modification the the poor man's Pultec. It has a set of 12 frequencies, a boost/cut switch and needs an on/off switch. Detials in the attached pdf.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: rmaier on November 12, 2011, 10:05:12 AM
PDF worked for me.
Thanks Ian. I have a feeling I'll be in touch again soon....
Do you have many eq boards left?

Ralph
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 12, 2011, 10:57:11 AM
Do you have many eq boards left?

Ralph

Yes, I have quite a few left.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: rmaier on November 12, 2011, 12:51:07 PM
Hahaha. In that case, please send me two more. Sorry about the frequent trips to the post office, but you'll have to stop being so generous with your work.

Cheers,

Ralph
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 13, 2011, 09:02:36 AM
Hahaha. In that case, please send me two more. Sorry about the frequent trips to the post office, but you'll have to stop being so generous with your work.

Cheers,

Ralph

My pleasure Ralph and thanks for volunteering to be the first to try out the mod. I'll post your PCBs on Monday.


Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on November 16, 2011, 05:51:48 AM
I am always a bit of a visual guy so... am I correct that with three boards it should be able to build something like the attachment?
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 16, 2011, 06:54:43 AM
I am always a bit of a visual guy so... am I correct that with three boards it should be able to build something like the attachment?

The mid is switched from boost to cut rather than having separate boost and cut pots - you can do either but not both at the same time like you can with the hi and lo. So I envisaged the mid as one pot and one frequency switch plus a toggle labelled boost/cut. You could use separate pots for mid boost and cut but you would still need to switch between them.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on November 16, 2011, 03:37:16 PM
see.. that is why I should have pictures :-)

so something like this or should it still have the on off switch?
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: MatthisD on November 16, 2011, 04:07:44 PM
Yes you need the switch because;

"It also means that the
minimum boost is not quite no boost at all so I have included a switch in series with the pot to allow
the mid boost/cut to be switched right off"

see.. that is why I should have pictures :-)

so something like this or should it still have the on off switch?
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on November 16, 2011, 04:11:14 PM
yeah I had read that.. that is why is was in there in the first place... somehow I thought I had mistaken that since I completely overlooked the cut/boost switch! Tnx!
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 16, 2011, 04:13:13 PM
see.. that is why I should have pictures :-)

so something like this or should it still have the on off switch?

As Mathis said you need an on/off switch but you could incorporate it into the boost/cut pot if you wish.

I would  have the cut boost switch configured so UP id boost and DOWn is cut but that is just a personal prerefence.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on November 16, 2011, 04:18:07 PM
aahh smart.. a three way switch!
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on November 16, 2011, 04:57:43 PM
Ian, would you mind if I started a feeler to see how much interest there is in dedicated mid boards? I am definitely going to build a version with the mid, and incorporate it in another project, but I would feel a bit silly about adjusting the standard boards, having to buy extra standard boards and then you running out of them again...

If not, 4 standard boards for me please :-)
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 16, 2011, 06:09:26 PM
Ian, would you mind if I started a feeler to see how much interest there is in dedicated mid boards?

That's fine by me. The modifications to the PCB layout will not be very hard. I assume you would want the inductor on the PCB as well. If there is enough interest I am happy to design such a PCB.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: gemini86 on November 17, 2011, 01:15:41 AM
I really like the ON-off-ON switch... keeps it clean...
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: baadc0de on November 17, 2011, 04:54:51 AM
Ian, would you mind if I started a feeler to see how much interest there is in dedicated mid boards? I am definitely going to build a version with the mid, and incorporate it in another project, but I would feel a bit silly about adjusting the standard boards, having to buy extra standard boards and then you running out of them again...

If not, 4 standard boards for me please :-)

If there will be a mids board, I'm definitely in for 4
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on November 17, 2011, 05:45:42 AM
feeler for the mid boards: http://www.groupdiy.com/index.php?topic=46575.0 (http://www.groupdiy.com/index.php?topic=46575.0)

@Baadc0de: noted you for four!

@Ian: would it be possible to use two mid boards and reserve one for boost and one for cut?
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 17, 2011, 12:38:31 PM
@Ian: would it be possible to use two mid boards and reserve one for boost and one for cut?

Are you thinking of doing both mid  boost and mid  cut at the same time?

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on November 17, 2011, 12:59:45 PM
well.. considering the broad freq spectrum of the mid section... it crossed my mind yes...
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 17, 2011, 03:46:31 PM
well.. considering the broad freq spectrum of the mid section... it crossed my mind yes...

I have to confess I am a little uneasy about the broad range of the mid section combined with simultaneous mid boost and cut. The reason is that the original poor man's Pultec design tries to ensure that the load on the input transformer is never much less than 10K. With the mid overlapping some of the high boost  and  hi cut frequencies then with simultaneous mid boost/cut and hi boost/cut at the same frequency this load could drop below 5K which the transformer will reflect to the input. Now that is not going to be a problem for most modern gear driving this circuit but some of my tube circuits would struggle.

From the circuit point of view I see no reason not to build it with separate mid boost and mid  cut circuits (though you will need two inductors). I guess configured that way it would make a rather interesting 4 band semi-parametric EQ. If you do decide to do it, it might be interesting to use a different inductor for the cut in order to broaden the Q somewhat. If you use a larger inductor for the mid cut you can get to some quite interesting low frequencies. I thought perhaps 150Hz and 180Hz high Q mid cut would be useful for hum elimination for example.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on November 17, 2011, 03:58:40 PM
hmmm sounds interesting... I would need some help from you however... but I put myself for 6 mid boards on the list... So I am ready for that!
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 17, 2011, 05:17:38 PM
hmmm sounds interesting... I would need some help from you however... but I put myself for 6 mid boards on the list... So I am ready for that!

No problem. Happy to help.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on November 17, 2011, 05:21:47 PM
Happy to hear that! BTW, I think you can start designing the dedicated mid boards... 39 orders so far :-)
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: baadc0de on November 18, 2011, 07:12:44 AM
Well, it's extra hard to squeeze all the items for 4 channel in 3U in there and I still need to add the on/off switch, an indicator light / LED, some switches to put turn the mids off etc..

Also this is way over the "poorman" moniker because I thought maybe I could use some jensens as I/O.. and use almost exclusively styros for caps.
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: baadc0de on November 18, 2011, 07:43:32 AM
And a 4U option that my girlfriend likes much better :P
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 18, 2011, 07:51:17 AM
And a 4U option that my girlfriend likes much better :P

That is certainly a beast of an EQ. I am not sure the mid PCB will fit to your front panel at present because it looks like being 3.5 inches wide. Check out the thread on the mid PCB in the Blacj Market for more details - and of course feel free to comment too.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: baadc0de on November 18, 2011, 08:02:54 AM
Yeah, all thanks goes to you Ian for making it available.. none of this would be happening if not for your efforts and I really appreciate it! It really looks like a beast of an EQ and with your nice 6CG7 makeup stage, styro caps, jensen trafos.. it's bound to sound awesome. Can't wait to get this crunching on some material. I'm thinking this would also be useful to EQ quadraphonic masters hmm :) since I also have two straight ahead pultec clones, I could pultec-ize a 5.1 mastering chain lol :)

That said, I'll be looking to mount the components on the back of the PCB and use the shallowest possible pots and switches so they don't touch the PCB. We'll see how that goes and if nothing else I could always offboard the switches and just mount the mid PCBs to the case not the front panel. I have a big roll of 6-wire ribbon cable and I'm not afraid to use it haha :) though I'd prefer if it doesn't come to this. I see maybe the inductor wouldn't work if I mounted it on the back of the PCB, so I guess it's either a different (rotated) inductor or offboarding it or offboarding the swtich, whatever works..
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 18, 2011, 09:14:16 AM
I see maybe the inductor wouldn't work if I mounted it on the back of the PCB, so I guess it's either a different (rotated) inductor or offboarding it or offboarding the swtich, whatever works..

Actually, after posting about the size of the new PCB I also wondered if we could fit the inductor on the back behind the switch. I think that provided you fit the switch first and crop its tags quote short it should be possible to then fit the inductor on the back.I'll give it some thought.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on November 19, 2011, 05:47:18 PM
Wow! I agree with the girlfriend! Could you post it in the mid feeler thread for inspiration?
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: baadc0de on November 23, 2011, 04:26:22 AM
For a 4U Poshman, which I define as 4 channels of dual mids + hi / low, please find attached a FPD file. You will have to add on/off power switches and any LED indicators if you wish, as well as your logo etc.. this is kind of like a template for those who are interested in such a panel. Enjoy :)

IMPORTANT NOTE: the forum software does not allow upload of an FPD file. Download this file and change its extension to FPD instead of PDF!
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 23, 2011, 05:07:57 AM
For a 4U Poshman, which I define as 4 channels of dual mids + hi / low, please find attached a FPD file. You will have to add on/off power switches and any LED indicators if you wish, as well as your logo etc.. this is kind of like a template for those who are interested in such a panel. Enjoy :)


That is an incredibly detailed panel. I notice that each scale is a .plt file. I have used FPD a bit but never to create something that intricate. Can you say a little about how you create these .plt files? I was just wondering if we should think about setting up a repository of such files for everyone to use.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on November 23, 2011, 05:25:24 AM
can't open or download the file.... grrr.. could you email it to erikb1971 at gmail dot com?
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 23, 2011, 06:57:03 AM
can't open or download the file.... grrr.. could you email it to erikb1971 at gmail dot com?

Done,

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on November 23, 2011, 08:30:35 AM
TNX.. and it IS looking good! Would you mind if I rebuild it to my 3u stereo version?
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 23, 2011, 10:06:55 AM
TNX.. and it IS looking good! Would you mind if I rebuild it to my 3u stereo version?

I don't mind and I think baadc0de intended to donate it to the group.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ej_whyte on November 23, 2011, 12:15:54 PM
For a 4U Poshman, which I define as 4 channels of dual mids + hi / low, please find attached a FPD file. You will have to add on/off power switches and any LED indicators if you wish, as well as your logo etc.. this is kind of like a template for those who are interested in such a panel. Enjoy :)


That is an incredibly detailed panel. I notice that each scale is a .plt file. I have used FPD a bit but never to create something that intricate. Can you say a little about how you create these .plt files? I was just wondering if we should think about setting up a repository of such files for everyone to use.

Cheers

Ian


http://www.frontpanelexpress.com/fileadmin/pdf/tips_tricks/jskala_english.jar (http://www.frontpanelexpress.com/fileadmin/pdf/tips_tricks/jskala_english.jar)

Very useful  :) Although it looks a bit confusing and usually takes a couple of attempts to get it right, have a play and you will soon work it out, shame there is no preview box though.

Create a file, import HPGL in to FPD and then it gives you the colour & thickness options. Make sure you set the origin to follow the origin of the imported HPGL, makes aligning the scale much easier.

Cheers
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on November 23, 2011, 12:20:31 PM
first sketch of stereo 2U posh men's eq... love the way you can switch the mid sections off or off and how there is a double boost mid, but no cut..... sloppy work, better later!:-)
updated.. that is better
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 23, 2011, 04:19:55 PM
first sketch of stereo 2U posh men's eq... love the way you can switch the mid sections off or off and how there is a double boost mid, but no cut..... sloppy work, better later!:-)

I am not sure about the double boost you have there. The idea was you could have a single frequency control that could be switched to either boost or cut OR you could have two frequency controls, one assigned to cut and the other to boost. Although you could have two mid boosts you would have to be very careful how you used it. If you set the boosts to the same frequency you would not get double the dB boost. The boosts are quite wide so you might well get two separate boosts where the frequencies are an octave apart for example but much closer and they will start to interact. I am not saying don't do but just be careful if you do. An interesting alternative might be to look at a pair of mid boost/cuts with no-overlapping frequencies using two different inductors. That way the lower end could be extended to the region of 150Hz and include frequencies like 150Hz and 180Hz which are useful for removing hum. So one control could cover from 150Hz to 1.5Khz and the second from 1.6KHz to 16KHz for example.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: erikb1971 on November 23, 2011, 04:22:41 PM
Oh I am sorry you did not get my sarcasm on my own sloppy design... they were not meant to be double boost, nor off-off switches. It was a matter of thoughtless copy pasting and no checking... it has been corrected!
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: baadc0de on November 23, 2011, 04:55:59 PM
Yeah, Ian is right, the FPD file is a gift to this wonderful forum of wonderful people :)

also, the dials are created with the jskala application linked above, though for more complex designs, I turn to the scale designer in FrontDesigner 3.0. It's not really a nice app, but the dial designer is great, has a good preview, etc..

I'll attach here another FPD file of another intricate design that uses dials created in FrontDesigner 3.0. The symbols were made in corel draw .. badly haha

EDIT: again you will have to rename the PDF to FPD ;)
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ej_whyte on November 23, 2011, 05:36:19 PM
first sketch of stereo 2U posh men's eq... love the way you can switch the mid sections off or off and how there is a double boost mid, but no cut..... sloppy work, better later!:-)

I am not sure about the double boost you have there. The idea was you could have a single frequency control that could be switched to either boost or cut OR you could have two frequency controls, one assigned to cut and the other to boost. Although you could have two mid boosts you would have to be very careful how you used it. If you set the boosts to the same frequency you would not get double the dB boost. The boosts are quite wide so you might well get two separate boosts where the frequencies are an octave apart for example but much closer and they will start to interact. I am not saying don't do but just be careful if you do. An interesting alternative might be to look at a pair of mid boost/cuts with no-overlapping frequencies using two different inductors. That way the lower end could be extended to the region of 150Hz and include frequencies like 150Hz and 180Hz which are useful for removing hum. So one control could cover from 150Hz to 1.5Khz and the second from 1.6KHz to 16KHz for example.

Cheers

Ian

I was planning on having 2 mid bands, both boost/cut switchable, haven't decided whether they will have identical overlapping freqs. or split bands yet. Is this still feasible as long as I dont put both bands in boost at close frequencies as described?

Cheers
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on November 23, 2011, 05:37:54 PM

I was planning on having 2 mid bands, both boost/cut switchable, haven't decided whether they will have identical overlapping freqs. or split bands yet. Is this still feasible as long as I dont put both bands in boost at close frequencies as described?

Cheers

Yes, and the same applies for cut as well.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: baadc0de on December 29, 2011, 06:29:17 PM
Is this design feasible then?

Large picture:
http://dl.dropbox.com/u/12679166/PoshMan4U.png

FPD file:
http://dl.dropbox.com/u/12679166/PoshMan4U.fpd
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on December 30, 2011, 11:39:50 AM
Is this design feasible then?

Large picture:
http://dl.dropbox.com/u/12679166/PoshMan4U.png

FPD file:
http://dl.dropbox.com/u/12679166/PoshMan4U.fpd

Yes, that's pretty much what I had in mind for the four band semi-parametric.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: mikeyB on May 10, 2013, 12:09:29 PM
Hi Ian - just been reading your notes regarding an eq with 2 non-overlapping bands. Do the 2 bands connect to the same places in the network (effectively in parallel) each with a 1k and 4k7 back into the network?

Thanks in advance
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on May 11, 2013, 06:58:49 AM
Hi Ian - just been reading your notes regarding an eq with 2 non-overlapping bands. Do the 2 bands connect to the same places in the network (effectively in parallel) each with a 1k and 4k7 back into the network?

Thanks in advance

Yes, they do. It is because they are in parallel that the frequency bands should not overlap.

Cheers

ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: mikeyB on May 15, 2013, 10:30:21 AM
Great stuff - gotta give this a go!
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: mikeyB on May 15, 2013, 11:41:48 AM
Hi Ian - you mentioned somewhere in the notes about scaling the capacitors down by a factor of 4 if you scale the pots up by a factor of 4.
Is this the case for the mid boost band as well regarding the inductor. I notice the carnhill has a max of 1H whereas a standard pultec inductor has a max typically 267mH (roughly a quarter).

Can you use this general rule of thumb?
I have quite a few pots in magnitudes of 22 (ie 2k2/22k/220k) and was thinking of trying these out. I would then be talking of a network with input imp of 2k5? Cap values would be halved from an original network of 600R.

Thanks as usual for such a great project.
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on May 15, 2013, 12:09:06 PM
Hi Ian - you mentioned somewhere in the notes about scaling the capacitors down by a factor of 4 if you scale the pots up by a factor of 4.
Is this the case for the mid boost band as well regarding the inductor. I notice the carnhill has a max of 1H whereas a standard pultec inductor has a max typically 267mH (roughly a quarter).

Can you use this general rule of thumb?
I have quite a few pots in magnitudes of 22 (ie 2k2/22k/220k) and was thinking of trying these out. I would then be talking of a network with input imp of 2k5? Cap values would be halved from an original network of 600R.

Thanks as usual for such a great project.

Broadly speaking yes. The RC parts of the EQ all scale this way and the mid inductor must scale the same to keep the Q as the original. The only components that go contrary to this are the caps in the mid boost/cut which need to go in the opposite direction to its inductor for the resonant frequency to be the same.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: gemini86 on May 28, 2013, 07:58:29 PM
I have a few questions. To preface; I'm planning to build a 4 band unit, non overlapping centers.

1) am I correct to assume the each mid band is connected the same?
2) according to my simulations, the bandwidth of the boost frequency is much wider than cut. This can be changed by lowering the 4k7 resistor. Does doing this cause any problems with impedances or anything?
3) There's nothing stopping me from using individual inductors vs a multi tap inductor, correct?

Thanks!
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on May 29, 2013, 04:56:02 AM
I have a few questions. To preface; I'm planning to build a 4 band unit, non overlapping centers.

1) am I correct to assume the each mid band is connected the same?
2) according to my simulations, the bandwidth of the boost frequency is much wider than cut. This can be changed by lowering the 4k7 resistor. Does doing this cause any problems with impedances or anything?
3) There's nothing stopping me from using individual inductors vs a multi tap inductor, correct?

Thanks!

1. Yes (see reply #87)
2. The bandwidth of the cut is much narrower than the boost. This is one of the limitations of a simple circuit like this. It is designed for a boost Q of between 1 and 3 which gives a cut Q several times the boost Q. In most cases this is not a problem as gentle boost and sharpish cut are what is normally required. Lowering the 4K7 resistor will sharpen the boost. However it does lower the minimum impedance the EQ presents to the transformer and to the driving source. I was attempting to ensure this never fell below 4K7 for a 10K:10K tranformer (and another reason why bands shold not overlap). If your sources can drive 600 ohms then you could reduce the  4K7 resistors and use a 600:600 transformer at the input, or, if you are going the semiconductor route your op amp unbalancing input circuit will handle this with no problem.
3. You can use individual inductors. Another way to solve the Q problem is to use different inductor values for boost and cut and switch them with the boost/cut switch but that's not really a poor man's EQ ( but that's what I did on the REDD EQ).

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: gemini86 on May 29, 2013, 12:09:46 PM
Thanks Ian. Last question, is the cut supposed to be limited to about 6dB?
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: ruffrecords on May 29, 2013, 02:04:13 PM
Thanks Ian. Last question, is the cut supposed to be limited to about 6dB?

No, It depends on the total resistance of the RLC circuit when the pot is fully off. This is the sum of the pot's off resistance, the inductor resistance and the 470 ohm resistor. The 47K lin hi boost pot and the 4K7 lin hi cut pot form a basic pot divicer with a loss od just over 20dB. You get mid boost by partially shorting out the 47K pot and you get gcut by partially shorting out the 4K7 pot and specific frequencies determined by the LC values. If you short out the 4K7 with a 470 ohm resistor you get another 20dB of attenuation which is the theoretical maximum cut. Unless your inductor resistance is very high you should be able to get more than 6dB cut.

Cheers

Ian
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: gemini86 on May 29, 2013, 04:53:32 PM
Thanks, Ian. Reading through your posts has taught me a ton about passive eq and how it all works. You're a scholar and a gentleman.
Title: Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
Post by: mikeyB on August 09, 2013, 09:29:39 PM
Hi Ian,
I ain't too great on spreadsheets but have just spent the last couple of days getting my head around the capcalc.xls
Do you have a cap/inductor calc spreadsheet for the mid sections?
You may recall, i want to try some different Z networks as i have orders of pots in the 2.2 magnitudes.
I've tried to create a spreadsheet for this, but alas, a working spreadsheet evades me!!
Any help would be much appreciated!!

Thanks - Mike