erikb1971

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
We are getting way past relatively easy


gemini86

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.
- Rodney

"...you better call Kenny Loggins, 'cause you're in the danger zone."

erikb1971

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!
We are getting way past relatively easy

dirtyhanfri

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
Working on it...

erikb1971

yeah! "Go Ian, Go Ian, Go Ian"
We are getting way past relatively easy

baadc0de

yeah! "Go Ian, Go Ian, Go Ian"

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

erikb1971

well.. eehhh gemini made it public :-)
We are getting way past relatively easy

dirtyhanfri

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.
Working on it...

baadc0de

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 :)

erikb1971

Of course no pressure! Luckily Ian does not strike me as anyone who would be influenced by pressure.... But silently.. fingers crossed!
We are getting way past relatively easy


dirtyhanfri

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #10 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!
Working on it...

gemini86

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #11 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

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.

- Rodney

"...you better call Kenny Loggins, 'cause you're in the danger zone."

MatthisD

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #12 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?

« Last Edit: October 16, 2011, 10:19:37 AM by MatthisD »

ruffrecords

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #13 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
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

rmaier

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #14 on: October 16, 2011, 08:26:39 PM »
At the risk of stating the obvious, I'd say this sounds like a killer idea!
"Even if you don't know where the destination is of the sounds that you originate, it's worth getting it right at the start." - Rupert Neve

erikb1971

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #15 on: October 17, 2011, 04:13:52 AM »
go Ian, go Ian, go Ian!
We are getting way past relatively easy

MatthisD

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #16 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?

ruffrecords

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #17 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
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

ruffrecords

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #18 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

www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

ruffrecords

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #19 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
« Last Edit: November 09, 2011, 05:45:26 PM by ruffrecords »
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'


 

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