MatthisD

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #20 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


ruffrecords

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #21 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
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


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

abbey road d enfer

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #22 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.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

ruffrecords

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #23 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
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


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

MatthisD

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #24 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

MatthisD

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #25 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.

ruffrecords

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #26 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
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


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

noulou

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #27 on: October 21, 2011, 07:35:59 AM »
Hey Ian,

Maybe this will help

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


Nikos

ruffrecords

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #28 on: October 21, 2011, 09:12:49 AM »
Hey Ian,

Maybe this will help

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


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

CJ

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #29 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
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html


Michael Tibes

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #30 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!
« Last Edit: October 22, 2011, 07:55:13 AM by Michael Tibes »

ruffrecords

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #31 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
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


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

MatthisD

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #32 on: November 01, 2011, 09:06:25 AM »
cheers Ian, I'll be giving it a shot!

baadc0de

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #33 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?

ruffrecords

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #34 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
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


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

baadc0de

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #35 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! :)

baadc0de

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #36 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?

ruffrecords

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #37 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
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 #38 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
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


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

ChrioN

Re: any ideas for adding a sweepable mid to the Poor Man's Pultec EQP1-A ?
« Reply #39 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
"All PCB traces are curved to emulate the electrical response of point-to-point wiring" -Drip


 

Related Topics

  Subject / Started by Replies Last post
8 Replies
3753 Views
Last post July 07, 2008, 11:56:08 AM
by bodega
388 Replies
75929 Views
Last post August 04, 2016, 11:33:06 AM
by ViciusbeatZ
991 Replies
163298 Views
Last post April 26, 2019, 04:17:05 PM
by ruffrecords
3 Replies
1194 Views
Last post May 27, 2018, 06:48:06 PM
by mylesgm