Poor Man's Pultec EQP1-A Build Support Thread

[ruffrecords]

I tried disconnecting the mid boards as you described, and it still is malfunctioning in the same manner. I've triple checked my wiring to my updated Daves dual mid guide, and everything is as listed. I am using stepped Lorlin switches instead of pots. Man this thing is kicking my butt! I will try again tomorrow.

OK, keeping the mid disconnected, set all the controls to off. Feed a 0dBu signal in, and with the VR1 fully clockwise, measure the output level, Let me know the result.

Can you post the schematics of your switched pots?

Cheers

Ian

 

[duantro]

I tried disconnecting the mid boards as you described, and it still is malfunctioning in the same manner. I've triple checked my wiring to my updated Daves dual mid guide, and everything is as listed. I am using stepped Lorlin switches instead of pots. Man this thing is kicking my butt! I will try again tomorrow.

OK, keeping the mid disconnected, set all the controls to off. Feed a 0dBu signal in, and with the VR1 fully clockwise, measure the output level, Let me know the result.

Can you post the schematics of your switched pots?

Cheers
Ian

I will try to check signal voltage tonight. Here is an updated modified dave's wiring diagram, again, AS MINE IS HOOKED UP, with direction, resistance, and type of stepped switches labeled.

Edit: corrected typo on high cuts to 4k7
Edit: corrected labeling of high board connections on v1.3

 

[ruffrecords]

Hi cut should be 4.7K lin not 47K.

Cheers

Ian

 

[mikeyB]

Hi Ian,
You may have seen some of the other pultecs on this forum that have the lo band frequency select switches seperated.
I am thinking of trying this to enable more (12)frequencies for both boost and cut.
i understand that the cut frequencies are an octave above the boost frequencies and this spacing is maintained with the standard 2x6 way switching.
Would i encounter any loading problems if i seperate the switches.
I'm guessing that if the 2 actual frequencies get close to one another, they will just cancel out.
Would there be any loading problems when they are far apart (eg boost set at 30Hz and cut set at 200Hz(400Hz actually))?
I would incorporate this arrangement for the hi end also.

Once again - thanks for this great project

 

[duantro]

Hi cut should be 4.7K lin not 47K.

Cheers

Ian

My high cuts are 4k7 Lin. My mistake, it was a typo from being in a hurry. I have corrected previous image to version 1.3.
When fed a 0dbu (0.755V) 1khz signal, measured at the input xlr jack at pin 2 and 3, the results are about 240mV on output xlr jacks. This is done with vr1 fully clockwise.

 

[ruffrecords]

My high cuts are 4k7 Lin. My mistake, it was a typo from being in a hurry. I have corrected previous image to version 1.2.
When fed a 0dbu (0.755V) 1khz signal, measured at the input xlr jack at pin 2 and 3, the results are about 240mV on output xlr jacks. This is done with vr1 fully clockwise.

The EQ has a nominal loss of 20dB and the tube gain make up has a gain of close to 26dB so with all the EQ controls off and VR1 fully clockwise you should measure a gain of about 6dB which means the output should have been  about 1.5V. The fact you get only 240mV means there is an additional loss of about 14dB somewhere.

Cheers

Ian

 

[ruffrecords]

Hi Ian,
You may have seen some of the other pultecs on this forum that have the lo band frequency select switches seperated.
I am thinking of trying this to enable more (12)frequencies for both boost and cut.
i understand that the cut frequencies are an octave above the boost frequencies and this spacing is maintained with the standard 2x6 way switching.
Would i encounter any loading problems if i seperate the switches.
I'm guessing that if the 2 actual frequencies get close to one another, they will just cancel out.
Would there be any loading problems when they are far apart (eg boost set at 30Hz and cut set at 200Hz(400Hz actually))?
I would incorporate this arrangement for the hi end also.

Once again - thanks for this great project

There should be no loading problems when using this technique for the lo boost/cut because in both cases the boost is achieved by increasing an impedance so loading is if anything reduced. However, the interaction of the lo boost and cut is modified by the 56K resistor. With no lo boost, this resistor is connected to ground (by the boost pot) and, in conjunction with the 470K pot, determines the amount of lo cut that can be achieved. As the lo boost is increased, this 56K becomes increasingly disconnected from ground and more connected across the 570L low cut pot. This has the effect of changing both reducing the amount of cut and the frequency at which it occurs. The result is the well known Pultec 'bump' where you get an initial cut of about 6dB then as the frequency reduces further this cut is replace by a boost at a lower frequency. The turnover frequencies being an octave apart are also instrumental in achieving the bump. So, if you separate the lo boost and cut frequencies you will alter this interaction so, even if the two frequencies are the same, they may not cancel out. If you remove the 56K resistor then the lo boost and cut are independent and should act as you would expect.

The hi boost and cut work by reducing impedance so when you apply both, the loading on the input transformer increases. This is mitigated a bit by the frequencies being an octave apart in the original design. If you make them identical then maximum loading will occur at the highest frequencies. The loading is limited by the 4K7 and 470 ohm series resistors connected to the wipers of these pots. In the worst case the loading should be no less than 5K which is simply reflected by the input transformer to the input of the EQ. As long as your source is happy driving 5K it should not be a problem.

Cheers

Ian

 

[dirtyhanfri]

Mine is working nicely.

The issue was VR1, was 4k7 instead 470k. Now it works as expected. I've been making sweep tests and he curves looks juat similar to the ones in your graphics.

Also noise is so low, -103dbfs in one channel, didn't measured the other

 

[ruffrecords]

Mine is working nicely.

The issue was VR1, was 4k7 instead 470k. Now it works as expected. I've been making sweep tests and he curves looks just similar to the ones in your graphics.

Also noise is so low, -103dbfs in one channel, didn't measured the other

Is that noise measurement using my tube gain make up stage?

Cheers

Ian

 

[dirtyhanfri]

Yes, @100 Hz. But measured with the input and output connected to the converters using voxengo Span. It was quieter @50 Hz. Maybe it's not the most accurate measurement.

I forgot to say Thanks again for this great project Ian.

 

[mus1k_freak]

Heres my work in progress a poor mans pultec....pedal  8)

 

[duantro]

My high cuts are 4k7 Lin. My mistake, it was a typo from being in a hurry. I have corrected previous image to version 1.2.
When fed a 0dbu (0.755V) 1khz signal, measured at the input xlr jack at pin 2 and 3, the results are about 240mV on output xlr jacks. This is done with vr1 fully clockwise.

The EQ has a nominal loss of 20dB and the tube gain make up has a gain of close to 26dB so with all the EQ controls off and VR1 fully clockwise you should measure a gain of about 6dB which means the output should have been  about 1.5V. The fact you get only 240mV means there is an additional loss of about 14dB somewhere.

Cheers

Ian

I have found that I have missed the brown wire , on the hook up guide, that connects the High cut ccw and Low boost cw pins. After connecting it I'm getting around .650V on output. I'm not quite up to the correct output level here, and my Low boost and the rest of the cuts still don't work. It's got to be something on my Low boost or High cut boards or connections that are incorrect. I've been tracing the .755V thru the circuit and can't decipher where the signal should show up on these sections (Low boost and High cuts). any suggestions as to where I could check for this?

 

[dirtyhanfri]

Here is my finished unit, It needs the front pannel, but I'm deciding on what to do, I'd like to engrave it

 

[duantro]

I'm assuming that this switch diagram is the current one? I found it in the drawing board thread.
I'm getting signal to the High boost side (T1 and T2 side), but nothing on the High cut side (T3 and T4).
Where should the signal be arriving to the High cut side from the High boost side?
I have R1 as 4k7, R2 as 430r, nothing on R3, L1 jumpered, and no dotted line jumper.

 

[wave]

Here is my finished unit, It needs the front pannel, but I'm deciding on what to do, I'd like to engrave it

Looks great Hanfri! Hey did you etch your PSU PCB from the file that I drew up? How is it working for you? Can you post a complete shot of it?

Thanks!
Dave

 

[dirtyhanfri]

Looks great Hanfri! Hey did you etch your PSU PCB from the file that I drew up? How is it working for you? Can you post a complete shot of it?

Thanks!
Dave

Yes, I use your layout, thanks!

I'll take some closer pics tomorrow.

 

[adeptusmajor]

Heres my work in progress a poor mans pultec....pedal  8)

That's awesome, but I would probably stomp that tube into oblivion before I got halfway through a set in a dark club.

 

[ruffrecords]

I'm assuming that this switch diagram is the current one? I found it in the drawing board thread.
I'm getting signal to the High boost side (T1 and T2 side), but nothing on the High cut side (T3 and T4).
Where should the signal be arriving to the High cut side from the High boost side?
I have R1 as 4k7, R2 as 430r, nothing on R3, L1 jumpered, and no dotted line jumper.

This is the diagram that shows how the hi and lo boards are wired together:

As you can see the input signal goes to T1 on the hi PCB (boost) and to the hi boost pot. This pot forms a pot divider with the low cut pot so the signal at the junction if the hi boost and hi cut pots should be about 20dB down at high frequencies because one is a 47K pot and the other is a 4K7 pot. The low boost pot and the lo boost caps are also in series with the lo cut pot BUT at high frequencies these caps are short circuits so the low boost pot does not affect the high frequencies.

As you turn up the hi boost, the caps between T1 and T2 start to short out the hi boost pot so the attenuation is reduced. At very high frequencies the caps are a short circuit so the 47K pot has the 4K7 R1 in parallel with it. This with the 4K7 of the hi cut leads to a 6dB loss instead of the nominal 20dB so this gives a 14dB boost. As you turn up the hi cut, the caps between T3 and T4 start to short out the hi cut pot thus increasing the attenuation. At very high frequencies these caps are short circuits so the 4Ky hi cut pot now as the 470R R2 in parallel with it so the total attenuation is nearly 40dB making a cut of 20dB relative to the nominal 20dB loss. If you turn the lo boost and cut pots fully anti clockwise you should be able to see the above effects happening.

Cheers

Ian

 

[JohnWatkins]

Hi Guys,  I'm starting to acquire parts to build a couple of these, so now might be as good a time as any to start with the noob questions.  Apologies if these have been covered already and I missed it in my reading.

1)  The BOM lists R1 as being a fixed resistor with a linear taper.  Is this a typo or an animal I'm not familiar with?

2)  My tube makeup PCBs are in the mail right now, but I'd like to get the tube sockets ordered ahead of time.  Will any 9-pin miniature PC mount socket fit?

Thanks in advance for any help.

John

EDIT:  Never mind on both counts.  Found the answers earlier in the thread.

 

[duantro]

Thanks for the description Ian. I've sent .775V 1k signal in, that measures .680V at bypass switch and on CCW of High boost pot/switch. At CW pin of High boost switch (the divider between the High boost and cut that should be -20db lower) my signal is 26mv. Am I correct that this isn't what I should be seeing? I apologize, I'm a horrible trouble shooter.