Poor Man's Pultec EQP1-A Build Support Thread

[Michael Tibes]

Even if there is no improvement, does the sound of the hum change when you rotate the transformer? I'd expect a 'sweet spot' with less hum. I had the problem with a 1073 build (900 modules in a dual 19" case) and couldn't solve it completely, not even with an expensive 'high-end' power transformer and mu-metal sheets from don audio. In that case I had a mic pre and an eq module and in the end I swapped the mic pre with the eq module, so that the filter coil was further away from the power transformer. The mic transformer was shielded and didn't have a problem. You could put the power transformer into a separate case. I hate that, but I hate hum even more. Otherwise the mu metal wrapped around the transformer helped, it just didn't help enough in my particular case. Maybe it would be sufficient in your case.

I don't know if rotating the pcb with the coil is an option (mounting it at 90 degrees?), it might be worth a try as well.

I'm curious how you solve this, I'm about to start building 2 stereo units myself.

Michael

 

[ruffrecords]

Hi Ian,

OK, I swapped the gain make up amps around.  Unfortunately, the hum stays with the channel.

Also pulled the switch and transformer primary wire away from the board, hanging off the back side. No dice. Still got the hum. =(

Cheers,
Chris

OK, so now we know the gain make up amps are innocent and it is not the mains lead. What happens if you short the input at the XLR (short pins 2&3)?

Cheers

ian

 

[fazeka]

I was afraid you were going to say that. 

To be honest, I don't know if the sound of the hum changed. I didn't want to rotate the transformer with the unit on, so I would rotate, turn on, listen, turn off, rotate, turn on, listen, turn off, etc. I did this quite a bit of times, as much as I could given the length of the leads on the transformer would allow me. And all variations in between.

I don't know how I could rotate the PCB with the coil as it has to be in that position to allow the rotary switches to enter through the front panel. Unless I am misunderstanding?

Only thing I can think of at this point like you say is to put the mains transformer outside the unit... unless there's something else I'm overlooking/haven't thought of?

 

[fazeka]

Hi Ian,

Nothing. No effect. Hum is still there.

OK, so now we know the gain make up amps are innocent and it is not the mains lead. What happens if you short the input at the XLR (short pins 2&3)?

 

[Michael Tibes]

I didn't realize before that the switches are soldered to the pcb, this makes it hard to move them around. I'd just take out the power transformer, put 2m leads on it and see what happens if it is out of the picture. Should be easy to improvise, you don't need a proper switch and so on. Just add a power lead, make sure the connections are all safely isolated and you're ready to go. Simple but safe.

Michael

 

[fazeka]

OK, I can move the transformer out... I have some diecast Hammond enclosures in my spares...

Question: how do I handle the grounding with moving the trafo out?

Do I:

1. Do not take the ground off the Classic Solo to the external case IEC with the power transformer?
2. Take the ground off the Classic Solo to the external case IEC with the power transformer?
3.  If #2, does that get connected to the external case chassis?

Cheers,
Chris

 

[ruffrecords]

The puzzling thing for me is I don't thing this is H field (magnetic interference). The lo cut is purely RC based, no inductors involved. The resistor increases as the cut increases so it will be more susceptible to E field (electric field). There is an inductor in the hi boost circuit immediately before the low cut but if the inductor was picking up hum then increasing the lo cut should reduce not increase it.

The only other thing it could possibly be is a ground loop in channel 2.  Are there any differences at in the grounding schemes of the two channels?

Cheers

Ian

 

[ruffrecords]

OK, I can move the transformer out... I have some diecast Hammond enclosures in my spares...

Question: how do I handle the grounding with moving the trafo out?

Mains in to Hammond transformer box. Safety earth connected directly to box. You need to continue the chassis/safety earth to the main box so you need a screened cable for the ac from the transformer to the main box. Cable screen connects to Hammond box at transformer end and to chassis at other end as close as you can to where it comes in.

Or are thinking of simply mounting ithe transformer on the outside of the rear panel of the main box and covering it with a hammond box?

Cheers

Ian

 

[fazeka]

Hi Ian,

The puzzling thing for me is I don't thing this is H field (magnetic interference). The lo cut is purely RC based, no inductors involved. The resistor increases as the cut increases so it will be more susceptible to E field (electric field). There is an inductor in the hi boost circuit immediately before the low cut but if the inductor was picking up hum then increasing the lo cut should reduce not increase it.

What you say makes sense, but yeah, I dunno...

The only other thing it could possibly be is a ground loop in channel 2.  Are there any differences at in the grounding schemes of the two channels?

I will check the grounding again/compare to channel 1.

Cheers,
Chris

 

[fazeka]

Or are thinking of simply mounting ithe transformer on the outside of the rear panel of the main box and covering it with a hammond box?

The Antek is too big to mount 90 degrees/sideways (if that is what you meant) to any part of the chassis including the rear panel (only 2RU). I wish I had that as an option.

If I can't find an issue in the grounding on channel 2 and I have to move the transformer out, it's kind of a pain in the a** to do all that work getting the transformer in it's own box and wired up to the mother ship... not too mention that my "Pultec" would then be in two pieces...

Cheers,
Chris

 

[ruffrecords]

@Chris,

I think I have a spare 60mm diameter transformer from one of the early prototypes. Email me your postal address and I will ship it out to you. At least that will give you the freedom to fit it inside the box in any orientation.

Cheers

ian

 

[krabbencutter]

Hi everyone,

I'm currently trying to follow this poor man's pultec schematic (http://www.ianbell.ukfsn.org/pics/PMEQP1-AMKII.png) and set everything up on a breadboard with some fixed capacitors for 100Hz and 10k to just get the basics working. However I'm pretty new to this whole DIY Audio world. I've started from Scratch several times now but it seems I'm missing something (probably something very very basic  :-X ) because my pots are acting and interacting in very weird ways. I'm measuring the Circuit with my presonus Studio26, which Outputs a line Level Signal and the receiving Input is set to Hi-Z. I'm using DDMF Plugindoctor to look at the frequency Response.

Here's a Picture of my breadboard: https://1drv.ms/u/s!Aob75XFkRCIipvw-N1cpjKXE2UG-qw
and because that's Pretty messy to look at, I tried to create a hopefully-not-so-messy Version with fritzing:
https://1drv.ms/u/s!Aob75XFkRCIipvw9k4qelxmOOT15Ig
The pots/circuit setions from left to Right are Hi boost (47k lin, 4700pf, 4k7R), Hi cut (4k7 lin, 0.022uf, 470R)  Lo boost (47k log, 0.068uf) and Lo cut (470k log, 3300pf, 56kR).

After wiring everything up the volume is dropping by 20dB, as is expected. The Hi-Boost and -Cut knobs however are only increasing the whole signal and my low cut is doing nothing at all. The Low-Boost works correctly but only if the high cut is not zero.

Can somebody maybe point me in the right direction?  Let me know if I need to provide any additional information!

 

[ruffrecords]

On your frizting board you seem to have the RC for bot the hi boost and hi cut across the pot rather than form the top to the wiper.

Cheers

Ian

 

[krabbencutter]

(Oops) Thanks a lot, that did the trick! I also had to swap out my 470k pot. They only had one with an additional switch at my local electronics store and it seems that it doesn't play nice with a breadboard, because the switching pins are in the same row as the outer pins. Speaking of "trick", I've attached a screenshot of that famous pultec curve (and some 50Hz hum  : )

Now to get a line output level I'll simply add a TL071 in a non-inverting amplifier configuration and to compensate for the 20dB gain loss I need a ratio of 10, correct?
Regarding the input stage I'm still a bit confused about the bridging transformer, Impedance and load.  I have one of these lying around: http://www.produktinfo.conrad.com/datenblaetter/725000-749999/739643-da-01-en-UEBERTRAGER_1_55V_1500MH.pdf

Would that somehow work? It will probably not have a favorable impact on sound quality. But as I'm still trying to catch up with all the stuff from my physics class 10+ years ago, my main goal here is to get it working and to understand the concepts. I guess a transformer would also help eliminating the ground loop?

 

[ruffrecords]

Here is a link to the recommended TL072 circuits for the PMEQP1A and all my other passive EQs:

https://drive.google.com/open?id=0B_n67A1hN3qtSkg5MW9wRDBWS1k

This and lots of other info is in the PMEQP1A folder in the DIY tab of my web site:

http://www.customtubeconsoles.com/diy

Regarding the transformer you mentioned, looking at its datasheet I do not think it will be suitable. First it is a 5:1 step up transformer and secondly its inductance is far too low to have a decent bass response.  think this transformer is more likely designed for use in a power supply.

What you need is a 10K:10K audio transformer. These are generally not cheap. Good ones from Cinemag, Jensen, Sowter and Lundahl will cost around $75 or more. OEP do some relatively inexpensive ones that are still capable of good performance. Something like their Z21807C would be a good choice.

Alternatively you can use another op amp to unbalance the input.

Cheers

Ian

 

[krabbencutter]

Thanks a lot Ian,

I've built the EQ section today As this is my first project and I have a Eurorack formfactor in mind, I've chosen only two frequencies for each band (30, 100 / 10k, 16k). Instead of Switching potentiometers I'm using simple two-way stereo switches and have also linked the cut/boost frequency for each band. The passive EQ board is working pretty well (if you ignore the horrible ground hum  on the screenshot  ;D) and I will take a look at the input/output stages next.

 

[ruffrecords]

Looks like you are making good progress.

Cheers

Ian

 

[ilfungo]

Hi
Could this schematics work as makeup stage for poor man's pultec?
It's correct?
thanks!

I've a low frequency attenuation on output ( from 100hz go down)....

 

[ruffrecords]

Loss of low frequency usually means the EQ is loaded too much. The circuit you posted should work OK. I assume the pot slide is meant to connect to the top of the 47uF.

Cheers

Ian

 

[ilfungo]

Thanks Ian
yes the pot slide is connected to the top of 47uf cap.
Sorry I'm not an expert :
"Loss of low frequency usually means the EQ is loaded too much" ?
Can you explain what could be?
I'm using a this circuit with Carnhill 2281...
Thanks!