Poor Man's Pultec EQP1-A Build Support Thread

[pacorro]

Hi from Spain
I'm a newbie and I'm trying to build a stereo poor man's Pultec, basic version: no mids, no psu, no make-up. Just low and high 100% passive.

I have a couple of doubts. I'm doing some researches here, and reading a lot, but it's 20 pages and it's not easy to find what I'm looking for.

Firstly, I built the 4 pcb's and I don't know if I have to do something with the L1 position. Have I make a bridge, or leave it free?

Secondly, I took a wiring's guide from the info page and customize to my purposes. I've been comparing it with the schematic and have a doubt about the ground.
Attached is my wiring guide and the schematic.

Have I put a wire from the low boost to the ground? (light green wire)

As I said, I'm a newbie, so take it into consideration.

Thanks in advance.

 

[pacorro]

Here is a snap of the Hi (left) and Lo (right) switch assemblies complete. What appear to be resistors on the Lo PCB are zero ohm resistors for the links.

Cheers

Ian
http://www.groupdiy.com/index.php?action=dlattach;topic=44213.0;attach=7417;image

???
I was thinking the resistors 1 & 2 was only in the HI pcb leaving the R3 free, while in the LO pcb I must put only the R3 and not the R1 & R2, according to what Ian said in another moment:
"The two resistors, R1 and R1 are only needed for the hi boost and cut and can be wire links or zero ohm resistors for the lo boost/cut. The terminals (pads really) T1 thru' T4 are the only external connections necessary. The only other component not on the PCB right now is the 56K resistor needed for the lo boost/cut."
Have I put a link in R1 and R2 in the low pcb? Have I put a link wire in R3 in the High pcb?
And what's about the L1? Have I put a resistor on it? A link?

Sorry, I'm sure this must had been discussed before. And remember...I'm a newbie.

 

[ruffrecords]

Hi from Spain
I'm a newbie and I'm trying to build a stereo poor man's Pultec, basic version: no mids, no psu, no make-up. Just low and high 100% passive.

I have a couple of doubts. I'm doing some researches here, and reading a lot, but it's 20 pages and it's not easy to find what I'm looking for.

Firstly, I built the 4 pcb's and I don't know if I have to do something with the L1 position. Have I make a bridge, or leave it free?

Secondly, I took a wiring's guide from the info page and customize to my purposes. I've been comparing it with the schematic and have a doubt about the ground.

Hi,

L1 was a late addition for some people who wanted a possibility of a peaking response at one frequency. For the normal PMEQP1A you need to replace it with a wire link.

Your grounding scheme is OK for unbalanced operation.

Cheers

Ian

 

[ruffrecords]

???
I was thinking the resistors 1 & 2 was only in the HI pcb leaving the R3 free, while in the LO pcb I must put only the R3 and not the R1 & R2. Now I'm very very confused.
And what's about the L1? Have I put a resistor on it? Wich value, please?

You fir R1 and R2 only to the HI board. One the low board you need to fit a wire link. I have some zero ohm resistors I use for this so it looks as though I have resistors fitted but they are really short circuits.

For the regular PMEQP1A, L1 needs to be replaced by a wire link.

Hope that helps.

Cheers

ian

 

[pacorro]

thaks a lot, Ian 

 

[pacorro]

Ian... I'm looking for the stuff to build the make up...
I just don't know wich preset pots I need for the VR1 and VR2.
would you give me an example, please?

Is something like this?
http://es.farnell.com/vishay-sfernice/t7ya102mb40/trimmer-pot-1k-1turn-thd/dp/1141645

http://es.farnell.com/vishay-sfernice/t63xb102kt20/trimmer-pot-1k-15turn-thd/dp/1141431

http://es.farnell.com/vishay-sfernice/t7yb474mb40/trimmer-470k/dp/1141671?whydiditmatch=rel_default&matchedProduct=preset+pot+470k&matchedProduct=preset+pot+470k&whydiditmatch=rel_default

http://es.farnell.com/te-connectivity-citec/cb10lv474m/trimmer-top-adjust-470k/dp/1227545


Thanks in advance

 

[pacorro]

Ian... I am trying to avoid the makeup stage. The in/out transformers are 350 euro, plus the psu and so on. I think is very expensive.

My best option is making the Eq 100% passive and going from the output to a couple of instrument inputs of a mic preamp. You said I need at least 200k of input impedance. This is very affordable, since there are very low price IC based preamps.

I found another cheap option, but I'd like you to confirm that it's gonna work...
I can get a Behringer active DI like this
http://www.behringer.com/EN/Products/DI4000.aspx

The input impedance is 220k, and the output impedance is 600 ohms. In this device there is a option of adding 20dB to the input signal, that is just what I'm looking for.
Am I wrong, or the Ultra DI is ok for my purposes?
Wich option do you think is better... the preamp through the DI Inpunts, the DI box or building the makeup stage (although is 10 times more expensive) ?

Thanks

 

[ruffrecords]

Thanks Ian.
I think I understand where the pins for Chrion's inductors are.
Red dot is zero and clockwise is tap 1,2,3, etc with the ground pins on the outside.

I'm confused at the pcb fix. I understand that I don't need to cut any traces. I think I do not need to connect the little corresponding numbers with a jumper.
Am I correct that the letters "S,B,D,E,...on the boards" would have originally mated with "Tap 0,1,2,3..."
If so then the inductor connections 0,1,2,3,... goes to the little numbers in the "fix photo", except 0 =screen, correct? And would it be best to use the number outside of the caps?

The new Screen pad appears to goes to cut/ boost switch.

So....where would the newly labeled "ind" go to?


I hope you can follow all of this, as I'm getting confused just reading it back ???


edit: Chrion discontinued his pcbs so I'll just be soldering wire direct to the pins.

OK, I have had a look at this. Assuming Chrion's inductors start at 0 and the first tap is at 1, the second at 2 and so on, then you need to wire Chrion's inductor as shown in this picture:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/PMEQP1A/midbottomchrion.jpg

The pin labelled 'screen' is for the connection from the metal clips of Chrion's inductors.

Apart from that, all the connections on the PCB retain the exact function as intended in the original design.

Hope that helps.

Cheers

Ian

 

[ruffrecords]

Ian... I am trying to avoid the makeup stage. The in/out transformers are 350 euro, plus the psu and so on. I think is very expensive.

My best option is making the Eq 100% passive and going from the output to a couple of instrument inputs of a mic preamp. You said I need at least 200k of input impedance. This is very affordable, since there are very low price IC based preamps.

I found another cheap option, but I'd like you to confirm that it's gonna work...
I can get a Behringer active DI like this
http://www.behringer.com/EN/Products/DI4000.aspx

The input impedance is 220k, and the output impedance is 600 ohms. In this device there is a option of adding 20dB to the input signal, that is just what I'm looking for.
Am I wrong, or the Ultra DI is ok for my purposes?
Wich option do you think is better... the preamp through the DI Inpunts, the DI box or building the makeup stage (although is 10 times more expensive) ?

Thanks

You do need a gain make up stage but it does not have to be my tube design or use the expensive transformers. Many people have built versions with op amp input buffers and op amp balanced output drivers that are much cheaper that the tube plus transformers option. I think someone here even offered a PCB for this or there are several commercial PCBs you could use.

You can use an external amplifier provided it has a high enough input impedance and you use short screened cables from the EQ output to the make up amp input. Either the DI inputs of a preamp or the Behringer should work fine.

The best option is to have the make up with the EQ and you should be able to do this cheaply with op amps.

Cheers

Ian

 

[ruffrecords]

Ian... I'm looking for the stuff to build the make up...
I just don't know wich preset pots I need for the VR1 and VR2.
would you give me an example, please?

Is something like this?
http://es.farnell.com/vishay-sfernice/t7ya102mb40/trimmer-pot-1k-1turn-thd/dp/1141645

http://es.farnell.com/vishay-sfernice/t63xb102kt20/trimmer-pot-1k-15turn-thd/dp/1141431

http://es.farnell.com/vishay-sfernice/t7yb474mb40/trimmer-470k/dp/1141671?whydiditmatch=rel_default&matchedProduct=preset+pot+470k&matchedProduct=preset+pot+470k&whydiditmatch=rel_default

http://es.farnell.com/te-connectivity-citec/cb10lv474m/trimmer-top-adjust-470k/dp/1227545


Thanks in advance

Any one of the first three pots will be fine but I would go for the second one because it is a multi-turn pot and it will allow you to make more accurate adjustments. The last one appears to be too large to fit the PCB.

Cheers

Ian

 

[duantro]

Thanks Ian.
I think I understand where the pins for Chrion's inductors are.
Red dot is zero and clockwise is tap 1,2,3, etc with the ground pins on the outside.

I'm confused at the pcb fix. I understand that I don't need to cut any traces. I think I do not need to connect the little corresponding numbers with a jumper.
Am I correct that the letters "S,B,D,E,...on the boards" would have originally mated with "Tap 0,1,2,3..."
If so then the inductor connections 0,1,2,3,... goes to the little numbers in the "fix photo", except 0 =screen, correct? And would it be best to use the number outside of the caps?

The new Screen pad appears to goes to cut/ boost switch.

So....where would the newly labeled "ind" go to?


I hope you can follow all of this, as I'm getting confused just reading it back ???


edit: Chrion discontinued his pcbs so I'll just be soldering wire direct to the pins.

OK, I have had a look at this. Assuming Chrion's inductors start at 0 and the first tap is at 1, the second at 2 and so on, then you need to wire Chrion's inductor as shown in this picture:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/PMEQP1A/midbottomchrion.jpg

The pin labelled 'screen' is for the connection from the metal clips of Chrion's inductors.

Apart from that, all the connections on the PCB retain the exact function as intended in the original design.

Hope that helps.

Cheers

Ian

Thanks for looking at it Ian.
I've included the pinout that Chrion has posted in his thread, for visual reference.
I think I understand it know.
1) S (screen) on the pm mid boards, go to the ground pins on Chrion's boards in his drawing.
2) IND on your boards goes to the cut/ boost switches.

 

[ruffrecords]

That looks right to me.

Cheers

Ian

 

[pacorro]

I've just finished my two Pultecs, and they're working perfectly!!

I've taken some graphs.
Ian, this thing is AMAZING!!

Signal passing through the circuit:

Low 20hz

High 3khz

Hi 16k

The signal falls over 10dB, so I really think makeup is not needed when coming from the daw at line level

I bought the Behringer DI and works perfect. Just adds a few low level harmonics, being the third one the greater. In this pic I sent a 1khz sine through the Eq. At the right side of the sine you'll see the harmonic distortion added by the makeup.

The only thing that I'm worried about is some noise in the low field (50hz) that comes from the circuit.
This graph shows the noise. In this plot there is not any makeup in the signal, just the Eq.

Did I made something wrong, or this is normal?
I don't think this will be a problem. Some devices like the La2a or 1176 shows the same kind of low noise and this is not audible. Just wondering why it happens.

Great job, Ian. I think I'll build a few more Pultecs. It was only 92 euro for 2 channels + the enclosure. Ridiculous price for that amazing Eq.
In the next days I'll make a extensive review.

 

[ruffrecords]

Well done! That was a quick build.

You seem to have a 6dB loss at 20Hz when just passing signal. I am not sure what is causing this because with all the controls set to minimum there are just resistors in the circuit.

The noise you are seeing looks like 50Hz hum and its harmonics. The output of the EQ is relatively high impedance and hence is susceptible to picking up hum. However, if you mount the EQ in a metal enclosure and use a good quality screened cable at the output and input then there should be very little hum pick up.

Cheers

Ian

 

[pacorro]

ah, ok. Yes, in this test I was using a 3m cable from the output to the AD converter. This may be causing the hum.
I have the same fall at 20hz in both channels, so maybe I did something wrong. I'm not worried about it. Some classic gear shows this kind of frequency response just passing the signal throug it.

Thanks man. I'll buy more pcb's soon!!

 

[pacorro]

Look at this beauty

This was my first project ever, and I'm very satisfied.

Here is a step-by-step guide (in spanish)
http://www.audiopeludo.com/index.php?ind=blog&op=home&idu=3&singlepost=549

 

[ruffrecords]

Excellent blog!! It loses something when Google translates it into English but otherwise it is a very good introduction for beginners. Well done.

I want to set up a gallery of pictures of my projects that have been built by other people. Could you send me some pics to my email address??

Cheers

Ian

 

[pacorro]

Ian, I did a deep test and I have some questions for you.

Firstly, the Low cut is very progressive and smooth in the first 3/4 of the pot. But once you reach the finals steps the cut becomes very hard, just like a HPF. Is this because the Lo cut pot is log? Wouldn't be better to put a linear pot on it?

The real frequencies never match with the references.
For instance, there is a cut in 20hz.

The cut comes until 800hz, that is a lot for a low cut, even at smooth settings.

When I did a boost in 20hz I've found a peak at 150hz.

Don't misundestand me... The Eq sounds great and I'm very happy with it. I'm just wondering why those results. The Highs are lower than expected, and the lows are higher than expected. If I set 10k I get 2k, if I set 20hz I get 150hz... I guess this is the Pultec style, correct me if I'm wrong.
Maybe I did something wrong in my building? I was very carefully. Some of you are getting similar results?
Cheers

 

[pacorro]

ah, ok, I saw your freq response graphs and are similar to mine, so it's the natural response of the Eq and not a problem in my device.
Sorry 

 

[mylesgm]

nice build pacorro!  I've got my pcbs set aside for a build next year, do you mid sharing the faceplate and where you got it made?