Trim output for EQP-1A amp?

Hi!
Just build p2p pultec based on EQP-1A filter and amp. Sound amazing!
But I found that it can be helpful to have a trim output for compensating volume of EQ'ed signal and unEQed .
EQP-1A amp have a constant gain of about 22dB. I think that changing it's gain in a range of 15-30dB would be super cool!
I could imagine that it's possible to change couple resistors with pots or rotary switches that will control it's gain, not?
Any ideas?)

One way to ensure the gain remains the same with the EQ in or out is to use a separate attenuator to bypass the EQ and set it to have the same loss as the the EQ. Here is a drawing of how it is done in the poor man's EQP1A:

https://drive.google.com/open?id=0B_n67A1hN3qtMGJEM1I3aDJ5UEk&authuser=0

Cheers

Ian

ruffrecords said:

One way to ensure the gain remains the same with the EQ in or out is to use a separate attenuator to bypass the EQ and set it to have the same loss as the the EQ.
Ian

Click to expand...

Ian is always first

Well, looks like method that you describe actual for ACTIVE bypass (signal not goes through EQ but still goes through amp as it in standard EQP-1A sch)
I forgot to mention, I would use relay bypass that will bypass ALL unit. So it's not necessary to compensate gain in active bypass mode.
As pultec is an EQ with which you are crank low and bottom frequencies more often then attenuate , so EQed signal would be slightly louder the unEQed and for trimming EQed signal it's necessary to attenuate gain of output amp.

May be it's possible to insert pot before interstage transformer of the amp for attenuating a signal? See image1.
But for my opinion it's not the best idea, because you contribute to degrade a signal that goes to the amp with fixed gain.
Better to control gain IN the amp.

If you have built an EQP1A based on the original schematics with its built in EQ IN/OUT switch then the gain with the EQ out should be exactly the same as with the EQ in and all the controls flat. In that case there is no need to alter the amp gain unless the overall gain is not unity under these two conditions. If it is not then probably the simplest way to tweak the gain over a small range is to alter the amplifier's  feedback resistor.

Cheers

Ian

ruffrecords said:

If you have built an EQP1A based on the original schematics with its built in EQ IN/OUT switch then the gain with the EQ out should be exactly the same as with the EQ in and all the controls flat. In that case there is no need to alter the amp gain unless the overall gain is not unity under these two conditions. If it is not then probably the simplest way to tweak the gain over a small range is to alter the amplifier's  feedback resistor.
Ian

Click to expand...

Yes, gain is same in EQ IN/OUT.

feedback resistors - two 360Ohm?

dirty1_1garry said:

ruffrecords said:

If you have built an EQP1A based on the original schematics with its built in EQ IN/OUT switch then the gain with the EQ out should be exactly the same as with the EQ in and all the controls flat. In that case there is no need to alter the amp gain unless the overall gain is not unity under these two conditions. If it is not then probably the simplest way to tweak the gain over a small range is to alter the amplifier's  feedback resistor.
Ian

Click to expand...

Yes, gain is same in EQ IN/OUT.

feedback resistors - two 360Ohm?

Click to expand...

Those are the ones but they are a bit tricky to adjust as they also are involved in setting the dc bias for the first stage. What gain are you getting from in to out with the EQ switched out?

Cheers

Ian

ruffrecords said:

Those are the ones but they are a bit tricky to adjust as they also are involved in setting the dc bias for the first stage. What gain are you getting from in to out with the EQ switched out?

Cheers

Ian

Click to expand...

Gain meassure:
I send 1k tone signal with zero level in pro tools to pultec. My DA converter is set to +12dBu. Then I bring signal back from pultec to AD convertor with same settings equal +12dBu level. As result I get a difference of -3dB at OUT position or at IN position with zero boost or attenuate on both bands. So my pultec "eat" 3dB. I don't think that it's so bad, because turning on this unit mean that I will boost a lot and that it will bring a signal up.

What transformers are you using?

MatthisD said:

What transformers are you using?

Click to expand...

I used sowter as input, interstage and output. Hammond for power. Orange Drops as filter caps.

dirty1_1garry said:

I used sowter as input, interstage and output. Hammond for power. Orange Drops as filter caps.

Click to expand...

See here, 1.85dBu insertion loss with 600ohm load.
http://www.sowter.co.uk/specs/3603.htm

The output transformer may also have a little more loss than the Peerless would have so that could explain the problem or part of it.

MatthisD said:

The output transformer may also have a little more loss than the Peerless would have so that could explain the problem or part of it.

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Gain loss is not a problem, I just want to know how it's possible to control the average gain of amp for attenuating the output signal when boosting hard))

dirty1_1garry said:

MatthisD said:

The output transformer may also have a little more loss than the Peerless would have so that could explain the problem or part of it.

Click to expand...

Gain loss is not a problem, I just want to know how it's possible to control the average gain of amp for attenuating the output signal when boosting hard))

Click to expand...

If that is all you want to do then the simplest solution is to add a balanced attenuator at the output.

Cheers

Ian

ruffrecords said:

If that is all you want to do then the simplest solution is to add a balanced attenuator at the output.
Ian

Click to expand...

That was my first idea, but then I thought... may be it's possible to control the gain of amp?)) Well may be output attenuator is the best solution without changing amp if that's not so simple.

dirty1_1garry said:

ruffrecords said:

If that is all you want to do then the simplest solution is to add a balanced attenuator at the output.
Ian

Click to expand...

That was my first idea, but then I thought... may be it's possible to control the gain of amp?)) Well may be output attenuator is the best solution without changing amp if that's not so simple.

Click to expand...

Changing the amp gain is not easy. A balanced attenuator at the input would probably be best although I am not sure why you don'y just turn down the level feeding it if you are worried about boosted levels being too high.

Cheers

Ian

What Hammond Power Trafo do you use?

Cheers

Bassniac said:

What Hammond Power Trafo do you use?

Cheers

Click to expand...

Hammond 370FX
I find it in Drip BOM. But I can say that it's too large, it's possible to use smaller one with less amperage.