2520 makeup stage on a P2P EQP1-a3

[kishibashi]

I've read every thread I could find so far on here regarding makeup gain stage on the solid state pultec. I think I've read myself in circles at this point. It's my understanding that I can use a 325 card for the makeup gain. Does this have the xfmr on it or without as I know I can get the sowter input and output as well. Little confused. I also saw Jeff at capi has a booster board with the 325 circuit which is perfect as I want a stereo unit... do I tap off of that into the sowter output?
Sorry for the likely stupid questions

 

[abechap024]

Well you can think of it as 2 seperate pieces of gear....

There is your passive pultec, you could use it without any makeup gain (a few old eqs like that) but be sure to strap 600ohm resistors across the input and output, because it is passive, the frequencies depend on what impedance the circuit runs at....
It was designed for 600 ohm termination so remeber that as you build on your make up stage. (just keep a 600ohm resistor strapped across the output and input)

So then you have your makeup stage, you want the gain to be reasonable.....I forget how much the pultec circuit knocks down the level, but whatever that value is, just set the gain of the opamp for that....its just a line amp, nothing fancy.

And if your using a 2520, you can set the impedance of the circuit lower than if you were using say a ne5534.

But I know there are a lot of wiring diagrams around, that take all the experimentation out of it, but at least you know it will work!

 

[mitsos]

The original solid state EQP-1A used a 2520 for makeup and an HS56 for output trafo. Our 500 version, the EQP-1S5, works exactly like this.  See attached schemo, it's not mine, it's been floating around here for years.  You can use any 312/325 type preamp card for this, as well as pretty much any "compatible" output transformer. 

 

[abechap024]

Ok, perfect thats the one I've seen before too. and I guess only the input has to be loaded with 600R to preform properly.

 

[mg73]

I´m getting approx 4db over unity gain with this cuircuit.
Using A20 trafo´s wired similar to the HS56V.
Is that normal?

 

[Harpo]

I´m getting approx 4db over unity gain with this cuircuit.
Using A20 trafo´s wired similar to the HS56V.
Is that normal?

So adjust to the 4dB lesser gain needed.
Loss factor for 4dB is power(10;4/20)=1.585.
Decrease feedback resistor 20K5 in this non-inverting gain stage 1+20500/1470 to about 12K4
or place a maybe 50K trimmer in parallel to this 20K5, initial value about centered (31K4) and dial in your needed target value to archieve unity gain.

 

[mg73]

So adjust to the 4dB lesser gain needed.
Loss factor for 4dB is power(10;4/20)=1.585.
Decrease feedback resistor 20K5 in this non-inverting gain stage 1+20500/1470 to about 12K4
or place a maybe 50K trimmer in parallel to this 20K5, initial value about centered (31K4) and dial in your needed target value to archieve unity gain.

Thanks,yes I know the different procedure to lower the gain
but I was more wondering about the original SS pultec schematic that seems to give an extra of 4-5db than untiy gain...

 

[thomasdf]

Anybody knows what that's cap's value right next to the 100K resistor, between 2520 "+" and ground ? I can't read or understand what it is and found no clue in the API documents.

 

[thomasdf]

Mmh, it may be a 10 MMF / 10 pico Farad cap ...

 

[Rob Flinn]

I've done this using a jensen JE123 for the o/p transformer. Sounds very nice.

 

[thomasdf]

I plan to use Altec octal transformers for in and out.

 

[hageir]

Quick question, could this be used with a Langevin 251a? (I was thinking a AM10 DOA to keep it "in the family" hehe)

 

[thomasdf]

If it's just a makeup amp, I can't see why not ? You just need the impedance to match, and you're all set.

 

[jokeramik]

Why do you use an output capacitor with a symmetrically fed OPA with DC voltage coupled back?

 

[emrr]

Quick question, could this be used with a Langevin 251a? (I was thinking a AM10 DOA to keep it "in the family" hehe)

990 is the “ideal” op amp there.