pt-80 delay idea/mod on SA571 compander.

[Levispeights]

I'm going to try to keep this short.

What I would like to do:
Increase the gain/volume of the delayed signal over the dry signal on my pt-80 delay. Currently, when the 'delay level' knob is maxed, the first delay note is the same volume as the dry note. I would prefer for the delay signal to be as loud as the dry signal when the 'delay level' knob is only at 50%. That way I could get the delay signal to be even louder then the dry signal.

How to do it:
One idea I had was to put a simple 'gain stage' after pins 6 and 7 of the SA571 compander and just before the delay level pot. However I would like to add as little to the circuit as possible.

Another idea, but I can't figure out how to do it, is to increase the output gain of the SA571 compander. I have looked at the datasheet and the application notes and there is an op-amp in there, i just don't know how to make it give me more gain. Can you set the gain of the op-amp in the SA571 compander or is it always unity gain?

Any help anyone could offer would be fantastic! Thanks in advance!
Levi

[slacker]

Probably the easiest way is to make the 10k resistor between the delay level and pin 2 of the TL072 smaller. Together with the 10k between pins 2 and 1 this sets the gain for the delayed signal, stock it's a gain of 1 (10k/10K) if you make it 5k then you'll have a gain of 2 (10k/5k) so 50% on the level pot will be the same as 100% now, 100% will be louder. This is going from the GGG schematic.

[Levispeights]

Of course!!! I feel very stupid right now. Slacker, you sir, are a gentleman and a scholar.

Just out of curiosity now, IS there a way to increase the gain of the op-amp in the SA571? I know it's internally set up as an inverting op-amp and on the pt-80 it looks like it's set up as a buffer to me.

[R O Tiree]

To raise the output of the expander section of the PT-80 circuit, you need to connect a resistor between pins 6 and 7.  Try 8k2 initially for a 6dB boost.  If that's not enough try 15k, which will get you a boost of about 10dB.  The downside is that the noise will also be louder by the same amount.

IMPORTANT! If you do this, you will also have to change the value of the resistor between pin 5 and GND:

8k2 from pin 6-7, 51k from pin 5-GND
15k from pin 6-7, 100k from pin 5-GND

The way the circuit is set up with that 10k from pin 5-GND, the output from pin 7 is biased at 6.6V (which is a bit weird...).  If you insert an 8k2 between pins 6-7, you will bias the output at nearly 9V!!  That's gonna clip like a bastid, if it even works at all - the opamp inside is basically a 741, so not the best.  Raising that pin 5 resistor puts the bias back (where it should be?) at around 4.5V.

If you need to try other values, give me a shout.

Of course, the other problem is that the more repeats you have, the louder the signal going back around to the PT2399 input.  Given that the limit (from the spec sheet) is 2V RMS, you're going to overload it very fast = more clipping, and not the "good" kind.

[Levispeights]

Ah! OK, now I see. I did try putting a resistor between pin 6 and 7 like you would do to a regular op-amp but I never messed around with the 10K coming off of pin 5. Thanks so much! I have been trying to wrap my mind around how the 571 works and I think a lightbulb just popped on! Thanks a ton!