BTDR-3H Reverb Circuit - Volume Stability

[mimmotronics]

I'm building a reverb based on the 1776 effects BTDR-3H Rub-a-Dub Deluxe circuit. I've made some changes and it sounds great. I'm testing the volume now...the problem is the output is at unity only when my guitar is at full volume. As I roll down my guitar's volume knob the output of the circuit falls dramatically. I have to constantly adjust the volume knob to bring the circuit's output back to unity with the bypass signal.

Essentially, I'm trying to build this so that if the effect's volume knob is at 12:00 the effect output is at or near unity with the bypassed signal...any ideas?

Also, I forgot to label R5 in the schematic before taking the picture, it is 10k.

Schematic:

http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/BTDR-3H+Circuit.JPG.html

[mimmotronics]

I've figured most of the problem. The schematic here vs. before shows what I changed. The output is much more closer to unity now! There's still a slight adjust on the effect volume knob (maybe an eighth to quarter turn's worth) I need to make in order to compensate for higher/lower guitar volume. I suspect that the way the mixing stage is implemented might be the problem for the need to volume adjust..any thoughts?

[samhay]

The input impedance is 10k. This is very low for a circuit designed to be driven directly by a passive guitar pickup.
Also, as you adjust the guitar volume, this will change the impedance the circuit sees. Using an inverting op-amp as input stage, the gain of this first stage  will vary with series input impedance...

What happens if you put another effect between guitar and reverb?

[mimmotronics]

The input impedance is 10k. This is very low for a circuit designed to be driven directly by a passive guitar pickup.
Also, as you adjust the guitar volume, this will change the impedance the circuit sees. Using an inverting op-amp as input stage, the gain of this first stage  will vary with series input impedance...

What happens if you put another effect between guitar and reverb?

It works much better thanks to your observation!

I was under the impression that the 1M before the input cap served not only as a static discharge path, but also as the input impedance for the signal...cool to learn that's not entirely the case.

I hooked up an SD-1 between guitar and reverb and the signal passed through quite nicely, I didn't at all have to adjust the effect's level pot to compensate for a change in guitar volume. I figure that the 1k output impedance of the SD-1 (by the logic above..is this correct?) is much less than the output impedance of my guitar, so the signal transfer (in relation to a change in the guitar's output impedance via the volume knob) is better because of that.

What I did was replace the 10k with a 220k resistor and the 56k with a 560k (I decreased the gain of the first op amp stage a bit). Sounds great now, thanks samhay!

[PRR]

> impression that the 1M before the input cap ....... input impedance for the signal...

"Inverting Opamp". Your R2 is, as said, the input impedance (parallel with your 1Meg, but no real difference). Under NFB, the impedance at the "-" input is "zero". (<10r for any decent opamp over the audio band.)

Yes, 220K is a more suitable value for guitar. However the 220K in series with the source adds a non-negligible hiss source.

The usual way to take a low-output high-impedance transducer (g-pickup) is the Non-Inverting amp configuration.

[samhay]

Happy to help and I too would use a non-inverting input stage as Paul suggests.