Need help with FX Loop master and switchable master volumes

[Gaz]

Jana, you're right, I didn't remember the two masters would be in parallel! Regardless of which arrangement I end up with, would you be able to give me some guidance on setting up the loop for a sensible voltage output? What should I be aiming for max? Thanks.

[Jana]

Disclaimer: I have just started my second cup of coffee and am working hard at getting brain cell number 2 online.

You would want a 100 to 150 mv level for guitar pedal type effects. I *think* you would want a 1 volt level for line level effects.

In response to one of your previous questions about clipping in the effects driver tube--I don't think you would want to do that. If you do, then you may never be able to get a clean sound with the clean channel.

(Hopefully others with more experience with loops can chime in. I don't put loops in my amps and haven't messed with them for quite a long time).

[d95err]

An idea is to have separate fx loops for each channel. Two parallell send jacks from the buffer stage. Then return jack->master volume->switch for each channel, then a passive mixer stage followed by the return gain stage.

You'd need two separate delay boxes if you need delay on both channels though.

[Gaz]

Okay, I think I've got with thanks to Jana. First off, I made a mistake initially, and accidentally had the driver wired up like I did before (AC coupled cathode follower), and so my voltage was still way too high.

I wired it up like Jana's drawing, and was able to get it down to a reasonable level by just using the the 1M/1M masters in parallel (so equivalent to a 1M/500K divider).

I already had the send control wired up, but realized that it was pretty useless, because it order to get unity gain, the send level had to be all the way up. Any lower setting would be less than unity, and since there were no longer master volumes after the loop, there was no way to compensate.

So here's where I think I'm being really clever....

In the drawing below I made the loop so that when nothing is plugged in a 220K/220K divider is formed. I compensated with more make up gain so that even with the divider in place there is unity gain through the loop.

Now, when you plug into the loop, the 220K top leg of the divider is removed, and there is just a 220K load on the return side (no audible difference from 470K). This doubles the voltage gain going into the loop. By doing this, unity gain on the Send control is at 12 o'clock, rather than all the way up. Doing this gives some more headroom if the master are dialed back really far, or a line level effect is being used (assuming it has an output control to dial the gain back to unity).

The only downside I can think of is that it's possible to overdrive the PI past unity gain if something is plugged into the loop and the send is turned way up. But hey, maybe it could be used as a cool boost effect...