Check out this FX loop

[bancika]

Slightly OT, but I made this schematic for opamp FX loop I plan to put into my next tube amp.

What do you think about it, is there something horribly wrong, cause I used various different schematics and data sheets to assemble it.
I'm still on the fence about using that gain stage at all because maybe I'll have enough juice to drive EL91's with phase inverter only
Tnx

[Meanderthal]

 Looks fine to me except 2 puzzlers-the send/return jacks,(I think you got the tip/ring mixed up, or mean switching jacks or something) and the photoresistor..?

If the gain worries you, you can always replace the 10k and/ or 1k resistor with a pot...

[dschwartz]

yeah, i strongly recommend u to put a send level and return level, that way you can adjust the gain at your pace.

one thing i dont undestand.. why is that LDR at the output? a mute function or what??

[db]

You need to provide a ground reference for the + input on the return op-amp otherwise it'll be floating around quite merrily.

Also, I would be a little careful about the 47nF on the send.  It will be OK driving an impedance of 100K or so, but if much less than this you may lose some bottom end.  It's probably OK, but I would use a 470nF just to cover your bases (no pun intended).

[bancika]

oh yeah, taken out of context that optocoupler doesn't mean anything. I'll have two separate channels with dedicated FX loops so optos are just for switching channels. Should I reference + input on return side with 1M resistor?
CircuitMaker doesn't have switching jacks so I used stereo...in reality it's switching mono jack.
How should I implement level trimmer, like voltage divider between ground and output with wiper connected to - input? Which value should I use?

Edit, if someone is curious, here's schematic for the rest of amp



Thanks

[d95err]

Some notes:

You probably want some kind of trimmer for the gain of the return stage too. That way, you could easily adjust it for line- or stompbox level signals in the loop.

If the signal at the input, or return goes past 15V peak, the opamps could be damaged (depending on the type of opamp). This could happen particularly at the input if the trimmer is set too high (unless the rest of your design is built to ensure the signal can never reach past 15V peak there). To protect opamps in these cases, limiting diodes are often used, like in this loop design:
http://forum.musikding.de/gallery/displayimage.php?pid=449&fullsize=1
Note: These diodes are easily mistaken for regular clipping diodes. However, they only clip if the signal voltage comes within the forward voltage of the diode (e.g. 0.6V) of the supply.

I've been thinking about building something similar into my tube amp (or the next build...). In that case, I'd try to use trimmers adjustable from the inside of the send/return jacks, like the "Turbo Jax" thing or whatever it's called. (Can't find it now. Anyway its an add-on FET booster for tube amps that is built into the input jack, with a trimmer mounted on the inside of the jack so you can adjust it from the outside with a screwdriver).

[bancika]

so that's why these diodes are for...handy. I saw that schematic and didn't know what they do. I'll definitely add them. Do I need similar thing at the input because with input trimmer set high voltage can get way over 15V.
Tnx

[d95err]

so that's why these diodes are for...handy. I saw that schematic and didn't know what they do. I'll definitely add them. Do I need similar thing at the input because with input trimmer set high voltage can get way over 15V.
Tnx

If you can find out what the maximum amplitude can be at the input of the fx loop (measure the circuit or use a simulator). Then you could add fixed resistors to ensure that it will always be below 15V. Then you could add the trimmer to adjust downwards from there. Or,  add similar limiting diodes at the input too.

[bancika]

I have updated schematic with, please check it out
www.storm-software.co.yu/temp/newamp.pdf