Design procedure for effects loop voltage output

[Enzo]

I'll try to find a quick example here, but look for some amp with a single loop switch that selects either -10db or +4db. It switches both send and return at the same time, so it doesn;t matter when the loop is empty. In the -10db setting it drops the send to the lowr level and includes the boost at the return. FLip the switch to +4db and it leaves the send at high level and does not include the return boost. So it isn't a matter of turning down the send and not having enough return.

The schematics are hard to read, but I think the TSL100 has a switchable loop level. Note the switchable voltage divider in the send, and the switchable op amp gain in the return buffer. SW3 sections a and b. They switch together, so set one, you set the other.

[Enzo]

Here, easier to see on the TSL601, see page 3 and sw1

http://www.webphix.com/schematic%20h..._tsl60_60w.pdf

[Chuck H]

I fully agree that the line/low level switch with analogous return circuit is very best way to go. It's still a little confusing for some players, but they'll figure it out. It's just a switch. The only rub is that all the amps that do this use silicooties! If it's a series loop that interrupts the "all tube" signal chain. Not a problem for me personally, but an all tube option for the mojo inclined should be worked out. I'm sorry that I don't have one to present. I was just bringing up the point in case it matters.

EDIT: I could certainly work out such a circuit on the bench. I just don't have one that I can pull out of my pocket right now. And for anyone needing to do something like this I wanted to mention that all the low impedance circuits for padding effects loops are really overkill. A loop should work fine with any impedance under about 50k. Cords up to ten foot and no notable loss of signal integrity. Just padding a gain stage to line level can get you below that. Don't fuss with adding a "buffer" stage. Just pad the last preamp stage you want to drive the loop. Start with a 10k load resistor and add series resistance behind it until it meets your voltage criteria. It'll work fine. I promise. Then you just need a recovery stage.

[Gaz]

I fully agree that the line/low level switch with analogous return circuit is very best way to go. It's still a little confusing for some players, but they'll figure it out. It's just a switch. The only rub is that all the amps that do this use silicooties! If it's a series loop that interrupts the "all tube" signal chain. Not a problem for me personally, but an all tube option for the mojo inclined should be worked out. I'm sorry that I don't have one to present. I was just bringing up the point in case it matters.

EDIT: I could certainly work out such a circuit on the bench. I just don't have one that I can pull out of my pocket right now. And for anyone needing to do something like this I wanted to mention that all the low impedance circuits for padding effects loops are really overkill. A loop should work fine with any impedance under about 50k. Cords up to ten foot and no notable loss of signal integrity. Just padding a gain stage to line level can get you below that. Don't fuss with adding a "buffer" stage. Just pad the last preamp stage you want to drive the loop. Start with a 10k load resistor and add series resistance behind it until it meets your voltage criteria. It'll work fine. I promise. Then you just need a recovery stage.

Cool, Chuck. I have seen lots of very expensive amps pad down the signal just like you described.

I still having a bit of trouble figuring out how to set the voltage with the pre-loop master volumes. It looks like in the 5150 there's no additional padding at all!


I tried this Soldano circuit, but with the master all the way up, the loop was spitting out 3.4VAC rms, and the recovery signal voltage into the PI was lower than the input by about half. At more typical settings, the effects loop voltage was okay for pedal use, but there was a lot of excess voltage on tap.

I wondering if it would be better to set the voltage higher like Soldano, so it's right with lower master settings, or rather to make it so the "everything all the way up" setting is still below line level?