Delay pedal + fx loop = huge volume drop

[stormbringer]

Hey. Been busy lately, but built a custom pedal for a friend, channel switching for his engl fireball 100 and a valvewizard smalltime delay.

He loves how it sounds, but when put in the fx loop (parallell, full wet) the volume gets lowered alot.

I know that line level loops can cause this, but accirding to the manual the send/returns are -20 to +/-0 with -10 nominal. So it shouldnt drop that much? Manual http://www.englamps.de/uploads/tx_ddfproducts/E635-OM.pdf.

The pedal does not distort, no input clipping and looks like unity gain more or less? But, as i read the schematic, the output is inverted, and being a parallell fx loop, maybe this is the issue? Or is it an impedance/cable length issue? But doesnt really hear tone degradation.

Schematic http://www.valvewizard.co.uk/smalltime.html.

Naturally i could just increase the opamp gain on the output. But i feel i want to understand the issue and use the least amount of band aid instead of proper design. If the issue is the inverted signal,i will add a volume pot and an another inverting opamp. Oh, and the buffer tubes should be fine, the amp is brand new.

[Roly]

The owners manual is coy about Fx levels and impedances, but if the problem is cancellation then the "Dry-Fx Balance" control should give highish levels of Dry at one end and highish levels of Fx at the other, falling to a minimum at some mid setting.

An obvious first test is bridge Fx Send and Return with a known good lead and confirm that you have good output in the fully Wet setting.

Since the amp seems to have a Dry/Wet mix already you could try making the delay wet-only by lifting the 10k between the output of the first op-amp and the input of the second, i.e. its own dry path.

HTH

[stormbringer]

Thanx Roly, i have instructed him to test the amp with just a Cable in send/return. will see what the outcome is.