New to FX loops - Send and return controls needed?

[Gaz]

I'm putting together a two channel design with a shared effects loop, and I'm concerned that either I'm not going to have enough control over the levels, or I'm going to have redundant controls. Below is what I have so far.

I'm wondering:

1. If I need the Send control even though I have the loop after the master volumes.

2. If I need a return control before the recovery stage, and/or an additional global master volume after the loop.

I want to keep it as simple as possible, but I've never really used a loop before to know what really necessary.

BTW, I'll be using pedals mostly with this loop.


Thanks for any suggestions!

[exclamationmark]

One thing that could possible happen with no attenuator before the send stage, is that you could feed an excessive amount of voltage into your pedals with either channels controls dimed, making effects clip and generally sound horrible. Also, the recovery stage could also start clipping, making effects such as reverb and delay sound really messy. So a level send and return might be a good idea.

They way I generally like to do things is find the max voltage the cathode follower spits out and attenuate it down to line-ish levels (your two channels should have about the same max headroom at clipping unless plate resistors or B+ is vastly different). Also, do the attenuation BEFORE the cathode follower, unless you want to stick extra resistance on it's output, somewhat defeating the purpose. Assuming a centre biased(ish) 12ax7 recovery stage, you'd probably want to limit the signal reaching it to 3v pk-pk. I add the master control after the effects loop, but you can do either/or. You should still have plenty of drive for the power stage.

Alternatively for my rack mount pre-amp units, I just limit the output to line levels, as most power amp units accept this standard, and stick whatever effects in between the two units. You could apply this idea to your design (ie, master volume before effects loop), and just make sure to 1). ensure your pedals aren't seeing excessive voltage and 2). make sure your recovery stage doesn't clip. Pretty easy to do with some careful attenuation, with no need for send and return levels.

[Gaz]

Thanks for your thoughts.

Okay, my thinking was in-line with yours as far as send control, so I think I'll hold on to that. The only issue I see with putting the attenuation before the CF is that the send control will always be in the circuit, like an additional master volume, correct?

[exclamationmark]

Yes, it pretty much acts like a fixed voltage divider 100% of the time. You can leave your circuit be, and just have the send level as a trimpot hidden away somewhere once you calibrate it. The extra impedance won't affect the circuit much anyway.

To clarify; what I do on my pre-amps where the master volume comes before the cathode follower is simply stick a resistor on top of the potentiometer that forms a voltage divider with the pot. ie, sticking a 50k resistor on top of a 50k pot will limit the maximum output to 50% of normal. Not sure how it will react with the tone stack in front of it though - You might have to play with the resistor a bit to get it just right. I just make the potentiometer plus the resistor value equal to 1 meg, and it performs as expected with little influence from tone stack loading.