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FX Loop Bypass - Signal Loss
#1
Hi Fellow TUT Followers,

Attached is a schematic showing the FX Loop I used in an amp that I built for a local pro. (The preamp shown is not in his amp, which has switchable Clean and Drive modes.)
He wanted a serial loop, for a Delay pedal, followed by a Reverb pedal (fitted inside the cab), instead of built in reverb and a panel knob to control the reverb level.
I achieved this with the attached circuit.

He recently asked if I could make a foot switchable bypass for the serial loop so he could mount his Delay pedal in the cab also and then just use the amp foot switch box, instead of adding the delay pedal out front. I was able to do this by repurposing one foot switch's function that he didn't use and changing that relay's wiring, as shown and adding a panel switch, as well.
Initially I had it wired so that the loop return was switched in and out, but that meant I lost the Delay Tails.
I rewired it to switch the send in and out, which retained the tails, but the Delay pedal output Z is loading down the bypass signal and there is a volume drop; it works great with no pedal, which is not the object of the exercise.

I would like to find a way to have the Tails and no signal loss in bypass, but I am struggling to find a solution.
If any of you can suggest a suitable way to fix this, I will be very grateful. Otherwise I will change back to loop return switching and he will have no Delay Tails when bypassing the loop.

Cheers, Noel


.pdf   40C110 - FX Loop Bypass. 2302021.pdf (Size: 709.88 KB / Downloads: 5)
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#2
I'd suggest moving over to a parallel/mixing loop. That way the dry signal is always the same and only the wet signal is manipulated. That would solve your issue.
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#3
Hi NGW

Really what you have is two cascaded FX loops where the second is a dedicated reverb block.

As Makinrose suggests, the reverb mixer should be redesigned as a proper mixer. This allows getting rid of the anti-parallel reverb control and using a simpler normally-oriented level control for the reverb output. Its mixing resistance can be low from the pot wiper into the mix triode, just as you need a low mixing resistor value from the input of the pedal.Set the reverb pedal to 100% wet.

Since the goal is to switch the send of the external FX loop AND to have remote control of the loop, do all the signal switching with relay contacts and move any panel switches to the relay coil control circuit. I don't see the point to S4 as it is wired as one of its positions makes RLY1 nonfunctional. In this context, one relay pole will switch the Send jack tip, tying it to the Send pot wiper or to ground. The second relay pole ties the input of the Send pot to the tip of the Return jack, but...

This is where there is a snag in the series loop. The output impedance of the effect is likely low and this loads the return jack node as far as being able to mix the effect input signal with the effect output signal, The obvious solution is to add a proper mixing stage here. Do you really need the CF? If the Send pot were a higher value and/or isolated from the preceding level control with a resistor, then the CF can be re purposed. The series-R into the Send control is functionally required anyway if the loop is strictly for pedals.

Ideally a CF would be positioned after the Send pot and drive the send jack directly as this maximises the benefit of having the CF. FX switching would be at the input to the CF.

Have fun
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