Fx loop with Mono (L) / R send and Mono (L) / R return


2019-10-19 9:36 pm
Hi everybody!

I am trying to add an FX loop to my Marshall Code 25 amp.
After a lot of research I figured out that the sound is passed from the preamp/DSP board to the power amp board as analogic sound on 4 cables.

Two of them are ground, the other two are the left / right signal.

I would like to fit an FX loop as follows:

SEND section:
When two jacks are plugged in the send it should send stereo signal directly from the preamp

When one jack is plugged in the L port it should sum up the L and R channel and send out mono signal.

RETURN section:
When two jacks are present than accept stereo return
When only the L jack is present, than split the signal and feed L and R on the corresponding cables.

I've seen quite a lot of amps being able to do that, but I can't find any schematic.

Any support is welcomed!

Find the schematic for some product that has the feature, rather than looking for the circuit itself. I recall a lot of keyboards had outputs wired that way. Some had cross wired L and R, while others had a R jack and a L/MONO jack. A sort of half-duplex if you will.

Mixers tend no to be wired that way, as they prefer a MONO button on the panel and pan pots for L/R distribution.

In guitar amps, L/R returns are easy enough. In my experience, there are some true stereo preamps, but also common is the preamp out is mono, and returns stereo.


2010-05-02 10:00 pm
What happens when You insert the jacks ? should there be no sound coming from power Amp ?
You need four switched jacks. Look at mixer schematics, since they usually employ this technique on the input jacks. Input L usually gets connected to both channels. One of the contacts on channel R is connected to channel L and when a jacks gets inserted, that connection is broken... and R signal is input. Some crossovers also use this technique for 2 way / 3 way switching. For example the old Rane AC22 comes to me in mind. You could go away with two jacks and adapter cables also or fitting some kind of a switch.


2019-10-19 9:36 pm

Thank you for your replies!
Just to clarify: the FX loop allows you to plug external effects (EE) in your amp. These EE receive the preamp signal and feed into the amp.

Given the guitar pre and amp structure is mono, an FX loop is relatively easy to build with a normal mono jack wired after the preamp as send and a switched mono jack before the amp section.

When a jack is plugged in the SEND it basically monitors what happens on that circuit.
When a jack is plugged in the RETURN it breaks the internal amp input and feeds directly in the amp.

What I am trying to build is similar to what you would find for example on the back of the Fender Rumble studio amp https://cdn11.bigcommerce.com/s-8wy...000_amp_back_001_nr__85674.1520293292.jpg?c=2 or on the back of a Fender Mustang 100.

The Rane AC22 looks similar, but I think when taken in detail, it does a different job.

I think I managed to pull out a Receive schematic, but I am stuck at the send part...


The send part is a bit more complicated as on one hand i have to do a summation of two channels and on the other, I have to send the channels separately...
Thank you, we are familiar with FX loops.

I suggested looking at some keyboards as various models already have L and R outputs cross wired so either jack alone gets mono, but plug in both you get L and R. Others had a L/MONO and R like your image.

For inputs, crosswiring the cutout contacts gets you MONO from one plug into either jack. I see no reason you couldn't do something similar for sends by adding a couple mix resistors in the crosswiring.