adding an fx-loop

[gurutzeta]

Hello,

I own a Marshall 30watt transistor amp, Marshall MG30RCD, and even though I do like how its distortion sounds, I end up not using it because of the lack of an effects loop, since I do like some modulation after it.
Is there any easy way I can modify my amp to have an effects loop?
Does anyone have schems for this amp?

Thanks

[cd]

Is there any easy way I can modify my amp to have an effects loop?

IMHO, no.  The actual circuitry itself would be trivial - one dual opamp would be enough for a send and return buffer.  What is extremely difficult is shoehorning everything itno your amp.  To do an FX loop right, you need to drill six holes - two for the send/return jacks, two for the send/return level controls, one for a series/parallel switch, and one for a line/guitar level (-10/+4dB) switch.  You can maybe get away with four holes (send/return level controls only), or maybe three if you use a stereo jack for the send/return (like Matchless).   Not worth the effort IMHO.

If you want o keep it simple, I would make a line out box that connects to the speaker output, then feed the signal from that to your FX and back through the amp's CD/tape input.

[gurutzeta]

You mention that the actual circuit is trivial, would it be just a matter of cutting the pcb line (or cable) that goes from the line out jack to the power amp circuitry and putting all the buffers, mixers and switches that you mention?

My thought is that I could remove the CD input jack of the amp (I do not use it anyway) and use it as a "power amp input". I could put all the circuitry that you mention in a box outside the amp, in a stompbox fashion, and have that box connected to the line out and "power amp input" and, of course, to my pedals loop.

Thanks for the reply cd, I will investigate the solution you proposed also.

[cd]

Hold on now - I didn't realize your amp had a line out jack!  If that's the case, forget about adding any circuitry or anything - just use the line out as the "send" and the CD input as the "return".  You can make an external box with some pots in it if you want to control the levels and whatnot, but you won't need to add anything inside the amp.

[gurutzeta]

Hold on now - I didn't realize your amp had a line out jack!  

Forgot to mention :roll: sorry...

If that's the case, forget about adding any circuitry or anything - just use the line out as the "send" and the CD input as the "return".  You can make an external box with some pots in it if you want to control the levels and whatnot, but you won't need to add anything inside the amp.

But I guess I will still need to do something similar to what I explained above of cutting the pcb line that goes from the "line out" to the power amp so that the preamp signal does not get mixed with whatever is fed through the return (CD input). Remember that, as the amp is right now, the CD input is mixed with the preamp signal... Did I explain myself?

Thanks again for your help cd.

[Johnny G]

to the best of my knowledge it the same with all the MG series, (is on mine) but the line out jack is switched so that when you plug something in (headphones, line out to recording console or FX in this case) it automatically cuts the connection between preamp and power amp.

only thing is i they have speaker emulation on the line out (not very good mind you but not too bad) so that could play buggery if your then putting it back through the poweramp/speaker

other thing is you might wanna find out what sort of signal the CD line in can take since it could be that it clips if you send it to strong a signal

hope thats all helpful, this is something i was thinking of doing with my little MG15DFX untill i gotmyself a proper distortion box and realised that it was infinitlly better than the onboard distortion. im tempted to open it up when i get a new amp and see whats what

[gurutzeta]

to the best of my knowledge it the same with all the MG series, (is on mine) but the line out jack is switched so that when you plug something in (headphones, line out to recording console or FX in this case) it automatically cuts the connection between preamp and power amp.

Nop, my amp has a headphone outpu, which is switched, and a line out which is not switched.

only thing is i they have speaker emulation on the line out (not very good mind you but not too bad) so that could play buggery if your then putting it back through the poweramp/speaker

Not sure about this one, but I think the simulation is only in the headphone output also. Maybe your amp is slightly different to mine. :?