Behringer XENXY 502 adaptation

[patrick398]

I was playing around with the crappy little behringer 502 mixer someone gave me last night and really like the sound of it when the gain and levels are slammed. Thought i'd try and adapt it and turn it into a pedal.

I just started breadboarding it but hit a couple of problems early on.
It runs on +-15v, but that's not an issue, i'll try run it from +9v with 4.5v vref and if it doesn't sound good i'll try get a 7660s involved for more juice.

I can make sense of most of the schematic, it's just the 'Mic/Line In' that has me slightly confused. This is the input i want to use, since it sounds the best and has EQ. My confusion is with the stereo input jack, and the four transistors before the op amp.
I tried using a mono input jack and disregarded T1 and T3 but then realised they connect to the gain pot, which is connected on it's other end to T2 and T4 which i have included. T1 & T3 and T2 & T4 have the same configuration of components around them, so i'm guessing the pairs are functioning identically but they each go to an opposite op amp input...more confusion 

Basically i'm just wondering how i can convert this part of the circuit for use with a mono input jack and omit the xlr part.

Hope this all makes sense



Thanks

[bluebunny]

My confusion is with the stereo input jack, and the four transistors before the op amp.

It's a stereo input jack because it's a balanced input (just as you have on the XLR input), the hot and cold halves (eventually, via the four transistors) hitting the non-inverting and inverting inputs of the opamp IC6B, which is configured as a differential amplifier.  T1 and T3 send in-phase and out-of-phase signals (respectively) to the opamp.  T2 and T4 do a similar thing with the cold input, but the other way round.

(...I think!)

[patrick398]

Thanks Marc, that makes sense. My issue is with the gain pot config now. Presumably using a mono jack negates the need for two of those transistors but i'm unsure how one set is feeding the other through the gain control. I might just breadboard it with a stereo jack and see if i can get it semi-functioning at least.
Thanks again

[patrick398]

Also, how crucial are those transistors likely to be? I'm assuming they're 2SA1316 and MPSA-06. I subbed in 5087 and 5088 but can't seem to get any signal out of IC6B, haven't tried the rest of the circuit yet, thought i'd try and get the gain section singing first

[bluebunny]

This is getting waaay above my pay-grade, Patrick!  But I don't think removing half the transistors is the answer with the way the opamp is configured.  I'm trying to write words to explain my fuzzy thinking, but it's coming out illogical (captain).  Tired bunny...      I'd wait patiently for someone knowledgeable and awake!

[Derringer]

Thanks Marc, that makes sense. My issue is with the gain pot config now. Presumably using a mono jack negates the need for two of those transistors but i'm unsure how one set is feeding the other through the gain control. I might just breadboard it with a stereo jack and see if i can get it semi-functioning at least.
Thanks again

I "think" that gain pot just mixes the in and out of phase signals. The greater the resistance, the more gain, as resistance is lowered, the out of phase signals cancel out and lower the gain.

It looks to me like you can omit T2 and T4 since they amplify the signal from the ring of the balanced 1/4" input. With a regular guitar TS cable, that node is just grounded.

From the emiter of T3, run a DC blocking cap, then a standard volume control pot with the bottom lug connected to whatever your 1/2V+ reference is. Then run the output of that volume into the non-inverting input of an opamp set up as a buffer. Pickup the output of that buffer at C10 and copy the rest of the schem.

[patrick398]

I "think" that gain pot just mixes the in and out of phase signals. The greater the resistance, the more gain, as resistance is lowered, the out of phase signals cancel out and lower the gain.

It looks to me like you can omit T2 and T4 since they amplify the signal from the ring of the balanced 1/4" input. With a regular guitar TS cable, that node is just grounded.

From the emiter of T3, run a DC blocking cap, then a standard volume control pot with the bottom lug connected to whatever your 1/2V+ reference is. Then run the output of that volume into the non-inverting input of an opamp set up as a buffer. Pickup the output of that buffer at C10 and copy the rest of the schem.

On the actual 502 unit, i'm using a mono guitar cable. If what you say is correct, i don't see how the gain pot would function. If the ring is grounded and T2 and T4 are out of play, i can't see how the gain pot would do anything since it doesn't connect to either op amp input except through those transistors.
Thank you for the reply, i'm sure there's just something i'm not understanding.

Either way your suggestion will give me another angle to work from. Hit a bit of a brick wall with this one. Can't decide whether the transistors are critical or i'm being an idiot and setting it up wrong on the breadboard...9 times out of 10 it's the latter so i'll persevere

[bluebunny]

i'm using a mono guitar cable.

This is just feeding an unbalanced signal into a balanced input.  You're grounding the "cold" phase.