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rkhanso 30th November 2011 10:51 AM

how to combine 2 amps' outputs correctly?
I am NOT an audio engineer and barely understand some of what is mentioned on this forum. If this has a simple answer, sorry.

I have built a Ruby guitar amp in an Altoids can - based on an LM386. It's the simple Ruby with a headphone jack added. It works great. It has a single 1/4" input from the guitar and a 1/4" output for a speaker cabinet. I've added a headphone jack with a resistor and capacitor to reduce the output at the headphones a bit so I don't go deaf. It has a volume and gain control and runs off a 9v battery. That's it....pretty basic.

I'd like to combine that with an O2 (before finding the O2, I was going to use a CMoy) so that both of the outputs can be heard in a headphone and I can play along with music. I'd like the mono guitar from the Ruby to be in both channels if possible.

Remembering that I'm not an audio engineer, what is the proper way to accomplish this (if at all) so that the amps outputs don't cause problem with each other? Some sort of isolation of the outputs by a transformer? A resistor or diode method? A mixer? I'd like it all to be contained in a single enclosure.

The Ruby amp needs to keep the volume/gain controls as this determines the 'sound' of the guitar. If it needs a mixer to do this, is there a good DIY mixer? I found a schematic for one, but it uses TL072s.

Thanks for any help.

wwenze 1st December 2011 01:06 AM

10k resister in series with amp's output, then connect to the input of O2 in together with the other source.

ethanolson 6th December 2011 09:49 PM

The O2 already has 10kΩ on the input and 20kΩ isn't needed or especially beneficial so no need to add resistance to the Ruby. So you should be able to whip up your own DIY mono to stereo cable, plug it in to the O2 and get rocking (with the Ruby up at full volume and the O2 managing the actual headphone volume).

rkhanso 6th December 2011 11:45 PM


Originally Posted by ethanolson (
The O2 already has 10kΩ on the input and 20kΩ isn't needed or especially beneficial so no need to add resistance to the Ruby. So you should be able to whip up your own DIY mono to stereo cable, plug it in to the O2 and get rocking (with the Ruby up at full volume and the O2 managing the actual headphone volume).

Now, I'm not an electronics expert by any means. So, if I'm way off here, I'm sorry.

I scanned the schematic for the O2. I don't see any 10k resistors inline with the input of the pre-amp. I do see 10k resistors (R14 and R20) to ground, but not in the audio path before the amplifier.

Aren't the resistors in the audio path R3 and R7? Somewhere between 100-300 ohms?

As stated in on the 'details' page for the O2:

INPUT TERMINATION & FILTERING: R7 and R3 serve multiple purposes and can be any value from 100 ohms to about 330 ohms (normally they can be the same value as the high gain resistors R19 and R23i.e. 274 ohms). They provide series input current limiting protection for U1 for when the amp is powered off or the inputs are overloaded. They also form an RC filter with C11 and C12 to prevent significant RF from making it to the op amp inputs where it could be demodulated and create excessive DC and noises. And finally they enhance the ESD capability of the amp. R14 and R20 set the input impedance to 10K which, as discussed in the last article, is optimal.

ethanolson 7th December 2011 02:36 AM

In simple terms the audio electricity has to go somewhere, either into the amp or to ground. The ground is the path of least resistance and by preventing that to the tune of 10k Ohm then there's "pressure" in the amp which is felt on the send. So the audio has to go through the stages before getting dumped into headphones or to ground.

I think that's not exactly accurate but that visualization helps me and I hope it helps you as well. Remember that the ground has to balance the input signal with equivalent output and that balance has been impeded on the return circuit.

mrbell321 19th January 2012 04:46 PM

Hi, I'm new here. I'm not an EE, or audio expert in any way so feel free to tell me I'm wrong in the harshest possible way(but please explain, if you do; I am trying to learn)

I don't see how the proposed solution solves the problem. The problem, as I understand it, is there are two low-level signal sources that need amplification and mixing to a single output. The proposed solution, to my eyes, looks to chain the amps together. This leads to two problems: 1) The input of the O2 is taken solely by the output of the Ruby,(unless the O2 has 2 inputs, that's I don't see, but would also negate the initial problem) which leads to an overall, single input and 2) Potentially overdriving the O2.

I hope someone can tell me why the output of Ruby and the output of the O2 cannot be mixed together with a simple, passive resistor network?

ethanolson 20th January 2012 05:04 AM

The solution was in whipping up a DIY cable (previously mentioned). For specifics, I would have a triple-ended cable. 1. mono plug from ruby splitting to stereo, 2. stereo input end for music source, 3. mini TRS to feed O2.

mrbell321 20th January 2012 02:10 PM

Ah... I see... When you said stereo to mono cable, I wasn't thinking 3 ended. So I guess this the cable works as wiring portion of a mixer, and the internal resistance of the devices works to make it effective?

What about overdriving? maybe I'll go read up more on the O2...


rkhanso 21st August 2012 04:32 PM

Still thinking about this....

Would it be simpler to just take the output of the Ruby guitar amp and the MP3 player and run them through a panning pot to be mixed before they go to the CMoy amp input? This way I can eliminate a couple volume pots. The volume and gain on the Ruby guitar amp will give the desired volume and level of distortion, the volume built into the mp3 player will control it's volume, and the panning pot will control the balance between the 2 signals as it enters the CMoy, which would then amplify to the desired volume at the headphone output.

I'm thinking there may not need to be a resistor on the output of the Ruby guitar amp if I did it this way. (Actually, I did put a resistor at the headphone output of the Ruby amp to lower the level at the headphones).

But, I'm wondering about the inputs into the panning pot. There is one stereo signal and one mono signal, for a total of 3. Do they make pots that will work to pan 2 signals on one side and one signal on the other? Is that a 3-gang pot? And, where would I get one? And, what value should it be? I'm thinking it should be linear?

Or, is a buffer/amp in place of the panning pot desired to really separate the signals so not to damage the Ruby amp or (more likely) MP3 or audio source?

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