Help on stereo mixer design?

Hey all - I'm new here, and fairly new to DIY electronics.  Over the last couple years or so I've been building more (some passive loopers/channel switchers, etc, mostly based on designs from beavisaudioresearch.com) as well modifying microphones and amplifiers, with the help of kits/instructions, and am slowly but steadily growing more comfortable behind a soldering iron. 

I successfully built a small 4 channel in to 1 out simple mixer based on JD Sleep's revision of an RG Keen circuit from generalguitargadgets.com.  This circuit uses a TL072 dual op amp. 

I'm hoping to make a stereo version, 4 channels in (two pairs) to 2 out, and I'm wondering if I could use a TL074 quad op amp to make this happen. 

I've drawn up a version of what I think this circuit could look like.  Would anyone be willing to take a look and see if it makes any sense/will work?

My knowledge of circuit design is based solely on what I've gleaned from following other people's designs - my understanding of the underlying principles is sorely lacking, so I'm really just duplicating all resistor/capacitor values from the dual op amp design but trying to apply them to this quad version.  Does that make sense? 

I should mention this circuit is essentially an output mixer - its part of a larger mixer/channel switcher device I'm working on  - so I won't be using volume pots on any of the inputs.  Does that affect the presence/values of the caps and resistors pre IC? 

Any feedback or ideas are most welcome! 

Thanks in advance. 

Looks basically OK to me. You can probably eliminate one of the voltage dividers that provide bias and run all four op amps from the same pot divider. The 0.1uF input are probably a little on the low side and give sa -3dB point at nearly 16Hz. I would suggest changing them to 1uF.

There are lots of things you can do in a stereo mixer design. What you have is about as simple as it gets and  will mix two stereo sources into a stereo output. It may well be exactly what you need but if you want it to do more than then then by all means ask.

Cheers

Ian

Hey Ian - thanks so much for your reply.

When you refer to eliminating one of the voltage dividers, am I correct in assuming you're referring to the circuitry around R11-14 and C5 and C6?  I had wondered whether I might be able to just tie IC pins 3+5 together with 10+12, rather than give each their own path.  Is that what you are suggesting?  Essentially eliminating R13, R14, and C6, and running IC 10+12 along with 3+5?

I don't know how far down this road you want to go, but in case you're interested: the larger project this design is an element of is basically a channel mixer/router for my live set-up.  The way I've been conceiving of the overall design, I'm using the TL072 circuit I described in my first post as in input mixer, summing 4 separate line level signals (I play several electronic instruments through one rig) into a single output.  That output gets fed through a number of FX pedals, then split into one or the other channel of a stereo loop pedal (via an SPDT toggle) and then gets sent to one of two amps on stage (via DPDT On-On-On toggles).  The whole thing is getting built into a stompbox design which lives next to me on stage.  Basically my goal is to create a set-up where I can play any of my four instruments through either of the channels in my loop pedal and then send either of those signals out to either or both of my amps.  I know it might seem a little unnecessarily elaborate, but I play in a duo where I essentially serve the role of one man band so I'm trying to give myself as many options as possible to create a bigger sound, as well as allow for situations where I may only use one amp or the other. 

My plan is to use this second circuit as the final output stage, where the four possibilities of signal path (Loop 1 to Amp 1, Loop 1 to Amp 2, Loop 2 to Amp 1, Loop 2 to Amp 2) get summed down to a single stereo output where one channel gets sent to Amp 1 and the other to Amp 2.  Hence the very basic nature of the circuit/lack of need for EQ/pan/etc. 

ANYWAY - again, many thanks to you for taking the time to respond.  Any further advice/clarifications are much appreciated!

-  Raky

disjectia said:

Hey Ian - thanks so much for your reply.

When you refer to eliminating one of the voltage dividers, am I correct in assuming you're referring to the circuitry around R11-14 and C5 and C6?  I had wondered whether I might be able to just tie IC pins 3+5 together with 10+12, rather than give each their own path.  Is that what you are suggesting?  Essentially eliminating R13, R14, and C6, and running IC 10+12 along with 3+5?

Click to expand...

Yes, that is exactly what I am suggesting. All that circuit does is bias the + inputs to half the supply voltage. You only need the one of them to supply all four op amps.

I don't know how far down this road you want to go, but in case you're interested: the larger project this design is an element of is basically a channel mixer/router for my live set-up. 

Click to expand...

A picture is worth a thousand words. It sounds very much like you have thought this through and are only having problems with details. However, I usually find that drawing a block diagram showing the signal sources and what the go through until they reach the final outputs is a good way to clarify my thoughts and communicate them to others if need be.

Cheers

Ian