Comments, suggestions, and reality checks on line mixer

Hello all and thanks in advance for the help.

I'm building an unbalanced line mixer. It will have 6 stereo unbalanced inputs and 2 stereo unbalanced outputs. I want to be able to send any of the 6 channels to either/both of the outputs using separate pots (so looking down at the mixer you'll see two rows of 6 knobs, one controlling the mix for mix buss A, and the other row of pots controlling the mix for mix buss B).

I have attached the schematic, and it is also available here: http://twin-x.com/groupdiy/displayimage.php?pos=-3400.

The top part of the schematic shows the line input section, taken mostly from a Rane mixer, except that I switched the unity-gain input buffer to an inverting op amp with (what I believe) is 6db max. gain.

The bottom part of the schematic shows the mix buss (this would be doubled for the two mix busses). It is unity gain (?) with output attenuation.

My questions are:

1) Is this a workable configuration for a line mixer? Am I missing anything major here?

2) The input stage has three capacitors. This is how the Rane schematic had it. I was thinking that I could get rid of at least C1 and R1 without too many problems. Are the other decoupling capacitors recommended?

3) Do I even really need an input stage to drive the two mix busses? Or is this too equipment-dependent to say.

I will be breadboarding this and experimenting, but I'd like to get some feedback as well.

Many thanks,

Andrew

From a quick glance I'd make R13 smaller, and I spell bus with one s, but opinions vary.

JR

Yes, lose R1 and C1. You might not need those 27pFs either.

Also, if your buffer only has 6dB of gain at full input, do you really need an input attenuator?

Seems if you're going to control the bus levels with independent pots, you can do without it - unless you expect particularly hot inputs.

Just a thought ...



EDIT: I'm not convinced you need R13 either. Put C4 there instead.

I'd remove C2 and R5. R4 at 10R is probably a typo. Remove the K from R13, or you lose your +6dB gain in this voltage divider. R13 for current limiting looks a little pointless for a TL072 (probably not 1st choice in this circuit). With 10K for R12, lead compensation 100pF might suit better for C5. YMMV.

Thanks for the input everybody. The consensus seems to be to thin out the caps and reduce R13 or remove it.

I probably won't be using TL072, and I was planning on using NE5532 (or 5534s) for the op amps. Would this change much?  Do I need the compensation capacitors if I am using a 5532?

wrt/ removing the input attenuator: I was planning on that being the level control for each channel. Now, however, I'm thinking that this is better done after the input stage before the mix bus.

Thanks again for the help.

Have a great day.

Andrew