4-Way Guitar Selector /w Gain -- Plz help me by reviewing this design

[jeghers]

All,

Here is my first draft for the design of my 4-Guitar Selector Switch with Gain. I am using OPA2134 op-amps for the audio buffering and CD4066 analog switches for the guitar selections. I am considering using a better analog switch to reduce THD. The design goals are described at the end of this posting.

I would be most grateful for any review/comments/suggestions you all might have. If there is a way I can improve signal purity, simplify, conserve power, whatever, I'd love to know anything I've overlooked.

Also, there are specific questions I am wondering about - if anybody has answers to these questions I'd be most grateful again.
-----------------------------------------------------------------------------------

Here are links to download my schematics, both in BMP and JPG format. JPGs are a little over 200k, BMPs are close to 400k. The schematic is in two parts: the audio path and the digital control...

Audio Path:

http://www.t4p.com/ftp/pub/3-Way%20G...dio%20path.bmp

http://www.t4p.com/ftp/pub/3-Way%20G...dio%20path.jpg

Digital Control

http://www.t4p.com/ftp/pub/3-Way%20G...%20control.bmp

http://www.t4p.com/ftp/pub/3-Way%20G...%20control.jpg
-----------------------------------------------------------------------------------

Here are questions I am hoping someone can help me with:

- Are my electrolytic cap polaities reasonable?

- I hear CD4066 switches still introduce some THD (enuf to negate the benefit of nicer op-amps), any other analog switches that might do better?

- Any better place in the audio path to place the analog switches? Any better way to turn the guitars on/off instead of CD4066s?

- Unity gain buffers offer hi-impedance inputs for the guitars, final op-amp mixer provides gain. Reasonable?

- Is the biasing of the op-amp inputs ok?

- Any decoupling caps that could be dispensed with? Any more needed? Are the values reasonable?

- I plan bypass caps between both power rails and ground, what values would be good?

- I plan to support both battery and extrnal power, would that affect what the bypass cap values should be?

- Would it be worthwhile to try using a single 9v power source, not bipolar (+9v/gnd/-9v)? I'd have to introduce a Vref with a power-draining resistor network, which I don't like.

- How much gain could I count on getting before the signal clips using just +9 and gnd? How much gain using bipolar (+9v/gnd/-9v)?

- Any ideas to reduce noise? To keep op-amps stable? To reduce power consumption?

- Any obvious flaws in the digital control? In OR mode, only one guitar at a time can ever be selected, in AND mode, any combination of guitars can be toggled on/off.
-----------------------------------------------------------------------------------

Here are the design goals in a nutshell:

- 4 guitars in, one low impedance output to a pedal/amp/whatever.

- Op amps allow high impedance guitar inputs so guitar tone is not adversely affected.

- The device is meant to NOT affect or "color" the tone of the guitars in any way.

- Each guitar input has a gain control (e.g. a volume knob)

- One more volume knob boosts overall gain somewhere between 3 and 12 db (I've not decided how much yet).

- Non-popping (debounced) solid-state switching with analog switches (probably CD4066s).

- Powered by either external or battery. I am willing to design for 2 9v batteries, but I'd prefer just one 9v battery if I can get away with it. Power consumption is important but not paramount.

- Digital logic (with capacitor-debounced Schmidt triggers will provide with exclusive (1-of-4) or multiple (more than one combined) guitar selection.

Thans so much in advance,
/Mark