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.
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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
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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.
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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
