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.
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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:

https://www.t4p.com/ftp/pub/3-Way Guitar Selector with Gain - Audio path.bmp

https://www.t4p.com/ftp/pub/3-Way Guitar Selector with Gain - Audio path.jpg

Digital Control

https://www.t4p.com/ftp/pub/3-Way Guitar Selector with Gain - Digital control.bmp

https://www.t4p.com/ftp/pub/3-Way Guitar Selector with Gain - Digital control.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

 

[Andrew Leigh]

Hi,

I am curious about what you are trying to achieve conceptually so a couple of questions.

Why do you want this configuration of inputs? Are you wanting to pick up any guitar and play without having to plug in, like the rest of us do?

What are you driving, solid state or tubes?

Most of us drive through any combination of pedals like distortion, compressors, flangers, chorus, delay, reverb. Why is TDH such a concern, are you desiring a crystal clean sound?

Do you currently have active electronics in your guitars?

Why are you wanting to introduce more gain before the amp, are you wanting to drive the amp into distortion? This may work for tubes but you could get some nasty surprises with solid state. You don't want colouration of the sound but I fear driving the amp harder than it expects could be a problem.

You also want to control the gain in this circuit, what is your guitar volume control going to do, stay set at max? Are you trying to even out and balance the signal levels of each guitar before they reach the amp.


Cheers
Andrew

 

[jeghers]

Good questions, let me try to answer best I can...

Why do you want this configuration of inputs? Are you wanting to pick up any guitar and play without having to plug in, like the rest of us do?

Hmmm, well first of all, I don't believe all the rest of us plug and unplug their guitars -- I know many do, but I am sure a lot also use the kind of AB selector switch like what I have today. So far I have played with 2 guitars and my 2-way AB switch has made changing guitars between songs much quicker and easier. But now I have a third guitar and I'd like to begin using it in gigs soon, so a 2-way selector is not enough. Since nobody made a 3-way selector that I liked for a reasonable price, I've decided to make my own. And since the varying outputs of my guitars has been a problem already, I want individual volume controls so I can make all the guitars have the same level. And since I am making my own device, I might as well cover all bases, so I decided to use really good audio op-amps and support 4 guitars instead of 3 (you just never know about the future )...

What are you driving, solid state or tubes?

Solid state today, hopefully tubes next year.

Most of us drive through any combination of pedals like distortion, compressors, flangers, chorus, delay, reverb. Why is TDH such a concern, are you desiring a crystal clean sound?

I want the selector device to not alter the sound of the guitars since A) I don't want to lose their pristine sound when playing clean and B) other devices like my pedal will alter the sound when I choose to do so. Seems to me a good guitar selector box does not change the guitar tone -- that's the job of some other device in the chain (like my Boss GT10 pedal).

Do you currently have active electronics in your guitars?

No, the guitars all have passive electronics. That is why I want very high impedance inputs on the selector device.

Why are you wanting to introduce more gain before the amp, are you wanting to drive the amp into distortion?

No, I am not doing this for overdrive, my pedal already does that, and it is certainly better. I just want a little bit of gain to adjust for the case of feeding into a less sensitive device. Unity gain might be fine to use, but this way I'll have the option of a little more (or a little less) if I ever need it.

You also want to control the gain in this circuit, what is your guitar volume control going to do, stay set at max? Are you trying to even out and balance the signal levels of each guitar before they reach the amp?

I'll either have all the guitars volumes at max, or more likely, I'll have them all at 75% so I can still turn them up (temporarily) at times where I need to punch through. The volume controls on the selector device will be used to even out the volumes of all the guitars.

Hope that helps...

/Mark

 

[Andrew Leigh]

Hi,

I can understand the need for a switch box for a variety of guitars, was curious as to why you needed it.

Have not played for years but still have my VOX AC 30 T/B (1967 ish) and my old Ibanez Musician close to a SG lookalike.

I can't help you electronically but have a look at the attached GIF's perhaps they will help.

The first circuit would require that have all your guitar volumes set to zero. When ready pick up the guitar dial to 100%, when finished dial the volume back to zero and repeat. What I would do with this circuit is to fit a bypass switch across the gain resistor, add a variable resistor of suitable value so that you can preset the gain to want to create a volume pedal for lead breaks.

The second GIF has three circuits for your perusal.

Have fun
Cheers
Andrew