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Old 26th July 2007, 04:56 AM   #1
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Default DIY amp channel switching

Hello all,
I am designing an amplier for use with my guitar and want to include multiple channels for the sake of convienience. What is the easiest way to go about setting up multiple channels and to enable switching between them? Also, which is the preferable method: building multiple preamps and switching between them or sharing gain stages and changing their characteristics? Any help would be greatly appreciated.
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Old 26th July 2007, 04:51 PM   #2
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I would try (this is just a suggestion and is by no means the only way to attempt this) building a preamp and then use some multiplexer
( 74151, 74153, 74153, 4051 has more inputs then needed but still should work http://pdf1.alldatasheet.com/datashe.../UTC/4051.html )
then switch between a clean section with tone controls and your overdrive circuit with its own tone. you would need 2 of these devices one before the desired channel section and one after it feeding into the power amp section. this in my mind was the first way i thought how to do it, hope this helps

Let us know how things go

Dave
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Old 28th July 2007, 11:03 AM   #3
wxn is offline wxn  Lithuania
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Just make sure you use those 4051 in current switching mode (aka virtual ground) if you don't want additional distortion.
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Old 2nd August 2007, 06:12 AM   #4
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Default Check out the Carvin Quad-X preamp

I have a Carvin Quad-X preamp and can attest that it is probably
one of the most versatile multichannel amps ever made. It has four
solid state front end channels, each with their own EQs and effects,
and then each stage has it's own vacuum tube cascades (up to 11 tube
stages) which makes this preamp highly configurable.
Search Google and read some of the reviews, I think you'll be impressed.

Might be worth checking out the schematics... and then picking and
choosing whatever suits your needs.

Carvin hosts complete schematics in their Museum Library.
You might wanna look at the Quad-X Amp Block Diagragm PDF, and
go from there.

Museum Library

Hope this helps.

I forgot. Here's a link to the user's manual
Quad X Pre-Amp Users Manual
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Old 2nd August 2007, 11:42 PM   #5
teemuk is offline teemuk  Finland
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Default Re: DIY amp channel switching

Quote:
Originally posted by AmpKiller66
Also, which is the preferable method: building multiple preamps and switching between them or sharing gain stages and changing their characteristics?
I guess there is no simple answer to that...

In some cases you can get the desired outcome simply by changing characteristics of a single gain stage: i.e. features like alternate gains, treble boosting etc. are something that you can switch on and off pretty easily. Realizing these in one circuit is usually much less hassle than building several nearly identical circuits.

In some cases this won't work. A perfect example is, for instance, the distinction between distortion and clean channel. Of course, one can build a distortion channel simply by boosting the gain of a clean channel but usually this works pretty badly. For more control over the tone you wish these channels to have separate tone controls and likely a separate volume control as well. In this case you can pretty much talk about two individual preamps.

...Or perhaps not: Naturally both channels can share some stages if there is a need for that. For example: They can share some transparent, clean gain stages at the input or output and of course stuff like circuits for master EQ, reverb, effects loop, line out with speaker emulation etc. .

It all depends on the application.


By the way, what do you mean by "multiple" channels?

Is that like three channels - i.e. clean, overdrive and lead (usually a very simple arrangement with few FETs and transistors) - or are you referring to something like 4 - 10 channels (for which you are likely starting to need a microcontroller to control all the switching devices). I just wonder what we are talking about in here since the term "multiple" is not a very definite expression.

What signal levels are you planning to switch?

This is a pretty important detail because solid-state switches typically have serious limits for their maximum input signal amplitude. Tube amplifiers usually deal with such a high voltages that you can pretty much ignore all SS solutions and use only either LDRs or relays.

You know, people can give you more specific help if you share some details of the concerned application.

And as a final advice: This all has been done before hundreds of times. You can get plenty of inspiration if you spend some time googling and studying some amplifier schematics - preferably those of modern ones because channel switching became popular as late as in the early 80's. Fender has lots of schematics of their modern amps in their website's "support" section. Most of these utilize pretty nice switching arrangements. Dr. Tube website has Marshall schematics. Check out the Valvestate models and see how they realized the switching. Mesa Boogie did it with tubes and LDRs - schematics for many of their models can be found from Free Information Society's website. If you can find schematics for Crate models from the late 90's you can see how they realized a very simple switching arrangments for clean, OD and Lead. Lead was basically same channel as OD, just had a little bit more gain. These were just few examples.
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Old 3rd August 2007, 04:19 AM   #6
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I'm sorry if my original question seemed vague. I just didn't think that there were that many options. Anyway, I am planning to incorporate 3 channels (clean, overdrive, and an over-the-top lead channel) into an all tube amp, sharing as many gain stages as possible. The switching will most likely occur immediately after a switchable gain stage on the input. I am also looking for as little switching delay and noise as possible. I hope this clears up any confusion.
As for using existing schemes, I decided to check here before I went digging through schematics because of the sheer multitude of different arrangements and how each one seems to be adapted specifically for that circuit. I am simply looking for a basic, fundamental scheme that will work with many schematics.
I do greatly appreciate all of your help on this matter.
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