A/B/C Help

[msurdin]

Hey Guys

I saw a A/B/C box on the cornish site. I have discussed this with some people before. I drew a layout for an idea I had for making something similar...

I emailed Cornish and read about it on his site and It said all 3 turn on and off on their own so each switch controls just that output. C is the refrence output (so its never out of phase) A and B both have toggle switches to make those outputs in phase if they are out. One member here said its just an inverted buffer that would do that. By inverted does he mean just fliped so in is really out on that buffer board?

In my drawling I have it true bypass, how can I wire it so it is not true bypass? I have never used a switch becides the 3pdt...


Here is my drawing. It looks like a child drew it sorry about that.

Here is the cornish one. http://petecornish.co.uk/SAEABCBox.html


Thanks for your help

[GibsonGM]

I can't really help with the 'non true bypass' part, matt....but as for buffers - a buffer is a transistor (or opamp...) stage with a gain of 1.  It doesn't amplify or attenuate a signal (usually...).   A simple 1 transistor buffer will INVERT a signal, yielding an output that's 180 degrees out of phase with the input.  If you mixed the original signal with that output, the result would cancel and = 0.  So will an inverting opamp section with gain of 1.   So what you thought was an 'inverted buffer' may have really been an "Inverting Buffer".   
Not every schem for a buffer is inverting....some research in the forum and on Google will give you info about which one is right for your application.    Buffers are mainly used where you want a high input and low output impedance without amplification.   This keeps your signal from dropping highs between stages/effects, and keeps effects from bleeding into each other (the way Boss does it's 'one-touch' switching...). 

The buffer you described above might be used to invert a signal so it 'matches' the other signals it's blended with, but otherwise keeps the signal level at unity....

[Mark Hammer]

Look at page 15 here: https://secure4.vivid-design.com.au/jaycar2005/images_uploaded/TL071074.PDF

That's more or less what the Cornish thing is.  It provides three isolated outputs from one input.  The user can cancel each one individually.  What you've drawn is something entirely different, which is ONE IN and ONE OUT, with three selectable loops along the way.  Entirely different beast.

[soulsonic]

Look at page 15 here: https://secure4.vivid-design.com.au/jaycar2005/images_uploaded/TL071074.PDF

That's more or less what the Cornish thing is.  It provides three isolated outputs from one input.  The user can cancel each one individually.  What you've drawn is something entirely different, which is ONE IN and ONE OUT, with three selectable loops along the way.  Entirely different beast.

One difference is that Cornish has polarity reversing switches on two of the outputs. I do a circuit like this for my A/B/Both box and it also has polarity reverse... though I think mine is done differently than Cornish's because he has differential outputs on A and B, so he probably reverses polarity with the usual DPDT phase-reverse trick. Mine reverses polarity electronically in the opamp circuit.
I like the idea of ABC being three separate mutes... that's a clever and simple way of doing it - but it's a pain if you want to do several amps simultaneously. I'm doing a Quad splitter right now that has the setup as 4 "presets", so you can jump from having 1 amp on to all amps on by just hitting 1 switch.

Give me a day, and I can have an "workalike" of the Cornish up here.

[msurdin]

Having a work alike would be great.

Now I know my drawling was differnt then what it was.. The C was going to be looped at the top so It would just be killed...

Soulsonic- Are you going to show it in a drawling? I dont really want to trouble you but it would be easiest for me...


I know I'm gonna buffer the unit before the splitters... Should I buffer after also? Or is 1 buffer per unit enough?

Thanks guys!

[soulsonic]

Here it is. This is a work-alike based on the descriptions on the Pete Cornish website. It's fully buffered and has the same differential polarity-reversing outputs on A and B as his does.
http://aronnelson.com/gallery/soulsonic-fx/Cornish_Style_ABC

Can't really express this in a simple drawing because of the buffer circuits, but basically the switching is being done by muting the individual outputs to ground. You can't do this sort of arrangement without some kind of active buffer circuit because muting one output would end up muting other things. I guess you could do some weird switching thing that's entirely passive or potentially use transformers for the splitting, but the sort of arrangement shown in the schematic has worked very well for me and is probably the best way to go as far as sound quality is concerned.

If you don't want to bother with the differential outputs or polarity-reversing functions, just chop them out of the schematic and then the circuit is incredibly simple and pretty much like the one in the app note Mark Hammer put up. Main thing to learn about here is the proper mute-switching for selecting the outputs.

[soulsonic]

Here is a basic idea for a board design. If anyone wants, I can provide the original ExpressPCB file for it.
http://aronnelson.com/gallery/soulsonic-fx/Cornish_Style_ABC_Board

[soulsonic]

Okay, I've made a small correction to the schematic. I added small resistors in series with the outputs so that the opamps don't have a total short to ground when muted. This should make them much happier.
Here's the URL... I think it's the same as what the old one was.
http://aronnelson.com/gallery/soulsonic-fx/Cornish_Style_ABC

I'm not going to make any changes to the board... the board layout is mostly intended as a guide for hand-wiring it on a perfboard - it's not intended as a press n' peel.

[soulsonic]

*Bump*
...so it doesn't get lost before msurdin gets a chance to see it.