Buffered ABY Box with tuner output

[fabiomayo]

Hi, everyone.

I've been meaning to build me an amp switch. Here's what I've made so far,
can you please take a look? The PCB worries me, especially the Vr trace that
goes around a lot of the traces. Traces are bold (2 mm).

Below is a drawing. The box turned out to be 13 x 15 cm (5.12 x 5.9 inches)
which is just a little bigger than Barber pedals. The first switch selects
between a buffer tuner out (silent) or the amplifiers outputs. Then there's the
usual ABY switching scheme: A or B on one switch, both A+B on the other.
The B output has a phase switch to choose between a inverting or a non
inverting buffer. Buffer circuits come dircetly from AMZ website.
http://www.muzique.com/lab/buffers.htm



Here's the logic scheme:


Will it work?
Thank you.

[52 Bill]

I have not checked all of your circuit yet, but there is a trace missing in the feedback loop of the inverting circuit. The 5pf cap and the 10M resistor are not connected to the circuit.

What is the wire from the ring connector of the input jack to the power supply jack for? It does not seem to do anything.

I would also suggest that you look at grounding the inputs of the buffers that are switched out of circuit, to reduce the noise.

[fabiomayo]

Thanks Bill. Good catch, there is a trace missing on the 1M/5pF.
I'll correct it later. The ring wire was supposed to be a switch
for a battery supply which I omitted/deleted and forgot to delete
the wire.

I could easily (and cheaply) ground the B buffers using a DPDT
toggle. I don't know how to do it to the A and tuner side without
adding a pole (meaning two expensive 3PDT footswitches). Any ideas?

I managed to fit everything on a Barber sided box (phase switch
mounted sideways).

Thank you again.

[fabiomayo]

Updated the drawing (but forgot to delete the ring wire). I don't think
a battery will fit anyway...

[52 Bill]

fabio, it seems that this all will work, the only thing that I would change is the B channel inverting and non-inverting outputs being tied directly together with no isolation. I really like the idea of being able to phase switch the B output.

I don't know what you are using this for, but it seems that it is really over designed for an buffered A-B box. I always thought that the basic point of a guitar buffer, is to make the original high impedance output strong enough to drive long cables, multiple inputs, etc. You have designed a passive a-b box with buffered outputs. I guess that I would have designed it to buffer the signal first and then a-b switch the buffered signal. If you did it this way, you could eliminate one IC.

[fabiomayo]

Thank you very much for answering, Bill.

Please, forgive my newbieness but I have a few questions if you don't mind:

1) I don't quite follow the "B channel being tied directly together". They are tied at the end. Do you mean to use the same 1/2 opamp and just switch the circuit around it for phase inverting instead of using two buffers?

2) I could buffer the signal and then switch between amp outputs AB or tuner output (which don't need to be buffered but that way it gives the option of using it as yet another identical amp output). From the amp outputs i could choose between both or A only. Same switching logic. Ok, then how do I select B channel phase? Should I put a inverting buffer in series? Will that be ok (it should I guess)?

At first I just wanted a AB box on the front of my pedalboard to silent tune. Then I wanted an ABY box to run two amps. And thought about putting a buffer (only true bypass pedals on my board - around 6 - and medium length cables) as everyone says it helps . Then I figured all on one board at the end of the pedalboard (which will not be good for tuning if the effects are on).

I could put an unbuffered AB box first and a buffered ABY at the end. I don't know, all I want is to run a couple of amps together and silent tune. The buffer would just be a plus (and a fun little DIY project).

Thanks again!

[52 Bill]

"1) I don't quite follow the "B channel being tied directly together". They are tied at the end. Do you mean to use the same 1/2 opamp and just switch the circuit around it for phase inverting instead of using two buffers?"

I believe that you should not tie the 2 outputs directly to each other, the working amp will try to drive signal backwards into the non-working amp. It would be better in my opinion to tie the inputs together and switch the outputs with the phase switch.

"2) I could buffer the signal and then switch between amp outputs AB or tuner output (which don't need to be buffered but that way it gives the option of using it as yet another identical amp output). From the amp outputs i could choose between both or A only. Same switching logic. Ok, then how do I select B channel phase? Should I put a inverting buffer in series? Will that be ok (it should I guess)?"

Yes, drive all of the outputs with one non-inverting stage (tuner, A and B) via your footswitches. Then use the inverting stage in series, on the buffered signal for the phase function.

"At first I just wanted a AB box on the front of my pedalboard to silent tune. Then I wanted an ABY box to run two amps. And thought about putting a buffer (only true bypass pedals on my board - around 6 - and medium length cables) as everyone says it helps . Then I figured all on one board at the end of the pedalboard (which will not be good for tuning if the effects are on)."

To work with your pedalboard you should build an FX loop into it. Build a single non-inverting stage buffer with a tuner out and an output to your pedals. Then either send the pedal return signal to the switches or build a second non-inverting buffer for the signal return input.

Hope this helps.

[fabiomayo]

Thanks, Bill. That helps a lot. I'll make another drawing later on (been a long day today).
I understand what you say about the pedal loop but at this moment I just want mainly
a simple amp switcher and buffer. Maybe later on I'll add a buffered AB box at the beginning
just for silent tuning and better signal preservation.

[tubetonez]

Not sure about the buffer circuits, but I'd lean towards just using one buffer and passive switching. Instead of another buffer, think about making one of the amp outputs transformer isolated. Every time I've used two tube amps of any make, I've always had grounding problems.

Another note, I first built a "dual" A/B box, one switch was A/B (I don't use "Y") and the other went to a tuner. In live use, I invariable managed to hit the silent tune switch going for my big solo....I eventually separated to two A/B switches for this purpose and mount the silent tune switch in an isolated spot on my pedalboard. I used the small boxes like EJ.

[fabiomayo]

Hi everyone.

Bumping this thread with this update.

I decided to take your advice and make a separate AB box in the
beginning of the chain and an ABY buffered at the end. I think I
incorporated every suggestions and it does fit in a small box.

tubetonez, you make a good point regarding the transformer
isolated outputs. I'll this simpler one first. Maybe it'll work with
my amps (hopefully).

Here's the updated drawing.