BYOC Amp Selector

[nonost]

Hi. I'm going to build the BYOC ABY box. I'm building this instead of the RG Hum-free because I need it as small as possible. I have a few doubts. Here:

http://byocelectronics.com/ampselectorschematic.pdf

I guess the Output A is at unity regarding input. Am I right?

It looks like the top left opamp and bottom left are part of the same dual amp. But they put the +v supply on pin 4 and -v on pin 8. I was expecting the +v to pin 8 and -v to pin 4. Then, for the third amp (top right) they used the second half BUT this time they do put the +v to pin 8 (as I would expect). I don't know if it is a mistake or it's something I don't get.

My output A will be always ON, and I will toggle the B on/off. No leds whatsoever. I don't really understand the switching around r3 and r7. What's the best place to turn the output b on/off? I'm new with transformers.

There's also a isolation switch at the end that I don't understand. They don't explain it.

Thanks a lot!

[stallik]

If you select A on the first switch, toggling the Both switch will turn on B as well. The switch at output B is a ground lift (I think) in case you get hum with both amps on.
The BYOC circuit is new to me but appears to use a single isolation transformer whereas the RG hum free splitter uses two, one for each channel.

I've built a number of ABY boxes, including the RG. They all work fine but sometimes an active ABY is unnecessary. They're designed to eliminate problems such as hum and, if you don't have that issue, a simple switch would give you the smallest build possible

Having said that, if you're a gigging player, the supply at one venue might introduce such issues so keeping an active box in your bag might be worthwhile

[nonost]

It's going to be used with solid state amps. The two amps are the same. They don't cause problem, but I just want to be safe.

[idy]

I don't see the schematic issue you see... I see the top two opamps labeled both IC1b....there is the error. One should be IC2a or something. The lower one is labeled IC2b. Presume the missing fourth opamp is safely disabled in real life (usually +input to ground and - input to output.)

I see that both upper opamps have +9 to pin 8, normal. And the lower one has pin 4 to -9v, also normal. Again the -9 to the other opamp pin 4 is not shown, not perfect schematic technique.

Maybe the version you are looking at and the one you linked to are different?

[nonost]

Hi idy. Yeah, I think you're almost right. The two top amps are part of the same dual op amp. Although they should be named like IC1a and IC1b. The bottom one is named correctly as IC2b ('b' because is using the second half). What do you think?

So pin 4 is -9v and pin 8 is +9v in both ICs. Is that ok?

[idy]

Almost. I thought the top two were in different packages because both show +9v connections. Only one of those per package!

You will have to look at the actual PCB to decide how they really did it. This is not that rare an error....sometimes you see a schematic vs. PCB where the two opamps on an IC are switched around, i.e pins 1,2,3 and 5,6,7. Schematic may show the signal first going in pin 3 and out 1, PCB may have it going in 5 and out 7...

[nonost]

Looking at the PCB it looks like the bottom amp is the lonesome and that the two at the top are in the same package.




I guess it doesn't really matters as long as +9 goes to pin 8 and -9 to pin 4, does it?

Should I ground pins 1,2 & 3 of the unused amp?

edit: ok pin +input to ground and the -input to output

Cheers!

[nonost]

Ok. I don't understand this +v -v power thing.

+v goes to pin 8 and -v to pin 4. OK. If I feed the pin 8 with +v, which side is the one working?

I'm doing something wrong.

[nonost]

Ok, I'm retarded. I have to make BOTH connections: +v & -v.  Even if I only use one side. God, I don't know how my brain works. It's annoying.

I think I fried the opamp because I got one weird voltage. Let's change it.

[GibsonGM]

There's no 'side working' until each power supply pin is connected to its respective power point, I believe, Nono...this is a dual polarity supply setup.   Needs the +, the - and the ground to work, no matter if single or dual opamp.  I assume it won't go at all with just one connected, but maybe it would output SOMETHING like a big brrrrrraaaaaaazzzzzzz.....


Often you'll see an opamp that only requires a positive supply + ground, with a 'Vr' bias supply point shown ("voltage reference").  This isn't one.

[nonost]

Ey! I missed the reply!

Yeah, I was malfunctioning. I realized the whole thing some minutes after posting. With no post no happy idea. Murphy's law.

I changed the 10k in both op-amps loop so the signal is at unity. With 22k it's ok. Maybe even 27k would be better.

Thanks!