R.G. Keen's Hum Free A/B/Y

[roknjohn]

I want to try building this for myself and a friend as well, but I was wondering if it is possible to drive at least 3 outputs from the buffer. My buddy would like to drive four if possible. Are there any reasons why I can't just duplicate what's after the buffer, or are the some limitations to how much the buffer can drive? If so what modifications would need to be made to drive four outputs?

Any help or advice is appreciated as always.

[roknjohn]

BUMP

[R.G.]

It's hard to say, and depends on the opamp(s).

The real issue here is whether one opamp can successfully drive the transformers. Since we are using the opamp to extend the low frequency response by trading the opamp's excess current driving ability for primary inductance in the transformers, it is possible that the opamp might run out of extra current. I would not trust it for more than two transformers per opamps, but there's no reason you could not just use one opamp as an input buffer driving one opamp per isolated channel. That kind of setup should expand to tens if not hundreds of channels done properly.

[KMS]

Take a look at this thread
http://www.diystompboxes.com/smfforum/index.php?topic=32666.0

Also do a search for A/B/Y and read through all you can find that you think is relevant to your application.

This next link is my PCB ready for transfer layout for R.G.'s 1999 A/B/Y circuit with 8 outputs.

However there is no inclusion for switching or LED's in this PCB....because I did not need that for what I did.

http://aronnelson.com/gallery/Schematics-etc/Octaplus_GEO_ABY_from_R_G_Keen_s_1999_transformer_isolation_version

You can simply not install the transformers and parts that you do not need for any output range you want less than the 8 outputs shown.

What R.G. said above is good advise and I do not guarantee that the PCB and my design will work for your application (I guarantee nothing)......but it worked for me...note this is not battery setup at all.

I used 6 outputs then into my FX circuits and then mixed them back together in a DIY multi channel stereo output mixer.  I did my switching of the 6 channels in the mixer and LED's indicators in the mixer......but I used LED/LDR in combination with my switches by running the audio through the LDR and regular 9VDC power on the LEDpart of the LED/LDR where I also have big caps 440uf with LEDpart of the LED/LDR to give a slight fade effect when I switch on/off some channels and 220uf for a quick fade but still not a slam on and slam off like you would get with no cap.  That allowed me to use cheap switches too.

The unit (all in one big steel stomp box and with grounding in one central location inside the box) is totally silent......better than anyone would need...no pop....nada...none....zilch......and no hum....nada.....none....noneatall.

I built my own LED/LDRs and I have found that by cherry picking the LDRs from Radio Shack I can get the LDR range from 100ohm to 2Mohm and only use .010amp for the LED...but I have forgotten what LED I used.......Red and real bright rating. I can get 40ohm on the LDR using .040 amp on the LED but I chose to conserve some power and stuck with the .010amp setting.

Overkill?...Yes Maybe.........but it was a lot fun doing it and I learned a lot.

[roknjohn]

Thanks R.G., thats exactly what I was worried about. Was wondering if I would have to change the input buffer to a transistor type, but I'll take your recommendation of one opamp per output and go with that.

Thanks KMS for the links, didn't find those when I searched. Must be how I searched.

Probably start on it over the holidays, I'll let ya know what configurations I end up with, and how it all works.