Thanks to www.geofex.com for this for this A/B/Y switch with isolated outputs. Using a standard passive A/B/Y switch to switch to dual amp outputs will probably cause noticeable  hum in the signal to one or both amps.  This ground isolating switch solves the hum problem.

Before you build one of these, try a passive A/B or A/B/Y switch and see if it works, sometimes the simple solution is the best.

Our test results from this unit were very good.   The board features a charge pump to get the headroom of the bipolar powered circuit from a single battery.   We tried a lesser expensive 7660 charge pump, but unfortunately there was a “whining” sound when it was used in this layout, so use the MAX1044A.  If you prefer to use a bipolar power supply, you can leave the charge pump part of the circuit unpopulated and use the pads at the lower part of the board to hook it in.

The switching is a little “different”, but it is simple and effective.  In the photos you can see that we drilled a third hole for a A&B LED, but it was a mistake and it’s not needed. We left the LED in there, but it doesn’t do anything.

Part Information

We used a Hammond 1590BB sized enclosure, we didn’t put in a battery clip on our unit, but there is plenty of room for a battery if you plan it out.

Please note from the photos, that we used nylon washer sets to isolate the switchcraft ouput jacks from the enclosure ground plane. The washers we used are available from Mouser.  They work great.

561-SW375 – Nylon Shoulder Washer
561-D37562 – Nylon Flat Washer

If you can’t get the 42TM018 transformer in your part of the world, here are the specifications so you can find one similar:

Mouser Electronics part number 42TM018
Interstage audio transformers
Primary impedance 10k ohms
Secondary impedance 10k ohms
Primary resistance 550 ohms
Secondary resistance 500 ohms

The MAX1044 charge pump is available from Small Bear or Digikey.com