Monday, 20 February 2012

Audio: Universal Input Selector / A-B comparison PCB

When modifying/building audio gear its pretty handy to be able to check between either two inputs or two outputs.


Doing this quickly is the key before you forget what the other sounded like compared to the previous test.
So I decided to build a quick and dirty passive universal A To B test switch PCB......................









The Schematic





The PCB Layout (The Capacitors are optional)




It is universal in the fact its bi-directional and takes either phono (RCA) or Jack.This comes in handy when testing audio at line level or output level in the case of headphones.There are provisions for input/output capacitors which comes in handy when testing audio differences between using input capacitors and when none are used.


Various scenarios and test uses:

1)  Switching between 2 inputs such as iPod and PC

2)  2 different sources (eg comparing 2 different headphone amplifiers)  with a single pair of headphones 

3)  Switching compare between 2 different types of headphones quickly for audio differences

4)  Input capacitor testing (one type versus another of one type verses none)

5)  More stuff than I can think off at the moment :)

Eagle files inc PDFs are available in the files section




The Switch used is a DPDT (Dual Pole Dual Throw) ON-ON configuration meaning it swaps "ON" state between one side and the other switching 2 contacts at once (Left&Right channels).

As the switch is only dual pole the ground is not broken during switching a better choice would have been a rotary 3 pole (Switches Left,Right & Ground outputs) 2 conductor ( 2 different sources/inputs) make before break multiway selector switch but I did not have one in my parts bin.

I think it turned out well for what it is and its a pretty useful tool to have lying around in the lab.