If you have some transformers floating around, it would be simple enough to make a cheap linear supply to try out.  Add some beefy caps in a CRC or CLC setup... 

https://flic.kr/p/2kmuC9i

https://flic.kr/p/2kmqddS

I have used the holiday period to build this amp but I am unfortunately having a mains hum similar to some other builds. Not having used SMPS before, am I missing a trick? Is there some sort of filter that I need to insert to make it hum free?

wire the headphone grounds directly to the amp boards, not the power supply

5 minutes ago, kevin gilmore said:

wire the headphone grounds directly to the amp boards, not the power supply

Thanks Kevin. That is what I have done.

I have balanced inputs so the signal/chassis ground is isolated from the smps and amp board grounds. In other words the smps power ground and output signal negative is floating and not tied down.

I probably need to do a bit more experiments to try and identify the issue I.e. whether it is conducted noise from the SMPS, EMI pick up, earth loop etc.

I was just wondering whether there was some standard EMI suppression filter required with SMPS?

this is not a balanced input amplifier

has anyone fully built his amp up and would be willing to put together a second one for someone that is totally inept with a soldering iron and schematics, asking for a friend 🙂