High frequency hiss using Hifimediy and a miniDSP - any ideas?

[Phaedras]

Hey guys

I hope you'll be able to help me solve this. Its probably some blatant error, but I can't seem to figure it out..

I've connected a MiniDSP, a Hifimediy T3s amplifier and a Connexelectronics SMSP300r power supply as shown in the diagram attached.

The problem is i'm getting a very high frequency hiss/windy sound. Its only noticeable when no music is playing or music is turned down low.. If it was only there when the music was off I could live with it, but I can hear it when playing quiet music so I'd like to remove it..

If I set a LPF at something like 500 or a 1000hz on the miniDSP the hiss disappears.

Could it be that the problem arrises because Im running the Amp from the main output of the power supply and the miniDSP from the Auxiliary output?

Any other ideas?

Thank you very much in advance. Hope someone has a solution..

[theAnonymous1]

Have you tried running the MiniDSP from a different supply?

[Phaedras]

Originally Posted by theAnonymous1

Have you tried running the MiniDSP from a different supply?

Yea I just tried that. The sound got worse when using PC as supply along with being the sound source.. If I changed the sound input to my iPhone the hiss got back to being like it was with the power supply.

I did change the input resistors from 20k to 58k to increase gain as the input sens of the T3s is 1.5v and max output from the miniDSP is 0.9v.. Could this be causing it?

This is the sound - might be hard to hear as its recorded with my phone:

Minidsp hifimediy t3s by Michael Ewald Hansen on SoundCloud - Create, record and share your sounds for free

[theAnonymous1]

Originally Posted by Phaedras

.. The sound got worse when using PC as supply along with being the sound source..

Well that's kind of expected; a PC is a very noisy device. When I said try a different supply I meant more like an isolated DC wallwart or even batteries.

It sounds like the noise is from a bad grounding scheme. Raising the gain of the amp probably isn't the main source of the noise, but it will definitely make it more noticeable.

[paskal9]

i'm running the balanced minidsp to my subamp. there's a significant hum when the minidsp is grounded to the amp chasis. so i put a thick paper underneath the minidsp and used plastic screws to keep it in place. maybe you should try isolating the minidsp from the chasis if it's grounded. the board have the mounting screw contact connected to the ground plane.

if that doesn't fix the problem try and remove minidsp from the chain and see if it's minidsp that's causing the hiss.

[DrNick]

I've just had a first test today feeding a minidsp from the 12V output of a BK electronics MF300, which is driving a Dayton RSS390HF-4. To be fair, this output is only supposed to be for the LED power meter they also offer, but I thought I'd try it. The hiss is beyond audible, it's loud, like a detuned FM radio. I also tried the USB power option and that was better, but still like a cassette tape from 1985. I'm just watching the grand prix, but when it finishes, I'll try driving the minidsp from some wall-wart power supplies i have from old toys and mobile phones and see if one of them is better. I've tested the setup minus the minidsp and it is fine, but from some searching it seems others do have this issue too. Is it all power supply related?

[DrNick]

Update, I put the connector on the end of the power cord to my wife's old phone charger (it was a 'Sendo' and no neither have I) which can output 5V at 300 mA. It was fine, I could only barely hear a tiny bit of hiss with my ear in the cone 4 inches away. As a solution, having two mains cables isn't quite as elegant as I had hoped, but at least this shows that the hiss seems to be down to the quality of the power to the minidsp. I may look for a better option to fit in the space.

[JRKO]

A 12v 7ah SLA will last for ages between charges and no second cable