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DECKY999 19th February 2008 12:02 AM

Interesting hum problem
 
I am building a per-amp based on A-15 Liteaudio DIY kit and their relay input selector board. I placed everything in a steel box that is grounded to the mains but there is no signal ground connection to the box. In other words - two transformers (one for the relay borad, other for the pre-amp board) are creating virtual grounds that are interconnected but they are not connected to the box (ie mains earth). Signal is passing through the relays and a 100K pot to the preamp board and then to the output connectors. The pot is a motorized Alps.

The problem is: without any input and with my headphone amp (battery powered) connected to the output I hear very bad hum noise.

My qyestion is - where is the hum coming from since there are no obvious ground loops (at least not obvious to me).

Am I doing something wrong and will the hum go away if I connect the preamp to a properly grounded power amp?

jaycee 19th February 2008 12:44 AM

Short the input to ground and see if there is any hum then. If the hum goes away then it is probably picking up a field from something nearby.

Conrad Hoffman 19th February 2008 02:23 AM

You need to connect the box to signal ground, otherwise any voltage present on the box will be capacitively coupled to the circuit. Connected to mains ground it will certainly have undesired voltages on it. Sure fire recipe for hum. Sort of like using shielded cables with the shields floating or connected to mains ground. You may need some sort of low value resistor, diodes, thermistor, or other isolator, for best performance. I'll usually sacrifice a few dB and go with a nice safe direct connection.

DECKY999 19th February 2008 03:21 AM

What about letting the box to float (no earth connection) and connnecting the signal ground to the box. Would that be any better?

In other words - what is the quietest solution for the unbalanced output in this arrangement? I know that the decoupled signal ground is the safest (I am using that in my power amp).

Conrad Hoffman 19th February 2008 03:43 AM

Not using the safety ground is always the quietest, but neither safe or legal. Generally a preamp chassis should be connected to the circuit star ground at a single point, located as close to the entry of the lowest level signals as possible. That does not mean the signal grounds themselves should be connected there- they route separately.

jaycee 19th February 2008 08:28 AM

Connect 0V (ie your "star" ground) to saftey earth through a loop breaker - see here for good info. A pair of diodes in inverse parallel will do instead of a bridge - 1N5408 should be fine.

You could probably get away with just the resistor and capacitor too - the resistor would still let enough fault current flow to blow a fuse, and more than enough to trip an RCD/GFCI.

Nordic 19th February 2008 09:23 AM

The diodes are there to help the resitor survive long enough to trip the mains... otherwise it is smoke and flames...

decky 19th February 2008 10:32 AM

Thanks for that - I already new about that trick since I used it on my power amp.

I had to do it again since obviously my "floating ground idea" did not really work. Still a lot to be discovered.

It is working fine know - I am not sure what to expect when I connect it to the power amp.

I ll see.

Thanks again.

:)

jaycee 19th February 2008 02:03 PM

Nordic, I guess an alternative is to use an NTC resistor.. one used for soft-starting SMPS supplies should be ideal - ive loads of 10 ohm ones from dead monitors!


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