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Old 1st February 2007, 07:31 AM   #1
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Talking Got Chipamp Hum? Possible Solution

This applies to people using the Chipamp.com Dual Mono PCB boards and possibly others using a similar arrangement.

I have been pulling my hair out trying to solve the hum problem from my amp, using large csa copper wires to connect the chassis ground points and trying input capacitors... not to mention watching every thread on this website hoping an answer would become clear. I had my amp set up as shown below.
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Old 1st February 2007, 07:38 AM   #2
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Listening to the hum the amp produced is seemed all too similar to that of the transformer which got me thinking. I had all the same issues that other people had, connect one channel to the pre-amp and all was fine, connect the second and it all got noisy. This indicated that there was some interaction between the channels and the only two points of connection are the chassis ground wires, which when disconnected from each other worsen the hum and the connection through the power supply.

So with the amp unplugged I disconnected the chassis grounds and measured the resistance between the two RCAs to find that there was approximately 5M Ohms between them, confirming my suspision that the transformer noise was what I was hearing.

To validate this theory I connected both amps to a single rectfier board and voila, hum gone!

So there are two solutions to this problem, one is to use a single rectifier board and the other is to use the amps in true dual mono and use two transformers, one for each channel!
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Old 1st February 2007, 07:42 AM   #3
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If you have one of these kits or you are running two rectifier boards from a single transformer one of these approaches should solve your hum problem.

Vikash, if you could get hold of some nice potted 160VA transformers the would sell like hotcakes with the dual mono chipamp boards you sell (I'll be buying another 300VA one from you in the very near future.... nothing like overkill! )

I hope this helps a few of you that have been stuggling with this problem for a while.

I also look forward to hearing other people's experience of trying this solution and others.
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Old 1st February 2007, 07:53 AM   #4
Nordic is offline Nordic  South Africa
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It looks like a groundloop... I seee no ground disconnect network between ground and mains earth....

Do you only get the hum with the preamp connected?

Can you confirm your RCA sockets are isolated from the case.... i.e. no continuity...
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Old 1st February 2007, 08:02 AM   #5
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The problem exists whether the chassis grounds are connected to earth or not.

There is some noise when the preamp is completely disconnected, although this goes if the rca is shorted.

The RCAs are indeed isolated from the chassis. Though they are effectively connected when the chassis grounds are connected to earth, as indicated in the diagram.
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Old 1st February 2007, 08:09 AM   #6
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Are you using sheilded cable from the RCA's to the amp?
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Old 1st February 2007, 08:12 AM   #7
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No, just a pair of twisted silver coated copper cable.
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Old 1st February 2007, 08:17 AM   #8
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Just to clarify I am posting a solution, I have spent a lot of time reading through threads on this site which have suggested some extreme measures to eliminate the hum problem, including suggesting that the boards are badly designed and that you need to cut some of the traces. Now I am not being funny, but if you had just shelled out a load of cash on some PCBs it would be nice to have a solution that didn't involve hacking them to death in a potentially vain search for a solution that is relatively easy.

I think what is perhaps missing from the chipamp.com installation manual for these kits is to point out a suggested implementation to ensure interaction free operation, i.e. just use one rectifier for a pair of amp boards or use a transformer for each rectifier board, i.e. one per channel.
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Old 1st February 2007, 09:15 AM   #9
Stuey is offline Stuey  Australia
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Ian,

I bought the stereo boards from Chipamp (single rectifier board) and paid no special attention to earthing, and have no noise problems at all.

In fact, contrary to most advice on here, my chassis grounds are joined together and earthed to the main chassis case, about 15cm from the mains (safety) earth. The chassis is a simple sheet aluminium affair I knocked up in a couple of hours. Nothing else is earthed to the chassis except the body of the pot, via its attachment nut.

In fact, there IS noise if I leave the chassis grounds joined, but remove them from the connector on the chassis.

My implementation is most basic, with simply a pot on the inputs. If I turn the pot up full, there is the faintest hiss.

FYI, anyway...

Stuey
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Old 1st February 2007, 09:26 AM   #10
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Stuey,

Thanks for the post, you have confirmed one of my findings that the use of a single rectifier board works just fine and that the problem has nothing to do with the chassis ground and its connection to the safety earth.

Ian
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