6 Channel Amp Wiring Advice

[Peter Daniel]

The Earth lead from pcb is correct, does it make a difference if it's not connected to a chassis?

For testing amp without source connected, you need to use shorting RCA plugs. Separating rectifier boards from amp boards shouldn't make any difference.

[wintermute]

Originally Posted by thunk303

A few more test.

Pic 2) Output (no input short)
Pic 3) Output (input Shorted)

Hi Dean just going back to this one. When you have the speaker connected up how close do you need to get to hear the noise in pic3? I would say that is probably about as low as the noise on my LM3886 amp and I have to have my ear pretty much planted against the speaker (maybe a case of my speakers are less sensitive/I'm a bit deaf )

You may not be able to completely eliminate the noise. Once again I'd suggest disconnecting your input lead and shorting at the input to the amp board. What this I think will tell you is whether the noise is being picked up by the input wiring, or whether it is coming from the power supply rails directly into the amp.

Tony.

[thunk303]

Good morning Peter & Tony. Lets hope I haven't been chasing my own *** on this one.

Most of my testing has been done with no short at the input. As we saw from yesterday's pics, when shorted the scope noise roughly halves.

When I remove the earth wire completely at either end there is no change.

Pic 3 is with the input shorted which I have only listened to using my shed system. It is quieter but I will have to re-install the amp in my main system for a reference.

If this level is acceptable I will need to look at whats happening at the MiniDSP, like I said, chasing my own *** and learning the hard way.

I will report back asap.

Thanks for the patience gents.

Dean

[wintermute]

Hi Dean, my own experience is that it is difficult to eliminate noise completely (though I'm not saying it is impossible). My LM3886 based amp with input shorted is very quiet, when I connect up my HTPC not so quiet (but still not detectable from the listening position, I decided to live with it

With input shorted, my playmaster series 200 mosfet amp, is dead silent on one channel but the other one has noise (again at fairly low levels not enough to hear from the listening position). I've been trying to get rid of that for years because I don't like the fact that one channel has noise and the other doesn't but I have not managed to

Lots of variables could come into play as well, the efficiency of your speakers and also the ambient noise levels may be vastly different to mine so what might be completely acceptible to me, may not be to you.

Tony.

[thunk303]

I have been reading about adding a resistor at the RCA connector which may be worth giving a go. Does anybody know what value resistor to use and how to connect it??

I know I am skipping a few steps here but will try anything at this point.


Cheers
Dean

[AndrewT]

Adding a resistor to do what?

[Peter Daniel]

A better approach would be reducing the gain of the amp by replacing 680R resistors with 1k2. This should reduce any possible noise as well.

[thunk303]

Its about 2am in the morning now but I will have some time on Thursday to give a few things a go.

The first I will try is removing the miniDSP and trying Pre/CD combo and see If I still have issues.

Good night all.

Dean

[thunk303]

Just wanted to thank KaffiMann for his input.

I haven't been ignoring you mate, just trying to keep my focus in one direction to make sure I don't miss a step. Looks like I will be exploring your suggestions tomorrow and hopefully having some success.

More tomorrow arvo.

Dean

[thunk303]

Well, there you have it, I am a NOOB!

I built a passive pre box (ALPS pot in a box with a few in and outs) and connected my cd player through the pot too the LM4780 amp.

AND guess what..... No noise at the speakers.

I did say I was determined to learn the hard way, right.

I then connected the miniDSP in line and hey presto. Noise is back.

So the lesson for today. K.I.S.S. Only pull your amp apart twice over once you have tried a different input source.

My thanks to Peter, Tony, Andrew and KaffiMann for there patience and advice.

I am now looking at different power-supplies/earthing for the miniDSP to defeat the noise at that end.

Cheers all
Dean