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Old 25th December 2011, 12:16 PM   #71
wintermute is offline wintermute  Australia
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6 Channel Amp Wiring Advice
Hi Dean, what size are the filter caps on the amp board? From what I can see it is a very minimal design. I suspect 1000uF caps??

I know that my LM3886 had 100Hz spikes on the output (not as big as yours) and only had 1000uF / rail on the PS (though it also had local 100uF decoupling on the LM3886 pins).

Increasing the PS caps to 5700uF in total almost completely eliminated the 100Hz noise.

I'd say it's worth while trying to disconnect the alloy plates from the star earth block. I'm speculating here that rectifier pulses back through the transformer and the resulting EMI may be inducing current in the alloy plates which could be then polluting your earth.

Have you tried the scope measurements with 2, 4, and 6 channels running? Is there any difference?

Hope there wasn't too much damage from the flooding!! and not too big a cleanup.

Tony.
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Old 26th December 2011, 08:54 AM   #72
thunk303 is offline thunk303  Australia
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Ok, so I spent the last 6 hours digging out the shed after the flood so I could get to the workbench. The house is fine with no real damage but we have discovered were the drainage situation could be improved in a few places.

Amp testing time.... I am becoming more comfortable using the scope so this has been a good opportunity to learn something.

1. Single module running, 100Hz 6mV noise on scope and hum at speaker.
a) Add another psu/amp module = no change
b) Turn on all other modules = no change
c) Connect Alloy shield plates (between trany's) to star-earth = no change
d) Connect static shield from around trany's to star-earth = no change
e) Connect shielded wire from input to rca on back panel = no change
f) Drink a beer and scratch head = no change

I have been reading the other threads you posted Tony. I am looking at ordering some larger caps and testing the theory unless other suggestions are made.
I have to admit It will be a little disappointing to find out that the amp with the existing caps was prone to noise and have never heard mention of the possible issue on the AudioSector web site.
I know this is DIY but........

Cheers
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Old 26th December 2011, 09:25 AM   #73
wintermute is offline wintermute  Australia
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6 Channel Amp Wiring Advice
Hi Dean, this post A Grounding challenge , please help.. and the next one show the difference on my LM3886 with 1000uF / rail and 5700uF/rail on the powersupply.

Note that in general it is pretty noisy (in that the scope trace is thick), I don't have any rf filtering on the input, and I think it needs it. I don't think I'd worked out that I could put the scope into 2mV/div mode at that point

Good to hear the house isn't damaged, and nice to hear you are getting the hang of the scope. It took me a while too, and I'm sure I still have a lot to learn, but it is a very useful tool!

Tony.
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Old 26th December 2011, 09:53 AM   #74
AndrewT is offline AndrewT  Scotland
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Is that 6mVpp of noise on the scope?
Can you estimate what portion of that 6mVpp is HF noise and what is LF noise?
Can you measure that same noise with an AC DMM or voltmeter?
What does the voltmeter read?
Do any of the noise voltages change when you short and open the amp input?
Have you tried a dummy Rs load on the amp input?
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Old 26th December 2011, 03:54 PM   #75
Peter Daniel is offline Peter Daniel  Canada
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Quote:
Originally Posted by thunk303 View Post
.
I have to admit It will be a little disappointing to find out that the amp with the existing caps was prone to noise and have never heard mention of the possible issue on the AudioSector web site.
I know this is DIY but........
There's a simple way to check it: isolate one amp channel (which means disconnecting everything else except for one amp channel), ground the input and check if it hums. If it doesn't, the problem in your setup comes form grounding.

The amp will produce noise without input connected to the source or grounded.
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Old 5th January 2012, 04:01 PM   #76
syklab is offline syklab  United States
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Hi Dean and Peter,
Sorry for riding on your thread on this issue, I have similar hum issue with my 2 channels setup. I have 1 transformer with 2 secondary (2x 25VAC) feeding 2 x 3875 Chipamp modules with both having it's own PSU module.

Trans(1) -P1 feeding PSU1 of AMP1 and PSU2 of AMP2 on positive rail
Trans(1) -P2 feeding PSU1 of AMP1 and PSU2 of AMP2 on Negative rail

When I isolate one PSU-AMP module by disconnecting the power rails on one module, no hum, very quiet on the other module. This applies to both PSU-AMP module when I swap it so this shown the PSU-AMP module is working fine. Only when I connect both PSU-AMP to the same transformer then it hum. Please note that I have not connect the SC ground and no ground breaker yet, the whole thing is on a cutting board for modup.

I followed Peter's grouding method for each module, do I need to handle the start ground differently when connecting a single transformer to 2 PSU-AMP module? Any suggestion?

Thanks

Last edited by syklab; 5th January 2012 at 04:06 PM.
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Old 11th January 2012, 02:14 PM   #77
KaffiMann is offline KaffiMann  Norway
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6 Channel Amp Wiring Advice
Hey.

I first made my 6xLM4780 build with dual mono 12Ah battery banks, no-noise-at-all. Built a 2xLM3875 thingy with a regular trafo, really happy with it, no noise at all.

Decided my "chief" setup it took up too much space, replaced the battery banks with 4x300VA 2x24V trafo's. Now i have noise on all channels, so I will be going back to my dual battery banks, the battery banks will be built into a slightly ventilated wooden box and hidden behind my 305lTH, so not a big issue, as opposed to before when it was all out in the open.

Just saying I also have these problems.
I have tried everything to get rid of the hum, it seems impossible.

Edit: It seems like hum is worsening for each trafo i connect to the system, it is probably just feeding more potential noise into common. I have tried reversing the phase of each trafo i add, makes no difference.
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Old 17th January 2012, 04:29 AM   #78
thunk303 is offline thunk303  Australia
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OK, finally had enough time today to get the amp onto the test bench and disconnect everything but 1 channel. I then took an alligator lead and placed one end on the star earth block and the other on the outside of the rca connector.

I forgot to take a photo but the 100hz hum looks the same with the lead connected or not. So no change on that front.

Do you think its worth trying larger caps Peter (+others), say 4700uF and see what happens? If so, what brand would be best?

Cheers all. Dean

PS. I have had the amp running (with hum) and am having slow but great success with my miniDSP/OB rig. More bass than I have every had before from the 2 x 15's. In every other regard the AudioSector LM4780's have been operating flawlessly.
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Old 17th January 2012, 09:34 PM   #79
Peter Daniel is offline Peter Daniel  Canada
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Quote:
Originally Posted by thunk303 View Post
OK, finally had enough time today to get the amp onto the test bench and disconnect everything but 1 channel. I then took an alligator lead and placed one end on the star earth block and the other on the outside of the rca connector.
That does not ground the input, it needs to be connected to the "inside" of RCA connector.

If you are experiencing hum issues, larger cap will not help here.
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Old 18th January 2012, 12:48 AM   #80
thunk303 is offline thunk303  Australia
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Thanks for keeping an eye on this one Peter

I will get the amp back on the test bench and redo the input earthing test hopefully in the next few days.

I will also take a few photo's this time and post them so the next step might be a bit clearer.

Cheers again
Dean
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