another Hum issue LM3875

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sts9fan

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Joined 2006
So I have built up the first channel for my stereo LM3875 kit from audiosector. I decided to test it out and see if it works and it does but it is also humming like crazy. I currently only have the safety ground hooked up. The signal ground is not connected. It also has the hum with no source connected. I tried a cheater plug (not to use just as a test) and the hum was still there.
Now I generally have no idea what i am doing so I am not sure how to test this thing. I am not sure where to take measurements to see if I am getting proper voltage etc. Any help would be great.
Thanks
Kris
 
ianpengelly

ianpengelly

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Joined 2006
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I would be tempted to try and isolate the problem if the hum is very loud. Disconnect the rectifier board from the amo board and make sure the voltages are correct and are d.c.

If this checks out ok, then review the amp board and make sure all the components are correctly fitted and soldered.

If this is ok then check the input rca and speaker terminals are actually isolated from the chassis (do this by measuring the resistance between the case and the terminals)

If they are isolated then .... panic! :xeye:
 
ianpengelly

ianpengelly

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I don't mean to be funny gents, but if his hum problem is as severe as Kris is making out I somehow doubt it is due to wires being twisted or not, it sounds more significant than that. There is plenty of chipamps crammed into tiny spaces where there have been no hum issues, so given that there is relatively large amounts of space in this case cable proximities are very unlikely to be the cause.
 
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AndrewT

R.I.P.
Joined 2004
Hi,
another chipamp with no smoothing at the rectifiers. :xeye:

two reds and two blacks on the primary side of the transformer.
Where do those two black wires connect?
Have you connected the centre tap to chassis ground?

That soldered safety earth connection is unsafe.
Try to get something more secure. If the solder melts, the safety becomes disconnected and the chassis could become LIVE. :hot: :dead:

Welded to chassis is best, but for most of us, we have to settle for bolted.

I would prefer to see double insulation on all the mains wiring.
the reds could have a heat-shrink sleeve all the way from spade to transformer. If the blacks are primary side, these too would be better with double insulation.
Is one of the red cables fully into the insulator of the spade plug? Looks like it might terminate just outside the insulator.

The secondaries have the same colour codes. Could the two windings be swapped between the two bridge rectifiers?
 
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sts9fan

Member
Joined 2006
another chipamp with no smoothing at the rectifiers
Click to expand...

This is my first project. Its a kit that says it complete. I am trying to keep it simple the first time around. Then when I add stuff I can PERSONALLY say it made a difference. I will reattach the ground in a better manner.

I think you may have given me some other advice but I cannot figure out what it is.
 
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AndrewT

R.I.P.
Joined 2004
Hi,
I have looked at the three pics you linked to. I am not authorised to look at the first link.
I cannot see where the two black wires go. Are these primary side connections?

PS.
call the safety earth as it is. Avoids ambiguity.
This "ground" for all other connections is confusing enough.
 
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