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Old 17th November 2011, 06:39 PM   #1
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Default Hum issue at minimum volume with lm3875 chip amp

I have a small issue that I have noticed with my lm3875 chip amp. At mimimum volume there is a bit of hum that is noticble, but when i move the volume control up a bit it goes away and is dead quiet...just wondering what this might be?. The pot is a 50k Alps before the the lm3875 and internal signal cable is screened.
The dac that drives the amp is a nos tda1541.

cheers
Paul
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Old 17th November 2011, 06:47 PM   #2
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Hi

1 - Push the pot to the max with DAC connected = Hum or no ?
2 - Have a try shorting the input without the DAC connected, turn pot from the min to max = Hum ?
3 - Maybe your pot is faulty, try another...
Cheers
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Old 18th November 2011, 12:25 AM   #3
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Quote:
Originally Posted by korben69 View Post
Hi

1 - Push the pot to the max with DAC connected = Hum or no ?
2 - Have a try shorting the input without the DAC connected, turn pot from the min to max = Hum ?
3 - Maybe your pot is faulty, try another...
Cheers
Ok thanks..will try those tonight and let you know the outcome...i was thinking along the same lines.
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Old 18th November 2011, 11:23 AM   #4
AndrewT is online now AndrewT  Scotland
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Disconnect all the external links from your Power Amplifier.
Short the Signal Input to the Signal Return.
Measure the output noises, both AC and DC.
Report back.

A surplus RCA/Phono plug can be used as a shorting link.
Open up the back of the RCA plug. Solder a short between the Hot pin and the Cold Return. Insulate the back. Label the modified plug as Shorted or 0r0. Keep it/them for future testing. I have pairs of 0r0, 50r, 75r, 1k0, 10k, 100k for testing.
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Old 10th December 2011, 08:00 PM   #5
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Hi,

I have a similar issue with my lm3875 monoblocks without volume control or pots installed. When i connect a source via the rca connector i first get a huge hum which quickly reduces to a mild hum and hiss. When shorting the signal input i have no audible hum or hiss.

When measuring output noise i get 19mV DC and 0.09V AC. Is this normal?

Thanks,

Sander
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Old 10th December 2011, 09:13 PM   #6
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Could be a ground loop.
Have you used a star ground system ?
Or is the amp bought in ready made ?

I had a similar problem with a mixer and the ground line was too long causing hum.
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Old 11th December 2011, 09:06 AM   #7
AndrewT is online now AndrewT  Scotland
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Quote:
output noise i get 19mV DC and 0.09V AC. Is this normal?
this is not normal.
19mVdc is OK for offset.
0.09Vac is 90mVac. That is not acceptable.
It is probably hum.
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