Hum issue at minimum volume with lm3875 chip amp

Chilli6565 · 17th November 2011, 06:39 PM

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

korben69 · 17th November 2011, 06:47 PM

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

Chilli6565 · 18th November 2011, 12:25 AM

Quote:

Originally Posted by korben69

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.

AndrewT · 18th November 2011, 11:23 AM

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.

s666foto · 10th December 2011, 08:00 PM

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

nigelwright7557 · 10th December 2011, 09:13 PM

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.

AndrewT · 11th December 2011, 09:06 AM

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.

17th November 2011, 06:39 PM	#1
Chilli6565 diyAudio Member Join Date: Apr 2008 Location: Newcastle, Australia.	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 __________________ " I love cats....but I couldn't eat a full one. "

18th November 2011, 11:23 AM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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. __________________ regards Andrew T.

10th December 2011, 09:13 PM	#6
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	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. __________________ http://www.murtonpikesystems.co.uk PCBCAD40 pcb design software.

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17th November 2011, 06:47 PM	#2
korben69 diyAudio Member Join Date: Nov 2006 Location: Near Lyon	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

10th December 2011, 08:00 PM	#5
s666foto diyAudio Member Join Date: May 2011	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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