Not your usual hum problem. Please help.

[madisonears]

I am currently building my first LM1875 amp. So far, everything is going well, thanks to the many contributions I have read here.

My problem is with another chip amp that I purchased. I hope that one of you knowledgeable builders can provide some insight. The amp is a compact model from Ceratec. It is simply a power amp with a volume control. I don' t know the chip used because the chip is held against its heatsink by an aluminum clamping bar which obscures the label.

The problem is low level hum, but not the usual kind. I know it is not ground loop. The hum is present when volume is fully attenauted, but reduces gradually as volume is increased until it is completely absent with volume control at "noon". Hum appears again as volume control is advanced beyond noon (and far beyond where I usually use it) until, at full volume, hum is only slightly louder than at no volume.

Today I opened the amp and rerouted some signal wiring a little farther away from the trafo. The amp is so compact that it's impossible to get it any distance apart. I believe that, because the hum varies with volume control position, this is a basic circuit design problem, not one of design execution.

The volume pot is a small blue plastic pot, soldered to the board. I believe it is 2k because that's the highest resistance I measured between wiper and ground or signal. When the volume control is at noon, there is 255 ohms to ground and appx 1700 ohms to signal side. The only markings are the letters KIE on the backside.

I know this kind of thing is impossible to diagnose without a schematic, but right now I'm seeking ideas for where I should even start looking, or what to test. I have meters and a VERY basic understanding of circuitry.

Does anyone have any suggestions? I would greatly appreciate your help.

Peace,
Tom E

 

[AndrewT]

Hi,
does the output offset vary with the volume pot setting?

 

[madisonears]

Andrew:

Thanks for answering my post.

I have just now measured the dc offset with shorted inputs and no load connected.

R channel: 1.2mv at no volume, 1.2mv at half volume
L channel: 9.8mv at no volume, 9.8mv at half volume

The readings are very similar with normal input and speakers connected.

I noticed when I had the amp apart that the outputs are connected to the speaker terminals out of phase. I doubt that has anything to do with the hum, but it seems strange they would have done it that way. It's very clearly wrong.

Peace,
Tom E

 

[Nuuk]

I have just now measured the dc offset with shorted inputs and no load connected.

But what Andrew was asking is do those readings vary when you alter the pot setting. Also, isn't a 2K pot rather low?

I noticed when I had the amp apart that the outputs are connected to the speaker terminals out of phase. I doubt that has anything to do with the hum, but it seems strange they would have done it that way. It's very clearly wrong.

If it is an inverted GC, then the output wires to the speaker terminals should be reversed.

 

[madisonears]

Nuuk:

Thanks for your response. Did you read my post very carefully?

I stated offset at zero and half volume, as Andrew requested. The offset does not change at any setting of the volume control.

By out of phase, I didn't mean both channels, I meant one channel in relation to the other. Yesterday I checked phase at the speakers by listening, and, indeed, the connection gives properly phased sound of one channel's speaker to the other. Internally, one pair (channel) of terminals has blue wire on the red terminal and white to the black, the other channel has blue wire on the black terminal and white to the red. The blue wires come from one part of the amp, the white wires from a different part of the amp. I think that is unusual. Doesn't that mean that the two channels of the amp are out of phase to each other? Is that done to make more efficient use of the power supply? Could that be causing the hum?

Peace,
Tom E

 

[Nuuk]

Hi Tom,

Sorry, it wasn't clear to me that you were getting consistent readings across the pot range.

And yes, it the two channels are wired up differently you will have one speaker out of phase with the other and that is plain wrong.

 

[madisonears]

Nuuk:

Yes, it seems wrong, but it sounds right. When I hook up the speakers according to the speaker outputs, they sound like they are playing in phase, even though the internal wiring clearly appears to be "out of phase." If I switch the wires of one channel, the speakers sound out of phase, even though that makes the internal wiring "in phase."

This is one weird little amplifier!

By the way, Nuuk, I've visited your web site, and it proved to be very helpful in building my LM1875 amps. You have presented some very basic yet specific information in a way that is pretty easy to understand. Thank you.

Peace,
Tom E

 

[Nuuk]

This is one weird little amplifier!

AMEN! And I would go with your ears on this occasion.

By the way, Nuuk, I've visited your web site, and it proved to be very helpful in building my LM1875 amps. You have presented some very basic yet specific information in a way that is pretty easy to understand. Thank you.

Thank you. Always good to know that it has been useful.

 

[digi01]

The problem is low level hum, but not the usual kind. I know it is not ground loop. The hum is present when volume is fully attenauted, but reduces gradually as volume is increased until it is completely absent with volume control at "noon". Hum appears again as volume control is advanced beyond noon (and far beyond where I usually use it) until, at full volume, hum is only slightly louder than at no volume.

tom,
for product amp,i think the bug 85% caused by design execution.
in my experience,there are latent offset caused by grounding.so what you mean is both channel hum issue?

zang

 

[madisonears]

zang:

Thanks for your response.

I would suspect grounding, except that the hum goes away completely when the volume control is at the middle position. A grounding problem would cause hum at any volume position.

Peace,
Tom E

 

[digi01]

tom,
check the sginal grounding.
trying to cut off one channel of the rca plate to the amp,watch what happens.

another way is insert one 1k resistor between the pot and the amp.good luck,forward

greetings from beijing,
zang