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
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