I just picked up a Mcintosh C28 preamp. The line section works fine. In the Aux and Tuner mode there is no hum. When I turn the selector knob to phono, mic or tape head, there is a hum present. So there is a problem in the low level section. I checked the electrolytics on board 044-309 and they are good. Would appreciate any advice!
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ANYONE?
Since the hum is in both channels I replaced the power supply caps. The hum in just the phono, mic and tape head line is still there. Low level section only!
Since the hum is in both channels I replaced the power supply caps. The hum in just the phono, mic and tape head line is still there. Low level section only!
Hey I hav the same problem with my Marantz PM-26 amp. Im not bothered about it since i never use the phono inputs.
Did you short the inputs to ground when you tested it? Those are very high gain channels, so they may be sensitive to hum pickup.
OK...so you're saying to short the phono inputs to see if the hum goes away? If the hum stops, that means what? There was a turntable connected and the hum was there.
Just to be clear, I mean to short each one to ground, not to each other.
If there is no hum, then everything is fine.
If there is no hum, then everything is fine.
Just to eliminate assumptions.
May I assume that you have the ground wire from the turntable connected to the ground screw on the preamp?
Akitika GT-101 Audio Power Amplifier Kit
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May I assume that you have the ground wire from the turntable connected to the ground screw on the preamp?
Akitika GT-101 Audio Power Amplifier Kit
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I have not hooked a turntable up to this but the owner said that he did and with the ground wire connected. He said there was a hum.
Maybe he has a bad ground on the table?
Maybe he has a bad ground on the table?
Likely a ground or wiring problem on the TT, if the phono input is quiet when shorted the internal circuitry is working correctly.
The problem may be your turntable wiring or ground wire. This is common.
Does the turntable ground terminal on the preamp ground to chassis? Many recievers/amps/preamps are configured like this. It is infortunate.
If so, configure the turntable ground lug so it is connected to the noise ground of the RIAA preamp circuit, or to the star ground if there is one.
I replaced all PS caps. Does anyone know if these Mcintosh preamps are just like this? There is always a slight hum. Louder with the phono section. It is very noticeable without an audio source playing through it and increases with volume. Can not be heard with music playing. My turntable works fine with my Audio Research unit so I know it is not that and also the fact that the hum is there even in the Aux mode, but much lower. Could a faulty transformer be the cause?
Noise + Hum for the phono stage is -78dB. Maybe that's high enough for you to hear? If not, then you'd have to do some diagnosis in order to figure out what's going on. First check would be the amount of ripple on the DC rails. You say you replaced the power supply caps, so you got those special cans from where?
I kept the original can caps in place, disconnected them and mounted new radial caps under the chassis.
If the hum level is the same with the new capacitors, then the old ones were not bad. You might as well just undo what you just did. You have to do an investigation to determine the cause of the hum. I would suspect bad regulators if the caps are good, but you have to use some equipment (like a simple AC voltmeter) to figure it out. Your job is to figure out where the hum is coming from. It could be the power supply, or it could be some bad part in the circuit itself.
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