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24.27.41.140
In Reply to: RE: McIntosh C20 Pre-amp Right Channel Buzz posted by mr9iron on May 21, 2025 at 16:44:05
"at about max volume the buzz will dissipate a little."This implies the hum is being picked up at or after the Volume control. It's a 1M pot, so the signal at the wiper is at the highest operating impedance (and therefore most susceptible to hum pickup) at mid position. One way to prove this would be to ground the "hot" side of the pot (which is connected to the wiper of the Loudness pot) and check whether the hum persists. Each channel of the Volume pot connects to a coupling cap that drives the cathode follower output stage. The terminal of the cap connected to the follower has a small amount of DC voltage on it; the side connected to the Volume control should not. It's worth measuring this to be sure the cap isn't leaking.
Edits: 05/21/25
Triode, just an update. I have narrowed the issue down to the last pot (the one adjacent to the barrel switch) Specifically the ground tap, on that last pot (which is the right channel where the buzz is) It sort of acts like a cold joint, though I re-flowed it.
If I add just a little pressure to that tap, I can make the hum much more pronounced, It will also produce loud static and popping. It will improve a bit with pressure, but that low level buzz is the best it will perform. Again the other channel remains perfect.
Here is a little video of it:
https://drive.google.com/file/d/1dpRb_5YGieGAjTc4lSOuN0isjIg1k95U/view?usp=drive_link
Pressing or adding pressure to any of the other lugs on the other pots will not produce or change the noise. The "hot" taps on that last pot might affect the buzz but only in that they will flex the ground tap I believe.
I guess I have a defective pot, though it's new?
I did send a message to the supplier "HiFi Audio" in NYC to see if they have any insights or have had other reports of this.
Jeff
Hi Triode,
thanks for taking a look. I first tested by putting the "hot" side of the volume pot to ground, the side connected to the loudness pot. The buzz still persists, though is just slightly weaker and completely goes away when nearing full volume.
I checked for voltage leaks on those caps. They were replaced, but figured I would check anyway. 19.5 volts at the pins and no voltage on the side of the .022 cap going to the wiper of the volume pot.
There really isn't much after this just the .47s which I replaced and the RCA's themselves.
The solder joints look good on the new volume pot, I'd be surprised if the issue was at the pot itself.
I noticed the original wires that run from v7 to the .47's is just plain wire that is twisted left and right side together. McIntosh did not use shielded wire here, would that have been better to use there? Unsure if RF is getting in there, but there isn't much between the volume control and the output connections.
I really appreciate the help
Jeff
Is the shell of the Volume control well grounded? The preamp's bottom cover is making good electrical contact? The connection at the grid of the output tube is extremely high impedance due to the bootstrapped cathode follower, probably about 10 Meg. The Volume control is the only thing pulling this area of the signal path toward ground, so all of this wiring is susceptible to hum pickup. I would be hesitant to use shielded wiring due to the likelihood of high frequency attenuation.
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