Transistors can be microphonic. And that is about all I know about it. But transistors can surely oscillate. Put a 'scope on the output and that will tell you a lot about microphony vs oscillation. Then you might be able to identify a source using a 'scope.
Turntable Hum/Howl Issue (moved and reposted)
In hindsight I should have posted this in Analog not Misc.
Therefore here we go.
Original Posting:
Yesterday my system developed a hum when playing vinyl records. Not the standard 60 HZ hum or buzz but a sort of resonating hum or howl around 120 HZ. Nothing with the system has changed in years which makes this more difficult to troubleshoot. Yesterday I played a recent garage sale find record and around 45 or greater on the volume level the hum appeared. It will runaway unless I reduce the volume immediately. I can replicate the issue by resting the stylus on the record without spinning and tapping the turntable base and/or tapping the console that it sits on. Otherwise, dead quiet.
Set up is: VPI Classic 1 table with an Audio Technica OC9XSL Moving Coil Cartridge with 2 Grams Tracking Force. Preamp is a McIntosh C49 using built in Phono Preamp. A picture of the console it sits on should be in my System Profile. House is hardwood floor and built in the 1940’s. I have no idea where to start. I was thinking on moving to a Moving Magnet Cartridge. Perhaps less gain might help the problem.
My finds in the last 10 days:
I have been doing all the test with headphones to remove the speakers and floor vibrations out of the equation.
I rebooted the Mac Preamp as suggested. Boot methods used: Hard, Soft, holding power button on Etc.... Did not help.
Plugged Turntable into a different power sources, other rooms as well. Nothing.
Cleaned all connections. Tried different RCA Cables, nothing. Changed to a new Moving Magnet Cart (thinking less amplitude than a MC would lessen the howl, plus rule out a defective cart), no difference at all.
Placed Sorbothane pads under the Tables feet. Nothing.
Thinking it might be the Mac Phono Section I used 2 different Preamps, same issue. I rang out cartridge wires from the cartridge to the Preamp ends of the RCA's. All less than 1 ohm and no shorts to ground or to any metal on the table. Inspected the Turntables RCA Junction Box, looks great.
At this point, any slight tapping on the plinth is just like ringing a howl type bell. If the speakers are connected, the howl turns into a runaway.
I'm not to sure what to do at this point short of a new Turntable. No idea on what might be a good replacement table. I have a feeling if I send the table or any part of it to be serviced they will not be able to replicate the issue.
Out of desperation, I did order the HW 40 feet from VPI.
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- 26 posts total
- 26 posts total