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Help with GAS Thoebe Preamp: it hums

trickert

trickert

Member
Hi,
I have picked up a GAS Thoebe preamp. It has been recapped and is otherwise in great shape. I'm listening to vinyl through it, and it sounds excellent... with one snag. It hums. There's a low hum coming out of it. The hum gets lower and higher with the volume; when I turn the volume all the way down, the hum is gone.

My other preamp is a Yamaha C-60, and it doesn't hum. So I know the Thoebe is the culprit.

Suggestions?

Thanks!
 
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Phono RCA jacks are isolated from the chassis? Black washers between the jacks and chassis, whereas the other inputs and outputs are directly on the chassis.
 
Haven't tried other sources and haven't intended to. But I'll check the RCA jacks etc. as you suggested later this evening. I'll need to pop the cover off for that, I'm gathering. (Sorry--really a nube when it comes to the mechanicals end of things.)
 
You should be able to tell if the Phono RCA jacks are isolated by just looking at them. The four Phono jacks will have a black washer between them and the chassis, all others will not.
 
I'm not sure now, my Thoebe, early version, has isolated Phono RCAs. My Thaedra I doesn't have isolated Phono RCAs but my Thaedra II has them isolated. Not sure if it makes a difference now.
 
trickert said:
Haven't tried other sources and haven't intended to.
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Would sure help to isolate the problem. If the problem was isolated to just the phono or if it is influencing other inputs. That will take you down a totally different fault isolation path.
 
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trickert said:
So, I hooked up a CD player. No hum. It looks like it is solely through the phono inputs.
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The phono stage in these is very sensitive. Various things could be the culprit. Sometimes just a loose screw, sometimes a drifted resistor, sometimes the switch(es), sometimes the Gremlin eludes you..
 
A stock Thoebe will have only a MM card inside, MC is handled by the external Goliath powered by the Thoebe. The MC card from either Thaedras will fit in the MM location in the Thoebe to get MC a different way.

Getting to the chassis mounted RCA jacks will require desoldering ALL of the jacks and removing the mother board. Not hard but a task nevertheless. We need to figure out whether it's ground hum or power supply hum. What happens when you remove the TT ground?

Craig
 
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