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6J1 preamp

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music soothes the savage beast
Joined 2004
Paid Member
Bought this little 6J1 based preamp. It runs on 12 vac, and has small mains hum. Any suggestion how to get rid of the hum?
 

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Moderator
Joined 2011
Could be how the RCA grounds or volume control grounds are connected
to the power supply common. Maybe the filament rectifier circuit is adding
noise to the grounding. Try feeding the filaments DC from an external 12V
power supply, disconnecting the pcb filament supply by removing D1,
and see if the hum is reduced.
 
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music soothes the savage beast
Joined 2004
Paid Member
Thanks. I searched the forum for '6J1' repeatedly and it always ended with 'sorry no results'...not sure why.

I have some success with hum. I separated normal power supply, now 14 volts AC, from heater, now 12 volts DC. I use separate trafo. Hum went down from 3-4 mV to about 0.3 mV.

Since i separated these two supplies, i can raise the voltage, but i need to replace power supply caps, since they are rated to 35 volts.
 
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music soothes the savage beast
Joined 2004
Paid Member
Great, thanks Ketje, will test all.

I thought that R16 was only drawn incorrectly in the schematics, but in reality was ok. Will check that out.

Most important suggestion in your picture is the change from preamp to buffer. That of course will require some copper trace cutting. I will consider doing so.

Well, i am already using two separate trafos, so i got that already on my own. :)
 
Great, thanks Ketje, will test all.

I thought that R16 was only drawn incorrectly in the schematics, but in reality was ok. Will check that out.
If it's only in the schematic, no problem then.
Well, i am already using two separate trafos, so i got that already on my own.
The two times 12V is not two trafos, simply a 24V with center tap.
Mona
 
music soothes the savage beast
Joined 2004
Paid Member
Ketje, I understand perfectly what your picture shows...

I was describing what I did, not that any one cares. I use two separate adapters to power it. One 12 DC adapter just to power the heaters. One separate AC adapter to power the voltage multiplier. This way I can swap the AC adapter with increasing voltage, I use 18 volts AC now, giving me +/- 48volts volts for tubes. Sounds better this way, then measly +/- 28 volts.

I will be beefing up some filtration here and there, but otherwise I am almost done with it. No need to waste time on cheap project.
 
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