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Old 19th April 2010, 07:26 PM   #1
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Default Help With Left Channel Hum

Hi.

After enjoying some sonically significant upgrades to a Cary SLA 70A Signature EL-34 P-P amp which includes a complete re-tube, new filter caps, coupling caps and cathode bypass caps I'm dissapointed that a low level hum still exists in the left channel. Although you have to place your ear close to the speaker to hear it, I'm somewhat surprised that the issue wasn't resolved along with the recent component changes. The right channel is dead quiet, btw.

I've attached a schematic but need to warn you I am somewhat of a novice when it comes to utilizing them. Any advice would be welcome. Thanks.
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File Type: jpg Cary Audio Design SLA-70 schematic.jpg (55.4 KB, 78 views)
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Old 19th April 2010, 07:28 PM   #2
bigwill is online now bigwill  United Kingdom
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Does it go away with only one channel connected from the preamp?
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Old 19th April 2010, 07:32 PM   #3
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Originally Posted by bigwill View Post
Does it go away with only one channel connected from the preamp?
No, and forgot to mention that. It is originating somewhere in the power amp.
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Old 19th April 2010, 11:14 PM   #4
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The resistor in the filament wiring is marked SELECT so maybe you need to change that one. That's the one at the bottom of the diagram.

Else you may have a heater-cathode leakage on the 6SL7; did you swap them?

If the two channels are operating from separate power supplies perhaps one supply is not properly filtered; look at the 500V electrolytic capacitors.

Are the two units wired identically? You could have a ground loop injecting hum into the amplifier input.
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Old 21st April 2010, 10:24 AM   #5
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Originally Posted by bob91343 View Post
The resistor in the filament wiring is marked SELECT so maybe you need to change that one. That's the one at the bottom of the diagram.

Else you may have a heater-cathode leakage on the 6SL7; did you swap them?

If the two channels are operating from separate power supplies perhaps one supply is not properly filtered; look at the 500V electrolytic capacitors.

Are the two units wired identically? You could have a ground loop injecting hum into the amplifier input.
Both the inverters and filter caps are new. All the tubes have been replaced.
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