I have 23 mV RMS hum from this amp, about half that with feedback connected. I think it is probably related to the choke input power supply. I originally based the amp on the 1957 400 watt GEC design, but have now modified it to eliminate the cathode followers, because having a floating heater supply made the noise worse. I'm using CCS'ed paralleled 6SN7's for drivers now. I have tried everything I can think of to eliminate the noise but am getting nowhere. Can anyone suggest a systematic method to find the problem?
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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Did you use a swinging choke for the first choke?
ETA: Also, I've never used current sources, but are you sure they aren't the source of the noise? I have an old DIY phono stage a friend made that uses CCS's on the plates and I put it away because they get out of trim and are too noisy.
ETA: Also, I've never used current sources, but are you sure they aren't the source of the noise? I have an old DIY phono stage a friend made that uses CCS's on the plates and I put it away because they get out of trim and are too noisy.
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I have tried grounding the center tap, the noise gets worse. Now, there is a .1uf film cap to ground. Also caps from fil. to ground at the 1st voltage amp, and the 2nd driver tube.
Yes, first choke has a gap in the lams, was sold as usable for swinging choke.
Changing from cathode followers to CCS's reduced the noise in this case. Also the distortion is half at lower to medium drive levels.
I disconnected the cathode bypass cap on the first voltage amp, it didn't help the noise.
Tried grounding it at the center tap, and also tried both legs, more noise...
This is why I'm getting so frustrated, LOL. I have built dead quiet high gain MC phono pre's with less issues!
Ill get a scope pic a bit later, Im multitasking right now. I don't have a current schematic, sorry. I'll have to draw one up. PS is bridge rectifier, into 10 H swinging choke, into 200uf, into 1.5 H choke, into 200uf. Star grounding, heavy bus bar for volt amp, splitter, drivers.
Heres a scope pic of the noise at the speaker terms, 5ms per div, so 60hZ if I calculate correctly.
Heres a scope pic of the noise at the speaker terms, 5ms per div, so 60hZ if I calculate correctly.
An externally hosted image should be here but it was not working when we last tested it.
Ok, I'll snap another pic @ 2ms/div later today. It was a full sweep but the camera was too fast for it. I'm assuming if it is indeed 60 hz, the noise is likely from the AC filament supply....I have built a number of other amps that were very quet with just a small bypass cap from CT to ground. This one is stubborn....
It's a monoblock. Input shorted.
2ms/div----
2ms/div----
An externally hosted image should be here but it was not working when we last tested it.
It could be the input choke. They can generate a good amount of magnetic radiation. The hams used to tune them. I see on the GEC schematic that there's a tuning network across the input choke. You could try a .47 cap in front of it and see if that helps. You have to watch the resonance, though or the voltage will shoot up. Here's an old thread from Audiokarma:
choke input PS throws noise like crazy
Just a thought if all else fails.
So this is a monobloc. Crazy enough to see a choke (?) as large as an output transformer.
Input grounded ok, thanks.
Your waveform looks like a pure 60 Hz sine wave. This is the first step of "a
systematic method to find the problem" (post 1).
So this is not supply related after rectification (120 Hz). If you switch off the mains
for a short time, caps still loaded and tubes hot, does this hum disappear ?
Edit : post 23 points in a similar direction like I am thinking of. Short the choke for a
test or move it away.
Input grounded ok, thanks.
Your waveform looks like a pure 60 Hz sine wave. This is the first step of "a
systematic method to find the problem" (post 1).
So this is not supply related after rectification (120 Hz). If you switch off the mains
for a short time, caps still loaded and tubes hot, does this hum disappear ?
Edit : post 23 points in a similar direction like I am thinking of. Short the choke for a
test or move it away.
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All the PS components use a star ground with substantial conductors. Bridge rectifier neg. to first cap ground is very short 12 gauge solid copper. Front end uses a bus bar that returns to star ground point. I don't think its a ground issue, but you never know. I tired various ground layouts , didn't help.
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