Lynn Olson’s Karna amp : http://www.nutshellhifi.com/Karna.gif
I am about 90% complete with the build, and have a few questions for those well versed in the tricks of the trade specific to hum cancellation. My understanding is that the most significant hum generated by AC heating is 120Hz, which is greatly cancelled by the push pull interstage / output transformer(s). See http://www.geocities.com/dmitrynizh/filament-ac-harmonic.htm for some juicy details.
Fair enough, that addresses the 120Hz component of hum, leaving behind a small fundamental component, and possibly some IMD quantities. Although I am interested in reducing IMD, my questions are more specific to means of reducing the 60Hz component. It appears to me that wiring the filaments with the same polarity (pin 1 to pin 1, pin 4 to pin 4) will aid in canceling some of the 60Hz hum, but would not affect the 120Hz hum. Is this a correct statement ? Has anyone found the opposite to be true, wiring the filaments in reverse?
One of the things I will be experimenting with is the relative polarity of filament heating between stages. So heat the driver stage at a polarity opposite that of the output stage, compare it to heating in phase. Hopefully one setup would produce less hum ?? Anyone experiment with this?
BTW, if I fall flat on my face (which I’m good at doing) I may resort to DC heating, but only as a last resort; let’s try to keep the conversation isolated to AC.
Thanks.
I am about 90% complete with the build, and have a few questions for those well versed in the tricks of the trade specific to hum cancellation. My understanding is that the most significant hum generated by AC heating is 120Hz, which is greatly cancelled by the push pull interstage / output transformer(s). See http://www.geocities.com/dmitrynizh/filament-ac-harmonic.htm for some juicy details.
Fair enough, that addresses the 120Hz component of hum, leaving behind a small fundamental component, and possibly some IMD quantities. Although I am interested in reducing IMD, my questions are more specific to means of reducing the 60Hz component. It appears to me that wiring the filaments with the same polarity (pin 1 to pin 1, pin 4 to pin 4) will aid in canceling some of the 60Hz hum, but would not affect the 120Hz hum. Is this a correct statement ? Has anyone found the opposite to be true, wiring the filaments in reverse?
One of the things I will be experimenting with is the relative polarity of filament heating between stages. So heat the driver stage at a polarity opposite that of the output stage, compare it to heating in phase. Hopefully one setup would produce less hum ?? Anyone experiment with this?
BTW, if I fall flat on my face (which I’m good at doing) I may resort to DC heating, but only as a last resort; let’s try to keep the conversation isolated to AC.
Thanks.
Last night, I found technical notes at Emission Labs and I found it very interesting. Maybe it can help you resolve the hum issue.
http://www.emissionlabs.com/html/articles/Power-Supply-1/App-note1.htm
http://www.emissionlabs.com/html/articles/Power-Supply-2/In-rush-current.jpg
http://www.emissionlabs.com/html/articles/Power-Supply-1/App-note1.htm
http://www.emissionlabs.com/html/articles/Power-Supply-2/In-rush-current.jpg
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