test result of using a separated power supply for filament (the on-bard supply only does HV/B+)
I was hoping to solve the hum problem by using a dedicated filament supply, but it did not work. There was audio frequency noise in the HV line, some kind of oscillation maybe, (not just the 63khz sharp spikes). It seems that the onboard power IC is expecting the 12V line to have some kind of load, when the current draw goes up the hum/noise frequency increases, and wise-versa. The oscillation was around 10hz when the load resistor was 68 ohms (around 180mA draw). At 0mA draw it became some kind of motor-boating noise.
The "sweet spot" (relatively speaking) happened when the load resistor was 12 ohms (1A draw), but the hum was still noticeable.
I was hoping to solve the hum problem by using a dedicated filament supply, but it did not work. There was audio frequency noise in the HV line, some kind of oscillation maybe, (not just the 63khz sharp spikes). It seems that the onboard power IC is expecting the 12V line to have some kind of load, when the current draw goes up the hum/noise frequency increases, and wise-versa. The oscillation was around 10hz when the load resistor was 68 ohms (around 180mA draw). At 0mA draw it became some kind of motor-boating noise.
The "sweet spot" (relatively speaking) happened when the load resistor was 12 ohms (1A draw), but the hum was still noticeable.
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