finished my 6au6/807 amplifier

[ryuji]

built it as a 5 channel amplifier (subwoofer is run by a plate amp) this is the current circuit. It is virtually hum free (my iron bias transformer hums more then the speakers do). using a linear power supply for filaments. Best amplifier circuit i've built to date from a listening pov.. now just need to figure out how to muffle/silence that bias transformer and ill have a silent amplifier

[Jebem]

Nice simple design, I like it.
Is the bias transformer humming because it is an old component and the E-I laminated pieces are vibrating, or the rectifier/cap/bleed resistor load combination is overloading it?
Why not run the transformer without secondary load just to check it?

[ryuji]

It's a 20 VA or so transformer. My current theory is the around 2 volt(!) dc offset is upsetting my transformers. My linear 6.3v supply transformer hums a little bit too.

[ryuji]

For a little background on bias supply: I have a 54v or so transformer that is rectified into a negative voltage and then filtered by two 47 uF capacitors and fed into an array of 25 turn potentiometer voltage dividers.

It's a bit of a pain to get at the bias transformer (this amp was a rebuild) the fan cooling system waslouder then the hum plus speaker hum. That's the big catch with getting things silent. The quieter you get things the noisier you discover they really are.

[Jebem]

DC on the mains grid is a real pain specially on toroidal transformers (not so much on E-I ones).
So You have 4 pots in parallel. What is the resistor value of each one?
We need to make sure that the secondary load is not overloading the 20VA trafo.

[ryuji]

It is actually five potentiometers. the the potentiometers and 3k6 resistors are replicated in parallel. all other components are 1x. Diode used is hyperfast rated with ~0.2v voltage drop
This is the circuit im using. I left the reference to the designer in the image and cropped out all components that do not apply to my design. I built it a very long time ago(2 years) so i do not recall what size potentiometer i used. Ill inform you when i get home today if its relevant.


Any ideas of how to scrub 2-3v dc offset from line? the circuits i have discovered so far seem to be limited to 1.2v and just adding more diodes doesn't seem to wise as i'd be stacking up the AC voltage drop too. Guess im heading towards a isolation transformer?

[Jebem]

I had similar problems when the rectifier diode is leaking and the filter caps overload the transformer, but I assume this is not the case or else the bias would not work.

Well, if the Pots are 10Kohm, we have total load of (13k6 / 5) + 1k = 3k7. If the input is 72VAC then the rectified value will be a little bit above 72VDC (not 1.4 times because this is a half wave rectification and it is under load). But the in the worst case scenario we have a estimated secondary current of (72V x 1.4) / 3k7 = 27mA, and the load = (72V x 1.4) x 27mA = 2.7Watt.
However, if the pots are just 1K the load will be 1.9K, resulting in a consumed power of 5Watt.
So the 20VA transformer should be able to handle this kind of load.

An isolation transformer is the best option, I think, but an expensive solution, as it should be located away of the main hi-fi components and have enough power to handle all the equipment load.

[Jebem]

Or replace the humming transformers with E-I laminated ones and having more power capacity. It will be cheaper, I think.

[ryuji]

That 1k resistor sees about a 1~2 volt drop if i am remembering correctly so the total current is likely in the 2 mA ballpark. from what i can tell that 1k resistor is intended to protect the circuit from a short by serving as a current limiter the 3k6 resistors are to prevent you from blowing the bias up by bottoming out the potentiometers

[ryuji]

Originally Posted by Jebem

Or replace the humming transformers with E-I laminated ones and having more power capacity. It will be cheaper, I think.

It is actually a E-I style transformer thats humming. ironic enough; my 400 VA toroidial power transformer runs silently(it runs a bit on the warmer side tho ~120F)