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
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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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.
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.
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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?
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.
An externally hosted image should be here but it was not working when we last tested it.
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?
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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.
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.
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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
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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)
i measured the dc offset from the wall instead of my amp and its 44 millivolts which isnt that bad. the drop across that 1k resistor is 16 volts not 2 like i thought it was.
Did in fact make note of something tho turning it on when my room was wicked cold. the hum was almost non-existant but as the tubes etc warmed up it got worse. Perhaps your theory of the circuit being the issue is correct. ill have to closely inspect the transformer and circuit next time i got the amp taken down and ready for experimenting/servicing(probably tomorrow night)
This is my old circuit. in theory the new circuit is easier for the bias to power. im not sure why new design is humming if it didnt before. a closer look will show more when i got it taken apart i guess.
the hum isn't insanity driving loud. as i said its plausible fan noise covered it up. you can only hear it during real silent parts of music etc. would be great to get it gone so im still going to try
Did in fact make note of something tho turning it on when my room was wicked cold. the hum was almost non-existant but as the tubes etc warmed up it got worse. Perhaps your theory of the circuit being the issue is correct. ill have to closely inspect the transformer and circuit next time i got the amp taken down and ready for experimenting/servicing(probably tomorrow night)
This is my old circuit. in theory the new circuit is easier for the bias to power. im not sure why new design is humming if it didnt before. a closer look will show more when i got it taken apart i guess.
An externally hosted image should be here but it was not working when we last tested it.
the hum isn't insanity driving loud. as i said its plausible fan noise covered it up. you can only hear it during real silent parts of music etc. would be great to get it gone so im still going to try
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Ok, so You found the problem... 16V on 1K resistor means 16mA, wich translates into 100V x 16mA = 16Watt. A 20VA transformer is not powerful enough for the job.
My guess is that You are using potentiometers with very low resistor value, and You don't need to do this,a s the output control grid resistors are 100K. I would use 20K potentiometers and a full wave bridge rectifier, and by the way make sure the rectifier you are using right now is able to handle at least 150V reverse peak voltage.
My guess is that You are using potentiometers with very low resistor value, and You don't need to do this,a s the output control grid resistors are 100K. I would use 20K potentiometers and a full wave bridge rectifier, and by the way make sure the rectifier you are using right now is able to handle at least 150V reverse peak voltage.
You're off by one decimal place, the power consumed is actually just 1.6W total for a 100V supply, so no problem with a 20VA transformer.
Loose lams or windings are mostly likely at the root of this noise problem.
Actually this makes perfect sense since when the tubes are cold they are not conducting, as they warm up the load on the transformer increases rapidly and it will start to buzz as it delivers power to the amplifier circuitry.
So, i finally got around to pulling apart my psu area of my amplifier, and it turns out that the loud-ish hum was in fact a loose washer holding the bias transformer in. Funny how it was this time a simple answer
Im guessing thermal expansion etc was pushing the washer to a louder spot. theres a lot of warm stuff in my psu area
Im guessing thermal expansion etc was pushing the washer to a louder spot. theres a lot of warm stuff in my psu area
For some reason i had issues with voltage drift on my input. put capacitor and it still drifted, so 470k did the trick
stranger part is the drift isnt on the tube side, its on the source
amplifier hum is pretty much completely gone. my high end sound card is being upgraded by my friend so i only got crappy onboard sound to play test with. clear improvement either way
An externally hosted image should be here but it was not working when we last tested it.
stranger part is the drift isnt on the tube side, its on the source
amplifier hum is pretty much completely gone. my high end sound card is being upgraded by my friend so i only got crappy onboard sound to play test with. clear improvement either way
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