Sorry about that... it's kind of difficult to explain. I'll try it again though.
The power amp hums if it's input is not shorted (like connecting a cable to it and leaving it unplugged at the other end). In normal operation however (say connecting the DAC to it directly), it's sufficiently quiet, with only a slight hum that's audible if you have your ear against the speakers. That's ok.
Back to the ACF2. With it powered up, there's the annoying 50-cycle hum. As soon as you power it down, the hum stops. It takes about 30-40s for its caps to discharge, after which the power amp starts buzzing. I would assume that at this point the power amp sees the cable essentially unplugged (without any connection between the signal wire and it's return).
Hope that was clear enough.
The power amp hums if it's input is not shorted (like connecting a cable to it and leaving it unplugged at the other end). In normal operation however (say connecting the DAC to it directly), it's sufficiently quiet, with only a slight hum that's audible if you have your ear against the speakers. That's ok.
Back to the ACF2. With it powered up, there's the annoying 50-cycle hum. As soon as you power it down, the hum stops. It takes about 30-40s for its caps to discharge, after which the power amp starts buzzing. I would assume that at this point the power amp sees the cable essentially unplugged (without any connection between the signal wire and it's return).
Hope that was clear enough.
Referring to this YouTube Which hum are you getting?
To short your inputs this is what I mean..
That's not working for some reason!?
Two types of tube amplifier hum and how to determine the source. +YouTube.com
Highlight that last sentence and right click it to do a google search
Btw.A pair of crocodile leads will work just as well as the pdf detail.
To short your inputs this is what I mean..
That's not working for some reason!?
Two types of tube amplifier hum and how to determine the source. +YouTube.com
Highlight that last sentence and right click it to do a google search
Btw.A pair of crocodile leads will work just as well as the pdf detail.
Last edited:
Have you tried screening the signal input cables? They appear to be right next to the mains wiring, which is asking for trouble.
The power amp hum when the preamp has gone off is probably due to the preamp no longer providing a low impedance output, as you suspect. It implies a fault in the power amp - maybe that needs screened input cables too?
The power amp hum when the preamp has gone off is probably due to the preamp no longer providing a low impedance output, as you suspect. It implies a fault in the power amp - maybe that needs screened input cables too?
I suggest you try a ground loop isolator. It seems like a ground loop. I used to have these problems when I connected my computer AND cable TV for FM to the antenna connector. Lifting the computer ground solved the issue, as did an isolator. My final solution was to use optical from the computer to eliminate the loop.
20A Car RCA Audio Noise Filter Ground Loop Isolator Suppressor Killer Remover | eBay
20A Car RCA Audio Noise Filter Ground Loop Isolator Suppressor Killer Remover | eBay
@DF96 I’ve screened the input cables and one of the output cables, without any effect whatsoever. At this point I am convinced this is not an EM issue. As for the power amp, I wouldn’t start modifying it, as it was not DIY and it doesn’t bother me in regular usage.
@kodabmx What does that do really? I can’t imagine where a ground loop is formed as the ACF2 is buzzing without any input - just output. I’ve tried to connect the preamp to the poweramp with a DIY cable I made where I know the screening is tied to ground at one end only.
I’ll try to short the inputs right on the board, so I’ll take the input switch and wiring out of the equation as well.
@kodabmx What does that do really? I can’t imagine where a ground loop is formed as the ACF2 is buzzing without any input - just output. I’ve tried to connect the preamp to the poweramp with a DIY cable I made where I know the screening is tied to ground at one end only.
I’ll try to short the inputs right on the board, so I’ll take the input switch and wiring out of the equation as well.
Oh, so it’s an isolation transformer. That’s nice. I’d get one of those.
As for the mechanical buzzing, it’s definitely not the transformer. I took it off the enclosure and tried the amp with it’s bottom removed, so I can measure the voltages. As far as I could tell, the buzzing came from center/front part of the board. I have a suspicion it might be the big polypropylene bypass caps (C4). The buzzing only appears when the B supply is connected.
@pl802 I’ve just seen the youtube video. I can understand what the guy is saying, but my heaters are regulated DC, not AC... so capacitance between the heater and cathode shouldn’t pass anything.
As for the mechanical buzzing, it’s definitely not the transformer. I took it off the enclosure and tried the amp with it’s bottom removed, so I can measure the voltages. As far as I could tell, the buzzing came from center/front part of the board. I have a suspicion it might be the big polypropylene bypass caps (C4). The buzzing only appears when the B supply is connected.
@pl802 I’ve just seen the youtube video. I can understand what the guy is saying, but my heaters are regulated DC, not AC... so capacitance between the heater and cathode shouldn’t pass anything.
I know ceramic caps can "sing" due to a piezoelectric effect related to the dielectric. Anyway, I'll check that tomorrow. It's late now and I don't want to mess up anything.
Here's the recording I made, in mp3 format (in case you guys were having issues with the m4a): filehosting.org | Download | Noise.mp3
It's definitely 50Hz, I've measured it with a guitar tuner.
Here's the recording I made, in mp3 format (in case you guys were having issues with the m4a): filehosting.org | Download | Noise.mp3
It's definitely 50Hz, I've measured it with a guitar tuner.
I had the exact same experience with this board having both low level hum on the output and the board itself emits a slight buzzing sound and vibration. I tried everything discussed here in this thread and eventually got tired of messing with it and used the parts for another project.
Mine is an ACF-2 octal.
I still have the board in the basement and can possibly set it up with some junk box parts if needed. I honestly would love to get it quiet!
KC
Mine is an ACF-2 octal.
I still have the board in the basement and can possibly set it up with some junk box parts if needed. I honestly would love to get it quiet!
KC
That makes me happy and sad at the same time. Happy that I’m not the only one... sad that it’s starting to feel like a design flaw. I feel that the board buzzing and the output hum are connected. If we can fix the buzzing, we’ll get rid of the hum.
Except for the hum, how did you find the overall sound of it?
Except for the hum, how did you find the overall sound of it?
I'm fresh now and I have some ideas, that baked in my head overnight. The manual says 6SN7 tubes can sound best with a plate voltage of +/- 120V. I'm running them at about 135V right now, so I could go down. This would mean a higher value for R4/R7 which in theory reduces ripple on the B supply (I'm using a 250V secondary). This would mean a lower value for the cathode resistors, in order to keep the idle current the same.
I would be grateful however if someone could validate this theory. I'm a software engineer, so my EE skills are not really that sharp.
Also, side question: how can I calculate the output current of the amp?
I would be grateful however if someone could validate this theory. I'm a software engineer, so my EE skills are not really that sharp.
Also, side question: how can I calculate the output current of the amp?
Some updates:
I powered up the unit again and shorted the inputs on the board. Still hummin' like it doesn't know the words
)
Put my ear as close to the board as I could without risking frizzy hair. I'm pretty certain it's not the caps that buzz, but the rectifier diodes (or something in that area anyway). What would cause these to buzz like that?
@DF96: I was thinking that I could reduce ripple on the plate, but then again, that ripple would be 100Hz, not 50Hz, so that's not the cause.
I powered up the unit again and shorted the inputs on the board. Still hummin' like it doesn't know the words
)Put my ear as close to the board as I could without risking frizzy hair. I'm pretty certain it's not the caps that buzz, but the rectifier diodes (or something in that area anyway). What would cause these to buzz like that?
@DF96: I was thinking that I could reduce ripple on the plate, but then again, that ripple would be 100Hz, not 50Hz, so that's not the cause.
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