[riddley]

I've recently got hold of a Cyber-20 tube headphone amp. I like it a lot, but one thing I've noticed is that on the "high" impedence setting the amp has a very faint hum. The volume of the hum doesn't change with the volume setting of the amp; it's just noticeable when the source is silent and it's not really possible to discern it when music is playing. Nevertheless, I know that it's there and I worry about whether it interferes with the reproduction of the music.

This is my first foray into tube audio and I really don't know that much about it; is such a hum to be expected from tube equipment? Is it more likely to be a result of the design of the amp or is it possible I have a faulty model? I know that the Cyber-20 isn't that well known on these forums but I'm hoping one of the tube gurus out there can give me some general hints.

 

[ServinginEcuador]

I returned a Cyber-30 due to a hum issue with all cans and volumes. The -30 uses a 2a3 tube, which has an AC filament, and while not too difficult to filter out the hum it does take more than seems to have been put into the Cyber-30.

If the sound is coming thru both tubes it is the amp's fault. Try tapping lightly on the tubes and see if one is microphonic (passes the sound from the tapping into the headphones thru the tube like a microphone).

The other possibility is that it is just a poorly made amp.

 

[CJG888]

Did you get this resolved in the end? I've just taken delivery of my Cyber 20, and it does exactly the same thing - low frequency hum on both channels which becomes louder on the high impedance / gain setting (it doesn't change when the volume control is turned, and is almost inaudible on the low impedance setting).
 
Is this just to be expected, or was it due to faulty tubes or a faulty PSU capacitor? I have ordered some NOS tubes and hope that this may help, as both of the stock output tubes are microphonic.
 
Incidentally, I am using it with HD600s.
 
I would hate to have to pack it up and send it back to the dealer in Shenzhen....

 

[Il Mostro]

FWIW, my Cyber 20 has no such hum issues.  Sounds like the OP might have had transformer hum or a noisy tube at idle.  I have occasionally heard this malady with full size tube amps over the years.  Don't forget to upgrade the 12AU7.

 

[CJG888]

Thanks for the advice. I presume it is unlikely to be the output transformers though, as it only happens on the high impedance setting, and doesn't change with volume level. Do you think it could be a faulty driver tube? As I won't be able to pick up my NOS tubes until Christmas (when I will hopefully be back in the UK for a few days), I have ordered a JJ ECC82 here in China as a good current production tube, to see if it makes an improvement. I presume the EL84s are not at fault, because the hum is the same on both channels.
 
I have experimented a bit with the impedance switch. Is this really an impedance matching switch, or does it just increase the gain? In other words, is it OK to listen to HD600s on the "low" setting (although the instruction manual says that this is for cans up to 200 Ohms), or will this have a negative impact on the frequency response. I've tried, and found that the "low" setting gave, in addition to virtually zero hum, greater transparency and more detail (and a more open midrange), but that the "high" setting gave superior body and dynamics. This is especially noticeable on solo piano.
 
BTW: did you recable your AH-D1001s? Did it make a worthwhile difference?
 
 
 
 

 

[Uncle Erik]

Sounds like AC hum. Most likely a bad electrolytic cap in the power supply.

 

[CJG888]

That was precisely what I didn't want to hear!
 
Is this something I can test with a multimeter (after fully discharging the caps and earthing them via a resistor!) or do I just have to pack it all up, take it to the post office and hope for the best?
 
Is there any chance it could just be a noisy driver tube?

 

[CJG888]

Well, I've changed the driver tube. This has brought a significant improvement in clarity (and that's just a new JJ tube..... the Mullard should be good!), but the slight hum is still there. Why does it go away with the amp on the "low impedance" setting (when listening through HD600s)? Listening through my low-impedance GR07s, the hum is much louder!
 
I have read that sometimes reversing the mains polarity resolves this sort of problem (if the mains transformer is at fault). Is this true?
 
I appreciate that in markets like Germany, reversing the mains polarity is very easy to do (just turn the mains plug around 180 degrees). Indeed, most German audiophiles have become paranoid about checking their mains polarity in recent years

. In China, however, 3-pin plugs, similar to a scaled-down UK plug minus the fuse, are fitted. Are there any likely safety issues related to swapping over the live and neutral connections on a Chinese amp, or is the PSU circuit going to be identical to that used in European-market models?
 
If this fails, I guess I'll just have to send it back to the manufacturer as a warranty claim.....