Hey folks -
Some background info on this noise everybody is talking about...
What you are hearing is tube microphonics, not any type of electronic oscillation (even though it no doubt sounds unlike any "microphonic" tube you've heard before).
The submini tubes used have some very small mechanical elements that, when excited, vibrate like a tuning fork. I suspect that the filament (a tiny wire under tension) is the culprit here, but I'm not certain. That's why when you hit the amp, plug in headphones or the input cable, or even flip the power switch, you sometimes hear a high-pitched tone that slowly dies out over a few seconds. (Since the jacks and power switch are mounted on the PCB, they all transmit vibrations to the board, then on to the tubes).
Fellow tube dude Steve Bench noticed the same thing about this type of tube:
"Microphonics with these tubes at very low (phono) levels is odd. Rather than the boink rattle (etc) one expects, the only thing I get is a low level tone of about 3.5kHz when you whack the box. I would not have expected such a high Q mechanical circuit." (
http://members.aol.com/sbench/6088pre2.html)
I'm sure there is some variation from one tube to another, but all the amps do this to some extent. Personally I don't find it to be an issue, as I generally don't subject the amp to any vibration. I also hear it when I plug in a cable or flip the power sometimes. It's definitly audible, but is not so loud as to be terribly annoying (at least to me).
I tried many, many submini tube types in an attempt to minimize this noise. The ones used in these amps are the quietest low-filament-power tubes I could find as far as microphonics go. There are some tubes (like the 6418) that sound great, but when you ping them they continue to sing louder, for a very long time (like 10 seconds). The tubes we used also sing if you hit them, but generally their song dies out in a several seconds.
Since the volume control is before the tube stage, the volume control has no effect on this microphonic sound. You will hear it the same even if the volume is turned all the way down.
There's not much that can be done about the singing. It _might_ help to try and do some mechanical isolation of the tubes from the PCB - though I tried a few things and didn't have much sucess. We wound up just sticking the tubes down with foam tape. Maybe some majic dampening goo would help?
If you take the amp apart, be careful that the tips of the tubes don't come into contact with the inside of the extruded metal case when the PCB is slid back inside; if they do, that will make them more likely to get pinged.
There are no drop-in replacements for the tube that we used that I've been able to find (there are some euro types that _might_ work but I've been unable to get any). There are other tube types that can work in the circuit, but require changes in either the filament voltage, bias, and/or plate load resistors. The other options that I tried are either more microphonic, or burn more power, or don't sound very good. That - combined with the fact that there is only one manufacturer of this tube, and that it has a very long lifetime - is why the tubes are soldered in. Sorry, not much tube rolling opportunity here.
I've never noticed any noise on charging, though honestly I didn't intend to have the amp being listened to while charging, so I didn't do anything special to make that mode of operation quiet. I wouldn't be surprised if some power adapter noise gets through, especially when charging a very low battery (initially high charge current), and especially if using very efficient IEMs.
Well, hopefully that clears up some of the questions and concerns. I'll try and check back for more questions later.
Pete
