AMB Laboratories DIY Audio Forums

[bluebyte60]

This is second time it happened. I have no idea how to diagnosis. First time I order another set of pcb for replacement (I have a balanced version), after about few months later, the same thing happen to another channel, can anyone help me to give some light why?

[amb]

R33 would most likely burn up if Q24 fails from overheating and becomes short circuited. Are you using XLR jacks for your headphone output? Is this amp driving speakers?

[bluebyte60]

Hi, overheat might be possible that I had a another amp sit on top of the beta22. Yes, XLR is used but only use to drive headphone. Q24 did shorted, but not sure if there is any other parts are failing. Since it's 2nd times with exactly same spot, I wonder if there is any possible upstream can go wrong...

[amb]

Probably only R33 and Q24 got damaged. If, after replacing them, you can go through the initial setup procedure on the board, then everything should be well . The outer MOSFETs should normally run a little hotter than the inner ones.

[bluebyte60]

Hi, big tks for your asistance! After replace the Q21-Q24. Now it's start to work again. Do you see the need to install a fan simply just pull the hot air out? I have cases like this https://diyaudiostore.com/collections/dissipante. In the meantime, I stack another amp on top of beta22 like the attached, will that be a terrible idea?

[amb]

Are the output MOSFETs all mounted on the Dissipante case heatsinks? If so then an internal fan wouldn’t be much help, and the stacking of amps shouldn’t matter.

If you’re driving speakers, please don’t play at sustained high volumes. After all, β22 is a powerful headphone amp that happens to be robust enough to drive speakers, but only as a light duty power amp for the latter. The TO-220 MOSFETs are too small for high power use, even when used with large heatsinks, because the TO-220’s thermal resistance is not low enough.

[bluebyte60]

Tks for the info. The big heatsinks are not used but I did a measurement show that most of the MOSFETs are around 55 to 60 degree (C). Except one channel has Q21 to Q24 running around 150 degree. Will need to do a more comprehensive check.

[amb]

Do the initial setup procedure on that board.

[bluebyte60]

Big tks, now I figure it's the vr1-3 was not properly set