Hi SY,

I built and really enjoyed this phono amp; it worked well for me with HO MC, 47k input R and no SUT; it really made me grab for a record instead of digital source whenever practical :-)

Now, recently I noticed on right channel a very strange low frequency noise, not a consistent hum, but it is like:
tum-tum-tum, then second or two pause, then tum-tum, pause, then tum-tum-tum-tum,....

It is not vey loud, just noticeable on low volume level, but clearly noticeable between the songs when listening on higher volume. At first I was sure it was right channel only, but I notice it also on left channel, just much quieter.

I hooked my scope as was planning to go trough the stages to find out where it comes from but was not able to get anything on the scope; I guess since it is not a consistent frequency, but a bit random...

Would someone have any idea how to go about trouble shooting this; could it be one of the DC blocking capacitors going bad???

I did try swapping all the tubes, but there was no change.

Hi, thanks for quick reply!
I built it on PC boards I got from Jack; it worked perfectly for over a year!
Yes, the boards provide separate regulators for each channel, but it is strange that it is possible (just possible!) to hear this on the left channel as well if set top maximum volume; but this might be crosstalk between the channels?

Would you suggest a method to troubleshoot regulators or should I just replace Q7, Q8, Q9 and Q10 and see if this helps?

It is a very deep noise coming from the woofer cone; basically very similar to tapping the cone with finger. it is a bit random, like 3 taps, then short pause, then 4 taps, then pause, then 2 taps, pause, 4taps,....
Shorting the inputs does not change anything, so it is happening within the amp.

Hi, thanks, will definitely try relocating it and using different power socket.
It is strange though that only right channel is really effected; on left channel I can also hear it, but just barely on max volume setting.
I am using medical grade power input module that should filter RFI and EMI; wondering it might be actually this module that causes the problem, but, again, I would then expect both channels to be effected the same?

Hi, so, I sort it out, at least sort of.
Once I moved the phono amp and TT completely away from other equipment and plugged in different power outlet (before they ware all plugged into same power strip) the thumping sound went away.
What was really strange that first I tried moving just the phono amp, but this did not help; only once the TT was moved it stopped.
I am still struggling to understand what would cause it, especially since all the components remained unchanged, also in position, since first day, but the problem appeared only after a year....
Will try experimenting a bit more and report if I find out something interesting.

Update, some time later...finally found what caused the noise; it was my new router wireless transmitter; as soon as I turn wireless off on the router all the noise is gone. Old router was causing no problems, being in same location, but once it broke and I got a new one that seems to have stronger transmitter....
Anyway, relocated it and solved the problem!