hum in amp!

[woodturner-fran]

Hi all,

I recently bought this chinese amp. Sound from the speakers is very good, but there is a constant hum from the headphone output. The tubes are 6P1 for the 4 power tubes and 6N1 for the pre tubes. With headphones plugged in, you turn on the amp, dead silence then after 10 secs or so you get a rising buzz/hum it gets quite loud and then settles back down to a lower volume and then it remains constant in the background. It is independent of volume. Sounds like 50hz to me. I think the heaters are AC heated and heres another thing - for the life of me I can't see any rectification anywhere!

This thread has more about the amp (I'm fran in the thread)

and here are some pics (also in the thread)








I would be extremely grateful for any help that anyone can give!

Fran

[SY]

On the bottom picture, the rectifiers are on the lower left.

A schematic will surely help. 95% chance that it's a grounding issue, 5% that it's an oscillation issue.

[ray_moth]

The problems of hum have been extensively discussed in previous threads in this forum. Many helpful suggestions have been made for countering both smoothing hum (100Hz if your rectifiers are OK, or possibly 50Hz if one side is not working) and induced/pick-up hum from AC (50Hz). I suggest you do a search.

[SY]

One more thing- I'd go through and resolder about half the joints that I can see in the photos. Gray and blobby are not good things in a solder joint.

[woodturner-fran]

many thanks,


I did search but I am pretty new to this, and don't fully understand all the lingo! sorry for this! Thats why I posted with help for this amp in particular..... the learning curve is a straight line pointing to the sky!!!

OK, I see the diodes now - they're 4007, right?

I'm sorry I asked for a schematic but I/they don't have one....

I can easily resolder the joints - but can anyone tell me what I should change with regard to the grounding? What about the way theres no hum when you turn it on first and then the hum rises before settling back to a stable level?

fran

[bigwill]

Yeesh, who soldered that thing

[woodturner-fran]

Bit of an update...

I downloaded a frequency generator tonight and from what I can hear the hum is 100Hz, not 50Hz. I did read elsewhere that this might be due to inadequate smoothing caps on the B+ supply (have I got that right!) - could someone tell me would that be some of the big electrolytic caps on the bottom left hand side of the third picture? EDIT - meant to add in here that I think the hum is a bit louder on the left hand side, just enough to move the "balance" off to the left a tad.

On grounding - See on the PCB there is one silver trace that starts near those caps and goes to the front of the board and terminates at a brass screw top right? Well, I loosened that screw tonight and very quickly you get very loud hum! (through the headphones - but although you can't hear the hum through my speakers, you would hear this) Much louder than what I have at present....... this is making me think that it might be an inferior component rather than the thing not being grounded.

The other thing is that while the PCB is grounded to the chassis at that point, the chassis itself is not electrically earthed, as in back to the household supply. Is this safe?

thanks to everyone who is helping here....

Fran

[SY]

If it's at 100Hz, chances are that it's ripple noise. Could be inadequate filtering but I'll bet that it's still a grounding issue. Fix the bad solder joints first, then we can go through the grounding scheme.

A chassis not connected to earth ground is VERY hazardous. Correct that immediately. These guys should get some major butt kicks for selling something like that.

[woodturner-fran]

Ok, so I opened up the amp again and have just added a earth from the IEC socket to the chassis, man the paint is thick on that thing had to scrape through it with a screwdriver, thought I was through but it was just primer! Anyway, have it done now (and the hum hasn't gone away!)


Also went over the board in more detail. The soldering on mine is not as bad as the one above (which is the same amp from the same seller that a friend of mine has but with better pics that I could get). There are a few "dull" looking solder joints, but these are on the tube sockets.

Would you guys have any idea which capacitor I should change from the pics above? And what values of course....

I also did some measurements on voltages. The heaters are all at 6.3/6.4V. I have yet to measure voltages at the other pins (need to go and print out the data sheets!)

Fran

[SY]

The first grounding bit I'd look at is the power supply. The loop between the rectifier and filter cap carries very high ripple currents and is Suspect Number One when these things pop up. The B+ and ground ought to come directly from the filter cap terminals. And the transformer return ought to connect to exactly the same point on the negative terminal on the filter cap.