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
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)
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
I would be extremely grateful for any help that anyone can give!
Fran
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.
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
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
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
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
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.
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.
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
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
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.
SY: so OK, in the pics above the B+ is probably from those electrolytic caps - the 120uF/400V ones (I think the B+ is 270V??!) so if I understand this correctly:
The ground transformer winding should connect to the neg terminal of the cap. The pos transformer winding should connect to one side of the diode and then follow on to the pos terminal of the cap. Neg trace should then connect up with that silver trace that terminates at the brass screw in the pic above, pos will go via circuit to give the anode voltage which I have discovered for the 6P1 should be 250V.....
The other 2 caps there are rated at 200V 100uF, I must check the voltage on them too to see if they are carrying voltage above that.
Wavebourn: I can easily trace the headphone ground - if you look at the 3rd pic, the headphone output is located above and to the right of the centre screw holding down the PCB, about 2o'clock. Its a cable with black (can't see), red, and yellow/white twisted together - the yellow and white is the ground.....
I can resolder it of course, but where to?
Another thing I did is to hook up the speaker outputs to my headphones and right enough the hum is there too. I can't hear it through my speakers though (quad ESL57s).
Fran
The ground transformer winding should connect to the neg terminal of the cap. The pos transformer winding should connect to one side of the diode and then follow on to the pos terminal of the cap. Neg trace should then connect up with that silver trace that terminates at the brass screw in the pic above, pos will go via circuit to give the anode voltage which I have discovered for the 6P1 should be 250V.....
The other 2 caps there are rated at 200V 100uF, I must check the voltage on them too to see if they are carrying voltage above that.
Wavebourn: I can easily trace the headphone ground - if you look at the 3rd pic, the headphone output is located above and to the right of the centre screw holding down the PCB, about 2o'clock. Its a cable with black (can't see), red, and yellow/white twisted together - the yellow and white is the ground.....
I can resolder it of course, but where to?
Another thing I did is to hook up the speaker outputs to my headphones and right enough the hum is there too. I can't hear it through my speakers though (quad ESL57s).
Fran
woodturner-fran said:
Based on the picture you posted earlier, I'd say the rectifier is the half-wave doubler type. This means that none of the high-voltage wires of the transformer are connected to the power ground. My guess is that the hum you're experiencing may be caused by the undersized and badly designed power supply.
Also, it appears that the headphones output is connected to the driver tubes, which I think is a rather unorthodox approach. Off the top of my head, I'd suggest you try and disconnect the headphones jack from the chassis and then check the level of hum.
Regards,
Milan
Well, here's where you're going to have to trace out the circuitry and see what the topology of the rectification is. I saw something in there that could be a voltage doubler, but it's hard to say. If it is, the transformer doesn't go to ground, but you still have to avoid grounding anywhere along the loop that carries the ripple current and take it directly off the negative terminal of the bottom cap.
Anyway, trace out the power supply part of the circuit, sketch it up, and we can see what has to go where. If you can't do that, don't fool around with this, get it to a competent technician.
Willy-nilly component replacement is unlikely to solve the issue. That said, tighten down all the hardware that has any electrical connection to anything. The hum could be something as stupid as some ground contact not doing its job because the paint was inadequately scraped away...
Anyway, trace out the power supply part of the circuit, sketch it up, and we can see what has to go where. If you can't do that, don't fool around with this, get it to a competent technician.
Willy-nilly component replacement is unlikely to solve the issue. That said, tighten down all the hardware that has any electrical connection to anything. The hum could be something as stupid as some ground contact not doing its job because the paint was inadequately scraped away...
Thanks guys,
I will do my best to trace out the pcb, it'll probably be tonight (my time)when I get to do that. In the meantime I also got back onto the seller requesting a schematic, saying that I knew a bunch of people who bought this amp and most of them had the same hum issue. Maybe it'll make him go and dig one out just to save himself the hassle.
It looks simple enough, so I'll get working on it!
Fran
I will do my best to trace out the pcb, it'll probably be tonight (my time)when I get to do that. In the meantime I also got back onto the seller requesting a schematic, saying that I knew a bunch of people who bought this amp and most of them had the same hum issue. Maybe it'll make him go and dig one out just to save himself the hassle.
It looks simple enough, so I'll get working on it!
Fran
Christ I thought that it was going to be easy. I hope the thing displays below correctly!
I'm sure I've left stuff out. Heres one thing I noticed though..... the 2 6N1 tubes have the same 6.3V heater supply. However this is connected to the ground trace via a trim pot see again in pic 3 above these pots are near the volume pot at the top of the pic. Would this not allow some hum through. I thought the heater circuit should be completely separate?
Anyway, hope you can see the drawing ok.
Fran
I'm sure I've left stuff out. Heres one thing I noticed though..... the 2 6N1 tubes have the same 6.3V heater supply. However this is connected to the ground trace via a trim pot see again in pic 3 above these pots are near the volume pot at the top of the pic. Would this not allow some hum through. I thought the heater circuit should be completely separate?
Anyway, hope you can see the drawing ok.
Fran
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I'll try to redraw it with Eagle.
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