Hello Paul,
So it's just the amp, with the inputs shorted, and the two input grounds connected together. It buzzes !! Erm, not easy at all.
I don't know what to advise really. Does the amp have any history, has it been OK in the past, and has it had any work done on it. It's a bit like a classic case of the grounds being "contaminated" which is very common in DIY stuff when the wiring layout scheme isn't correct. Commercial stuff is usually fairly good on this.
Rotating the transformer is something that can be done ( no one ever seems to mention it actually ) and that can help find a null in any stray magnetic field but this sounds a bit more than that.
If it is a wiring problem this won't help at all.
I'll have a think -- tea time
So it's just the amp, with the inputs shorted, and the two input grounds connected together. It buzzes !! Erm, not easy at all.
I don't know what to advise really. Does the amp have any history, has it been OK in the past, and has it had any work done on it. It's a bit like a classic case of the grounds being "contaminated" which is very common in DIY stuff when the wiring layout scheme isn't correct. Commercial stuff is usually fairly good on this.
Rotating the transformer is something that can be done ( no one ever seems to mention it actually ) and that can help find a null in any stray magnetic field but this sounds a bit more than that.
If it is a wiring problem this won't help at all.
I'll have a think -- tea time
I bought the ID75 and the ID1P together around 12 months ago.Its been like this since day one.
The guy I bought them from says it didn't hum any more then other amps he had used.As he posted here about another issue which I havn't noticed and didn't mention any hum I would be inclined to believe him.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=93992&highlight=
It was pretty dusty when I got it (and looked untouched) due to the open mesh case.The original dealer stickers are on both units so I guess they were supplied as a pair.
I've basically stripped it down cleaned,replaced all the electrolytic caps/bridge rectifier/checked all wiring etc.There was one resistor that was a bit charred on each pcb so I replaced those.
The fact that each channel works noise free independently would suggest to me that the fault is something other then on the two PCB's......but i'm out of my depth here.
Paul.
The guy I bought them from says it didn't hum any more then other amps he had used.As he posted here about another issue which I havn't noticed and didn't mention any hum I would be inclined to believe him.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=93992&highlight=
It was pretty dusty when I got it (and looked untouched) due to the open mesh case.The original dealer stickers are on both units so I guess they were supplied as a pair.
I've basically stripped it down cleaned,replaced all the electrolytic caps/bridge rectifier/checked all wiring etc.There was one resistor that was a bit charred on each pcb so I replaced those.
The fact that each channel works noise free independently would suggest to me that the fault is something other then on the two PCB's......but i'm out of my depth here.
Paul.
Grounding and layout is a vast and complex subject. The two PCB's are fine on their own. The problem is (Possibly) in the way the wiring is configured. Again we are only guessing but it does seem likely from the tests you have done that the very act of connecting the two input grounds together ( and with a resistance between ground and each input -- be it shorting plug or the 22 k pot of the preamp ) is causing an internal ground loop for want of a better word.
Without literally pulling the amp apart and actually tracing it all out I'm afraid I can't say for sure.
It might be worth trying this. Go back to the shorting plugs again. On their own no buzz. Connect the grounds together and it buzzes. What happens if you connect the grounds via a low value resistor, say in the range 2.2 to 22 ohms -- it's not critical. Any buzz now ? What I am getting at is that it may be possible to "lift" one of the grounds, the low value resistor is enough to stop circulating currents yet does not affect the signal. It's a recognised trick -- even some of the big names use it.
Without literally pulling the amp apart and actually tracing it all out I'm afraid I can't say for sure.
It might be worth trying this. Go back to the shorting plugs again. On their own no buzz. Connect the grounds together and it buzzes. What happens if you connect the grounds via a low value resistor, say in the range 2.2 to 22 ohms -- it's not critical. Any buzz now ? What I am getting at is that it may be possible to "lift" one of the grounds, the low value resistor is enough to stop circulating currents yet does not affect the signal. It's a recognised trick -- even some of the big names use it.
Ok Mooly cheers..........
Resistor is left channel ground with passive connected = that channel still with buzz/hum but the right channel is pretty quiet.
Do I now need to try a resistor in the right channel as well ?
I've got to go out in a bit so it will probably be tomorrow now if I need to do this.Thanks
Paul.
Resistor is left channel ground with passive connected = that channel still with buzz/hum but the right channel is pretty quiet.
Do I now need to try a resistor in the right channel as well ?
I've got to go out in a bit so it will probably be tomorrow now if I need to do this.Thanks
Paul.
Is it a major job to lift the tranny out ? By just extending the wires it would help prove where the problem lies.
If you look at this drawing, each amp on it's own is fine. The problem comes when the inputs are joined together. Firstly by the grounds, but also by the center signal conductor. This occurs because the volume pot particularly at low levels "shorts" the center signal lead to ground.
Until they are joined the stray field from the transformer is not such a problem. When they are joined the field can then induce a current into the wiring -- it's like a "loop" around the tranny. This induced current causes the hum. Depending also on the resistance of any wires/PCB tracks etc this current also develops a volt drop along those wires. If the signal ground and feedback returns of the amp happen to be separated by some of this "resistance" then the amplifier will amplify that signal ( the volt drop ) as to the amp it appears as a valid input.
If the problem is down to the physical wiring scheme itself, then lifting the tranny away will not reduce the hum.
If the hum goes lifting it away then it is down to induction from the stray field.
Got your message as well Quite a bit reading.
If you look at this drawing, each amp on it's own is fine. The problem comes when the inputs are joined together. Firstly by the grounds, but also by the center signal conductor. This occurs because the volume pot particularly at low levels "shorts" the center signal lead to ground.
Until they are joined the stray field from the transformer is not such a problem. When they are joined the field can then induce a current into the wiring -- it's like a "loop" around the tranny. This induced current causes the hum. Depending also on the resistance of any wires/PCB tracks etc this current also develops a volt drop along those wires. If the signal ground and feedback returns of the amp happen to be separated by some of this "resistance" then the amplifier will amplify that signal ( the volt drop ) as to the amp it appears as a valid input.
If the problem is down to the physical wiring scheme itself, then lifting the tranny away will not reduce the hum.
If the hum goes lifting it away then it is down to induction from the stray field.
Got your message as well
Quite a bit reading.Attachments
Thanks mate.
Removing the transformer is alot of work,though I might try this at some point if I get the time.
What you say seems to makes sense,but what I don't understand is the amp did look all original...........surely it wouldn't of had this problem from new ?
If the other/older amp which is a bit off a mess wiring wise had the problem...............................perhaps I should build a pair of monoblocks out of them
Cheers,
Paul.
Removing the transformer is alot of work,though I might try this at some point if I get the time.
What you say seems to makes sense,but what I don't understand is the amp did look all original...........surely it wouldn't of had this problem from new ?
If the other/older amp which is a bit off a mess wiring wise had the problem...............................perhaps I should build a pair of monoblocks out of them
Cheers,
Paul.
quickie said:
That's the bit I don't understand either
The fact that it buzzes with a passive preamp, and even more importantly with the inputs (both signals and grounds) all tied means the problem is in the amp.
Rotating the tranny is a possible try out.
Sorry I can't add any more really, your going to have to "get technical" with it now.
The big question ! Is it induced hum ? Is it a wiring problem ? Is it both ?
Good luck with it
Have you compared the wiring between the two amps carefully as I know you say the other is OK. Nothing silly like the phono sockets not insulated from the chassis. Have you measured on ohms from the case and mains earth to phono earth on both amps. Unpug everything first -- not because of any hazard but because the tiniest circulating currents and voltages play havoc with resistance readings on most DVM's
At least we have established that the problem is 100% with the power amp...............I wasn't so sure of before
The only thing I can think of is the transforrmer buzzing in the ID75 is definately louder then in the other amp and the circuit/wiring are picking this up ?
I don't know if they are indentical between the two amps as of yet,they appear to be custom made Talema jobbies.The front end has its own higher voltage supply.
I'll get in some wire/crimps and get the thing out from the case at some point,though thinking about it removing the two heatsinks might be easier.
Thanks for all your help so far...........
Paul.
The only thing I can think of is the transforrmer buzzing in the ID75 is definately louder then in the other amp and the circuit/wiring are picking this up ?
I don't know if they are indentical between the two amps as of yet,they appear to be custom made Talema jobbies.The front end has its own higher voltage supply.
I'll get in some wire/crimps and get the thing out from the case at some point,though thinking about it removing the two heatsinks might be easier.
Thanks for all your help so far...........
Paul.
Hi Paul,
Buzzing transformers are another issue. Something else to read, this explains it quite well and how to resolve it.
http://sound.westhost.com/articles/xfmr-dc.htm
Buzzing transformers are another issue. Something else to read, this explains it quite well and how to resolve it.
http://sound.westhost.com/articles/xfmr-dc.htm
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