I am troubleshooting an amp for someone and noticed that although it's pretty quiet (normal hiss with hum only just audible with your ear on the dustcap) with no input, it hums if you connect a source, even a floating one such as an iPod, or even if you directly connect it to the amp's earth star!
The input valve's grid is tied to ground via 100k, and goes through the PCB to the star earth directly on its own trace.
I made the PCB for this amp and it was very quiet even during testing with a nest of clip leads.
The wiring all looks sane and I can't see anything that would cause a loop, and surely grounding the grid via a clip lead wouldn't cause a loop through the high impedance grid circuit.
Can anyone shed some light on what might be happening? I've never encountered this with amps I've wired myself, but this was done by a friend and he may have made a mistake...
The input valve's grid is tied to ground via 100k, and goes through the PCB to the star earth directly on its own trace.
I made the PCB for this amp and it was very quiet even during testing with a nest of clip leads.
The wiring all looks sane and I can't see anything that would cause a loop, and surely grounding the grid via a clip lead wouldn't cause a loop through the high impedance grid circuit.
Can anyone shed some light on what might be happening? I've never encountered this with amps I've wired myself, but this was done by a friend and he may have made a mistake...
Hi,
Is the power supply clean? How much ripple do you have?
It could also come from a magnetic coupling (output transformer too close to the mains transformer or smoothing choke).
If it hums when you connect a source, it could well be a loop somewhere.
Good luck.
Cheers,
Serge
Is the power supply clean? How much ripple do you have?
It could also come from a magnetic coupling (output transformer too close to the mains transformer or smoothing choke).
If it hums when you connect a source, it could well be a loop somewhere.
Good luck.
Cheers,
Serge
The power supply is fairly clean - it's just a CLC network fed with a GZ34 though. The amp is very quiet when the input is left floating, but as soon as you connect it to anything - there's a hum!
What I can't quite get my head around is the fact that if you connect a clip lead to the exact point the 100K grid resistor is earthed, poke a 33k resistor into the other clip and touch it to the input, it hums! How can you have a loop with 133k of resistance?? This seems to defy logic!
It seems even if I short the input terminal with a 33k resistor, it hums.
I cannot figure this out. Why are my rats nests so much quieter than this fairly neat amp?? 😕
I cannot figure this out. Why are my rats nests so much quieter than this fairly neat amp?? 😕
This is a problem with a ground loop. The loop is closed with a source connected to the amp due to a signal cables
Try to put a 10 ohm 1 w resistor in series from the circuit ground to the chassis ground of one (or both) channel.
I assume that the input pin jack are isolated from chassis
Ciao
Walter
Try to put a 10 ohm 1 w resistor in series from the circuit ground to the chassis ground of one (or both) channel.
I assume that the input pin jack are isolated from chassis
Ciao
Walter
Does this mystery amp of yours happen to use active hum cancellation ? If it does, this would explain the apparent absence of hum (incorrectly set null) with high-Z input and apparent presence with overcompensation using low-Z input.
If it doesn't, short input tube's grid to the point just below its cathode resistor. Does hum still appear or not ?
If it doesn't, short input tube's grid to the point just below its cathode resistor. Does hum still appear or not ?
It's non-raspy 50Hz, although it doesn't sound like a pure sine.
No. It's a monoblock 😉
Yep, with nylon washers that came with the phono sockets
Nope
Hmm, I haven't tried this, although I have tried bypassing the 100k grid resistor to earth and the hum appeared. It did not appear with a 33k resistor carefully placed in parallel by hand though...
Looks like rectifier's current goes to the ground. I would suggest that the PS has own ground bus from rectifier to the last filter cap, and the cap is connected to the star earth (not a center tab of a power tranny!)
Also, both power amp and a signal source may have some power filters that create a ground loop. Screw them. They are needed only to protect power lines from ripples if your amp has SMPS that generate them. However, if you can't do that line level transformers may help, but it is an expensive option.
Also, both power amp and a signal source may have some power filters that create a ground loop. Screw them. They are needed only to protect power lines from ripples if your amp has SMPS that generate them. However, if you can't do that line level transformers may help, but it is an expensive option.
I think you may have nailed it. It seems to be wired such that the first cap after the rectifier goes directly to the star earth, so the charging pulses will be going directly through it, although I would have thought that would have resulted in a raspier buzz. I will rewire the PS properly tomorrow
It definitely seems to be PSU related, when you cut the power with the input grounded you're just left with a gentle hiss and no hum at all until the valves cool down
If you haven't any hum without any cables connected to the source, it is a loop problem.
If you take one signal cable an connect the R channel to Left channel you got hum; the loop is closed.
When you connect the source to the amp (without care on ground layout) with two cables it easy to create a big loop due to the ground wire that haven't a zero resistence (in theory); this means you will have a common mode current.
If you cut this loop the problem disappaer; the method I told you (10 ohm resistor in series with the ground of each channel or only one) is used sometimes to do this.
There are two beautiful articles (in italian, I am sorry!!!) about these problems; they are published on May - June 1994 on Audioreview magazine; in the world never did this job in deep (with diagrams and solutions) to preserve and solve this type of issues. The name was "Interazioni deboli" - "Weak Interaction"
Ciao
Walter
If you take one signal cable an connect the R channel to Left channel you got hum; the loop is closed.
When you connect the source to the amp (without care on ground layout) with two cables it easy to create a big loop due to the ground wire that haven't a zero resistence (in theory); this means you will have a common mode current.
If you cut this loop the problem disappaer; the method I told you (10 ohm resistor in series with the ground of each channel or only one) is used sometimes to do this.
There are two beautiful articles (in italian, I am sorry!!!) about these problems; they are published on May - June 1994 on Audioreview magazine; in the world never did this job in deep (with diagrams and solutions) to preserve and solve this type of issues. The name was "Interazioni deboli" - "Weak Interaction"
Ciao
Walter
I've solved it! Thanks to Wavebourne's suggestion, I tied the negative lead of the first capacitor to the negative lead of the second (where the transformer centre tap also goes) instead of having it going there VIA the star earth.
It's a bit weirdly done but it's now as quiet as it should be! 😀
Thanks guys!
It's a bit weirdly done but it's now as quiet as it should be! 😀
Thanks guys!
ARGH WHAT THE HELL? IT HAS REAPPEARED
This is REALLY annoying me now. What did I do to make it go and what made it come back???
This is REALLY annoying me now. What did I do to make it go and what made it come back???
Power filter?
Try line level transformer, to isolate grounds from each other. Try any 1:1 transformer, even if it does not have good enough frequency response, it is just an experiment to check if the hum will go away or not.
Also, when I use notebook to record concerts I always disconnect it from power running on batteries, since I have unbalanced interface to it. Otherwise when it starts charging batteries it throws a garbage to the ground.
An externally hosted image should be here but it was not working when we last tested it.
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hello.
do you have an input cap (in front of the 100k grid res)?
heater voltage (6,3vac): often there are two res (typ. 100 ohms) connected from each 6,3vac line to ground................do you use them?
i guess it has something to do with the transformer..........50 hz humming that disappears if the transf. is off...........
greets
do you have an input cap (in front of the 100k grid res)?
heater voltage (6,3vac): often there are two res (typ. 100 ohms) connected from each 6,3vac line to ground................do you use them?
i guess it has something to do with the transformer..........50 hz humming that disappears if the transf. is off...........
greets
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