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Old 7th October 2012, 08:26 PM   #1
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Default Locating hum source

Hey there,

my amp test setup is finished und luckily the first power-up went without smoke and yielded some nice music output

See Rundmaus at work - PP1C without sand
for details...

Now, some slight buzz/hum is audible close to the speaker. It does not sound like a clean 50Hz/100Hz sine wave, instead somewhat 'sharper', sawtooth-like.
Also, it did not disappear instantly when I switched off the mains.

Conclusions so far: not heater-related, not due to B+ ripple

I am planning to follow these steps to locate the problem:

* check amplitude and waveform of the hum via oscilloscope

* ground the grids of the input/PI stage, if the hum is induced in the input wiring/volume pot circuitry, it should disappear

* if not, ground the grids of the finals, if the hum disappears, it's coming from the PI stage

* if it still hums, ask someone more experienced

Did I forget anything important? Any remarks concerning possible sources of not mains-related hum?

Thanks in advance,
Andreas
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Old 7th October 2012, 11:46 PM   #2
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The best way to determine where the hum is coming from is to o'scope and see what the waveform and frequency is. If double the line frequency and sawtooth like, it's AC ripple getting into the signal chain, usually because of ground loop problems. Could also be magnetic coupling from an AC ripple choke.

If the same as the line frequency, then it's likely magnetic coupling between a PTX and the OPT.
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Old 8th October 2012, 01:26 AM   #3
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by Miles Prower View Post
The best way to determine where the hum is coming from is to o'scope and see what the waveform and frequency is. If double the line frequency and sawtooth like, it's AC ripple getting into the signal chain, usually because of ground loop problems. Could also be magnetic coupling from an AC ripple choke.

If the same as the line frequency, then it's likely magnetic coupling between a PTX and the OPT.
He has already switched off the AC mains power and listened as the amp faded out. This is a VERY good test because an amp still running on no AC power can not have B+ ripple, Heater noise or magnetic coupling between transformers.

With the mains switch off the only source of hum is the AC mains wiring inside the chassis that connects the input IEC connector to the switch. Make sure that wwire is tightly twisted and tuck in the corner of the chassis and that there are no audio signal or power supply wires even close to the mains wiring. You can test this by unplugging the amp and listening vs, switching off the amp and listening.

The only thing left is that AC hum is getting in via the "environment". The best solution to that is "correct" value grid stoper resisters that are soldred directly to the tube socket and also using shielded wires on the signal wires inside the amp. Shielding really does help (but remember to ground only one end of the shield.)
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Old 8th October 2012, 03:47 AM   #4
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check your source isint inducing an noise and also try unplugging everything on the mains circuit you are powering the amp from and see if that resolves anything....
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Old 8th October 2012, 03:49 AM   #5
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Also, make sure that there is no space between signal and signal ground conductors, anywhere. Otherwise they are an antenna for time-varying magnetic fields. Tightly twist them together. (Do the same for all conductor pairs.)

A picture of the build and a schematic would be very helpful.
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Old 8th October 2012, 11:10 AM   #6
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Hey there,

Quote:
Originally Posted by gootee View Post
Also, make sure that there is no space between signal and signal ground conductors, anywhere. Otherwise they are an antenna for time-varying magnetic fields. Tightly twist them together. (Do the same for all conductor pairs.)
I tried to do that wherever possible, but not in all cases there's a signal ground wire there to be paired with the signal carrying one.

Quote:
A picture of the build and a schematic would be very helpful.
The 'build' is currently a not-very-tidy test setup, see the following post:

Rundmaus at work - PP1C without sand

Schematic can be found in the same thread, although it has been combined with a different power supply:

Rundmaus at work - PP1C without sand

Regards,
Andreas
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Old 8th October 2012, 04:11 PM   #7
pointy is offline pointy  United Kingdom
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Will you be using negative feed back
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Old 8th October 2012, 06:46 PM   #8
gootee is offline gootee  United States
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Quote:
Originally Posted by Rundmaus View Post
Hey there,



I tried to do that wherever possible, but not in all cases there's a signal ground wire there to be paired with the signal carrying one.



The 'build' is currently a not-very-tidy test setup, see the following post:

Rundmaus at work - PP1C without sand

Schematic can be found in the same thread, although it has been combined with a different power supply:

Rundmaus at work - PP1C without sand

Regards,
Andreas
Andreas,

You could try running the two signal grounds through wires, instead of through the chassis, back to the ground end of the last capacitor of the main power supply.

Those are the chassis gnd symbols on the schematic that are between the two 100K inout resistors and between the two 330k resistors for the second stage.

Can you post a schematic of the power supply?

If you have more than one smoothing cap for the main supply, then the first smoothing cap's ground should probably connect at or very near the transformer or rectifier ground, and the other caps' grounds should connect at a different spot, which is probably where the signal grounds from above should also connect. The idea is to not have the rectifier charging pulses sharing the same conductor path as anything else, since they will induce voltages across the conductor, which would then be arithmetically summed, to some degree, with the voltages back at the non-ground ends of whatever shares the conductive path. It is part of the concept of "star grounding", which it looks like is not implemented in your build. But doing it for only the signal grounds might help.

You also have some "enclosed loop areas". Just looking at the schematic: If you could put the 100k input Rs and the 100 Ohm Rs in a straight line, and then pull the 330Ks and 0,33 uFs from the next stage over and put them and their wiring right against the line of input resistors, and then put the two 4k7 Rs right next to each other (centered vertically on that part of the schematic) and run their four wires twisted together, straight to the left all the way to where the line of input resistors was, you could probably close up the loop area and not have antennas for the time-varying magnetic fields to induce currents in. There are probably several ways to accomplish the same thing. So there is probably a better way than I described. If you could also get the ground wires to follow their signal wires for as long as possible, it would be even better. Ideally, you might have wanted to fold the schematic in half, along the horizontal center line, and then implement it that way. But if you don't want to move tubes around at this point you should be able to contort the wiring to accomplish almost the same effect.

If we're lucky maybe DF96 or one of the other experts will chime in.

Read this thread: Thoughts of apparent noise on ground - due to power tx?

Cheers,

Tom

Last edited by gootee; 8th October 2012 at 07:02 PM.
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Old 8th October 2012, 09:58 PM   #9
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not liking what I am seeing in the schematic. you have 310 VAC and only 200uf of 450V capacitors. Whats you B+ voltage. 310 on a full wave rectifier could yield 465 volts which is above the 450V the caps are rated for.
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Old 8th October 2012, 10:31 PM   #10
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Quote:
Originally Posted by speakerfritz View Post
not liking what I am seeing in the schematic. you have 310 VAC and only 200uf of 450V capacitors. Whats you B+ voltage. 310 on a full wave rectifier could yield 465 volts which is above the 450V the caps are rated for.
This is the original PSU I did not use, actual schematic follows tomorrow! Now tired --> bed!
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