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electromagnetic hum problem
electromagnetic hum problem
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Old 14th February 2015, 02:30 AM   #1
VMUNIX is offline VMUNIX  Argentina
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Default electromagnetic hum problem

I built this some weeks ago and although it sounds good enough for me I'm getting quite a bit of hum from the power transformer. It was purpose machine wound although i don't think there's any shielding.

This pic shows the location of the transformer.
electromagnetic hum problem


So what I did is to extend the wires and locate the transformer about 40cm away from the chasis and the hum was gone, so I tried with that copper foil around it and it really didn't improve, not noticeable. When I closed the transformer near the pentodes I got the hum again, so I was considering making a metal box for it. Any suggestions ?

electromagnetic hum problem
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Old 14th February 2015, 02:44 AM   #2
Keit is offline Keit  Australia
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Three things straight off:-

1. Your copper foil looks very thin. The usual gauge is 1 mm or thicker.
2. Is that a steel chassis or aluminium? If steel, some transfomer flux may be circulating in the chassis. No ammount of foil will fix that. Try mounting the transformer with more non-magnetic spacers between it and the chassis.
3. How did you join the ends of the foil together? The join must be of very low resistance for the foil to do its job and short out the leakage flux. The foil should be cleaned of all fingerprints at the join with saucepan cleaner powder, overlapped a few mm, and soldered with a high wattage iron, not the usual low power iron for electron use.

Last edited by Keit; 14th February 2015 at 02:47 AM.
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Old 14th February 2015, 03:06 AM   #3
VMUNIX is offline VMUNIX  Argentina
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Quote:
Originally Posted by Keit View Post
Three things straight off:-

1. Your copper foil looks very thin. The usual gauge is 1 mm or thicker.
it's thin indeed, it's what I use to shield stages inside the chasis like separators.
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Originally Posted by Keit View Post
2. Is that a steel chassis or aluminium? If steel, some transfomer flux may be circulating in the chassis. No ammount of foil will fix that. Try mounting the transformer with more non-magnetic spacers between it and the chassis.
It's iron, but even holding the transformer with my hands close where it should be is enough to induct noise.
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3. How did you join the ends of the foil together? The join must be of very low resistance for the foil to do its job and short out the leakage flux. The foil should be cleaned of all fingerprints at the join with saucepan cleaner powder, overlapped a few mm, and soldered with a high wattage iron, not the usual low power iron for electron use.
Ups!, I did not solder the ends and perhaps there not even contact between them, so this one should be well joined as opposed to the inner shielding between the winding's, which by the way there isn't any in this one.
The shield should be grounded ?
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Old 14th February 2015, 04:51 AM   #4
Keit is offline Keit  Australia
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It won't make any difference to the shielding effect of the foil if it is grounded or not.

However, grounding a metal that is otherwise not electrically connected to anything is good practice - from a safety point of view and for minimising noise not magnetic in origin. From teh photo, it looks like you have a glass fibre board of some kind between the transformer core and the chassis. Assuming it is a thermoset and not a thermoplastic, that is good practice, but teh foil should go on teh core side of it, not the chassis side. It probably won't make a noticable difference though.

If your foil is thin, you can proably get away with a standard soldering iron.
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Old 14th February 2015, 09:00 AM   #5
DF96 is offline DF96  England
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The copper band around the transformer is not a shield for electric fields, which is what I suspect the OP thinks. Such a shield can be almost as thin as you like, and need not form a complete turn, but must be grounded.

This is a belly band, and it reduces external magnetic fields by acting as a very low resistance shorted turn. So it must be thick enough and have a very good low resistance connection at the join. It need not be grounded.

Basically the problem is that your transformer has far too much external flux. Custom winding does not guarantee a good quality result. Is it possible that the core is saturating? It looks quite small. One might expect a power transformer to be similar in size to the OPTs, not smaller.
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Old 14th February 2015, 09:09 AM   #6
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Have you tried rotating the transformers by 30degreess from square?
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Old 14th February 2015, 09:44 AM   #7
Keit is offline Keit  Australia
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Is it possible that the core is saturating? It looks quite small. One might expect a power transformer to be similar in size to the OPTs, not smaller.
If the core is saturating, it will certainly leak a lot more flux. But that is not likely to be his problem.

Don't overlook that it is excessive voltage that causes saturation, not the load. Flux density in a transformer is determined by voltage and self inductance, and is independent of load. So, even if the OP chose a transformer too small, so long as he chose the correct voltage, hum won't be his problem. An overheated transformer yes, regulation issues perhaps, but not hum.

It is quite common for the power transformer to be smaller than the output transformers. It appears that there are two output transfomers - hence the amp is two-channel. But only two large tubes, so its single ended - DC in the output transformers. That requires a large gapped core in the ouitput trans to retain sufficient inductance.

Quality output transformers tend to be large because they are designed to be efficient from 20 Hz to 20 kHz or so. That means plenty of copper window (= large core), an eye on core losses in design, and space consuming winding techniques to maximise coupling and minimise capacitance.

A power transformer only has to work at 50 Hz, so even though in a Class A amplifier typically has to carry 5 or 6 times the audio output power, the design emphasis is minimum cost.

But they OP probably grabbed whatever output trans he had available anyway.

Last edited by Keit; 14th February 2015 at 09:50 AM.
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Old 14th February 2015, 09:55 AM   #8
DF96 is offline DF96  England
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A quick check with a lamp limiter would show whether the transformer is saturating. Remove the valves and power up with a lamp limiter. If the lamp lights then saturation is probably occuring.

Who designed the transformer?
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Old 14th February 2015, 10:08 AM   #9
Keit is offline Keit  Australia
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The OP said he didn't solder the ends of the foil together, so as to make it indeed a very low resistance shorted turn. That's what he should do first. If it doesn't make sufficient improvement (it most likely will), he should increase the effective foil thickness by either replacing it or adding another layer.

Why go looking for a problem that his amplifier symptoms don't fit? If you present to a doctor with influenza, he doesn't order a urine protein test.

Last edited by Keit; 14th February 2015 at 10:10 AM.
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Old 14th February 2015, 10:26 AM   #10
infinia is offline infinia  United States
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I doubt the transformer is hardlimiting enough for a light bulb to light, otherwise excess heat would of been reported. if the foil doesn't help, next up, better isolating the mounting screws with fiber washers top and bottom.
remove all tubes and measure secondary voltage with a variac will give the most useful data, linearity etc.> if the sound changes with removed load then it's probably a buzzing winding, if it changes with input voltage then look at core saturation issues.
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