[Gareth Magennis]

Hi,

I was brought a 1U stereo mic pre for repair because one side had a hum. On
inspection, it was the mic amp PCB nearest the mains toroidal transformer
that was producing the hum, and loosening and rotating the transformer had a
big effect on the amount of hum induced in that channel, thus, I believe,
conclusive proof that the toroid is the culprit.

The customer claims that this hum suddenly appeared. He also mentioned that
he had a couple of Coles ribbon mics (4038) that he had stored on top of the
mic pre, and was wondering if their strong magnets had anything to do with
creating this hum.

So the questions are, is it possible for a toroidal transformer to suddenly
develop a magnetic "leak"? And, is it remotely possible for a strong
external magnet to somehow also induce such a leak (albeit through a rather
robust 1U chassis).


The hum is only noticable at full gain, and is way lower than the
accompanying hiss. Nevertheless, the customer claims he can hear it all
over his (classical) mixes and is not happy at having paid a large amount of
money for a humming unit, which is just out of warranty. (It sounds like a
100Hz PSU hum, not a 50Hz ground type noise by the way, that'd be 120 and 60
to you guys over the pond, I am in the UK).

It may also be worth noting that this unit is designed and built in the USA,
just in case the difference in mains frequency may be a factor, though my
suspicion is the hum has always been there.



Cheers,


Gareth.

[Scott Dorsey]

Gareth Magennis wrote:
I was brought a 1U stereo mic pre for repair because one side had a hum. On
inspection, it was the mic amp PCB nearest the mains toroidal transformer
that was producing the hum, and loosening and rotating the transformer had a
big effect on the amount of hum induced in that channel, thus, I believe,
conclusive proof that the toroid is the culprit.

If this was from a legitimate manufacturer, they should have adjusted the
transformer the same way you did, rotating it for lowest hum.

The customer claims that this hum suddenly appeared. He also mentioned that
he had a couple of Coles ribbon mics (4038) that he had stored on top of the
mic pre, and was wondering if their strong magnets had anything to do with
creating this hum.

I don't think they could have affected the transformer, although they might
have affected a transformer shield. Check the input transformers with a
magnetometer and see. If you don't have a magnetometer, and the shields are
removable, it won't hurt to put them in a bulk eraser.

So the questions are, is it possible for a toroidal transformer to suddenly
develop a magnetic "leak"? And, is it remotely possible for a strong
external magnet to somehow also induce such a leak (albeit through a rather
robust 1U chassis).

The only thing that I can think of which would cause a sudden magnetic
field leak is if the core was damaged or cracked. Was this dropped?
If so, though, that should also be running hot.

The hum is only noticable at full gain, and is way lower than the
accompanying hiss. Nevertheless, the customer claims he can hear it all
over his (classical) mixes and is not happy at having paid a large amount of
money for a humming unit, which is just out of warranty. (It sounds like a
100Hz PSU hum, not a 50Hz ground type noise by the way, that'd be 120 and 60
to you guys over the pond, I am in the UK).

It may also be worth noting that this unit is designed and built in the USA,
just in case the difference in mains frequency may be a factor, though my
suspicion is the hum has always been there.

The leakage at 50 Hz is probably not quite as bad as at 60 Hz but it's
probably a toss-up.

Adding some more shielding around the power transformer won't hurt.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Gareth Magennis]

"Soundhaspriority" wrote in message
...


"Gareth Magennis" wrote in message
...

So the questions are, is it possible for a toroidal transformer to
suddenly develop a magnetic "leak"? And, is it remotely possible for a
strong external magnet to somehow also induce such a leak (albeit through
a rather robust 1U chassis).

Toroidal transformers normally leak. The theory says B=0 externally, but
that is not practically true. A winding shift increases leakage. But by
rotating the transformer, you are repositioning the leads. That in itself
is significant.


The hum is only noticable at full gain, and is way lower than the
accompanying hiss. Nevertheless, the customer claims he can hear it all
over his (classical) mixes and is not happy at having paid a large amount
of money for a humming unit, which is just out of warranty. (It sounds
like a 100Hz PSU hum, not a 50Hz ground type noise by the way, that'd be
120 and 60 to you guys over the pond, I am in the UK).

It may also be worth noting that this unit is designed and built in the
USA, just in case the difference in mains frequency may be a factor,
though my suspicion is the hum has always been there.

I sympathize with your suspicion.

If it is in fact the case that the hum is new, in spite of your rotation
experiments, I would look at the regulation. The output impedance of the
supply will affect the vulnerability to induced currents.


Bob Morein
(310) 237-6511





I don't hve the unit here any more, the customer is intending shipping it to
the manufacturer in the States, after videoing the evidence on his iphone.
He has been in contact with the designer of this unit, who has been vey keen
to help, so I expect a good result, apart from the shipping bills!

By the way, the transformer actually has no screening at all, Scott. There
is a metal partition running from front to back of the 1U case separating
the transformer and PSU from the cards though.


Thanks for the replies, most educational.




Gareth.

[Scott Dorsey]

Gareth Magennis wrote:

I don't hve the unit here any more, the customer is intending shipping it to
the manufacturer in the States, after videoing the evidence on his iphone.
He has been in contact with the designer of this unit, who has been vey keen
to help, so I expect a good result, apart from the shipping bills!

You may well be seeing an engineering problem here, yes.

By the way, the transformer actually has no screening at all, Scott. There
is a metal partition running from front to back of the 1U case separating
the transformer and PSU from the cards though.

What about the input transformers? The way this stuff normally happens is
that the power transformer radiates field and the input transformers pick it
up. It's cheaper and easier to shield the input transformers than the power
transformer and also gives you isolation from the power amp in the rack space
above.

I have seen transformerless input stages pick up magnetic fields also, due
mostly to bad PC board layout. That's a different issue really, though.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."