[philicorda[_2_]]

I'm still messing around with my old A&H Mod3 desk.

The transformers on all the mic inputs are very microphonic, enough to be
annoying. Apparently, this can be caused by the core of the transformer
becoming magnetised, and loose windings vibrating in the field.

I've read on the net that it is possible to degauss an amplifier's output
transformer by connecting it to a signal generator and driving it into
saturation into a dummy load.

Anyone tried this with a mic transformer? Or a guess at what
level/frequency/load might work to degauss without cooking it?

[Scott Dorsey]

philicorda wrote:
I'm still messing around with my old A&H Mod3 desk.

The transformers on all the mic inputs are very microphonic, enough to be
annoying. Apparently, this can be caused by the core of the transformer
becoming magnetised, and loose windings vibrating in the field.

That's weird. I'd be more apt to suspect there is DC on a winding than
that the core is actually magnetized.

I've read on the net that it is possible to degauss an amplifier's output
transformer by connecting it to a signal generator and driving it into
saturation into a dummy load.

It's possible to do that, but FIRST you need to use a magnetometer to
make sure it's actually magnetized because you are apt to do more harm
than good otherwise.

If you don't have a magnetometer (and everyone with a tape deck should
have one), run a 60 Hz sine wave into it, crank the level up, and watch
the thing clip. If the transformer saturates at the same point on the
top and bottom of the waveform, it's not magnetized.

Anyone tried this with a mic transformer? Or a guess at what
level/frequency/load might work to degauss without cooking it?

I have degaussed sub-ouncers that way, and also in a tape degausser, and
both worked okay. But transformers don't just get magnetized on their
own. If you test one, and it IS magnetized, you need to find out how
it got that way. Leaky caps throwing DC on a winding is the most common
culprit.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[philicorda[_2_]]

On Mon, 26 Mar 2007 20:11:07 -0400, Scott Dorsey wrote:

philicorda wrote:
I'm still messing around with my old A&H Mod3 desk.

The transformers on all the mic inputs are very microphonic, enough to be
annoying. Apparently, this can be caused by the core of the transformer
becoming magnetised, and loose windings vibrating in the field.

That's weird. I'd be more apt to suspect there is DC on a winding than
that the core is actually magnetized.

I've read on the net that it is possible to degauss an amplifier's output
transformer by connecting it to a signal generator and driving it into
saturation into a dummy load.

It's possible to do that, but FIRST you need to use a magnetometer to
make sure it's actually magnetized because you are apt to do more harm
than good otherwise.

If you don't have a magnetometer (and everyone with a tape deck should
have one), run a 60 Hz sine wave into it, crank the level up, and watch
the thing clip. If the transformer saturates at the same point on the
top and bottom of the waveform, it's not magnetized.

No magnetometer unfortunately.

I removed a transformer from it's round metal case and rigged up a needle
on a fine balance. It was attracted to the transformer! No idea how
strongly it's magnetised, but it definitely has some kind of field.
The needle was not already magnetised as it did not attract to other metal
things.

The transformers are tiny (about 1.5cm square) and the windings are not
potted or anything. They are marked "Electronic Techniques (Anglia) 276-1".


Anyone tried this with a mic transformer? Or a guess at what
level/frequency/load might work to degauss without cooking it?

I have degaussed sub-ouncers that way, and also in a tape degausser, and
both worked okay. But transformers don't just get magnetized on their
own. If you test one, and it IS magnetized, you need to find out how
it got that way. Leaky caps throwing DC on a winding is the most common
culprit.

I measured at the primary and secondary of the transformer and don't see
any DC. The phantom is not on as I have the channel out of the mixer and
only the 24V audio power is connected.

I think the capacitors could well be leaky/going off though, as there is a
lack of bass in the desk in general. Taking an insert output or from the
group buss wires has more bass on some channels.

Swapping an input transformer for another one (possibly a sowter) makes
it not microphonic, though it also reduced the mic pre's gain quite a lot
too. The preamp is 3 transistors!

I'll try the 60hz sine next and see what happens. Not bothered about
ruining a couple of them experimenting with degaussing as I have
in mind replacing quite a few with ones from my junk pile. So some
channels might soon have that much sought after classic shure vocal master
transformer sound. As you say, It would be good to find out how they got
magnetised as I don't want to ruin my other nicer junk ones though.

There are some weird things about this desk... there are two extra
twisted wires parallelled with the transformer primary that go to two
otherwise unused pins on the channel module connector, but there is
nothing wired to them on the backplane. No idea why they are there, but
they are on every channel and don't look like someones modifications.


--scott

[Scott Dorsey]

philicorda wrote:

The transformers are tiny (about 1.5cm square) and the windings are not
potted or anything. They are marked "Electronic Techniques (Anglia) 276-1".

Uh-oh. You know, you might just be a lot better off replacing those with
higher quality transformers than bothering trying to demagnetize them.

I think the capacitors could well be leaky/going off though, as there is a
lack of bass in the desk in general. Taking an insert output or from the
group buss wires has more bass on some channels.

Remember the lack of bass is also a symptom of a magnetized transformer
too, though!

I'll try the 60hz sine next and see what happens. Not bothered about
ruining a couple of them experimenting with degaussing as I have
in mind replacing quite a few with ones from my junk pile. So some
channels might soon have that much sought after classic shure vocal master
transformer sound. As you say, It would be good to find out how they got
magnetised as I don't want to ruin my other nicer junk ones though.

Remember that if you're going to do it with a bench degausser that you
will have to take the things out of their mu-metal case, as the mu-metal
case will prevent the thing from being degaussed.

There are some weird things about this desk... there are two extra
twisted wires parallelled with the transformer primary that go to two
otherwise unused pins on the channel module connector, but there is
nothing wired to them on the backplane. No idea why they are there, but
they are on every channel and don't look like someones modifications.

Maybe it's because they go to extra taps that aren't being used, since
they used generic transformers. Maybe some channel strips use them, but
the channel strips your console was configured with don't.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Mike Rivers]

On Mar 27, 2:04 pm, philicorda
The transformers on all the mic inputs are very microphonic

The transformers are tiny (about 1.5cm square) and the windings are not
potted or anything.

The reason why they're microphonic is because they're not potted.


They are marked "Electronic Techniques (Anglia) 276-1".

Swapping an input transformer for another one (possibly a sowter) makes
it not microphonic, though it also reduced the mic pre's gain quite a lot
too.

Probably because it's not the same turns ratio as the original. There
may also be a difference in noise of the preamp due to a different
winding impedance.

There are some weird things about this desk... there are two extra
twisted wires parallelled with the transformer primary that go to two
otherwise unused pins on the channel module connector, but there is
nothing wired to them on the backplane.

Probably they're used on a different mixer that uses the same card.

[philicorda[_2_]]

On Tue, 27 Mar 2007 12:27:18 -0700, Mike Rivers wrote:

On Mar 27, 2:04 pm, philicorda
The transformers on all the mic inputs are very microphonic

The transformers are tiny (about 1.5cm square) and the windings are not
potted or anything.

The reason why they're microphonic is because they're not potted.

I think it exacerbates the problem, but surely it won't have
been this bad when the desk was new. You could probably do talkback by
shouting at the mixing desk.

I guess I could pot one like a guitar pickup and see what happens. I want
the problem to be magnetism though, as its more interesting and
probably less effort to fix.



They are marked "Electronic Techniques (Anglia) 276-1".

Swapping an input transformer for another one (possibly a sowter) makes
it not microphonic, though it also reduced the mic pre's gain quite a lot
too.

Probably because it's not the same turns ratio as the original. There
may also be a difference in noise of the preamp due to a different
winding impedance.

So, if the ratio is lower than the original, would that mean a lower
impedance on the secondary, and less noise?

Also.. turning the gain up high with the existing transfomers gives
a big wide boost around 2Khz for some reason. The other transformer I tried
does this less, though it's still audible.


There are some weird things about this desk... there are two extra
twisted wires parallelled with the transformer primary that go to two
otherwise unused pins on the channel module connector, but there is
nothing wired to them on the backplane.

Probably they're used on a different mixer that uses the same card.

That could be it. I think there was another model for PA use which uses the
same channel strips. Perhaps there was a pad permanently wired in on the PA
version.

[philicorda[_2_]]

On Tue, 27 Mar 2007 14:32:43 -0400, Scott Dorsey wrote:

philicorda wrote:

The transformers are tiny (about 1.5cm square) and the windings are not
potted or anything. They are marked "Electronic Techniques (Anglia) 276-1".

Uh-oh. You know, you might just be a lot better off replacing those with
higher quality transformers than bothering trying to demagnetize them.

They do sound quite funky. I tried recording a drum kit... I don't think
they can handle much level at all and so squish a bit. It's not worth my
while spending on this desk, it's more just a place to use all the junk
transformers and inductors and stuff I've collected.

It would be nice to sort the microphony and perhaps improve the grounding
a bit. For some reason the power supply is floating and negative does not
go to ground. The negative is wired to chassis ground at the desk end
though.


I think the capacitors could well be leaky/going off though, as there is a
lack of bass in the desk in general. Taking an insert output or from the
group buss wires has more bass on some channels.

Remember the lack of bass is also a symptom of a magnetized transformer
too, though!

Ahh. Didn't know that. I'll have to give another listen on the line inputs.


I'll try the 60hz sine next and see what happens. Not bothered about
ruining a couple of them experimenting with degaussing as I have
in mind replacing quite a few with ones from my junk pile. So some
channels might soon have that much sought after classic shure vocal master
transformer sound. As you say, It would be good to find out how they got
magnetised as I don't want to ruin my other nicer junk ones though.

Remember that if you're going to do it with a bench degausser that you
will have to take the things out of their mu-metal case, as the mu-metal
case will prevent the thing from being degaussed.

There are some weird things about this desk... there are two extra
twisted wires parallelled with the transformer primary that go to two
otherwise unused pins on the channel module connector, but there is
nothing wired to them on the backplane. No idea why they are there, but
they are on every channel and don't look like someones modifications.

Maybe it's because they go to extra taps that aren't being used, since
they used generic transformers. Maybe some channel strips use them, but
the channel strips your console was configured with don't.
--scott

[Mark]

On Mar 28, 6:05 pm, philicorda
wrote:
On Tue, 27 Mar 2007 14:32:43 -0400, Scott Dorsey wrote:
philicorda wrote:

The transformers are tiny (about 1.5cm square) and the windings are not
potted or anything. They are marked "Electronic Techniques (Anglia) 276-1".

Uh-oh. You know, you might just be a lot better off replacing those with
higher quality transformers than bothering trying to demagnetize them.

They do sound quite funky. I tried recording a drum kit... I don't think
they can handle much level at all and so squish a bit. It's not worth my
while spending on this desk, it's more just a place to use all the junk
transformers and inductors and stuff I've collected.

It would be nice to sort the microphony and perhaps improve the grounding
a bit. For some reason the power supply is floating and negative does not
go to ground. The negative is wired to chassis ground at the desk end
though.



I think the capacitors could well be leaky/going off though, as there is a
lack of bass in the desk in general. Taking an insert output or from the
group buss wires has more bass on some channels.

Remember the lack of bass is also a symptom of a magnetized transformer
too, though!

Ahh. Didn't know that. I'll have to give another listen on the line inputs.


Even though the caps may be good and there is no steady state DC on
either widning, the xformer may become magentized due to the power on
and power off transients which can sometimes put DC current through a
winding for a while when the coupling caps are charging and
discharging.
See if there is anything different at all about the turn on /off and
steady state voltages with this channel compared to the others.

You should be able to demag it by connecting an audio tone from a
gnerator to either winding and gradually reducing it to 0.


Mark

[Paul Stamler]

"philicorda" wrote in
message ...
Probably because it's not the same turns ratio as the original. There
may also be a difference in noise of the preamp due to a different
winding impedance.

So, if the ratio is lower than the original, would that mean a lower
impedance on the secondary, and less noise?

A lower impedance on the secondary, yes, along with a lower signal level.
Whether it meant less noise depends very much on the circuit design.

Bipolar transistors have two noise sources, voltage noise (e-sub-n) and
current noise (i-sub-n) in addition to the thermal noise of whatever's
feeding them. If you divide voltage noise by current noise, keeping track of
units, you get the source impedance at which that circuit adds the least
noise. For example, a circuit with a voltage noise of 3.5nV/rtHz and a
current noise of 0.5pA/rtHz will have an ideal source impedance of (3.5 e-9)
/ (0.5 e-12), or 7000 ohms. (This circuit is, in fact, a typical NE5534a
opamp.)

FETs and tubes work differently. For all intents and purposes, they have no
current noise. The higher the transformer ratio, the better they'll perform
in terms of noise, all other things being equal. (They never are, of
course.)

Peace,
Paul