Transformers attacked by magnets !

I saw this, interesting trick from another (or the same ?) CJ.
It's the last post:

http://recpit.prosoundweb.com/viewtopic.php?t=4348

Another trick for transformer hackers: I did this recently as I've made a passive transformer box for bass (600/600/600 xfmr, electric bass or whatever into the first two coils in series, third coil to balanced out). It actually eats a lot of bass, strangely enough! The resulting bass tone is all mids but sits really well in a mix. Well, the other day I tried putting a tiny samarium cobalt magnet on the transformer to saturate the core magnetically. This sounds GREAT on bass. It adds lots of even-order distortion, and the xfmr overloads way more easily, but the real gain comes from when you match the polarity to your bass- there'll be one polarity that's blatantly sweeter than the other, because a bass waveform isn't symmetrical. I'm using a P-bass copy here.

So, mess around with really cheap transformers, but don't forget to also mess around with magnets on them! The tone shaping possibilities are very cool for some things, and it's a kind of effect you CANNOT get with just EQ, because it's modifying the transfer function radically, not just altering the frequency response.

Chris Johnson

Click to expand...

I figure we could get the very same effect also by putting DC-current through a winding but using magnets was new for me.
I thought using DC was recently already suggested by I forgot who. But don't know if anyone has already hands-on experiences.

And with the recent thread in mind about putting ungapped TXs to work where gapped ones are required, one could think of an internal coarse compensating-current setting and a front-panel fine control.

Finally, using a magnet could probably make up for a great stage act :green: Guitar-player attacking his Twin Reverb with a big magnet and pulled off-stage by cops...

Bye,

Peter

[quote author="clintrubber"]I tried putting a tiny samarium cobalt magnet on the transformer to saturate the core magnetically. [/b]This sounds GREAT on bass. [/quote]
Yes, magnet on the core can adjust low end.
Mr. Sennheiser do it well in his popular and still the some microphone.
I can not remember type, it is that uses small horn with astatization coil
in front of the membrane.

If microphone have diaphragm mass, this mass is trough gyrator
transformed to capacitance. And you (with primary inductance)
have second order hipass. (with low freq resonancy).
It is good on microphones, where you can not modify membrane
mass to tune it (better way is to modify magnet placement by
some paper spacer.)

In the electric circuit, you can put capacitor in the series with primary,
and modify values of resonancy by it. And/or modify transformer with the magnet.

xvlk

from xvlk:

In the electric circuit, you can put capacitor in the series with primary,
and modify values of resonancy by it. And/or modify transformer with the magnet.

Click to expand...

So it's mainly/all here about the low-end, correct ? Could imagine that, but would the higher frequencies not be affected at all ? (assuming no saturating lo-freqs present at the same time)

And:
Must admit I don't see right away why the magnet gives even-orderdistortion, but then I haven't read all of RDH chapter 5 yet.

In the electric circuit, you can put capacitor in the series with primary,

Click to expand...

We're not allowed to :wink: I mean, for a Neve BA283-like output stage there can be no cap. So I was thinking about modifying the circuit of last week by an additional control that gives some more or less compensating current (within limits)
(modif. not yet drawn and the opamp-section can be simpler

Bye,

Peter



Bye,

Peter

Yes, magnet on the core can adjust low end.
Mr. Sennheiser do it well in his popular and still the some microphone.

Click to expand...

Sennheiser MD421 low-cut is made like this - moving a permanent magnet in relation to the transformer core.

Jakob E.

[quote author="clintrubber"]
Could imagine that, but would the higher frequencies not be affected at all ? (assuming no saturating lo-freqs present at the same time)
[/quote]
Higher frequencies can not saturate core in the
voltage transformer. Equivallent of this is shunt
inductance and the higher frequency, the smaller core flux.
[quote author="clintrubber"]
I don't see right away why the magnet gives even-orderdistortion,
[/quote]
Even order distortion is property of all single - ended stage.
You must keep in mind system transistor - transformer.

It is like DC bias magnetophone. If you have no bias, you have
mainly odd harmonics, if you add bias, you have weak odd and some even.

[quote author="clintrubber"]
Neve BA283-like output stage there can be no cap. So I was thinking about modifying the circuit of last week by an additional control that gives some more or less compensating current (within limits)
[/quote]
I not know neve gears, but to regulate transformer
current you not need the OA current hell,
Simple current mirror in the collector (to partially pick output current) do work. It is said "Synthetic load".
Some two transistors and potentiometer.
And it is good, that first (and control) transistor is 10 X smaller .
10 X smaller control current and smaller controling potentiometer.

xvlk

Every time I look at that subject line, all I can think of is the song "City Baby Attacked By Rats" by GBH.

Showing my age... :wink:

from NYD:

Every time I look at that subject line, all I can think of is the song "City Baby Attacked By Rats" by GBH.

Showing my age...

Click to expand...

:wink:

Weren't they around at the same time as MDC, DRI, SOD, MOD ?
Well, might be from before that and that these threeletterwords came around a bit later ?

GBH.... another great Clay Records band!

City baby.... REVEEEEEEENGE!!!

Keef

Hi xvlk,

Thanks for the info about hi-freqs & even-order distortion.

more from xvlk:

I not know neve gears, but to regulate transformer
current you not need the OA current hell,

Click to expand...

Hey, opamps are our friends -- :grin:
-- as long as we keep them out of the signal path. :wink:
But just kidding, that opamp-circuit is straightforward and
I don't expect problems with that opamp arrangement itself.

Simple current mirror in the collector (to partially pick output current) do work. It is said "Synthetic load".
Some two transistors and potentiometer.
And it is good, that first (and control) transistor is 10 X smaller .
10 X smaller control current and smaller controling potentiometer.

Click to expand...

I think I see where you are going, you want to give the
2N3055 an active load, correct ?
So the load is forcing a fixed current equal to the 2N3055
bias current so all AC must go to the TX-prim.

Please tell if you mean something else, would be
interesting to hear.


Much debate is however going on about that DC through the TX-prim,
be it justified or adding just more myth to the Neve-DC-classA-myth.

So while I do think your "Synthetic load" would result in the same as
the path I'd like to try, I just feel to keep the original circuit as
untouched as possible and to do the 'tricks' more from the outside.
It's just a feeling, since rationally speaking I think it basically
comes down to the same result.


OK, let's update the previous drawing
(as from the original thread http://www.groupdiy.com/index.php?topic=4597&postdays=0&postorder=asc&start=15)



Note it still runs the counteracting current through the tertiary winding
since we're using a ungapped TX.
In addition to that that current is made a bit variable,
plus & minus w.r.t. the counteracting current so say making a 85% ...115% variation possible (no idea yet how much is needed,
but OK).

Note the constant portion of this current obviously just offsets
the situation; if we were using a gapped TX the saturation-control
would do -15% ... +15%.
So here we're just combining two things into one:
saturation-control and being cheap by using an ungapped TX.

To get more feel for the circuit I've added some estimated values.

For ease of disposing the waisted power at the counteracting winding
a LM317-regulator+variable resistor was considered, giving a lower
component count. That resistor must both be variable and be able to
dissipate some power, so I went back to the opamp circuit but
giving it a variable resistor at another location.

For less waisted power there can also be a separate supply
(but it'll require more additional components, a separate winding etc).
But OK, that must stop at some point.
After all we're doing class A here, so let's ignore a few more
components and Watts and go for sound and possibilities -
and not for the most economical design for now.

Regards,

Peter

Minor detail: I just realized that the electrolytic cap for some kind
of softstart (as drawn at the lower-right) should be connected to
the voltage-divider, and not in // to the 10 Ohm resistor.

And in general: please shoot ! Would be nice to hear your comments.

Bye,

Peter

The circuit drawn will present a low-impedance load to the transformer - at all frequencies. When transformer tries to output an AC voltage, the voltage-to-current converter will try to fight against it - in effect shortening it out.

You will need a way to make your current drop DC-only. And a way that does not load the transformer at audible frequencies.

A DC-servo type of circuit.

Or, even better, feed the DC current to the trafo through a fairly large inductor to make it lowfreq-and-DC-only

Jakob E.

from Jakob:

The circuit drawn will present a low-impedance load to the transformer - at all frequencies. When transformer tries to output an AC voltage, the voltage-to-current converter will try to fight against it - in effect shortening it out.

Click to expand...

Hi Jakob, thanks for the response.

The circuit is a VCCS (at least it's meant to be :wink: ) and should in principle have a high source impedance. Output-impedance of the VCCS could be made higher by cascoding, but will like be not required.
(The LA-3A circuit that Juergen had found seems to realize the same function but only uses a fixed resistor so the BD-arrangement will likely be enough w.r.t. being high-ohmic.)

The opamp & BD-NPN are actually only there to give an easy adjustable current without having to use trimmable power resistors.

You will need a way to make your current drop DC-only. And a way that does not load the transformer at audible frequencies.

Click to expand...

The AC generated by the tert.-winding would see the impedance of the current source. I think the output-voltage will vary, but the DC-current will be maintained.

Maybe we could understand this by comparing it with an active stage (here the ter. winding) that has an active load (here the VCCS).

But I should do a simulation to see what actually happens.
Or better, build the thing ! I'll go after '283-PCBs.

Regards,

Peter

Man those black schematics take ten years to load, and we have dsl.... :twisted:

[quote author="CJ"]Man those black schematics take ten years to load, and we have dsl.... :twisted:[/quote]

:?: :?: Fast here!

Serious, does this slow down things ?
Hadn't heard about that before, sorry.

Give me minute, I'll fix it.

Kidding me ? :shock:

Anyhow, the polarity-switch has been flipped.

Bye,

Peter

OK, loading quicker, you dropped about 25% of the bytes when converted.
I think our server was slow this morning too many people surfing!
no bidg deal.

ctw, do you like the sound of the cobalts or alnico's best? :razz:

No problem, just good to know about those black pixels.

ctw, do you like the sound of the cobalts or alnico's best?

Click to expand...

Both tasted good, but one of them in partic. was a bit heavy on the stomach - forgot which one.

But seriously, I haven't pointed a soldering-iron to all this.
Want to get me some BA283-PCBs (like Fabios) and start trying
all this on the bench & on the ears.

BTW, interesting bit of THD-vs-materials info here:
Output Transformer Lamination Selection
http://www.cinemag.biz/application_notes/AN-104.pdf

Hadn't read about the material-side yet, but starting to grasp a little.
For you it'll be known territory though.

Bye,

Peter