[sdiy] Easy transformer question

[Stephen Begin]

Do you by any chance know where I can go to see schematics for the three
transformer version?

Also, when you are matching diodes, what exactly is it about them that has
to be matched?

thanks,
Steve

----- Original Message -----
From: "Czech Martin" <Martin.Czech at Micronas.com>
To: "Stephen Begin" <trypannon at hotmail.com>; "synth"
<synth-diy at dropmix.xs4all.nl>
Sent: Monday, March 10, 2003 5:07 AM
Subject: RE: [sdiy] Easy transformer question


I've toyed arround with such circuits a while ago:

At first, the real ring modulator circuit comes from
radio circuits, the useage of coils and transformers
is very common there, and due to the narrow bandwith
and high frequency it is easy to wind your own.

The schematic you mentioned has only two transformers,
the ones for the "audio" path.

The transformer ring modulator (real ringmodulator) circuit
lives from diode bridge biasing. That's why you need at least
two transformers with a center tap. Almost always these
taps are built using two windings in series, that's why
you see four wires.
Real ring modulators ofen have a third transformer in order
to drive the bridge. But this can be circumvented
by electronic means. However, the third transformer will enable
you to have symmetric wiring and it will prevent DC problems.

Sometimes there is no indication of winding polarity.
So you have a 50% chance of not functional circuit.
Better measure: drive the transformer primary with some (safe!)
AC voltage (I assume it is a 1:2 or 1:4 or so type).
Now connect the two secondary coils in series.
You should be able to measure a substantial AC voltage
on the secondary side, just as transformer ratio says.
If the voltage is very very small, the two secondary
coils are out of phase, try the other connection possibility.

Diodes: Ge diodes are expensive and hard to get.
It makes some difference in sound.
Matched quadruples of diodes are near to impossible to get.
The bridge offset depends on the diodes.
I.e. they should be all the same and thermally coupled.
Perhaps get a lot (a belt) of diodes and try to measure
diode voltage at two different currents (forward of course).
You can also add some resistance to each diode.

If the third transformer is replaced by electronic means,
there is a possibility to add DC voltage in order to cancel
feedthrough. However, feedthrough will vary with temperature
and aging, so a panel knob would be a good idea for that.


Of course you need good audio transformers.
These are not cheap. But this  is audio application,
not RF.
The problem is often in the low end. Farnell seems to carry
such items, with Flow ~ 40Hz or so. I got three of them.
Below that you'll get a lot of distortion and signal loss.

If the three transformer circuit is used, the modulator can of course
not go to DC.

m.c.




-----Original Message-----
From: Stephen Begin [mailto:trypannon at hotmail.com]
Sent: Samstag, 8. März 2003 00:11
To: synth
Subject: [sdiy] Easy transformer question


I finally got around to ordering the parts for a passive ring modulator, and
I remember somebody told me that the transformer they'd seen used was mouser
# 42tl028.  http://www.mouser.com//catalog/specsheets/164249.pdf

The circuit (http://www.synthfool.com/ringmod.html) shows two connections on
one side of the transformer and three on the other, but this transformer has
three connections on one side and four on the other, and I've been
unsuccessful in my search for some sort of transformer 101 type pages.
Basically what it boils down to is...do i only use two pins on the four pin
side, or do I pair some of the pins together, or what?

Thanks,
Steve