[sdiy] Urei 1176 old version single ended output - how does it work?

[jhaible at debitel.net]

Hi.

I have a question about something I don't understand.

I have built a clone of the Urei 1176 Limiter some years ago.
But there I have used the driver station of later versions,
which is an ordinary class AB power amp and output transformer.
(http://www.oldcrows.net/~jhaible/compressor/jh_compress.html)

But ever since I built this, I was wondering about the older
versions' output stage, which is single ended class A with a
very special transformer coupling.

Here are two links to show what I mean:
http://www.waltzingbear.com/Schematics/Urei/Urei_1176_page_2.htm
and
http://www.waltzingbear.com/Schematics/Urei/Urei_1176LN_series_D.htm

These circuits leave me puzzled.
I know that it's possible to build transformers that work directly
in single ended circuits - they carry the whole plate or collector 
DC current, they have an air gap to avoid satuaration, and they 
need to be bigger than a push-pull type or ac coupled transformer
of similar power rating and frequency response.
So far everything is clear. Most single ended tube amps work like
this, and there was a Neve (?) transistor amp like this as well.

But the Urei circuit has one primary transformer winding connected
to the collector of the power transistor, and another primary
winding connected to the emitter. So my first thought was they made
some clever circuit where two identical DC currents thru two indentical
transformer windings will make their magnetic field cancel (like
in a push-pull stage), such that no special air gap and oversized
transformer is needed. And if you look at the polarity of the
primary windings, apparently that's exactly what they are doing.
(The "black" wire apparently is tied to a positive supply voltage;
there must be a connection dot missing where it corosses a horizontal
line.)

So the DC current's influence will cancel. Good.
Only that the AC will cancel as well. 8-(
And here is where I don't understand it. It looks like a phase
splitter configuration, but it isn't. When the power transistor
is conducting more than its DC bias current, both collector and emitter
current will increase - same for decreasing - so if there was
a cancellation effect for DC, there will be the same cancellation
for AC. I can turn it around as much as I try, it's always that way.
Unless, of course, one of the primary windings would be _bypassed_
for AC, by connecting a capacitor in parallel to the winding.
Now there is such a capacitor, C14 in 
http://www.waltzingbear.com/Schematics/Urei/Urei_1176_page_2.htm ,
but it's way too small to bypass a considerable amount of AC
current.
So I'm really puzzled.

Of course not knowing the transformer data makes interpretation
of the circuit difficult, and the asumption of a DC currentn
compensation may be wrong from the start. I can't even read all
of the resistor's component values exactly.
The purple audio version of the schemos show readable component values,
and the connection dot mentioned above, at least:
http://www.purpleaudio.com/pdflib/PurpleMC76.pdf

I thought I'd share my thoughts; maybe somebody else on the
list can shed some light.

JH.


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