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

[JH.]

Thanks Rene, I wasn't aware there might be other benefits (than DC
compensation)
for such a configuration. Looking at the Quad schemos what you say makes
perfect sense: So the Urei  is most probably a typical single ended
configuration with air gap transformer.

This still makes me think about a circuit where one winding is bypassed
by a large electrolytic, in order to provide DC compensation. I don't
know if there would be any benefit over simple AC coupling (with a
similarly sized electrolytic), though.

JH.

> I think the winding does provide some local negative feedback, and also
> uses the collector AC current a second time. I would guess that the
> transformer is wound such that you transform the emitter output
> impedance and the collector output impedance to the impedance on the
> secondary.
> There are some tube amps which use such a split not quite plate, not
> quite cathode circuit. Namely the Quad II.
> http://www.drtube.com/schematics/quad/quad-ii.gif
>
> Here we have a push pull variant, where certainly the magnetic flux
> would cancel, but still they use this split circuit.
>
> Cheers,
>   René
>
> jhaible at debitel.net wrote:
> > 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.
> >
> >
> > -------------------------------------------------
> > debitel.net Webmail
> >
>
> -- 
> uzs159 at uni-bonn.de
> http://www.uni-bonn.de/~uzs159
>
>
>