Re: OPT equalising circuit

Phil Allison wrote:
> 
> "Alex"
> >
> > I noticed that in most cases a capacitor or a RC circuit is connected
> > across the primary of a tube OPT. It is apparently to compensate for
> > the leakage inductance of the transformer.
> 
> ** Partly for that  PLUS the fact that most speakers have a sharply
rising
> impedance curve at high and supersonic frequencies.   A nominal 8 ohm
> speaker at 250 Hz can easily be 100 ohms at 50 kHz.
> 
> > However, I am not sure how to correctly calculate this RC circuit.
> 
> ** You are not alone ....
> 
> > If I know my load impedance Rl,
> 
> ** No you don't.
> 
> You just know the  " nominal " impedance =  useless.
> 
> What you need to make sure of is the amp does not break  into HF
> oscillations ( parasitic or continuous)  when there is no load  OR  when
> under test with simulated ( or real ) speaker loads.
> 
> The vast majority of tube amps have no need of such a compensation
> etwork  -  as the OT has little leakage and the NFB reduces to low value
at
> supersonic frequencies.
> 
> ......   Phil

I don't know how many tube amps you have designed and or built or
repaired
but I've lost count. Many tube amps will oscillate at HF when connected
to 
a capacitor load.

In order to gain unconditional stability in very many tube amps and some
tolerance 
of capacitor loads, or having no load at all, or having an inductive
load wuth high Z 
at HF, is is VERY NECESSARY to place what is called a Zobel network of
R+C in series
across one or more positions in the tube amp.

The Zobel is more likely to be needed in an Ultralinear or beam tetrode
or pentode
output stage where the gain rises as the load on the tube rises because
of the leakage inductance.

The Zobel is less likely to be of any use in a triode output stage.

Transformer series and parallel resonances between stray lumped shunt C
and leakage inductance 
also confounds the designer, and the Zobel can act to damp the resonance
and the 
adverse loading they cause at the resonant F.

So just what value for R&C are to be used in an output stage?

Let us assume the amp is set up with an output tranny to power 5 ohms,
but present
a load to the tubes of 5k anode to anode.

In most cases I have had to tame, generally I have tried TWO Zobel
networks,
one across each half primary of the OPT, ie, from CT to each anode
connection.
Where 5k is the a-a load, the minimum R value is 5k / 4 = 1.25k because
that's the class B load 
when the amp works in class AB.
The next standard R value above is about right, say 1k5.

The value of C should be high enough to not adversely load down the amp 
too much at too low a frequency. Therefore I would use C which has equal
reactance to 
RLa-a / 4 at say 100kHz as a starting value. 0.001uF could be about
correct.

At 20kHz, the two x ( 0.001uF + 1k5 ) networks has Z = 16k a-a approx,
and this will not 
reduce the response into a resistance load very much.

A typical amount of leakage inductance of a poor quality 5k OPT coulod
be 50mH in series with the 
5k load. This is normal for very many tube amps.
At 100kHz, 50mH has reactance = 32k, a huge amount of series reactive
impedance 
effectively in series with the load seen by the anodes.

If a square wave with fast rise time is used to test the amp, and the
anode 
signals viewed, there can be a huge amount of overshoot and ringing at
ther anodes,
even with a resistive load at the output.
The ringing crap is transformed to the secondary, and when fed back 
can cause the amp to oscillate.

The 1.5k plus 0.001uF will act to reduce the ringing at the anodes.

Zobels used across the secondary usually have R = rated RL plus a C 
that makes the reactance of the C = R at 100 kHz.

So 4.7 ohms plus 0.33uF are typical values.

The other place for a Zobel are across the RL of the V1, 
so that where there is 47k load, you might use 4k7 plus 470pF.

This will reduce the gain of V1 at HF, and reduce the phase ****ft
where OLG > 1. 


But unless you have designed and built a lot of amps and know the art of
trying things 
and observing, and ending up with R&C values that 
ONLY stop the amp oscillating, and load the amp minimally,
so that the response with a pure R load is very litle affected, then
chances are you'll get it all wrong.

The obel networks make it possible to use ESL speakers without worry of
having a huge 
peak in the sine wave response between 8kHz and 32 kHz.

With any value of pure C loading from 0.05uF 5uF, the peaking caused
in the sine wave response due to having FB should not exceed 6dB.

This will indicate a good stability margin.

In fact pure C loads are rare. With most ESL, there is some C alright,
maybe 1uF to 5uF, but usually there is a series R equivalent to say 1.5
ohms
which provides enough damping to a tube amp which is in effect an active
bandpass filter
with more than 6dB attenuation beyond the LF and HF open loop poles.

See my website for numerous examples of Zobel networks 
in amplifier schematics.

http://www.turneraudio.com.au

Well damped amps do not sound bogged down by being overloaded at HF by
such networks.
They usually have clearer sounding HF.

Patrick Turner.