Re: OPT equalising circuit

In article <DJCTj.7816$ko5.5567@[EMAIL PROTECTED]
>,
 "Alex" <apogosso@[EMAIL PROTECTED]
> wrote:

> "John Byrns" <byrnsj@[EMAIL PROTECTED]
> wrote in message
> news:byrnsj-FE9DF8.20272704052008@[EMAIL PROTECTED]
> > In article <d_BSj.6957$ko5.2719@[EMAIL PROTECTED]
>,
> >  "Alex" <apogosso@[EMAIL PROTECTED]
> wrote:
> >
> > > Thanks, Partick.
> > >
> > > Your reply is constructive and informative, unlike the other one.
> > >
> > > My case is not a Hi-Fi amplifier, but a radio with a single-ended
stage.
> OPT
> > > is quite lousy. It is designed for Rl=7.5K (for 6M5, EL80 or 6F6
tube at
> > > 250V). Its leakage inductance, referred to primary is as large as
> 165mH --
> > > the windings are not interleaved.
> >
> > There is an interesting variation on this capacitor thing that you
will
> > see in some old radios.  Rather than using a single capacitor which
> > slowly rolls off the high frequency response, two capacitors are used,
> > one across the primary, and another across the secondary of the output
> > transformer.  The two capacitors, along with the leakage inductance of
> > the transformer, form a third order low pass filter.  This provides a
> > sharp cutoff to attenuate noise energy above the desired audio band
> > while providing flatter frequency response below cutoff in the desired
> > audio band, than a single capacitor can.
> 
> I have never seen that, but it is an interesting idea. Might be useful
on
> SW. Requires a set of switches to throw relatively large caps in
circuit.

The examples I have seen didn't use switches, the cutoff frequency was 
fixed at 5 kHz, or whatever frequency it might have been that the 
designer choose.  The capacitor values, as well as the amount of leakage 
inductance, are critical to obtaining a flat response below the cutoff 
frequency.  To provide several switch selectable filter cutoff 
frequencies, all with the desired response shape, it would be necessary 
to switch the leakage inductance as well as the capacitors!  Patrick, do 
you have any practical ideas of how one might provide several switch 
selectable values leakage inductance in an output transformer?  As far 
as simply switching the capacitors out, I would have to check the math, 
but I think that leads to a gradual roll off with the capacitors 
switched out, starting below the cutoff frequency with the capacitors 
switched into the circuit.

> As
> I understand, you need to turn the feedback off  for this 3-rd order
filter
> to have effect.

Yes it is hard to imagine NFB being used around this sort of filter 
circuit, the radios I have seen that used it did not have negative 
feedback.  Even if there weren't a stability problem, the NFB would tend 
to negate the desired effect of the filter.  NFB from the output tube 
plate would seem possible though.

> Turn the NFB off and the mechanical resonance of  the
> speaker will be undamped.

There may be some mechanical and acoustic damping of the speaker driver, 
and certainly you could use power triodes to drive the speaker and 
provide damping.  IIRC the radios I have seen this circuit used in, did 
mostly use triode output tubes.  Also the vast majority of tube radios, 
and small phono amplifiers, use pentode output tubes to drive the 
speaker without NFB, so it is hardly an unheard of situation.

> So, there needs to be a special circuit, gradually
> disabling the feedback with the frequency rising, but not affecting
gain. It
> can be done.

If the need for NFB is felt, one could take the feedback from the 
secondary at low frequencies, crossing over at higher frequencies to 
feedback taken from the plate.

> And what if we place more LC pads between the secondary and the
> speaker? Potentially can make the radio fit to receive SSB...

Adding "more LC pads" sort of misses the whole point of this circuit 
which is to take advantage of the already existing, read free, leakage 
inductance of the OPT to create a third order LPF. Adding "more LC pads" 
would mean actually paying real money for the additional inductors 
required.


Regards,

John Byrns

-- 
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