"John Byrns" wrote in message
...
In article
,
Patrick Turner wrote:
On Jun 24, 3:34 pm, John Byrns wrote:
I want to see a schematic with all test results before I make up my
mind on Alex's FB "trick." It could be a clever trick, or a swindle.
However the network introduces both a zero and pole into the response,
with
the
zero at a higher frequency than the pole. Remember this network is just
another
tool in your toolbox; it is not a cure all and requires some
sophistication
in
its application. Now the one thing I know about stabilizing the low
frequency
response of a feedback system is that it is all about correctly placing
the
poles, and zeros if there are any.
In any amp where there are say 2 CR coupled stages and a final stage
with LR then you have a recipe for LF instability and a poor margin of
stability at LF.
OK, I have worked through some of the math and understand more fully what
is
going on with Alex's feedback network. As I said the network introduces
both a
zero and a pole in the loop gain.
Ignoring the added pole for a moment, the zero can be placed so that it
exactly
cancels the effect of one of the three poles in the amplifier you describe
above, with 2 CR coupled stages and a final stage with LR, effectively
reducing
the number of low frequency poles by 1, making the LF stability problem
easier
to deal with. The zero effectively cancels both the phase shift and
amplitude
roll off caused by the pole that is being canceled. Unfortunately it is
impossible, at least so far as I know, to build a network with an isolated
zero
such as I have described, so an actual network, such as Alex's, must
include a
pole at a lower frequency. Hopefully this new pole won't cause us too
much
trouble if we place it at a very low frequency where the loop gain has
already
fallen well below 1.0 as a result of the other two remaining poles that
weren't
canceled.
Now the obvious question is, why bother with this extra complexity when we
could
simply directly move one of the 3 poles to a very low frequency, as would
probably be part of the normal pole staggering process anyway? I will
leave
that for others to comment on as I have not personally mucked about in my
workshop with amplifiers that have 3 LF poles. I suspect that one reason
may
have to do with LF overload when using a Bean Counter approved OPT.
I can see how Alex's network has the potential to resolve a problem I have
encountered when mucking about with simpler amplifiers having only 2
poles.
When using OPTs designed by Bean Counters, especially SE OPTs, there is a
tendency towards LF overload in the OPT and final tube(s). I have
attempted to
mitigate this problem by choosing a relatively high pole frequency for the
interstage coupling network to keep LF signals out of the OPT and final
tube(s).
This puts the interstage pole too close to the pole caused by the OPT
which then
causes a bump in the ³CLG² low frequency response, plus of course it isn't
really a very good solution to the LF overload problem. It occurs to me
that
Alex's feedback network might also offer a solution to the OPT saturation
problem in Bean Counter designed OPTs, just as it offers a solution to the
input
stage problem.
--
Regards,
John Byrns
Surf my web pages at, http://fmamradios.com/
You grasped the idea perfectly well John. This frequency compensation is
useful to prevent a lousy OPT and 6AQ5 overloading in a lousy boring audio
amp found in those boring AA5 style radios.
Below is the sequence of mods you do to improve the radio.
1. In an AM detector you remove a cap coupling a detector load (470K||100pF)
to the volume control (1M pot). In fact you are making the volume control
the detector load. This is for the detector to be able to handle nearly 100%
modulation. But now you det DC on the input of the audio amp.
2. You decouple this DC from the pot wiper by a 0.02...0.05uF, but you need
10M input impedance (grid leak) so that the AM detector is not loaded and
100% modulation is still handled.
3. On most stations your AM detector delivers 1...4V of audio, while
sensitivity of a typical boring feedbackless two-stage audio amp is
100...200mV. The radio always works with volume control close to minimum.
The speaker is boomy (not damped), distortion is high. You want to trade the
sensitivity excess for distortion and apply NFB, reducing sensitivity to
0.5...1V. You are enjoying tight crispy sound, but THAT IS WHERE PROBLEMS
BEGIN.
4. Now the bandwidth of your amp is in theory goes to 5...10Hz, but because
of the lousy OPT, these low frequencies do not reach your ears, but only
overload the 6AQ5, since the error signal becomes too large at low
frequencies.
5. Note that neither the american aggressive LF cutoff by reducing
interstage cap to 2000pF nor the Patrick's shelving does not work, since
still yjr NFB is pushing to maintaim unmaintainable LF output, the error
signal is large, the first stage is overloading, and (IMPORTANT!) since the
shelving is virtually a differentiator, it accentuates all the harmonics
generated in the overloaded 1-st stage and feeds them to the 6AQ5. Instead
of overloading the 6AQ5 we have emphasized distortion from the first stage.
6. That is where you need this RRC compensation in the feedback to roll the
low-frequency response of the amp in line with the capability of the lousy
OPT. Typically from 80....100Hz down. Applying the RRC divider provides
undistorted output down to 10...25Hz,
because the error signal remains under control.
7. However at the frequencies lower than 10...25Hz, where the NFB RRC
network levels out at 100% beta, the error signal continues to rise. This
VLF content caused by fadings, AGC knocked by atmospheric interference, etc.
is smaller, and some might stop here, but a purist might like to prevent
overloading even at VLF.
8. To do the above one needs to degenerate the first stage gain, rather than
to use a Partick's attenuator after it. To degenerate the 1-st stage gain
you need to place say 22K resistor in series with the 1-st stage cathode,
and shunt this resistor by a 0.22...0.47uF capacitor. Thus for medium
frequencies the 1-st stage will be working as usual, but ay low frequencies
the transconductance will be degenerating with the perfect linearity of the
first stage maintained. It is the same shelving, but implemented in a wise
linear mode. (Of course the grid leak can not be taken to GND any more, it
should be connected to the cathode or a tap in this 22K resistor.) At
infinitely low frequencies the gain of the first stage is to be degenerated
to 10...20 so that the 1-st stage output is just below the negative bias of
6AQ5 and the later is never overloaded.
Usually I apply mods up to #7 and sometimes #8 too if I can find enough free
solder lugs around the 1-st audio stage tube. Of course, all of the above
implies the cathode of the 1-st audio tube has to be free of duo-diode
functionality. That forces to use separate diodes for AGC and the AM
detector. In some cases instead of 6Q7, 6B8, 6AV6, etc. I might use a 6SL7
tube with one triode as the 1-st audio and the other triode as a diode for
the AM detector, and a silicon diode as an AGC detector.
Regards,
Alex