Donald Chester wrote:
Gary K4FMX said:
The best quality audio of all can be gotten from low level modulation
and a linear amplifier.
A linear amplifier has the same kind of distortion as a class-B modulator.
That's true except with a linear amp most of the distortion products
fall outside the audio bandwidth. 2nd and third harmonics etc. are
outside the audio bandwidth.
With tubes, the best quality audio can be had from low distortion plate
modulators such as class-A series or Heising modulation, or pushpull
plate modulators running class A or AB1.
Except for the distortion introduced by the modulation transformer.
Pulse-width series modulators produce perhaps the best audio.
I suspect the best quality of all comes from the new class-E rigs.
Agreed.
According to the tube manuals, class-B audio service has inherent
distortion levels on the order of 3-5%. It can be reduced with negative
feedback. My Gates BC1-T manual claims less than 2% distortion at 100%
modulation.
The signal driving a linear amplifier has its own distortion, since the
original signal has to be produced somehow. Pushpull class-A audio or
series modulation, with feedback, might be a good candidate for the
driver stage of a linear. If the linear is run properly in class AB1,
that would be near the best possible audio out of a tube transmitter,
even though the efficiency is not all that good.
With low level modulation and a linear amp it is much easier to produce
excellent audio than it is from high level plate modulation. Building a
low power (driver) low distortion AM transmitter has fewer problems than
high power low distortion transmitters. Class A direct coupled
modulation schemes can be accomplished much easier at low levels than at
high levels. Use of a balanced modulator can also eliminate the problems
associated with occasional over modulation that plagues high level
conventional modulation.
Speaking of efficiency, an AM linear or grid modulated amplifier has
close to the same overall efficiency as plate modulation, when
calculated from the ratio of power drawn from the a.c. mains, to rf
carrier output. A linear amplifier running AM has exactly the same
efficiency as when it runs SSB. It's just that the duty cycle is
different.
That's true. An SSB amplifier at a power output level of 1/4 its full
power has an efficiency level of exactly 50% of its full power out
efficiency.
If it is 66% efficient at full output it will be 33% efficient at 1/4
power output level. Just like it is with an AM signal as you say.
Actually, since with the human voice, the average power is 7-8 dB lower
than peak power (equivalent to around 30% modulation), the average
efficiency of a SSB linear is similar to that of an AM linear because
the efficiency of a linear is a function of the amplitude of the signal
(0% at idling current, and a maximum of about 67% at maximum peak output
just below the point of saturation or flat-topping). AM linears got
their reputation as "low efficiency" on AM because of the 100% duty
cycle carrier runs about 30% efficiency to allow enough headroom for the
positive peaks. With an AM linear, you can see the glow on the plates
DECREASE when you whistle into the mic to produce 100% tone modulation.
The DC input is the same regardless of modulation, but the rf output is
higher, since sideband energy is now included. That power has to come
from somewhere, so the efficiency of the amplifier goes up to generate
the sidebands.
The advantage of plate modulation with AM is the ease of tuning up and
QSY'ing. You simply dip the final and load to the desired carrier
output, while maintaining enough grid drive to assure class-C service.
With low-level modulation (linear or grid modulated), the rf drive level
and degree of antenna coupling are critical to the modulation linearity
of the final.
With a rice box type exciter and amp for low level modulation all one
has to do is turn the knob to insert full drive, tune both final and
load controls for maximum output and then reduce drive to 25% of full
output and you are good to go.
73
Gary K4FMX
