Jim,
For those of you folks that have pre-distortion capabilities, like
the ANAN xcvrs... and use an external tube linear amp. Has anyone
tried measuring the IMD results with pure signal turned on....and
also varying the bias and idle current of a tube amp, like a GG
triode ??
I haven't done it with a tube, but I have experimented this with solid
state amps. I don't think the results would be fundamentally different
with tubes, but some differences caused more by the external circuitry
than by the active device will probably show up.
PureSignal will keep the close-in intermod products, inside its
correction bandwidth of roughly 40kHz, nicely down while you reduce the
idling current, up to the point where you reduce it so much that that
the amp has _no_ gain for the smallest signals (biased into cutoff). At
that point PureSignal goes crazy, and the IMD shoots through the roof.
It cannot deal with class C amplifiers yet, not even with pure class B.
But it might, soon.
At the same time the IMD outside PureSignal's correction bandwidth goes
up unhindered, as you reduce the bias, and is worse than without PS. As
you reduce idling current, this IMD typically becomes excessive before
the point where PS goes crazy.
So you cannot go too far in reducing idling current, certainly not all
the way to cutoff, but you can get pretty close while maintaining an IMD
better than what a good class AB amp with high idling current and no
predistortion can provide.
Typical numbers for my solid-state test amplifier could be roughly like
these, and I hope the format of this table comes through:
----- no PS ----- ---- with PS ----
Ibias IMD3 high IMDs IMD3 high IMDs
=================================================================
2A -28dB -60dB -60dB -58dB
1A -25dB -55dB -60dB -53dB
0.5A -20dB -50dB -60dB -48dB
0.2A -14dB -45dB -58dB -38dB
In this table, "high IMDs" means the ones that fall outside of, but
close to, the correction bandwidth of PS. Those are the most
objectionable. I filled the table with typical values from memory, from
when I made the tests some months ago, rather than with actual test
data, which varies by band, drive level, etc. It shows the main trend.
Another thing I would like to mention in this context is that the
reduction of IMD inside the correction bandwidth that you can achieve
with the current version of PureSignal depends relatively little on the
bias and on the drive level, as PS even corrects for considerable
overdrive, but depends hugely on amplifier memory effects. The current
PS algorhythm cannot correct for memory effects.
In conventional amplifiers the distortion coming from memory effects is
usually swamped by distortion coming from small-signal nonlinearity
(bias-related) and large-signal nonlinearity (saturation-related). But
when you use PS to remove distortion coming from these two main causes,
the previously hidden distortion from memory effects becomes dominating.
"Memory effects" refers to the case when the characteristics of an
amplifier change according to what it was doing a moment ago. The main
cause of memory effects is unstable supply voltages (main supply and
bias). For lowest memory effects it's essential that all supply voltages
be tightly regulated (rarely the case with tube amps). An electrolytic
capacitor filtering the supply at the amplifier, with some inductance
and resistance between that and the regulated supply, causes terrible
memory effects! Because it generates a time-lagged voltage drop. Either
the capacitor must be so large that the voltage on it remains highly
constant over the whole envelope of the signal (hardly feasible in a big
amplifier), or it should be removed. Small (ceramic) caps are enough to
bypass RF currents, and the time constant they form with any resistance
or inductance in the supply wiring is of negligible length.
Memory effects can also rise from temperature fluctuations inside
semiconductors. I wouldn't expect this to be a problem with tubes, but
at the same time I would expect bad memory effects with tube amplifiers
coming from their typically unregulated plate supplies.
So, Jim, if you try driving a tube amp with your ANAN and you find that
PS doesn't cause a dramatic improvement in IMD inside its correction
bandwidth, you should suspect memory effects from unwanted supply
voltage modulation as the main cause. And the bias should be adjusted to
keep the IMD outside PS's correction bandwidth small enough. What
exactly is "small enough" of course depends on how far the closest
fellow ham is located, how good a ham neighbor you want tobe, and how
likely that other ham is to come over with a shotgun!
Manfred
========================
Visit my hobby homepage!
http://ludens.cl
========================
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|