f4cb3e86
From: apogosso@tpg.com.au
"Engineer" wrote in message
news:2c765374-a2bf-4be4-997e-aa9243ca56cf@c41g2000yqm.googlegroups.com...
> This one's for Patrick in Oz.
> Hi, Patrick,
> I've looked at your "standard" VLF stabilization coupling network for
> this amplifier class (0.05 parallel 1Mohm, with 220Kohm grid leak on
> the o/p tubes.) It appears to provide a forward gain roll-off below
> about 15 Hz with a shelf at about 1/6 of the normal gain (-15.5 dB)
> below about 3.2 Hz. Assuming my calculations are correct, what is the
> purpose of the 1 Mohm across the 0.05 uF? Surely just letting the roll-
> off continue at 6 dB/octave below 15 Hz would scotch any VLF
> oscillation. Is this a phase shift issue? The simple -6dB /octave
> roll-off leave a 90 degree phase advance in place below, say, 10 Hz
> whereas the shelf would appear to avoid that (I'm not certain as,
> regrettably, no pspice to hand at present!) I stabilized my
> Williamson clone with just 0.05 uF coupling caps to the 6L6's with an
> "aggressive" 100K grid leak (- 3dB at 32 Hz, but I may increase the
> 100K a bit....) Do I really need a VLF shelf below that?
> Thanks and cheers,
> Roger
Your agressive LF frequency compensation might cause undesirable effects.
1. 100K bias is bit too low -- loading preceeding stage (driver, phase
splitter) unnecessarily. Perhaps you can achieve the same result by 220K and
0.022uF coupling.
2. By agressively cutting LF from 32Hz you reduce loop gain in the working
range (20Hz) thus increasing output impedance, reducing speaker damping,
increasing intermodulation and distortion (if it matters at 20Hz?) This is
aggravated by the fact that the driver stage has to labour 3dB harder at
20Hz because of the attenuation in your agressive circuit.. So I bet you
will get overall results twice worse than Mr Turner would have done with his
smart shelving.
3. Though your amp might appear stable, most likely it will be peaking close
to oscillation at 8Hz or so where the +90deg lead from your agressive
circuit will combine with +90deg lead from your OPT at the 0dB loop gain
crossing. Any LF rumble might drive your amp into overload.
So I need to admit, shelving is wiser, because it does not take out precious
dBs from the loop gain in the whole audio range and gives a better phase
margin, and no peaking. Mr Turner makes a deep 12dB/octave nose dive below
15...20Hz by combining OPT effect with 220K/0.05uF effect, but then, closer
to 0dB crossing, he gently goes out of the deep nose dive, shelves the beast
out and happily crossing 0dB at 8dB/octave. Past that he can dive deep
again -- does not carem as no oscillation will occur below 0dB.
However, both shelving and "agressive" compensation have one drawback in
common. NFB gain is constant at all frequencies since NFB is simply a
resistive divider. Thus the NFB tries to have the amp's responce flat (at
low level of course) down to possibly 3Hz or so. This is crasy and
unnecessary.
For that reason, instead of shelving interstage coupling I shelf the NFB.
Instead of a resisive divider I would put a resistor (typically 100R) in
series with a capacitor (order of 47...100 uF). Interstage coupling is kept
a simple semi-agressive RC circuit (say 0.022uF and 330K). Overall loop gain
shelving effect is the same as in Mr Turner desihns, but because of the
deepening feedback at VLF the amp becomes a rumble filter itself. A drawback
of that method is that you need an electrolytic in the feedback, Some people
do not "trust" electrolytics as a frequency shaping components.
Regards,
Alex
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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