In article ,
Patrick Turner wrote:
On May 31, 1:06*pm, John Byrns wrote:
In article
,
*Patrick Turner wrote:
Well, I can't explain it any better than I have. If circuit gain is
increased by feeding back a fraction of an output signal of one stage
to an earlier stage and the gain is increased, THD increased,
bandwidth reduced, then I'd say PFB exists. We see feathers, a ducks
bill, wings and webbed feet, a ducks tail that wags and we say there's
a duck.
Unfortunately this is a completely incorrect description of how a generic
paraphase inverter operates, so the duck analogy doesn't apply.
Oh well, I can agree to dissagree
The point is that only one variant of the paraphase inverter circuit, the one
where V1 & V2 share an unbypassed common cathode resistor, as used in the QUAD
II, has the PFB you describe, the generic paraphase inverter and other variants
don't have the PFB you are talking about.
For example, take a look at this paraphase inverter circuit.
http://hhscott.com/pdf/250.pdf
Where is the PFB in this circuit? The 6 dB gain increase exists even though
there is no PFB in this circuit. The 6 dB gain increase is simply the result
adding the voltage mirror stage which doubles the total output signal, this is
typical of all paraphase inverter circuits.
The PFB in the QUAD II circuit can be eliminated by tying a large electrolytic
capacitor from ground to the common cathode of V1 & V2, the gain of the inverter
circuit will not change when the PFB is eliminated this way, although there will
be a problem applying global NFB with the capacitor in place. If you try this
experiment with a capacitor from ground to the common cathode of V1 & V2, be
sure and disconnect the global NFB first by disconnecting the 470 Ohm NFB
resistor from the secondary of the OPT and connecting it to ground before making
any measurements.
The paraphase tends to add gain "artificially" for a given number of
tubes, moreso than merely cascading them.
"artificial" gain is the best kind, it can be made so pure that its sound
is
simply sublime.
If only it was true. Paraphase and PFB usually have higher than
possible THD without the paraphase PFB. The sound of such stages
cannot be assumed to be sublime, and wherever higher THD is allowed to
breed unecessarily the sound drifts to mud.
It is as true as calling the 6 dB gain of the paraphase inverter "artificial",
when it is actually a direct result of adding the voltage mirror circuit.
And in the case of Quad-II, the KT66 grids are biased via 680k Rgs,
and one has a 2k7 at its bottom from which the signal to V2 pentode
grid is wrought.
Now that 2k7 is just what Walker chose, but it could be a range of
values; if larger, there's more PFB and more gain and weird things
happen when you increase that
2k7 too much because there is a definite limit to the amount of PFB
which can be applied without terrible instability.
I don't know if 2k7 is the correct value for this resistor, I assume that
Quad
choose correctly here, there is only one correct value for this resistor
where
the circuit will function properly, other values are not an option! *
Don't be so sure about this. Try building an input stage identical to
Quad-II in some amp you have laying around. You don't need pentodes; a
single 12AX7 will do, and where the Quad has 680k plus 2k7 try putting
a 5k pot to replace the 2k7, then add some NFB and make some
adjustments with the pot. After awhile the penny drops about the PFB,
too much is hell, just enough is all that's needed - about 6dB.
You'll soon find that probably optimal R value is where there is
minimum signal voltage across the 680 ohm common Rk to the two EF86,
or 1/2 12AX7, so the trick of the circuit is that the EF86 tend to
have a fake CCS connected to common cathodes. One may analyse the
effects of a distortion signal fed back along the signal path and what
happens to it in terms of gain. But the PFB increases the amount of
NFB applied beyond what it appears to be. There is more to Quad-II
input drive amp than 90% of people may realise.
You need to be a little more definite about what this faux QUAD II phase
inverter circuit using 12AX7s is supposed to look like? Is it just to be an
input stage without any global NFB? Assuming that we aren't worrying about the
complex provisions for global NFB in the QUAD circuit, having omitted them, it
is obvious by inspection that when the "R value" is adjusted for equal outputs
from the two plates of the inverter circuit, then the signal voltage on the
common cathodes will be minimized. Adding the QUAD style global negative feed
back circuit greatly complicates things, as global NFB is applied to V1 and both
global NFB and global PFB are applied to V2.
I don't think I would say "that the EF86 tend to have a fake CCS connected to
common cathodes", if anything I would call it a "fake piece of wire".
The QUAD II input stage has much more going on in it, at multiple levels, than
you seem realize, or seem have thought about. I'm not saying that your Turner
circuit isn't better, however you should have made an effort to more fully
understand the operation of the QUAD input circuit before ripping it out and
replacing it with your design.
Changing
the value of this resistor will change the amount of PFB as you say,
thereby
changing the gain of V1. *The problem with changing the value of this
resistor
away from the correct value, to some other value, is that in addition to
changing the amount of PFB, it also unbalances the phase inverter so that
the
two output tubes don't receive the same amount of drive voltage.
Very few Quad-II amps have well balanced drive. But I recall the 2k7
was not critical for balance. I suggest you try examining a Quad-II
circuit.
I have examined the QUAD II circuit, and did so long before you were even aware
of its existence, if some of your narrative here is to be believed. The 2k7 is
absolutely critical for proper balance in the QUAD II, however balance is also
dependent on the gain of V2 in addition to the value of the 2k7. I suspect that
the reason QUAD didn't use a self balancing type of paraphase circuit in the
QUAD II is because it would have required more components, including at least
one electrolytic capacitor for the cathode of V1 to maintain gain, and there
would have been a slight loss in open loop gain, less than 1.0 dB, due to
increased loading on the driver plates, and there is little doubt that QUAD was
pinching pennies in the QUAD II, so the increased cost, and very slight loss of
gain would have been intolerable.
--
Regards,
John Byrns
Surf my web pages at,
http://fmamradios.com/