Diff drive

[gianis]

It's negative feedback...

[smoking-amp]

The general in band phasing looks right to me, but at some low frequency the double RC couplings will cause two sets of 90 degree phase shifts, turning it into an oscillator. So at least one set of Cs need to be removed.

Using a single diffl stage to correct another power stage is commonly called a buffer circuit and was championed by J. Ross Macdonald back in the 50's. (search on "augmented cathode follower" here to find a link to his paper, or just search for his website. He's still around!) The single diffl is often used in instrumentation circuits. Shows up often on Tubecad too.

Edward Cherry used such a double diff amp control stage for a SS amplifier back in the 70's in the same way you have it shown. Sometimes the double is used in modern SS amps where a large bank(s) of paralleled output transistors are controlled in separate groups for class AB. (makes for easier control of DC biasing for crossover when using matched diffl transistors)

The double diffl should be working fine once the Cs are fixed, but it does waste a set of triodes. There is only one output signal to control (sum of the two output tubes), so only one diffl stage (or driver) is more commonly used to control that.

The double diffl. does provide a high impedance signal input at the diffl. grids versus the usual single diffl driver with resistive feedbacks to the grids. But the feedbacks can be moved to the cathodes to avoid that if thats a problem. (see RCA RC-30 handbook, page 696, 50 Watt HI-Fi audio amp.)

Don

[bigwill]

Thanks everyone for your informative replies

smoking-amp, your reply was especially interesting. I took your capacitor comment into account, and I suspected it would be a problem in the first place.

Here I have drawn a revised version, which eliminates two triodes and uses the output directly from the plate to feed the cathode current source to provide feedback - a very short feedback loop!

[smoking-amp]

but some issues yet

Feedback to the cathode is usually more linear than a full differential pair. (the cathode to grid voltage is the same factor for both the input and the feedback, whereas a full differential has some residual odd order distortion between grids) The low cathode impedance Zk being OK as long as some attenuation of the feedback signal is intended.

However, one still has to consider the variability of the cathode impedance with driver current when deriving the attenuation factor. Usually the two driver tube cathodes are connected together, so their 1st order impedance variation cancels. With the separate cathodes here and a CCS on each one you will probably see some significant Zk modulation on the gain. I would include a resistor (adj. pot initially) between the cathodes and adjust for minimum distortion.

(I don't know if there will be a null point, or if it will just tilt all the way to 0 Ohms) Since the output pentodes generate some characteristic 3/2 power distortion, their may be some advantage to the driver having a little opposite dist. (Well, I guess it can't go all the way to 0 Ohms across the cathodes, or there would be no feedback. The RCA design has 390 + 500 pot + 390 Ohms across the cathodes and then no CCS, just a tail resistor of 47 Ohms)

I see the RCA 50 Watt amplifier also included some feedbacks to the driver plates. The plates have a roughly 3/2 power law effect on the driver current, whereas the g1s and cathodes each have more like 2 power law effect (due to grid wire "island effect") So one can also tweek for lowest distortion apparently by adjusting the neg. fdbk partitioning between them.

details, details

Don

OH, on the previous issue of the double cap couplings in the loop, I guess one could stagger the RC constants sufficiently to avoid getting two 90 degree phase shifts simultaneously. Same issue comes up in a standard C coupled amplifier with multi-stages.

[smoking-amp]

Looking at the 2nd version here:

A center tapped inductor (CCS off the center tap optional) would work nicely (as long as its inductance is sufficiently high to not cause a phase problem) to replace the two driver tube CCSs. This would sum the two cathode Zk impedances to mostly null out their variation. Could also put a resistor across the inductor to set the feedback attenuation even more stably.

Its tempting to try the xfmr secondary (16 Ohm, 4 Ohm, 0 Ohm) for the CT inductor, but that introduces xfmr internal phase shift problems. Maybe if the secondary were a driven winding too (CFB using the secondary), the phase behavior would be a bit better. But likely would require a high performance (high bandwidth) xrfmr. Risky.

Don

A quick analysis (looking at one tube side only) of the gain modulation from the varying Zk's (if left uncorrected) appears to attenuate the neg. fdbk more when the driver tube is turned on and the output tube is turned off. So the loop will try to increase the output tube gain when its turning off, which is opposite to the natural tendency of the output tube. So this effect is causing similar even harmonic cancellation like an ordinary diffl. stage. But leaving an odd harmonic signature like an ordinary diffl. stage too.

[Bandersnatch]

Originally posted by smoking-amp
but some issues yet

Feedback to the cathode is usually more linear than a full differential pair. (the cathode to grid voltage is the same factor for both the input and the feedback, whereas a full differential has some residual odd order distortion between grids) The low cathode impedance Zk being OK as long as some attenuation of the feedback signal is intended.

The current comming down that FB resistor is current not seen going through the triode's plate load with a CCS in the cathode. Notice that in the RCA amp the driver was a pentode, and one running at about a quarter of the FB-delivered current.

I don't think the cathode Z has much to do with it; the current delivered through the FB resistor couldn't care much between being grounded or attached to the driver cathode I think.
cheers,
Douglas

[smoking-amp]

Hi Douglas,

Hmm, yes, I see what you are saying. The CCS can draw the extra DC current from the feedback, and any variation of feedback current has to go thru the triode unaltered, showing up as a linear voltage variation on the driver plate load. Seems that this should be strictly linear. Now I'm trying to figure out where my previous analysis went wrong. ( I was assuming that the cathode would present 1/gm as its impedance, and this varies as the current thru the tube varies. Hmmm.)

edit:
I guess I have to take into account the additional impedance from the plate (cathode to plate), rp, which has a signal on it too.


Just to throw in another interesting variation, (of the 2nd version) suppose we put the CCSs in as the driver plate loads, and use a resistor for the cathode pull-down. With constant current thru the triode, its gm has to remain constant. So Zk would presumeably be constant too. The feedback attenuation factor being determined by the resistors and constant 1/gm.

edit:
Have to take into account the rp here too, but that should be constant as well. (1/gm and rp in parallel being constant)

Don

[bigwill]

smoking-amp, a CCS in the anodes would be an interesting idea! This would further linearise the driver, and feedback could go into the cathode and the cathode could go through a resistor to provide bias....

[Svein_B]

Feedback to the cathode. Hm.
I wonder if it is time for another vote on the feedback being negative or positive ?

SveinB.


Edit: I guess it is still OK.

[lineup]

There is no main difference.

A differential pair for inverted and non-inverted inputs.
In this case the negative feedback to the cathode is buffered with another tube ( follower),
which will put the current into the cathode.
This is the inverted input tube doing buffering.

In the resistor to cathode case,
the negative feedback current is put to cathode without a buffer = directly.