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The more I look at this thing the trickier it gets.
Indeed. As soon as I saw that it was Dan Mayer's Tiger output stage plus added complication, I quit any hard analysis. And skimming the surface, it has Dan's faults "fixed" with extra gain, sort of a Super-Tiger. Dan's Tigers could bite, in analysis and in real life. Bryston has picked a hard path to walk.
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only providing about 100mV across any of the output Q B-E junctions.
My thumbs-only analysis suggests it needs 4 Vbe at the input, not 2 Vbe as the diamond-buffer front-end provides. This ignores some non-ignorable marginal factors. Designing a good Class AB stage is all about controlling milliVolts of bias under many-mV of thermal drift and many-Volts of signal. So you will bring it up to 3.9Vbe and still be underbiased, to 4.1 Vbe and it will suck current like electricity was free. If Jung's current sources are low tempco, then raising the input emitter resistors will bring it up to active current and then run-away.
If we really wanted this level of complexity: the Bryston is scaled for something like 100mA idle, 10 Amp peaks. If we are looking for 10mA peaks, we might start with all resistors 1,000 times higher. But that certainly does not look right. Or saying the Bryston drives 8Ω, we drive 600Ω, we might scale ~100 times higher. But certainly 1Ω output emitter resistors don't seem to fit 600Ω loads. Usually the bigger the better for stability and linearity. For efficiency, low is better. ~10% of load is a good target, hence 0.47Ω-1Ω resistors into 8Ω loads. And 30Ω-50Ω for 600Ω loads.
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I suspect there are some additional compensation components not shown
The Tiger, and indeed any quasi-complementary output's CE-CE side, wants to be violently unstable due to two CE stages in cascade with low/unity feedback. It can be very unpredictable in class AB: the wide swing of current causes wide swing in working Ft and device impedances, so it may idle clean and radiate CB waves at large output. The classic fix is some resistance in the input emitter, and a honking big cap C-B on the output device. That in effect sets output stage Ft low and constant (though at low bias currents the working Ft still falls).
However if the CE-CE stage can be tamed, it can give an output Z much less than the ~30Ω at 1mA that a CC stage gives open-loop. (OK, 13Ω in stacked push-pull, but probably double counting emitter resistors, so 30Ω/mA is about right.)
There is a 4-transistor push-pull gain stage that works sweet: Borbeley's Hafler DH-101 phono stage. But it only works Class A, and is current-feedback so it gets a little odd at low gain. To work predictably it has to have very high Beta in the second stage, since the feedback loop closes ports with current gain of just 1*Beta.
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the emitter R's in the Jung input Q's can be raised, although this also raises the output Z going forward.
A lot of interaction there. Indeed the Diamond Buffer wants those resistors small, OTOO 26mV. Even that is enough to complicate things... I think on paper these resistors should be zero, but in practice (with discrete parts) that can't work.
Without voltage gain, I still vote for a modified Jung. The LED bias current can be much less. Some LEDs won't like that; we can select LEDs or simply use a couple 1N914s. Straight diodes will drop idle current as temp rises, but that may not be a problem in a studio.
Another mod on the Jung is small resistors in the output collectors driving the B-E junction of a pair of Class-B boosters. If the Class-A output runs 1mA idle, you can touch 2mA peak, which will drive 10K loads to full voltage, and handle all low-level audio details in 600Ω, while still Class-A, and shift to A-B for high levels in 600Ω.
With a need for gain: it is hard to beat the better ICs for good device matching and insane complexity, thus potentially good performance for the supply current.
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Hmmm... this hasty hack -
- seems to have point-oh THD, 0.01% at 1V peak in 600Ω and only a little higher at 20V peak. Temperature stability seems too good to be true (output stage current actually declines a bit at 80C). Output impedance is quite low for ~2.2mA idle current: 5Ω-6Ω.
Output impedance actually wanders all over the place with signal level: if I reverse things and drive the output through 600Ω I get 2% THD with a large dose of 5th and significant out to the 11th. The THD is low in the forward direction only because Zout is so very low compared to the load. From working with a basic Diamond, I know the odd-order distortion is in the nature of the beast. There is a magic value to null the 3rd, but then the 5th stands out. This can be nulled but that leaves a 7th, and you run out of places to correct. I am amazed that this very-B biasing is not worse than it is. Remember it gets 33mA peaks from 2.2mA idle.
Frequency response is absurd: ~100MHz. Minor peaking there: gain rises a bit above unity, a bad sign for a fancy cathode follower.