Self regulating Class A

[MarcelvdG]

Nelson, have you got a patent on sliding bias class A or on non-linear common-mode loops used for instantaneous class AB control (or on both)? In the second (or third) case, can you give me a reference to the patent? The oldest article I've ever seen about class AB control loops is the Huijsing and Tol paper published in the Journal of Solid-State Circuits in 1976. I'd be very interested to read older material on this subject, if any.

I've never used the LT1166, but I have used class AB control loops made with a transistor array and some discrete transistors in an audio power amplifier. That works very nicely.

[Nelson Pass]

My patent 3,995,228 was submitted September 1975, and as
far as I can see describes a circuit which would do either, but
addresses Class A specifically. The basic circuit is similar to,
but predates the work of Cordell and separately, Hawksford.
They both obviously intended Class AB operation.

As an aside, I developed an extremely interesting circuit in
this area just as I was walking out the door at Threshold.
It remains a secret.

[MarcelvdG]

Nelson, thank you for giving me your patent number. The circuit on the first page of patent 3 995 228 looks very interesting indeed, but it doesn't seem to be a class AB control loop as described by Huijsing and Tol. If I understand it correctly, it eliminates cross-over distortion, but it doesn't eliminate thermal tracking problems due to varying output transistor temperatures.

[Nelson Pass]

More accurately, it doesn't do a very complete job of
either.

[Mark A. Gulbrandsen]

"but I was rather underwhelmed by the sound quality."
__________________________________________________ __

I would agree with Grey on this one. Although very sophisticated devices are available to control bias on demand as happens in the Krell's one would really need to control the bias a bit before it is needed to be sure the circuitry is stabilized before that level is required. Thats pretty impossible to do......

I believe the best solution is a combination of constant high bias class A, and water cooling. That way one can enjoy the best of both worlds.

I've owned a Krell FPB 300 and a KSA-80B albiet at different times and from my recollection the KSA 80B if left on all the time sounded much better overall than the FPB 300 but had less slam. The 300 was a very short time owned amp for me...I then traded it for a BAT tube amp. That is a whole other story though.........

On another note(pun intended):To me, the Pass designs still sound much more like real live music which I am very fortunate to be able to hear live quite frequently here in SLC. That has become far more important to me than the heat produced, or the slam capabilities of any particuluar amp

Mark

[mlloyd1]

That Nelson ... he's as bad as JMS, dropping those little pearls in the B5 newsgroup as he pops in and out

mlloyd1
(who still immensely enjoys watching his tapes of Babylon 5)

Originally posted by Nelson Pass
..... I developed an extremely interesting circuit .....
It remains a secret.

[Nelson Pass]

OK, I'll give you a hint. The trick is to track and hold (for a while
anyway) the minimum value of the sum of the currents through
the positive and negative halves of the output stage. Regardless
of the dynamics of output current, this value is the bias, and
can be used to control an AB bias with great precision. There is
a very cute circuit which uses a couple op amps tapped outside
of the signal path and not requiring an isolated supply which then
drives an opto-isolated transistor in the bias circuit.

The decay time of the track and hold is set at a value lower than
the thermal time constant of the heat sinks and slower than the
audio signal.

[GRollins]

Hmmm...as in a peak hold circuit with a resistor to bleed the cap down over a given time period? At least conceptually that doesn't sound too difficult.
It still seems that you're going to be faced with a lag problem. Take something like the Gladiator sound track that starts off on the soft side of normal and just keeps getting louder and louder and louder. Unless you purposefully build in a little overshoot on the assumption that louder is to come, you're still going to be playing follow-the-leader when the music exceeds your previous set point.
If you really want to get serious about this kind of thing, you're going to need to read the music onto a hard drive (we're assuming a digital signal here), scan the piece, calculate and record the necessary bias, then play. Only a single scan would be needed as long as the bias numbers were recorded permanently. All kinds of cute tricks could be brought to bear on the problem once you're working ahead of the music instead of behind it.
Didn't I read that Linn has a music server dingus based on hard drives? Might be a good platform for this sort of thing. Of course, if you give the PC people who live, eat, and breathe MP3 files a little nudge, they might play with it. On the other hand, given the sound quality of those files, it fits into the why bother? category.

Grey

[Variac]

These guys are have been discussing a PC CD drive based player.
Some have decided to buffer the output to reclock the data.
Would work fine with Grey's proposal. doesn't seem like any records are being set on making actual players...

DIY CD drive based on a computer CDROM

[Circlotron]

Originally posted by GRollins
It still seems that you're going to be faced with a lag problem. *snip* If you really want to get serious about this kind of thing, you're going to need to read the music onto a hard drive (we're assuming a digital signal here), scan the piece, calculate and record the necessary bias, then play.

Or just put X milliseconds of shift register upstream of your DAC, with the before-the-shift-register bitstream also sent to another DAC that is used purely for bias duty. *THEN* you really do know ahead of time what the signal is about to do.