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In the Forum: Melquiades Amplifier
In the Thread: Staxquiades project

Post Subject: Circuit instabilityPosted by N-set on: 9/26/2012

Actually I do not have the instability issue sorted out completely...
I'm talking about circuit instability now, not the tube instability. Let us imagine all tubes are rock stable.
As I see it there are at least two major sources of instability, in the sense of changing the OP of the output tubes,
in the DC scheme I try to implement:

A) Signal-related instability.
A1) Even harmonics generated by the driver will momentarily (at the timescale of the period of the fundamental)
rebias the output stage. This effect I belive is easy to minimize. First of all by the use of a linear driver (Hagtech 6E5 curves published here look
as straight as legs of a supermodel). Linearity of the output will also help here as it will be less sesnitive to small OP variations.
For PiPi amp this effect is moreover strongly supressed by even-harmonics cancellation
A2) Cathode blocking. If the driver is cathode biased (a very natural choice here, see below) and the cathode R decoupled, a strong signal will
momentarily charge the capacitor, it will hold the voltage for the time ~1/RC, moving the driver towards cutoff and by that rising the current of the output. So I'd say cathode decoupling is a no-no.

B) Supply line instability. This I belive is much more serious.
Change of the supply line will change all the voltages in the system but with different effects as driver and output will react differently.
The standard thing is to use cathode biased driver as it will be self-balancing. This is what's done e.g in the Axiom

http://www.chimeralabs.com/images/axiom.sch.jpg (with diodes instead of R)

but is really an old approach: http://dev.emcelettronica.com/amplifiers-builder-guide-10-watts-direct-coupled

I'm perhaps shooting my own foot badly trying to put the driver on the fixed bias as then the supply line variation is greatly
increased (!) and I go very much agoinst the current. Few ad hoc rescue scenarios:

* That the driver should be in a high-enough-current point to accomodate 10% line variation is clear (hence my 6E5 current is highish).
But one can hope to minimize the OP change effect in the output too, by using i) linear tube and ii) in the most linear region; the last is of course high current, so if the output is close to its max it can be fried. With headphones one does not need in theory much more current than to drive the C at a given f and V. My crazy high output satge current comes in the 1st order from trying to get a big A1 voltage swing, but the secondary effect is that the tube is far away from the cutoff but yet safe enough to accomodate the 10% line variation, so at lest in terms of survival and not cutting off it should be OK (the sims say so). If this will work sonically...no idea.

* Driver power supply passive. Huge cap and some resistance will create a mechanism to smoothout the line variations. And this is perhaps my main hope. Huge, HV caps were not available to the fathers of DC coupling, so the only way for them to stabilize the circuit was cathode bias.
I've put on paper 10.000uF cap and some effective 200R from the line side, giving the line time constant of 0.5s in the driver PS. Will this be enough...no idea

* Driver power supply active. Active regulation is another option. There are voices that particularly shunt regulators are sonically least intrusive or even beneficial. Adding a 6E5 as a shunt shoud not be a big problem.

* Line stabilization...this is a very dangerous territory as the "mad electricity" thread shows. It's very difficult to imagine what makes all those line regenerators audible even given the Romy-style PS with a lot of decoupling. THe one who resolves it will get an audio nobel!
One ad hoc idea is that as those regenerators seem to carry some sort of switching power supplies (loooking at PP2000 size it's way too small to have a linear supply of thet power), perhaps line dirt uses the switching freq. as the carier and gets spread all around and then is picked by some high imp. (esp. grid) circuits inside the amp....ony a naive theory, but checking the line before, after and the air with a spectrum analyzer can shed some light. I'm wondering what happens if one forgts slim, compact, attractive shape and tries to design a regenerator that would not use switching...

Summarizing, there is a serious point regarding the stability (esp. short-time scale as it's most audible) of my circuit. I keep thinking....

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