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Old 15th Jan 2021, 9:12 pm   #12
Radio Wrangler
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Default Re: Exotic speaker cable - problems.

That's quite standard, and comes under the general heading of 'A little knowledge is a dangerous thing'

What that little network on the output of an amplifier does is very important. The lack of one can drain your bank account. HOW that little network does it is quite subtle and to see how it has to be designed in order to work needs knowledge in the Bode and Laplace areas of engineering maths. Most batchelor's graduates ran away from the control theory modules of their courses, so framed paper on the wall is no surety of competence or understanding. Amusingly, it isn't actually hard, it just involves some weirdly alien concepts that can be hard to assimilate.

However, pontificating on how things can be done is not held back by any lack of technical knowledge. The results might just be affected, though.

Railing against feedback?

Well, if you don't understand something, you are scared of it, you do not feel in control of it. If you do understand it, it becomes a tool in your toolbox, to be used when appropriate. From here on, it gets funny, verrry funny.

So some pundits declared that feedback is evil, and that a non-evil amplifier must not have any. So amplifiers were 'designed' without any overall feedback loop and they all felt much happier and set to imagining thrusting new ways of describing the audible improvements they convinced themselves they must hear.

They'd removed the overt, obvious feedback path. They'd missed all the local feedback systems like stage degeneration.

An emitter follower is a feedback amplifier. The feedback path is one and the same with the signal path. through the transistor. Same with cathode followers and undecoupled cathode resistors.

So far, the anti-feedback people aren't even aware that they've missed most of the feedback in an amplifier. Even less are they aware that we've spotted their omission and are quietly having a jolly good laugh.

The next joke is that emitter followers, through the internal capacitances of the transistor and the inductances of their legs and circuit connections can go unstable at RF. No overt feedback path need apply. Nearly all transistor amps end in emitter followers as their final stages. One of the conditions for the oscillating follower problem is good RF capacitance to ground via low inductance on their output.... Ah, those damnable esoteric cables again. They work their magic even on pundit-certified feedback-free (well, the obvious feedback that is) amplifiers. The amplifier hoots its little head off, and the heat from cross-current sends up a smoke-based distress signal.

Serves them right?

Mostly.

There have been innocent errors in this area. Peter Walker is almost sainted because of his Quad II, and Peter Baxendall is up there in the pantheon as well. They made a mistake that wasn't a mistake at the time. Those triplets of transistors in the output of the Quad 303 are naturally unstable unless the TO3 transistor is dramatically slower than the two smaller devices driving it. Their mistake wasn't an error at the time, their circuit worked, but they failed to see that the devices they used had open ended specs and were being improved all the time. The TO3 transistors got improved more than the TO5 and TO18 drivers. The gap between the transition frequencies closed up. So nowadays it is a major problem to buy a replacement TO3 power transistor BAD enough to work in that circuit.

I stress that what they did was fine with the devices of the period and no-one was predicting quite the improvement that was coming.

But fire up a Quad 303, apply an 8 ohm load, maybe with a bit of ordinary cable capacitance, drive the amp somewhat, not even close to clipping, and look with a scope. You will likely see a short burst of RF oscillation (around 10MHz) hanging around one place on each cycle of the audio sine you're testing with. It looks like a smudge on the scope trace. Not all 303s do this, but a lot with replacement transistors do.

There seems to be a reciprocal law at work. The more boutique-y an amplifier is, the less of a clue the designer seems to have had, when you do a hard analysis.

It's as if the structural engineering for a bridge or large building was done on religious principles. What could possibly go wrong?

Anyway, the speaker cable problems are inextricably entwined with output stability networks and they are entwined with feedback design and your toe bone's connected to your.....

This thread has been about sensible, engineering based, answers to a perfectly real issue that samjmann raised. It's an area that I felt wasn't well enough understood in the world at large and has had tons of oofle dust dumped on it by a few decades' worth of shamen and mystics.

The way samjamm phased his question made me feel he wasn't just another cultist with a mind already made-up. It was a serious issue, asked carefully, and deserved the best serious answer I could give.

For the really silly bits of audiophoolery, there is the appropriately named "The audiophoolery thread" not very far away. It's where you'll find happy stories of 1800 mains cables, smart rocks and unicorn hide. It's where you can have a damned good laugh.

So let's keep this thread about high-profit cables and amplifier stability, please. This way we'll have a nice record for the next person to realise something funny is going on when fancy cables cause smoke, to be pointed towards. (And I don't have to type it all over again!)

David
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