[TenTec] Audio equipment power and characteristics OT.

[NL7VL]

Jerry,

That's quite a bit to digest, with a lot of it over my head, but I do
see things I agree with and some things I had not realized. I'd like to
make sure I get this if you don't mind.

> There are many factors in sound equipment, some due to active device 
> nonlinearities, some to clipping characteristics, and some to
> impedance matching.
> 
> Solid state circuits tend to swing almost all the way to the supply
> and ground or + and - supplies and clip hard. Tubes tend to go
> nonlinear before reaching the supply limits and so clip more gently,
> from voltage rise on one extreme to current saturation on the other
> extreme.

OK, so that's one reason why a solid state amp sounds muddy when it
clips and a tube amp will gradually distort. Turning a preamp up -
whether it be solid state or tube, will gently push the tube into its
nonlinear region.
 
> There tends to be a whole lot more feedback in solid state circuits, 
> especially in low level stages using OP amps with open loop gains 
> 100,000 times the closed loop gain. While that takes out almost all
> of the nonlinearities but retains the supply voltage hard clipping
> limits. Sometimes there's some feedback around tube output stages and
> output transformers, but not 99.9% like an solid state output stage.

Then the passive devices that couple the tube stages and contribute to
a tube amp's distortion characteristics? (Also at your last point,
too.) 

> With that high level of feedback, the solid state power amp's output 
> impedance is very low, far lower than the rated speaker impedance.
> With a tube PA, the load impedance is matched by the source impedance
> of the amplifier, in a solid state PA, the load impedance is the
> lowest impedance that doesn't overload the PA devices or the power
> supply. Its not the impedance that would have half the peak to peak
> voltage that the amp did open circuit like it would be on a matched
> tube PA. That low impedance adds much speaker damping so that the
> speaker cone position is controlled closely by the AC voltage. Which
> keeps a speaker with poor acoustics from ringing from transients as
> much.

As I think about this, the low end of the solid state tends to
reproduce more cleanly than the tube, and this is because low impedance
of the output transistors? 

> As for power rating, the advent of solid state has created new vistas
> of audio power rating specmanship. One of the amps I liked the best
> was good for a whole 0.2 watt but was class A solid state. It was
> clean. But most musical instrument and home stereo amps are rated at
> 50 to a few hundred watts. Speaker efficiency hasn't gone down,
> ratings have grown with "inspired" techniques. Most are peak power,
> not average, but peak to peak voltage squared divided by the load
> (which gives a number conveniently 4 times "RMS Power"), and if
> measured are measured with the supply the amp would be delivered with
> replaced by substantial voltage regulated supplies so the power out is
> what it would be on the leading edge of a key closing tone before the
> power supply drooped (leading to lower power and clipping). I suspect
> some is rated with a much lower load impedance than the amp is rated,
> but since the solid state amp is a constant voltage source with very
> low output impedance the amp does more power the lower the load
> impedance, until its power supply croaks or the output devices melt
> off their emitter lead inside the device package. The result of these
> subterfuges is that an amp that probably really delivers 10 watts,
> might be rated at 200 watts. Of course without the regulated supplies
> and extra output device cooling not shipped with the amp, it can only
> do that power if measured as peak to peak voltage squared divided by a
> load impedance of a fraction of an ohm (where normally rated for 4
> ohms minimum load Z) over one or two cycles of a 10 kHz keyed tone.
> After a very short time, power supply droop and output device heat
> takes over to limit the useful real output power.

I wonder if some of the unbelievably high solid state ratings are
taking into consideration the slewing power of the circuit - its
ability to handle very sharp transients like the crack of a snare drum
etc. Yesteryears speakers could not handle what is expected of them
these days. Even back a few years, it wasn't that difficult to fry the
voice coil with highs or separate the paper from the coil with lows.
The new speaker engineering really impresses me.
 
> All it takes for feedback to a guitar's strings is delay from
> circuits and the acoustics path between the speaker and the guitar to
> have one cycle or an integer multiple of one cycle time delay. It
> shouldn't matter much what the active devices are.

Hmm. I'll have to look into that one. I believe you, I've just never
considered the concept before.

> Tube preamps tend to not have the feedback of the solid state op amp 
> based preamp because its too hard to get excess gain in a tube amp 
> without getting hum and noise. To hard to get excess gain so there's
> no serious feedback beyond a partly unbypassed cathode resistor. But
> that makes the stage more susceptible to heater cathode leakage
> injection 60 Hz hum.

I see. But what about microphonics in tubes? I swear at times I can tell
when someone is running tubes, whether it's CW or SSB. I can't imagine a
solid state amp with those "ringing" attributes. And to me, it has the
same quality that my Hammond M-3 would have when I would get a little
ruff with it - or a guitar pick plucking strings over a really hot
humbucker pickup. 

> The there may be come effects from the coupling and bypass
> capacitors, tubes using paper or mylar caps while solid state with
> inherent low impedance uses miniature electrolytics of dubious
> quality and not quite perfect approximations of real capacitors.

So this get back to the point above - that some (of the older?) passive
components can contribute to tube feedback effects through unintended
coupling - yes?

-- 
 Neal, NL7VL

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