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.
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.
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 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.
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.
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.
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.
73, Jerry, K0CQ
On 2/7/2011 4:03 PM, NL7VL wrote:
> O.T. for just a bit ...
>
> I thought it was amazing that my Leslie Model 147 with a 40 watt rms
> tube amp could easily keep up with the (new at the time) 100-150 Peavey
> amps or PAs. That 40 watt tube amp and its exquisitely engineered tone
> cabinet could literally shake the plaster loose from a wall.
>
> I'm sure if I when to hydrogen audio I could get a better explanation,
> but I have always believed that tubes sound "better" with music amps
> for guitars, etc, because of the nature of the tube's ability to clip
> harmonically, whereas a solid state amp will just distort everything
> evenly to the point where it sounds like mud. There is just no way a
> guitarist (Ok - Hendrix) could stick his axe into the amp and get a
> string's harmonics to resonate and feed back without a tube amp.
>
> Also there is a certain coloration or warmth that is inherent with tube
> audio. But I would never use tubes to create a low-distortion audio
> amplifier. If I want the coloration, then I add a tube pre-amp.
>
>
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|