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Re: bmw-digest V5 #179

> From: Chris Nolke/HNS <[email protected]>
> Date: 3 Sep 96 18:51:58 EDT
> Subject: re:amp math
> 
....
> >compared to group 1 amps. The drawback is worse linearity resulting 
> >in more distortion. In theory peak power could be 12 V^2/4 ohms = 
> >whopping 36 W (pretty close to 35, isn't it). Rms power could be (12 
> >V/1.41)^2/4 ohms = 18 W. In practice a good design results something 
> >like 14 W per channel with a reasonable level of THD.
> 
> Your theory on class A amps is good, but your power calculations have two 
> problems...
> V^2/R assumes a non-reactive resistance.  Speaker coils are, by def., very 
> inductive, and four ohms applies to reactance at 1000 Hz.  For straight DC, 8 
> ohm speakers generally measure about 5.3 ohms.  The power depends alot on the 
> frequency (assuming only one).

More math... I used resistance to simplify things. In reality 
speakers are not purely resistive like you said. Impedance is the 
vector sum of resistance and reactance. Impedance in a speaker varies 
greatly according to frequency. To advertise power reading to a 
resistive load is in a way correct. Power is usually measured to 
resistive artificial load (big resistors) instead of speakers. Some 
magazines test amps with a reactive load also. Class A refers to the way 
an amp works. My classification has nothing to do with it.

> Second, class A amps often do have high voltage power supplies, relying on 
> current for the power through transformers.  The transformers (however) are 
> rarely put into Head units, as they are inefficient and generate heat.  
> Generally what you say will apply to head units, but external amplifiers do all 
> sorts of things.  I owned a Precision Power amplifier that was a triple 
> Darlington array, and boy oh boy did it put out.  Two fans though.

I am not sure what you mean hear. Transformers can be used in output 
stage like tube amplifiers do. The reason to use them is high output 
impedance which is not a problem with transistors. Class A means the 
way the output stage works. Class A amps consume a lot of power even 
on idle but they a considered to be very linear (= low distortion). 
Darlington array is just a way the output stage is connected. 
Darlington transistors have two transistors in series to produce high 
gain.

Maybe this discussion should go to rec.car.audio soon or else we get 
'Zumolled'.

Jari