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re:amp math

>To cut to the chase what I did is as follows:
>
>Sony ES CDX-X810DSP [head unit] - cd receiver (in-dash) with built-in
>amp at 35 watts RMS power at 1% THD into 4 channels, digital signal
>processor, etc. Sony ES CDX-91 - 10 cd changer (in trunk). MB Quart
>Soundset up front - Model QM 325.21 - 3-way speaker system designed
>for BMW E36 2-door model, including: 5.25" woofer, 4" midrange with
>coaxially mounted .75" titanium tweeter. Left rear speakers alone for
>now. Installed cost approx. $1800.

wow... thats pretty strange....

I put almost Exactly the same system in my Audi.  I have the same head unit, 
with the joy-stick.  The internal amplifiers are not used.  In the front are a 
pair of MB Quart seperates (4" and seperate titanium tweeter).  In the back, 
the factory system contained MB Quart Seperates that were amplified by 30 W 
amps in the door.  I left these.  The head unit feeds a Precision Power two way 
passive crossover and the rear amps.  From the X-over, the highs go to two 
channels of a JBL amp, the lows go to the other two channels of the amp that 
are bridged.  I believe the amp claims 100 w x 4 into 4 ohms.  I gave it 1 
farad of capacitance on the power in.  The lows are handles by a parallel pair 
of Infinity Kappa 10" in an isobaric box.  I listen (generally) to electronic 
music and it handles it well.  The Audi keeps the sound in the car, as I find 
people who share their bass annoying as hell.  I think you will graduate to an 
external amplifier.  They are far superior due to better cooling.

>Watch for words PEAK, RMS, THD and how many channels there are in 
>manufacturers power readings. A good example is the BMW amp. 200 W is 
>20 W x 10 channels. Even the 20 W is quite high for an amp like this.

Halleyujah!   Anyways...  There is a BIG difference between Peak and RMS 
power.  Capacitors within the power supply allow an amp to access decent 
current and provide peak power well in excess of what it can do over several 
milliseconds.  GOOD companies ALWAYS advertize their RMS power.

>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).
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.

NEVER listen to the amount of power a stereo claims to have as a system.   
There are FAR too many variables.  LISTEN to it and judge yourself.  In college 
I put on dances with a pair of Klipsch speakers and a "60 watt per channel" 
Adcom amp.  It was GREAT.  Efficiency is never addressed in these system 
quotes.  Ive heard 1200 W car stereos that sound like crap.

chris (who is off to hawaii now)