[net.audio] How do you get 60W out of a car stereo with 12 V????

pd@eisx.UUCP (P. Devanbu) (10/10/83)

As far as I know, the most power you can get with a 12V supply, even
using double amplifier 180 deg out of phase, is 9W rms into 8 oms
or 18W into 4 Ohm. How the (#*$?!! do they manage 60W per channel amps ?
Is that a fake ??


Prem Devanbu
......eisx!pd

tgg@hou5e.UUCP (10/11/83)

You can have megawatts from 12v amplifiers - you need a voltage converter.
(I have listened to the box of a high powered Sony car amplifier before
and have heard the switcher in the converter wind up).

jgc@sunrise.UUCP (Jim Constantine) (10/12/83)

There are basically two methods for obtaining higher power outputs :

1) for intermediate power 7-20 W, Use what is called a "bridge" amplifier.
   They work like this :

   input
   ----.---{ 180 deg. phase shift}-------{amp}-------|
       |                                             |   /
       |                                            |---/   speaker
       |                                            |---\
       |                                             |   \
       ----------------------------------{amp}-------|

   Note: neither side of the speaker is connected to ground.
   
2) For large power outputs a power inverter is neccessary to convert
   the +13.5VDC input voltage to something larger, usually  + and - 35VDC.

dave@rocksvax.UUCP (Dave Sewhuk) (10/12/83)

Only 2 ways to exceed those numbers:
    1) Build a DC to DC converter to generate a > 12V supply for final
	transistors.  If done this way there is a possibility that some
	RF interference could be generated in the converter.

    2) Use a transformer to take "step-up" voltage going out to speakers.
	You can do this because the output impeadance of most amplifiers is
	low.  Always adds low frequency distortion unless there is a lot
	of iron in transformers. 1 required per channel.

I have seen those amps that claim 100W per channel and they are nothing more
than a redefinition of the term watt.  Algebraically represented as follows:
	Power(r) = V*I*r where is is a constant value usually much greater
	than 1, and named after its user.
I beleive they can stick any number they want on auto equipment.  The FTC only
has rules for advertising and specifying home equipment but car equipment 
is not under those rules.   Buyers beware!! If the unit is over 10W per channel
is must fall into one of those 2 categories above if there is any truth to 
their claim for power.

-----
Arpa: Sewhuk.HENR@PARC-MAXC.ARPA
uucp: {allegra, rochester, amd70, sunybcs}!rocksvax!dave

guy@rlgvax.UUCP (Guy Harris) (10/13/83)

Another nasty trick for claiming high power on an amplifier is to claim the
power at the peak of the power curve as the "official" power; I always
appreciate it when they give the power at 20Hz and 20kHz.

	Guy Harris
	{seismo,mcnc,we13,brl-bmd,allegra}!rlgvax!guy