From: jeh@cmkrnl.com
Newsgroups: rec.audio.car
Subject: Re: ohms and watts and such
Date: 17 Nov 92 21:31:27 PST
Organization: Kernel Mode Consulting, San Diego, CA

In article <1e9njgINN1fh@bigboote.WPI.EDU>, walkerr@bigwpi.WPI.EDU 
(Robert A Walker) writes:
> OK if i have an amp that puts out 60 watts a channel
> and i run one channel to a dual voice coil sub with 8 ohms resiatance per
> coil, and i run these in parelell.  Is the total output 60 watts, oh yeah
> if the 60 watts is at 8 ohms.  What does my total resistance become.  

Paralleling two 8-ohm voice coils yields a net load of 4 ohms.  

Whether or not the power stays at 60 watts depends on the design of the amp.
Lowering the resistance (impedance, actually) allows more current to flow for a
given voltage.  Since power (watts) = voltage (volts) * current (amps), and
current = voltage / resistance (impedance in this case), lowering the impedance
=> more amps => more watts -- IF the amp has additional current capability. 

But if, when the amp is running 60 W into an 8-ohm load, the amp is already
supplying as much current as it can -- ie if its power supply is "maxed out" at
that point -- lowering the impedance (by paralleling two voice coils, or simply
using a single speaker with 4-ohm impedance) won't pull any more power out of
the amp. 

For a moment, think of the amp as a battery.  (this analogy has serious flaws,
but it's good enough for now.)  Connect a light bulb to the battery and the
battery puts out a certain amount of current.  Connect another light bulb in
parallel and (assuming that the battery has the current available) the same
amount of current will flow through the second light bulb as well, so you get
twice  the power usage (watts) and twice the light.  You can keep adding light
bulbs until you're drawing as much current as the battery can supply.  If you
keep adding light bulbs after that, each add'l bulb will make the other ones
get dimmer. 

> I've noticed that onthe specs of some amps that they put out 100 watts at 8 ohms
> 200 at 4 400 at 2 and so on, do all amps follow this pattern?  

Again -- it depends on the design of the amp.  

> If my amp
> can handle only a four ohm load, how should i connect a dual voice coil
> sub with 8 ohms per coil?  

When an amp is rated to handle a four ohm load, it will handle larger (higher
impedance) loads just fine.  

> I presently have them in parellel and they sound good, 

given that the amp is rated to drive four ohm loads, this is fine. 

> putting the two coils on each channel in series would give 16
> ohms and would sound lousy, or at least quieter.  

yep.  Look at the math.  What you've done is double the impedance.  This cuts
the current in half,  which cuts the power (watts) in half.  Of course that's 
only a difference of 3 dB, which is well past the threshold of "noticeability",
but it's hardly a major change. 

> Am I hopelessly lost or am i on the right track?  

You're on the right track.  You will get at least 60 W into the four-ohm load,
maybe more.  Probably not 120 W, though -- most car audio amps' power supplies
aren't designed with much margin. 

	--- Jamie Hanrahan, Kernel Mode Consulting, San Diego CA
Internet:  jeh@cmkrnl.com, hanrahan@eisner.decus.org, or jeh@crash.cts.com
Uucp:  ...{crash,eisner,uunet}!cmkrnl!jeh