Radios that have + and - lines for each speaker have bridge amplifiers.
There are actually two ampifiers, one for each speaker connection, that
are feed out of phase. So when one speaker terminal is driven high, the
other is driven low. This produces twice the voltage swing and hence
four times the power (power equals voltage squared divided by the
resistance). This type of amplifier does not use coupling capacitors
because neither speaker lead goes to ground. So both + and - outputs
are 'hot' at half the battery voltage. If you connect one of these
outputs to a grounded speaker, the speaker will draw a great deal of
current and the sound will be distorted. The radio will also run hot
under this condition. (And water wetter won't help.)
I'm suprised any radio on the market today does not have this kind of
audio ampilier. Silicon is so cheap these days, you're almost getting
four times the audio power for free.
BTW. the minus designation has nothing to do with ground/chassis. The
+ and - are just there so you can hook up all the speakers identically
in-phase.
Assuming a 4 ohm speaker:
Old fashioned grounded speaker amplifier:
((12Vpp/2)*.707)^2/4 = 4.5 Watts RMS
New bridge amplifier with + and - outputs:
((24vpp/2)*.707)^2/4 = 18 Watts RMS
>Glen,
> I don't know about the Kraco unit specifically, but recent
>installations of Sony and Blaupunkt radios have had a + and a - for each
>speaker. The speakers have two connections on them. The - does not go
>to a chassis ground--maybe it should be thought of as a "return" wire for
>the speakers electrical supply from the radio.
> Perhaps a call to Kraco would get you an owners manual for the radio,
>or a trip to the Kmart where they sell them would give you a chance to
>look at one.
>Bob
>On Thu, 14 Aug 1997 21:50:31 -0500 glen.christian@juno.com (glen
>christian) writes:
>>howdy fellow mg'ers,
>>i got me a little problem. i took my radio out today to see why it
>>wasn't sounding well (besides the fact that it is a cheapo kraco unit)
>>and found the speaker wires to be in a bit of a mess. seems like the
>>dpo knew less than me about installing radios. anyway, this particular
>>radio has 4 wires coming out the back. one for 12vdc (car is neg.
>>ground btw), one for ground, the other two for the "+" connection for
>>the left and right speakers. all the other radios i've ever worked
>>with have both a "+" and "-" connection for each speaker. i am
>>assuming that this type of wiring i have here requires me to connect
>>the "-" from each speaker to ground. can anyone confirm this? thanx in
>>advance.
>>
>>Glen Christian
>>Mechanical Engineer Graduate in search of Work
>>"So many cars, So little time"
>
>
Bob Donahue 'Still Stuck in the '50s'
rdonahue@holli.com
52 MG-TD #17639 under DIY restoration, NEMGTR #11470
71 MGB still in shop, 20 months and counting
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