<VV> RE: radio speaker

[Tony]

At 0555 04/03/2004 -0500, Paula Tilsley wrote:
>Question one)  Will using a 4 ohm instead of a 10 ohm damage the radio?
> 
>Rickie
> 
>Using the 4 Ohm speaker will not damage the radio.  If anything, it
>would just be easier to blow the speaker.  

Actually no.   The output circuitry in '60s vintage Delco radios is Class-A
transformer (autoformer) coupled.  The 10 ohm speaker is a closer match to
the output impedance of the big goofy germanium output transistor.   A 4
ohm speaker would be a mis-match and tend to clip the output voltage peaks
lowering the effective power out and increasing distortion.  You won't
damage the 4 ohm speaker if it's of any substance at all.   Remember that
those older single ended Class-A 12 volt output circuits were only good for
around 4-5 watts tops and any mismatch would effectively lower that figure.   


>However, the closer the
>impedance, the better the sound quality will be.  

Absolutely.   In the case of these Class-A output circuits, try to stay as
close to 10 ohms as possible.   It's the only effective way to guarantee
adequate power transfer from the output circuit to the speaker.  


>You could add a
>resistor in-line to match the speaker impedance to the radio but it
>would probably be easier to get close with an 8 ohm speaker and call it
>a day.   

I'd not do this under any circumstance.   The resistor would simply make
the output circuit "see" a 10 ohm load and much of the power would simply
go up as heat radiated by that resistor.  Volume from the speaker would
drop dramatically.  Distortion would increase equally.  You'd be better off
with a 4 ohm mismatch, performance-wise.  

Now:  An 8 ohm speaker is a pretty decent match and should work out pretty
well.   There's not a lot of difference between the 8 and 10 ohm speakers.
 And, 8 ohm speakers are still available fairly easily... as are 10 ohm
Delco speakers for that matter, if you look around.   


>Do not use a speaker with a higher impedance than the radio, as
>it will make the amp work too hard and could potentionally damage the
>radio.

In the case of these Delcos, this isn't exactly true since the output
transistor still has a source to ground via the autoformer/transformer.
And, there remains the point of not finding many speakers with an impedance
of more than 10 ohms or so unless you run across some specialty speakers
made for specific applications... I've seen 16 ohm speakers before but not
in a long time.   For that matter, 8 ohm speakers are starting to get kinda
scarce compared tom the new industry standard of 4 ohms, intended to be a
good match to DC-coupled output stage amplifiers running push-pull class
AB2 output circuits which is what's seen in most audio output circuitry
these days although the trends are now towards MOSFET devices than planar
based transistors... the basic class of operation hasn't changed,
however... although with stacked outputs to increase output power, even
lower impedance loads such as 2 ohms will sometimes be used... providing
the output circuitry can handle the current requirements.   You can get
away with this when using DC coupled output circuitry but not with
transformers which are dependent upon a good impedance match for best
operation.   Go higher or lower and efficiency drops off quick.    


tony..   

...factory/warranty Delco audio sound systems tech, 1975-1992 and dabble in
it to this day