[Vintage-Audio] Dual Voice Coil For Woofer?

[Matt Johnson]

> The 15 inch speaker has two voice coils, one for the
> right channel and a
> second one for the left channel. Somehow the two
> signals are put in phase
> and fed to a single speaker. 	
> 	
> How did they cause the two voice coils to work with
> one speaker?

Its simply two voice coils wound on the single former
that are electrically isolated from each other. The
signals applied to each are magnetically summed
allowing each amplifier channel to be added without
electrically connecting them.
 	
> 	
> Was doing this common or unusual?

Quite common actually, many passive subs use this
arrangement. It is even more common is car audio
though. Often people will connect both coils to a
common amplifier channel to obtain a lower drive
impedance (provided they have a current capable amp!).
Of course it is vital that proper phasing is observed
since low bass is usually in phase. Connect the
channels out of phase and the net drive to the cone is
zero!

The shame in this situation is that there is a high
level crossover between the amp and the passive
woofer. To really drive a sub the voice coil needs to
be connected directly to the amp with no reactance in
line to harm damping factor. Also having a low level
crossover ahead of the amps will allow the amps to
deal with their respective frequency bands reducing IM
distortion between the seperate systems. I had a
friend of mine that had an older Cerwin Vega 12"
passive sub like this that removed the passive
crossover replacing it with a plate with connectors on
it. When he drove the system with a pair of amplifiers
and a low level crossover I was simply amazed at the
performance. My JBL 2235H still did much better
though!

Regards,

Matt


	
		
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