Using HC12 for bass

[John Vanderlee]

My friend, the retired Allen rep, always says to add bass use pair of HC12 on a separate bass only channel, and that has been very effective. So I got curious - why not remove the tweeter and midrange and just blank up the holes? That way we can keep those drivers as spares. But then I thought the effect that their impedances might have on the crossover, and would the lack of them mess up the woofer response?
Then again, a B20? I believe, is nothing more than a 15" woofer in a box without a complex crossover? Or just a cap across its terminals?
Curious minds etc...

[toodles]

You can do as you suggest and it will not cause problems with the crossover. Allen used a parallel crossover design in this model (and most, as far as I know) which means each crossover section is treated independently and does not interact with the other sections.

The B20 has only a capacitor in series with the woofer to limit bass response below 15 Hz--this is a protective measure so that any turn-on thump or disconnection of amp inputs with them powered on doesn't try to destroy the woofer. A cap across the terminals would act as a direct short on the amp and probably damage the amp--never do that.

[John Vanderlee]

Thanks toodles! You're invaluable!

John

[myorgan]

John,

While it might be possible, I wonder if it would accurately reproduce the sound. While the bass channels do comprise primarily of frequencies in the bass range, certain aspects of those stops would require at least a mid-range to accurately reproduce the sound. An example of this would be the chiff effect for the Bourdon/Flute stops.

For that reason, on all my Allens, I either use HC-12 speakers alone for the 32' channels; or actually use a sub-woofer crossover to send the lower frequencies to a dedicated B-20 or B-40 sub, and the remainder of the signal is channeled through the companion HC-12. That way, I'm sure I get the best reproduction of the sound.

Michael

[John Vanderlee]

Actually, in the 4channel setup we used an Allen active low pass after the Preamp to create an extra bass (fifth) channel with its own amp. It drove 2 HC12 with the tweeters and Midranges removed and blanked over. It sounds awesome! This formerly underpowered ADC4500 system is now going back into the theater with 8 HC18's, 4 HR200, 4 HC15's and the 2 HC12's mentioned above. All driven by 9 channels of ADC amps. I'll report on how it sounds there after Tuesday.

[toodles]

If using the Allen low pass filter before the amp, then there is not any need to have the midrange and tweeter in the speaker system--those frequencies have already been crossed over to the main (non-bass) channel prior to the amp.

[jbird604]

Being a minimalist when it comes to circuitry, I'd be inclined to strip out the crossover components when using an HC-12 in place of a B-20, where there is an active crossover in the organ itself. Probably just an excess of caution, but I always fear that any crossover components that aren't really needed may be draining at least some power, and you need all the power you can get for truly good bass.

[John Vanderlee]

Probably right to an extent, John. But the speakers represent most of the load which is where the power is consumed, I felt - and upon hearing as well - that the lack of those drivers provided more power available to the 15 "drivers. Of course everything is relative and I don't think they robbed THAT much.
Besides it's too late ;-) we installed them in the theater yesterday.

[jbird604]

Yes, at least you removed the mid and tweeter, which were useless at the very least. I'm sure there's little harm in leaving the crossover in there, but the series coil in the woofer circuit may have some resistance, which might very slightly affect the available power to the woofer. I also dislike having capacitors in parallel with a coil when not necessary, but that is a minor quibble and probably serves to clean up the response a bit anyway.

[John Vanderlee]

That's good point.
...a good point!

[toodles]

I agree with John--better to remove all sections of the crossover (or at least disconnect them) that are not in use.