New GPA Altec crossover with 511B horn and 4ohm woofer

**MerlinVSM** · August 9th, 2012, 12:38 PM

I have the Altec 511B horn, the GPA 902 driver and an Acoustic Elegance Dipole 15inch woofer per speaker. I got curious when i saw the new N1200-8A crossover from Great Plains Audio. Would it be a good idea to use this new crossover with the above units as the AE can not be wired for 8ohm, only 4ohm or 16ohm.

Well, I asked Bill at GPA about he's thoughts and quote "The crossover will perfectly with the 511 horn and 902 that is what we designed it for but I do not know about your 15 inch speaker, it is looking for a 8 ohm load so I would not recommend it."

Anyone here that have any input ?

Can the crossover be easily modified be be used AE woofer 4 or 16ohm wired ?

**Old Guy** · August 9th, 2012, 01:44 PM

Hard to predict. Generally an upward mismatch is better than a downward one, so will likely sound better with 16 ohm woof than 4.

**MerlinVSM** · August 9th, 2012, 02:31 PM

What does it actually mean that it is a mismatch ? Different frequency response between the units, output etc. compared to 8 ohm load ?

**bowtie427ss** · August 9th, 2012, 02:39 PM

Originally Posted by MerlinVSM

What does it actually mean that it is a mismatch ? Different frequency response between the units, output etc. compared to 8 ohm load ?

The impedance of the load determines the actual knee point of the filter for a given capacitor and inductor value.

The GPA network was engineered assuming 8 ohm loads, a load of higher or lower impedance will shift the intended crossover point.

I believe what OG eludes to is that if you use a 16 ohm load where 8 was engineered, you will have an "overlap" where the upper roll off of the LF will begin above the roll off of the HF, which is more desirable than having the upper roll off of the LF begin too far below the roll off of the HF leaving a "hole" or dip in response in the XO region.

**MerlinVSM** · August 9th, 2012, 03:15 PM

An alternative I assume may be to go for two woofers ?

I orginally planned to use an active Behringer DCX2496 crossover but doupt it will be up to the task for a great hifi sound after listening to it. It may need to be modified.

The new GPA passive crossover was therefore evaluated as an alternative.

**Old Guy** · August 9th, 2012, 03:16 PM

Just to be my stickler self, a little more complicated. IOW a 4 ohm load on an 8 ohm crossover will shift the elements of the xover. But not in a LINEAR fashion...it will have the components operating at unintended freqs, but not necessarily the same freq. So the response will get very not flat. What I am trying to say is it won't shift to say 500 or 1200 instead of 800. The separate components (coil, cap) will be operating at something other than 800, and not the same frequency.

How bad the resulting response is, you would have to experiment.

**bowtie427ss** · August 9th, 2012, 02:31 PM

You really need to analyze the impedance of the driver in the enclosure, and determine the impedance at crossover frequency as they're doing in the "400lb center channel speaker" thread.

**aditya** · August 10th, 2012, 05:42 AM

Originally Posted by MerlinVSM

Anyone here that have any input ?

Can the crossover be easily modified be be used AE woofer 4 or 16ohm wired ?

Good/serious xovers are always driver specific, but that does not mean we can not experiment.

I have an idea...

This particular AE woofer is a OB driver (Qes at 1), and also quite efficient (102 db).
Xovers for OB woofer configurations start their slopes quite early, nearly an octave earlier (to fight out the rising FR slope).
One can use the '4 ohms mode' of the woofer, and disconnect the cap across the LF o/p of the xover.
This should make things quite good, as the HF side is left untouched.

Try and let us know.

**Old Guy** · August 11th, 2012, 06:00 AM

Originally Posted by aditya

One can use the '4 ohms mode' of the woofer, and disconnect the cap across the LF o/p of the xover.
This should make things quite good, as the HF side is left untouched.

Try and let us know.

Worth trying. Assuming a textbook design, the inductor will be slightly large, rolling off the high mid in the woofer. Only at 6 db, but might have a small dip.

I would really recommend DIY. Not that hard.

**bowtie427ss** · August 11th, 2012, 08:25 AM

I would really recommend DIY. Not that hard.

Seems most logical.

Aren't those Acoustic Elegance drivers basically made to order?

Since they are aimed primarily at the hifi DIY market, it would stand to reason that each would ship with QC analysis that included a graphic representation of each driver's impedance curve. By the smallest chance, do they?