Crate PA Cab PS-12H Crossover ?

[TomCarlos]

Friends....

I just bought two (old) Crate PS-12H PA Cabinets. The cab features a 12" woofer and PHT-404 horn. You see the same horns in many cabinets.

The cab is an 8 ohm enclosure. The woofer is 8 ohms. The horn is 8 ohms. They are wired in parallel except the horn has a 2uf cap in series with it. So how do you get 8 ohms out of this cabinet? Is it because of the cap?

For the hell of it, I would like to install a real crossover circuit. I guess I can buy just about any type of 2 way so long as it supports an 8 ohm woofer and 8 ohm horn? I know the horn cutoff frequency is 3K.

Any advice?

Thanks, Tom

[olddawg]

Friends....

I just bought two (old) Crate PS-12H PA Cabinets. The cab features a 12" woofer and PHT-404 horn. You see the same horns in many cabinets.

The cab is an 8 ohm enclosure. The woofer is 8 ohms. The horn is 8 ohms. They are wired in parallel except the horn has a 2uf cap in series with it. So how do you get 8 ohms out of this cabinet? Is it because of the cap?

For the hell of it, I would like to install a real crossover circuit. I guess I can buy just about any type of 2 way so long as it supports an 8 ohm woofer and 8 ohm horn? I know the horn cutoff frequency is 3K.

Any advice?

Thanks, Tom

That's pretty much standard proceedure for cheap stuff. The 2uf cap is just a treble pass filter. You are running the woofer full range. Most of the energy is being used in the lows and mids. The system is rated at 8 ohms because thats what the amp sees at 1KHZ. (the standard measurement spec is at 1KHZ) Don't bother with a crossover. It will not sound any better and you will lose efficiency. It is what it is.

[TomCarlos]

I appreciate the reply... thanks. Tom

[Enzo]

I agree. If you wire an Xover, your woofer will no longer be full range, and you will lose some mids.

Or you could continue with the full range woofer and add a more involved Xover for the tweeter only. But then we are back to WHY? That cap is a simple 6db per octave crossover already. What do you gain by rolling the thing off at 12db or 18db?


Now if you want to do it just as a fun project, fine. You want to accommodate the 8 ohm drivers, but the Xover must also be able to handle the powr level of the cab. Don;t put a 30w Xover in a 100w cab.

Parts Express has a bunch of made up Xover boards, plus they have bare boards ready for you to install your own parts. They have charts of what value Caps and inductors to use for any given frequency.

[TomCarlos]

Ok.... Got it and thanks for the help.

Yes, this is a learning experiment. I found the following site:

Crossover Design Chart and Inductance vs. Frequency Calculator(Low-pass)

The horn in my 2 way cabinet is a standard PHT-404. It's 8 ohms, 20 watt, with a Cutoff Requency of 3Khz. So I ran the calculator (and using an 8 ohm woofer) and I see that I would need a 6.6uf (6.8uf is the standard size) and a 424uh Inductor. Yes, that seems more complicated than the simple 2uf which is the only crossover device in the cabinet right now. But again, it might be fun to compare the simple solution versus the two component approach to see if there really is a difference (with two cabs set up side by side).

And yes, I found the Xovers on Parts Express. It might be easier to buy the cap, inductor, and mount them on my own board.

Tom

[J M Fahey]

I don't think the sound will be "better", it might even turn the other way, but in any case it would be great to add a very common and cheap 12V 15W car lamp in series with the horn as protection.

[TomCarlos]

By the way, is this the correct formula for calculating crossover for a simple one cap circuit)?

Frequency = (.159)/Capacitor*Speaker Impedance)

If that is the case, I don't see how a 2uf cap, at 8 ohms, can be set for 3KHz. According to the table on this site, 2uf will yield a Xover around 9Khz.

I think I am confused!!!

Tom

[J M Fahey]

Hi Tom Carlos.
Your formula is right.
Yet if your horn has a 3 KHz cutoff (for acoustical reasons), it will easily burn with a poor and cheap 6dB per octave simple capacitor.
Solutions?
1) add the proper coil (expensive, not an off the shelf product) to turn it into a better 12dB per octave crossover or even better, an extra capacitor to achieve 18 dB per octave (your accountant will kill you) or
2) Just use a capacitor 1/2 or 1/4 the calculated value.
Does it sound bad? Not at all.

[TomCarlos]

Ok... I need to revisit this. The Horn Spec is for 3Khz. In the perfect world, if I was to use a cap so that the frequency cutover came out to 3Khz and nothing below 3Khz made it's way through to the horn, I would be done. But in fact, that is not the case. At 6db, frequencies below 3Khz would sneak through. So when you say use a cap of "Half" or "Quarter" value, what you're saying is to chose a cap so the calculated cutover is at 3Khz plus 1.5Khz, so around 4.5Khz? That would make sense. So using a 4.7uf would give me a cutover of 4.2Khz and slowly roll off frequencies below that.

Do I have that correct? Thanks!!

Tom

[J M Fahey]

Even more than that: 6dB per octave is way too slow, you will have destructive power at 1.5Khz, so the cheap , accepted solution is to use a cap of 1/2 or even better 1/4 its theorical value.
If you calculate, say, 10uF, you may use 4.7uF but 2.2uF is much better.
Here "less is more" because tweeters and drivers, really burn just by looking at them (ask any soundman)
Good thing is that it still sounds good, remember you do not cut highs going to woofers, and they still have some useful response in the transition area.
It works!! (Your box is an example)

[TomCarlos]

Thank you J.M.

That makes perfect sense.

Tom