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Simplest Cab Crossover

B

ByrdyBass

May 23, 2024
32
34
#1
Howdy all,

I have a PA speaker that I've been using as a cab for many years with a 15" speaker and a tweeter/horn. For a reason that escapes me, I disconnected the tweeter a long time ago and the jack is wired directly to the speaker. Both speaker and tweeter are 8 ohms, and the amp I use has a single 8 ohm output.

I'd like to reconnect the tweeter if possible. Does anyone have, or could anyone throw together, the simplest possible crossover circuit to reintegrate the tweeter? There are two jacks on the back as well, I assume for an in/out config. If there's any other info you need for this, let me know and I can do my best. The tweeter is this model Fostex Horn Tweeter 025H30 | #1823383770, and the speaker is unidentified but both are 8 ohm for sure.

Thanks.
 
#2
If I understand, the woofer is run full range. That may be normal as some lower end cabs do not have a full crossover...no low pass filter (LPF) for the woofer.

The tweeter will require an appropriate high pass filter (HPF). If the original circuit is till in the cab, simply hook it back up.

AFAIK it was common to use an 18dB per octave filter in this applicatoin. Ideally you would determine which cab you have and find specs that list the crossover frequency. A guess is something like 3.5kHz.

You can buy a ready made 3.5kHz with 18dB/octave cutoff here:
www.parts-express.com

Eminence PXB:3K5 High Pass Crossover Board 3,500 Hz

Eminence PXB:3K5 High Pass Crossover Board 3,500 HzEminence PXB board-only crossovers are a lower cost alternative to the "cabinet ready" models and are targeted specifically for custom cabinet projects as they do not include the terminal cup, input connectors or L-pad found on the "cabinet...
www.parts-express.com
This unit is rated for 400W.

You could probably find cheaper ones, but you do tend to get what you pay for. This one is rated 250W:
Dayton Audio 3.5k-HPF-4 High Pass Speaker Crossover 3,500 Hz 12 dB/Octave

You could also make one, but it's helpful if you have an actual clue of what sort of components to use and how to assemble it. You can find online calculators that will give you component values and a circuit diagram.

I can't say this is good or bad, but here is an example:
1774970811996.png

You input the impedance of the driver and desired crossover frequency, and the calculator tells you component values. There are lots of these online...this is just the first one found.

Ref: Speaker Crossover Calculators by V-Cap
 
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#3
Wasnex said:
If I understand, the woofer is run full range. That may be normal as some lower end cabs do not have a full crossover...no low pass filter (LPF) for the woofer.

The tweeter will require an appropriate high pass filter (HPF). If the original circuit is till in the cab, simply hook it back up.

AFAIK it was common to use an 18dB per octave filter in this applicatoin. Ideally you would determine which cab you have and find specs that list the crossover frequency. A guess is something like 3.5kHz.

You can buy a ready made 3.5kHz with 18dB/octave cutoff here:
www.parts-express.com

Eminence PXB:3K5 High Pass Crossover Board 3,500 Hz

Eminence PXB:3K5 High Pass Crossover Board 3,500 HzEminence PXB board-only crossovers are a lower cost alternative to the "cabinet ready" models and are targeted specifically for custom cabinet projects as they do not include the terminal cup, input connectors or L-pad found on the "cabinet...
www.parts-express.com
This unit is rated for 400W.

You could probably find cheaper ones, but you do tend to get what you pay for. This one is rated 250W:
Dayton Audio 3.5k-HPF-4 High Pass Speaker Crossover 3,500 Hz 12 dB/Octave

You could also make one, but it's helpful if you have an actual clue of what sort of components to use and how to assemble it. You can find online calculators that will give you component values and a circuit diagram.

I can't say this is good or bad, but here is an example:
View attachment 7464260
You input the impedance of the driver and desired crossover frequency, and the calculator tells you component values. There are lots of these online...this is just the first one found.

Ref: Speaker Crossover Calculators by V-Cap
Click to expand...
I doubt if the coil on that Fostex driver will handle more than 10 or 15W RMS. A 250W crossover is overkill.
 
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#4
All very interesting... Identifying the cab is a bit tough. Here's a reddit thread about it:


Looks like some of their speakers had a "horn level" knob but I don't think this one ever did. I know this might seem like a worthless project from looking at those speakers, but it honestly doesn't sounded half bad with the little amp head I have, and I'm on a super tight budget right now, so buying a different cab isn't my first choice. If something like that eminence crossover could reconnect it for $50-$60, might be worth it to me.

I'm not sure if reconnecting what was in there before is the right move either. Here's a pic of the only components that were in there when I got it; just a resistor and cap. Unfortunately I can't tell you how it was wired in; t'was disconnected long ago.

IMG_2667.JPG
 
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#5
byacey said:
I doubt if the coil on that Fostex driver will handle more than 10 or 15W RMS. A 250W crossover is overkill.
Click to expand...
Might be <10W.

I could be wrong, but my understanding is you rate the crossover to the power rating of the box, rather than how much power the tweeter is actually expected to get.

I believe the Eminence crossover has a fuse that can be selected to protect the tweeter. AFAIK, the Dayton crossover does not have this feature.
 
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#6
ByrdyBass said:
All very interesting... Identifying the cab is a bit tough. Here's a reddit thread about it:


Looks like some of their speakers had a "horn level" knob but I don't think this one ever did. I know this might seem like a worthless project from looking at those speakers, but it honestly doesn't sounded half bad with the little amp head I have, and I'm on a super tight budget right now, so buying a different cab isn't my first choice. If something like that eminence crossover could reconnect it for $50-$60, might be worth it to me.

I'm not sure if reconnecting what was in there before is the right move either. Here's a pic of the only components that were in there when I got it; just a resistor and cap. Unfortunately I can't tell you how it was wired in; t'was disconnected long ago.

View attachment 7464271
Click to expand...



That looks like a serious bodge.

My guess is that is effectively this circuit:
1774973802571.png

Looks like a 10 ohm resistor in parallel with the driver. AFAIK resistor will drop the power and lower the impedance to a bit above 4 ohms.

Looks like a pair of 2.2uF capacitors in parallel. Capacitors in parallel add, so you get 4.4uF.

AFAIK this is sort of a rough estimate:
1774974087118.png


Effectively a 6dB per octave HPF at ~9K.

The higher cutoff frequency is required since the rolloff is only 6dB/octave.

My guess is the cab originally had a L-pad to set the level of the tweeter...and possibly a full crossover (HPF and LPF).


Eminence used to make a PX-LPAD 100W L-Pad, but it may be discontinued. Parts Express does not appear to carry the Eminence L-Pad., but they have several others including one that looks suspiciously like the PX-LPAD 🤔 .

I would get one rated for considerably more than the tweeter, to reduce the chances of burning it out.

Here is the Eminence wiring diagram.
1774974628555.png


The resistance of the L-Pad needs to match the impedance of the driver.


I suggest you consider what that cab is worth...IMHO not much. FYI I considered it crap when it was brand new. Spending money to get it running is not going to add much value. It's only worth it if you can spare the cash and expect to enjoy and learn from the experience. FYI, I am done similar projects with some old acoustic monitor guts my brother gave me, so don't think I am making fun you.
 
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#7
byacey said:
I doubt if the coil on that Fostex driver will handle more than 10 or 15W RMS. A 250W crossover is overkill.
Click to expand...
Correct, the way "packaged" crossovers are marketed at the lower end of the retail spectrum is "system power" which is generally optimistic in many regards.

Wasnex said:
That looks like a serious bodge.

My guess is that is effectively this circuit:
View attachment 7464285
Looks like a 10 ohm resistor in parallel with the driver. AFAIK resistor will drop the power and lower the impedance to a bit above 4 ohms.

Looks like a pair of 2.2uF capacitors in parallel. Capacitors in parallel add, so you get 4.4uF.

AFAIK this is sort of a rough estimate:
View attachment 7464288

Effectively a 6dB per octave HPF at ~9K.

The higher cutoff frequency is required since the rolloff is only 6dB/octave.

My guess is the cab originally had a L-pad to set the level of the tweeter...and possibly a full crossover (HPF and LPF).


Eminence used to make a PX-LPAD 100W L-Pad, but it may be discontinued. Parts Express does not appear to carry the Eminence L-Pad., but they have several others including one that looks suspiciously like the PX-LPAD 🤔 .

I would get one rated for considerably more than the tweeter, to reduce the chances of burning it out.

Here is the Eminence wiring diagram.
View attachment 7464302

The resistance of the L-Pad needs to match the impedance of the driver.


I suggest you consider what that cab is worth...IMHO not much. FYI I considered it crap when it was brand new. Spending money to get it running is not going to add much value. It's only worth it if you can spare the cash and expect to enjoy and learn from the experience. FYI, I am done similar projects with some old acoustic monitor guts my brother gave me, so don't think I am making fun you.
Click to expand...
I think you will find that the 10 ohm resistor is in SERIES with the horn, not parallel.
 
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#11
Again, all very interesting and helpful! And uh, actually from looking at the wires I think it was something like this:

-------| |-------- +/-
|
HP
|
-----/\/\/\/------- +/-

Only things I can't confirm are +/- from the jack because the wires were snipped right at the base. Also the tweeter has no markings for +/-. I tried the "9v battery trick" from a google search but I think I'm going to have to unbolt the horn from the tweeter so I can see which way the cone is going.
 
#12
That's series wiring of the crossover filter.

Whoa, you will never see the diaphragm move, if it does, it's destroyed!

Don't worry about the polarity, it means nothing in the scheme of things for your speaker.

Rule of thumb: if it's not broken, don't break it. If it is broken, don't break it more.
 
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#15
ByrdyBass said:
that 9v "trick" is all over the internet.... Do you think I should just put it all back in as is then? Also I'm not sure about it being two caps in parallel, I think it's just something like this:

View attachment 7464384

Capacitor, Axial Polyester, 2.2uF, 250V – 5045-12098-00 – RTBB

www.rtbb.com.au www.rtbb.com.au
Click to expand...

If it's one 2.2uF cap vs two in parallel, it just raises the cut off frequency, which is probably a good thing for the health of the driver.

1774981683986.png



The 9V test polarity test is NOT for horn drivers.
 
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#18
Wasnex said:
If it's one 2.2uF cap vs two in parallel, it just raises the cut off frequency, which is probably a good thing for the health of the driver.

View attachment 7464399


The 9V test polarity test is NOT for horn drivers.
Click to expand...
Duly noted on the 9v test! And nice. I think I'm just gonna wire this back together with that circuit and see what it sounds like. Appreciate all the advice and insight!
 
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