Prebuilt BFM Crossover from Speaker Hardware Question

[Ben Esparza]

I was wondering if this prebuilt crossover for BFM cabs would work if the tweeter(s) were 8 ohms? Leland Crooks...can you tell me? If it is meant for 4 ohm tweets, how and what do I used to make it work for 8 ohms? Thanks.

[BurningSkies]

Why not contact Leland or Bill directly?

[Ben Esparza]

Originally Posted by BurningSkies

Why not contact Leland or Bill directly?

Okay then...Leland...can you answer my question?!

[poorbassist15]

Bill's on TB too. I'm sure he'll be here soon.

[BurningSkies]

questions@speakerhardware.com

[BurningSkies]

billfitzmaurice@myfairpoint.net

[GrowlerBox]

Or better yet, on the BFM forum billfitzmaurice.info

[will33]

Originally Posted by Ben Esparza

I was wondering if this prebuilt crossover for BFM cabs would work if the tweeter(s) were 8 ohms? Leland Crooks...can you tell me? If it is meant for 4 ohm tweets, how and what do I used to make it work for 8 ohms? Thanks.

The hp section of that is designed for a 4ohm high section, that's why there's a 4ohm resistor across the + and - tweet connections (piezo's don't have voice coils. For an 8ohm high section, you'd pretty much remove that resistor and halve the cap and coil values assuming your hf driver is close to it's nominal impedance at that frequency. Not sure what the 30ohm resistor is for...probably padding or smoothing.

Can play with this


Crossover Design Calculators

[billfitzmaurice]

Originally Posted by will33

The hp section of that is designed for a 4ohm high section, that's why there's a 4ohm resistor across the + and - tweet connections (piezo's don't have voice coils. For an 8ohm high section, you'd pretty much remove that resistor and halve the cap and coil values assuming your hf driver is close to it's nominal impedance at that frequency. Not sure what the 30ohm resistor is for...probably padding or smoothing.

Can play with this


Crossover Design Calculators

Totally incorrect, I'm afraid. The 4 ohm resistor is to maintain an acceptable minimum load to the amp at high frequencies, where the capacitive load of the piezo array can appear as a dead short. The 30 ohm resistor presents a resistive load to the high pass filter, which otherwise would not work at all.

[will33]

I use 4ohm resistors in the + line to piezo's for that reason but what's with the 30 ohm to make the filter work then? I know there's a good reason for it but what, smaller part values, beefier design. I assumed you could put an 8ohm across there and figure values for an 8ohm hpf.

[billfitzmaurice]

Originally Posted by will33

I assumed you could put an 8ohm across there and figure values for an 8ohm hpf.

If you do it will cost at least 6dB of sensitivity, as most of the HF signal power will be dissipated in the resistor. Some sources recommend doing so to allow the use of an off the shelf crossover, with no regard to the insertion loss that will result. Not a good idea IMO. To do it right you've got to design it from scratch to work with high impedance piezos.

[will33]

Cool, that's good to know, thanks.

[Ben Esparza]

Back to my original question...will the HP section of the crossover work for an 8 ohm tweeter array?

[Rune Bivrin]

The crossover is specifically designed for piezo tweeters. They don't have an impedance in the same way a regular tweeter does (although there's certainly a capacitive impedance present.) It will not work at all with a regular tweeter.

[Ben Esparza]

Originally Posted by Rune Bivrin

The crossover is specifically designed for piezo tweeters. They don't have an impedance in the same way a regular tweeter does (although there's certainly a capacitive impedance present.) It will not work at all with a regular tweeter.

Elaborate on "not work at all". An 8 ohm tweeter horn will not get any signal, it won't split the signal at 2kHz, or what?

[Rune Bivrin]

Originally Posted by Ben Esparza

Elaborate on "not work at all". An 8 ohm tweeter horn will not get any signal, it won't split the signal at 2kHz, or what?

Without doing the maths I'd say you'll get an attenuated signal due to the 4 ohm resistor and a far too high crossover frequency due to the high pass cap being calculated for a 30 ohm resistor.

If you're not going to use piezo tweeters, don't use a crossover designed for piezo tweeters. It's that simple.

You can use the separate High-pass and low-pass filters that Leland sells at SpeakerHardware if you want to. Those should do what you need.

[billfitzmaurice]

Originally Posted by Ben Esparza

Back to my original question...will the HP section of the crossover work for an 8 ohm tweeter array?

Which of my cabs are you using it in? All of the information you require is in the plans.

[Ben Esparza]

Originally Posted by billfitzmaurice

Which of my cabs are you using it in? All of the information you require is in the plans.

I'm not using it for one of your cabs. I plan on building an SWR Baby Blue Monitor clone with an 8 ohm cone tweeter. I'm using all the speakers that came stock on that cab. The stock design crossover splits on that cab at 2kHz. All this crossover talk is foreign to me (please excuse my ignorance) and I was hoping I could use the 4 ohm woofer version of that crossover (the cab woofer design is 2x8 = 4 ohms) and send the HP signal to the 5" cone woofer.

(I am now awaiting the flying daggers)

[rpsands]

Selenium makes some 4 ohm crossovers off-the-shelf that would probably work OK for this application. Google it.

None of Bill's will to my knowledge work that great. Probably not terribly. ONLY if you use the compression driver crossover, but that will still be on the high side.

For a 5" mid, you typically will want closer to 1200-1600hz. The Eminence stock 1600 would probably work if you were using an 8 ohm woofer section.

* better would be to design your own crossover, but there's a lot of legwork involved in that

[billfitzmaurice]

Originally Posted by Ben Esparza

I'm not using it for one of your cabs.

Then you can't use it. You might as well be asking if you can use a BMW transmission in a Toyota.