Crossover/Horn Trouble!

[Keith Medlock]

While putting new woofers in my Fender Rumble 350 combo, I accidentally broke the cheap piezo horn. So I figured it would be a foot time to pop in a better driver. I used an Eminence APT:50 driver with an APT:80S horn flare and a PBX:3K5 crossover. I wired it up and can only hear static out of the driver and horrible crackling and popping from the woofers! I bypassed the R131 47 ohm 10% -5w attenuation resister and it just got louder (duh). What did I do wrong?

[Keith Medlock]

Doh! It just occurred to me looking at the schematics that the woofers don't have a crossover. The woofer and tweeter overlap between about 1.5k to 3.5k. Is this the problem? The woofers are sucking up the power trying to play the high freq. so the tweeter can't be heard? And is it screwing with the impedance the amp sees when this overlap happens because it amounts to 3x8 ohm speakers in parallel playing at once? Im lost...

[gbono]

I believe this is schematic for your amp http://support.fender.com/schematics...atic_Rev-B.pdf

You should be able to get sound out of the amp with just the 2x10's connected and the HF (horn) switch turned off. I'm assuming the amp worked before you modified things?

The crossover is a high pass not a 2 way and should be connected "between the HF driver and amp's horn output - though you've changed the impedance the amp is going to see.

[olddawg]

Usually with a piezo horn you do not need a crossover at all and that seems to be the case with the schematic. I always put a high pass cap of around 4.7 or 2.2uf to one though if I'm making a cheap monitor or something just in case. Truth is you might be better of with the piezo horn for this application and may not see a stunning improvement. But, since you already have it, you could try running your driver with just a 2.2uf non-polarized cap on the positive terminal of the driver and just connect it like the piezo with no further modifications to the circuit. Just a simple hi pass filter paralleled off of the full range output to the speakers. And like I said, might go as low as 4.7uf.

[Keith Medlock]

Sorry, I wasn't clear. I have sound coming from the woofers, but on addition to the normal sound I have the crackling/popping and nothing but the crackling/popping coming from the horn.

[Keith Medlock]

The PXB:3K5 crossover I have hooked up to it now has a 12mf and 4mf cap on it and designed for a 3.5k crossosover point with 18db/octave slope. Would just using a cap like you suggest be better? I just got this crossover because it was the one recommended for the driver. I'll jot down the schematic of how its wired now so you have a better idea.

[Keith Medlock]

Schematic

Here's the way its wired right now.

[Steve Conner]

If you're getting normal sound from the woofers and crackling from the horn, you probably made a bad solder joint or other bad connection when installing the crossover. Or maybe the fuse on the crossover is bad or loose in its holder, give it a wiggle.

The crossover was a good choice. I wouldn't recommend just using a single capacitor as it can allow too much midrange energy into the horn which causes distortion and risks burning it out. You'd probably get away with it on bass guitar, but it's better to do things right.

[g1]

Have you tested the crossover on it's own? New does not always = good/working.

[Keith Medlock]

If you're getting normal sound from the woofers and crackling from the horn, you probably made a bad solder joint or other bad connection when installing the crossover. Or maybe the fuse on the crossover is bad or loose in its holder, give it a wiggle.

The crossover was a good choice. I wouldn't recommend just using a single capacitor as it can allow too much midrange energy into the horn which causes distortion and risks burning it out. You'd probably get away with it on bass guitar, but it's better to do things right.

You nailed it! The fuse which is actually some kind of lamp was defective. I bypassed it, checked all solder joints and removed the 47ohm resister from the circuit. It sounds good now. Only problem is the horn is TOO loud. Any advice on what size resister and eh her in the circuit to put it?

[Steve Conner]

The lamp is normally in series with the tweeter voice coil, where it provides some worthwhile protection. Are you sure you didn't get the input and output terminals mixed up?

Often the resistor is a high wattage variable one (sometimes called a "rheostat" or "L-pad") allowing you to vary the tweeter level.

Considering the horn tends to be much more efficient than the woofers, 47 ohms might not have been too far off the mark. Maybe 10 or 22.

There is no theoretically "right" place to put a single resistor. No matter where you put it, it will upset the impedance seen by the crossover network. The place you put it before is as good as any.

[Keith Medlock]

The lamp is normally in series with the tweeter voice coil, where it provides some worthwhile protection. Are you sure you didn't get the input and output terminals mixed up?

Often the resistor is a high wattage variable one (sometimes called a "rheostat" or "L-pad") allowing you to vary the tweeter level.

Considering the horn tends to be much more efficient than the woofers, 47 ohms might not have been too far off the mark. Maybe 10 or 22.

There is no theoretically "right" place to put a single resistor. No matter where you put it, it will upset the impedance seen by the crossover network. The place you put it before is as good as any.

Gotcha! I just ordered an Lpad. Check out the schematic I posted above. The lamp is in series, but it starts compressing the signal at 250w and reaches full cut at 400w. Since my driver is only 50w rated anyway and honestly, if its gonna blow with the minimal work it'll be doing, I would rather it blow and replace with a higher rated one rather than it being attenuated by the lamp.

My only real concern now if for the amp. If it had two parallel 8ohm woofers with a parallel piezo with 47ohm resister wired in series with it, does replacing the piezo with the 8ohm tweeter driver, xover and LPad work the amp too hard? There is that approx. 1500hz overlap between the woofers and tweeter. Sorry, Im new to all this and going through a steep learning curve at the moment!

[Keith Medlock]

Okay, I think I answered my own question. Looking at the charts for the two drivers, the tweeters impedance is relatively flat, but at the point the woofer starts to overlap it, the woofers impedance is already so high that the amp shouldn't care. The woofer is at 8 ohms around 250hz and then impedance just climbs from there.

[Steve Conner]

You got it. BTW, if you're using a L-pad, you probably ought to put it in between the crossover and the tweeter. If you put it on the other side of the crossover, it'll see the bass energy too and might overheat.