stupid x-over question

cracker75 · #1 04-22-2010, 09:26 PM

Is it OK to disconnect one speaker from a crossover, to listen to the other one in isolation?

I have a 3-way cab that distorts at very low volumes and want to figure out if it's the woofer or the mid.

Does having no load on one set of terminals upset the x-over or the amp?

DukeLeJeune · #2 04-22-2010, 11:47 PM

NEXT DAY EDIT: What I had originally posted here was INCORRECT.

My thanks to Passinwind and Bill Fitzmaurice for pointing out my error in ignoring the virtual short that circuit resonance will present. Although this dip to a virtual short is narrow in frequency, it's still potentially harmful to an amp. It may well be that the reason I haven't had any problems is that I'm doing crossover measurements at fairly low power levels, and come to think of it I usually have damping resistors in the circuit that would prevent a near-short. However, I'm going to start clipping a resistor in place as a dummy load just in case. Brainfart on my part not to recognize this.

To cracker75, to be safe substitute a resistor of somewhere between 8 and 30 ohms (exact value not critical), rated at 5 to 10 watts, connected in the circuit where the driver used to be.

T-Bird · #3 04-23-2010, 12:06 AM

Hi.

What Duke said.

For troubleshooting purposes there shouldn't be any problems.

Not all (cheap) cross-overs take the reduced load kindly though, for optimal results a dummy load could be used.

Regards
Sam

testing1two · #4 04-23-2010, 12:15 AM

I know this should go without saying, but take extra care that the two disconnected leads do not touch each other or anything conductive.

Passinwind · #5 04-23-2010, 09:12 AM

My understanding is that you will be creating something pretty close to a short at the electrical crossover frequency. Here's a SPICE model showing the effect of disconnecting the mid driver in a fEarful 12/6 cab using greenboy's crossover design. At 1 volt input the amp will see a 10 amp draw. The inductor must sink all that current to ground as well. Go past a few volts and the amp will likely go into protect mode, or worse. Bill Fitzmaurice has mentioned this is in several threads in regard to permanently disconnecting tweeters and being a Spock type I wanted to see for myself. Corrections requested and gladly taken.

The good new is that unless you are using an unfiltered swept sine wave generator for testing the current draw will be much, much less than the graph shows. If Duke says he tests this way (by disconnecting a driver) with no problems, I believe him. Just wanted to point out that it isn't a slam-dunk "no worries." I'll try a model using an actual bass signal one of these days -- just need to bone up on my SPICE chops a little first.

cracker75 · #6 04-23-2010, 12:44 PM

Thanks guys.....I have a vague understanding of x-overs, but know nothing about electricity.

Time to go figure out why my cab sounds like crap.

billfitzmaurice · #7 04-23-2010, 01:30 PM

Quote:

Originally Posted by DukeLeJeune

I can't think of how disconnecting one of the drivers for a test like you describe could cause a problem. I do it all the time when working on a crossover design.

Depending on the crossover topology it could result in the amp seeing a very low impedance at the corner frequency, even a dead short in some cases. Explained here:
http://www.bcae1.com/xoorder.htm

if in doubt use a dummy load in place of the driver.

cracker75 · #8 04-23-2010, 02:17 PM

Quote:

Originally Posted by billfitzmaurice

Depending on the crossover topology it could result in the amp seeing a very low impedance at the corner frequency, even a dead short in some cases. Explained here:
http://www.bcae1.com/xoorder.htm

if in doubt use a dummy load in place of the driver.

I don't even know what to use for a dummy-load.

JackANSI · #9 04-23-2010, 02:31 PM

http://www.tedweber.com/atten.htm

cracker75 · #10 04-23-2010, 02:41 PM

So f__k it. I'll open this can 'o worms.

I recently purchased two used/demo cabs from Marc Serio at Dr. Bass. Though his reputation for un-filled orders is well known, those fortunate few who receive their cabs seem pretty happy with them.

Well, I've received mine and they sound like crap.

I bought a 1080, which has a BP102 10" driver, an Alpha 8MR 8" mid, and a QSL tweeter (haven't looked it up yet).

Also a 112 cab with a Delta 12LFA, and a selenium tweeter(also haven't got informed yet).

The 1080 distorts heavily at even the lowest of volumes, and the 112 does too, though at more "average" levels. Both boxes have mild rattles, and the 112 has something in it that squeaks at moderate SPL's.

Marc supposedly replaced all the drivers in both cabs before shipping them, and considering that I checked, and they were not only replaced but upgraded, I can't figure out where the distortion is coming from.

I A/B'd them both with an Eden 210, and the distortion wasn't coming from the amp or the bass, in fact, it sounded just fine.

I did a bit of reading and just tried the "push test" on the driver cones to see if I could feel any rubbing (ie blown voice coil) and they seem fine. I also read that the x-overs could be fried and that's what could be introducing the distortion.

I'm a little hesitant to return them because then it could be months (if not longer) before I see them again.

Any suggestions on how to diagnose the problem?

Marc has offered to drop-ship me new drivers, but if that's not the problem, it would be a waste. It seems as though he wants me to have awesome cabs, and I don't want to bad-mouth the guy just yet. I just want them to work, and I'm hoping "crap" is not their signature sound.

PS: I have no means for electrical work besides a soldering gun.

JackANSI · #11 04-23-2010, 02:44 PM

Have him send a driver for the one and a crossover for the other maybe?

COBRARI · #12 04-23-2010, 02:49 PM

I think a "dummy load" would just be an 8 ohm resistor rated at a couple hundred watts connected to a 1/4 jack.
I am sure that a music shop would sell this as a simple cheap unit to use for just the purpose stated.

AlexanderB · #13 04-23-2010, 04:36 PM

A "couple of hundred Watts" of resistor is very expensive and unnecessary... A standard 5W 8 Ohm unit would be fine for some brief trouble shooting. I do not know the cross over design and frequency of this design, but the bass does not deliver huge amounts of energy above a few hundred Hertz.

/Alexander

DukeLeJeune · #14 04-23-2010, 05:54 PM

My post above was mistaken, and I have posted a correction in it. Passinwind and Bill Fitzmaurice are correct.

As for a dummy resistor to take the place of the disconnected driver, I'd suggest anywhere between 8 and 30 ohms, rated at 10 watts if you're going to be hitting it with much power.