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Two Cabs "singing" square waves.. Crossover Broken?

Standalone

Standalone

Jan 17, 2005
5,955
7,609
New Haven
www.jsbmusic.net
#1
I thought my amp was causing the problem, but have definitely isolated the issue to two of my four cabs. It's happening with my GK and with an LMII that I bought and returned.

There is a low level, synth-sounding electronic distortion going on in both my Yamaha BBT 110s and a used Flite that I picked up here on TB. It is not happening with two other cabs I own (see sig). It is happening with different amps, too.

Both are small cabs that I got used and I'm sure have been pushed. (The Yamaha has certainly been pushed a little playing small bar gigs by yours truly.)

They both have the same sound, a sound that is audible especially at low volumes, and is drowned out as the volume is turned up. The distortion/singing tone "plays" under each note, and modulates slightly as the note decays, just as though I were playing the note through my Moog.

I'm thinking that this is a crossover problem, and I'd like to fix it myself.

Is there a way to be certain that it is a crossover problem? If so, how do I choose a crossover-- what frequencies would work well with that Yamaha and with the Flite 2x8+dome tweeter... ?

Can I purchase a 3 way x-over for the Flite and leave the lowest frequencies out so that I can run Full range to both my cabs rather than biamping? (The flite gets farty when pushed)

StudioFall2008024.jpg


Thanks!

UPDATE: There's NO x-over in the flite, just a resistor on one of the two wires that go to what is not a tweeter, but a small driver. Hmmm :/
 
#2
I'd first remove the crossover from the circuit on both cabs and just run wire from the woofers to a simple jack, that's it. Then see if the sound is still there. That would allow you to remove the ? from the end of your sentence and put a period there. Is your 2x8 made to be a full range cab or is it designed to be the top in a bi-amp setup? Get a 3-way crossover and leave the bottom end out and it won't sound like a bass anymore. I'd try a high-pass filter set low like 50-60hz to help with the farting. You'd still want to let it play the 80hz to get some representation of your low E unless it really is a "topbox". I'm not familiar with that particular cab.
 
#3
If you get the same sound from both cabs, I'd first see if you can borrow another cab from someone first to see if it's the head. It just seems weird to me that you would have the same sound from two completely different types of cabinets. Edit: never mind, just read the FIRST SENTENCE of your post again... Have you ruled out cables?
 
#4
I suspect that leaving sections of a three way crossover without a load could probably cause damage to the crossover. Besides if you leave out the low driver where are you going to get your low end from?

Are you actually trying to gig with a ten and two eights? I would have thought that that set up would be insufficient. I gig with a pair of 2x10s. I could at times just use one but I prefer to treat my speakers more gently so I always use 2. How are you connecting these cabs to the GK? The only time I have had something like you describe it turned out to be a faulty active crossover in the amp.

Paul
 
#5
My band plays jazz/ samba stuff in relatively small venues. The guitar amp that I'm going up against is a roland cube 60 or some such. I've gigged the yamaha 110 solo and it's been loud enough, and hasn't sounded strained. I'm betting that those late- 4th set R&R tunes for the drunks probably pushed the cab too far and made the x-over unhappy, though.

I usually gig with my little GK 210.
2008_10_6Studioandamps010.jpg

But I'd like to play with other setups, too. Or just repair them to my satisfaction and sell them on. I double on bass, so two big cabs plus everything else is a tough proposition. (as you can see, I have the PA to haul, too...)

I have two other cabs, both of which don't exhibit the problem. One is a GB 212 Neox, so I'm sure that it would be clear enough at all frequencies to show any problem.

I suspected the same about using a 3-way crossover as a LP filter/2-way would lead to problems.

The 800RB has a 300 W 4ohm min. amp and a seprate 100 W 8 ohm min. amp. They can be run biamped or full range. I got the flite to use as the 8 ohm top box.

It's not the 8 ohm amp that's blown (a common thing I've heard about with GK 800's as folks try to run it at 4 ohms...) because it's happening with the 4 ohm yamaha through the 4 ohm amp.

It's also definitely not the amp as I had the same problem with the LM11 I had in the other week.

If anyone has any x-over frequency suggestions for those particular tweeters and drivers, I'd love to hear them. I have no idea. 3kHz? 4.5? Any other sources than Parts Express? Thanks!
 
#6
will33 said:
I'd first remove the crossover from the circuit on both cabs and just run wire from the woofers to a simple jack, that's it. Then see if the sound is still there. That would allow you to remove the ? from the end of your sentence and put a period there. Is your 2x8 made to be a full range cab or is it designed to be the top in a bi-amp setup? Get a 3-way crossover and leave the bottom end out and it won't sound like a bass anymore. I'd try a high-pass filter set low like 50-60hz to help with the farting. You'd still want to let it play the 80hz to get some representation of your low E unless it really is a "topbox". I'm not familiar with that particular cab.
Click to expand...
I had been thinking that an x-over that cut the bass at 375 Hz or so would still allow those 2x8's to support the low mids, and then my bass eq knob could boost the bass to my 210 or 110 without farting out the 2x8. But I'm tending to agree with paul, that there would be damage to the x-over from it not having anywhere to send that energy.
 
#10
Well, at any rate, it's pretty clear that it's the cabs. I'll try some cable switching, but I've been hooking up and unhooking and re-wiring everything with my band's live rig cabling and doubt that's it.

I guess I'm left to wire directly to the woofers and see what's up. I'd still love to get a good link or some info on how to choose an x-over. :help:
 
#12
Rick, those were my thoughts exactly. An earlier post of mine was asking about the head.

But like I said, a brand new Little Mark did the same thing. Three basses have done the exact same thing-- my Marcus, my '81 Ibanez, and my Steinberger Spirit.

My Genz Benz cab is plenty hi-fi to pick up the sound that's going on here, but what convinced me was getting that other amp in here.

I've even considered the power in my studio as a possible issue, but I've eliminated that, too.
 
#15
That's a capacitor. Or at least something disguised as one :D

Open up the Yamaha, pull the woofer off of the crossover, and wire it up to a piece of speaker cable, then wire that to the GK. See if that makes the nose.

Also, just speaking from my knowledge of electronics (I've not seen this problem occur as you describe) a crossover failure would very likely not do this. These cabs use passive crossovers, which are just networks of (usually) an inductor and capacitor (or resistor and capacitor sometimes). Any failure of these (almost always the capacitor as they are decidedly less robust than the other two passive components) would either kill the sound or change the frequency of the crossover. A square wave tone being induced in the drivers probably comes from some sort of interference.

Just to be safe, take one of the cabs (probably the 2x8) over to a friend's house, and hook it up to their rig, so you have different power, different head, different cables, different bass (although I realize you tried all your basses) and see if it's still a problem.
 
#17
Well, Standalone, you certainly did your troubleshooting homework.

That's indeed a 6.8uF (microfarad) capacitor. It's a high-pass filter to prevent lows from reaching the small driver. (Caps block low frequencies.)

It looks to be an electrolytic capacitor, which is pretty bad when compared to a polypropylene one. Typically, one should use a poly cap in this application, because the signal is passing through it to the tweeter.

See [Invalid or Expired Link Removed] for a good poly capacitor. (A higher voltage rating is OK, just not lower than what's in there.)

If the other cab is configured similarly, you might have the answer. Just swap in a poly cap of the proper value in each cabinet.
 
#18
Standalone said:
Yeah, I knew I had the wrong word for it. :D Thanks, though. What does the capacitor do in a setup like this?
Click to expand...

Capacitors and inductors change their impedance (in layman's terms, basically resistance) depending on frequency. Thus, crossovers are built using combinations of capacitors/inductors and resistors. This forms a "filter" that blocks out low (or high, depending on how you lay out the circuit) frequencies. Basically, at a rate depending on the capacitance, the capacitor will tend towards infinite resistance (an open circuit) as the frequency gets lower, and zero resistance (a short circuit) as the frequency gets higher. Using this principle (which is reversed for inductors) you can build circuits that filter out low or high frequencies. This concept is used in EQs, crossovers, notch filters, feedback elimination, etc, etc...


Edit: the suggestion for upgrading the capacitor is not a bad one...
 
#19
The cap may indeed be the cause of the distortion.

When you button it up, seal the small driver enclosure (including around its wires) with any handy caulking or a thick bead of hot glue. Avoid silicone, because it contains acetic acid.

Ensure there's an airtight seal when you replace the jack plate.

The [Invalid or Expired Link Removed] from Parts Express is magic for this purpose.
 
#20
Thank you gentlemen. I also just opened up the Yamaha and found a happy little crossover in there. I didn't have the spare bits or the time to rig up a wire to check the woofer of the Yamaha.

I should rig up some testing wires with alligator clips and such for this purpose, rather than getting the soldering iron out every time I want to mess with something...

I tracked the problem in the Flite to ONE of the woofers-- the sound is very clearly emmanating from one of them. I'm thinking it's a blown woofer, but the sound is SO moog-ish that it's hard to be convinced that the issue is in the woofer mechanically. It does not increase or change with volume changes...

I'll pull the speaker out and then try to re-seat it to see if there is any change.

The yamaha's distortion problem is only audible on the decay of notes, a similar, very electronic sound.

Are blown woofers ever known to behave like this? When I've heard bad/blown speakers (like the ones in my saturn), they sound worse when turned up, not better like these.

Again, many thanks Rick and cda.

Can I test the capacitor with my multimeter? Can I test the woofers?
 

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