Behringer amp new speaker woes?

[GibsonGM]

Hi,

I just replaced the speaker in my bass player's Behringer BXL3000 amp.  Old one was 'open', so the coil fried.  He thinks it was years of abuse.   Anyway, got the new one in, and now I have some fairly gross low end distortion.   Which is making me think "Hey, maybe one of the output semiconductors is bad, which when driven hard, caused the old speaker to blow!".   The new Eminence speaker is 300W, old one was rated 200W, and Musician's Friend says it's a drop-in replacement, BTW.
So I am wondering how to proceed from here...no schematic or voltages to be found apparently.  I did measure about 200mV DC on the speaker, which is not a lot but could indicate a failing output cap?? Seems worse than that, tho, like possibly operating 'out of class' a little.   
Any ideas appreciated!
Thanks,

~Mike 

[wavley]

Nothing will cook a speaker faster than DC, it sees that low impedance and burns things right up.

[petemoore]

  Have it serviced by qualified technician, or know that possible death is the result of not understanding and developing amp safety.
  The two tasks that come first for effective debugging are:
  Schematic, try to find it, take the time, then maybe have to draw it or parts of it in order to apply any debugging techniques to it.
  Inspection, you should be able to see right away if the relevant components have adequate access for repair/replacement, these units were designed for assembly line contstruction, I haven't looked inside to see if it can be easily or possibly repaired.
  If upon inspection it appears a schematic won't help component access, and component access looks like it will limit the ability to repair the circuit, reconsidering the repair and need for schematic might be something to consider.
  I can't see inside but have looked at 'fast Mfr'd' items and had to turn away because of the component layout tightness and 'everything tied down with wires'.
  Once past the stage of deciding you need a schematic and cant find one [not sure I've looked for a B schem yet], you may be able to draw one up, or at least enough to start debugging.

[GibsonGM]

After some more trial & error, here's what I get.  BTW, this should've gone in OT, sorry about that!!

Holding the speaker OUT of the cabinet, on edge as it would normally be, no buzz; sounds great.  It is mechanical...the new speaker mounts "from the outside" rather than going against the baffle on the inside with its gaskets. The gaskets are on the outside (front) of the spkr, and I need them on the back, lol.  So the real thing might be the need to "gasket out" the speaker from behind, to keep it from slapping the small amount of hole behind it.  I'm pretty sure the edge of the sound cone is contacting the rim of the hole.  Does it make sense to jack it out a little?  I have a little gasket material from a 12" spkr I could adapt to a 15", think that's worthwhile here?  I hope this idea is comprehendable...

Electrically, I think she's ok.  The original spkr. had LOTS of mileage and was being abused when it died.    I'll measure the DC again, but it was VERY small leakage.  Connecting/disconnecting spkr made no pops or deflection.   Seems like it's in the mounting, please help, gig Sat. and he wants to use this amp!     )   

Thanks guys,

~Mike  )

[PRR]

200mV DC on a 200W stage-speaker is odd but will NOT hurt the speaker.

I don't see how a front-mounted speaker can slap its hole.

[petemoore]

  The back of the cone can slap the wood corner of the cutout very near the surround ?
  Shimming sounds like a plan for that.
  At least it'll help determine the answer to the slapping, at most it'll make the amp sound with bass again.
 

[wavley]

Why don't you just pull the front gasket off the old speaker and white glue it to the back of your new speaker.

No gasket certainly = rattle it's not the speaker slapping, but the vibration of metal against wood.

I used to recone speakers for a living and elmer's glue is the right thing to use for gasket glue.

[GibsonGM]

Hey, thanks! I thought that might work.  I gave him back his old speaker, maybe he still has the gasket?  Or could use the nice new one on the front.  I will hook it up again, re-measure my DC.   I checked for DC when the old one was attached which was burned out (not that that makes a diff., but you never know!).       

Just out of curiosity:  If one should get a very low DC on a spkr, wouldn't putting a BFC in line with the signal cut it, or at least offer the spk. some protection?  Like 1000uF wouldn't have an audio impact....I know it's not a cure for a leaking cap elsewhere, but....

[Rob Strand]

Re gaskets:  if the old gaskets aren't continuous around the circumference the gaps near the mounting screws can cause air leaks and hence produce a wooshing sound.   Some places sell speaker sealing tapes which also act as a gasket.

Re DC offset:  Don't put the cap in.  There's nothing wrong with 200mV offset.   Some amps have a DC offset adjustment, if that amp doesn't then the problem doesn't need to be solved. 

Amps that use output caps are usually single supply types, which have a large DC output - the cap necessarily blocks the DC.  However, the DC also biases the cap.  Amps without caps won't provide that bias, so you could get some reverse polarity at low frequencies.  Ideally the cap needs to be a non-polar, which is not available is large sizes.

[PRR]

> wouldn't putting a BFC in line with the signal cut it

Yes BUT the required value ("BF") will force you to an Electrolytic, you don't know which-way the DC will be, so you need a bi-polar electro, which is at least twice the size and cost. And for full protection it must be able to stand the full rail-voltage, which on a 200W amp could be over 70V.

And being a "protection" part, we want to spec it VERY conservatively. If we estimate a 70V cap, then get a 75V jolt and the cap fails, we not only lose the speaker we lose the "protection" cap.

This starts to look like a very-BFC.

> 1000uF wouldn't have an audio impact....

In hi-fi we would not use anything that small. At 40Hz it adds 4 ohms reactance. In 8 ohm load that will cause a perhaps-audible droop at 40Hz. Also hi-fi guys like "damping factor" over 40, and 4 ohms added gives damping factor near 2.... the speaker bass-resonance gets flubby instead of tight.

Stage-amp users are not hyper-critical; they know a few feet over makes a big change on the lowest note, more than any change a BFC could cause. Still, you don't need ANY less bass, and you don't need added variable to jiggle on stage.

So I'm thinking 3,300uFd 100V non-polar, which is a very big glop of damp Aluminum. And maybe not a stock part anywhere, so you need two 6,800uFd 100V polar caps. That's pretty "BF".

> about 200mV DC on the speaker, ...could indicate a failing output cap??

As Rob said.

I doubt any Bee-ringer has an output cap. Even in stage-amps, you don't want extra stuff in the load path sucking any power.

I did know a Yamaha 100W with output cap, but that's early 1980s. And IIRC they used 3,300uFd.

It is equally cheap to build a +/- power supply, rig the amp so its DC is "zero" output. "Zero" could actually be a 10mV offset times a gain of 20, or some such math. In hi-fi where speakers are more delicate, we try to trim down nearer 20mV. But a stage-speaker has GOBS of travel and heat dissipation. We are going to be banging 60V peaks on the speaker. 200mV or 0.2V at idle is 1/300th of available travel and 1/100,000th of permissible heat.

You may still have a bad (rubbing) speaker or a half-sick amp. You just haven't proved it here. Can you try this speaker with another amp, on Bass, LOUD? Can you try this amp with another speaker, even with a guitar and guitar speaker? (It may be unwise to play loud bass through a guitar speaker.)

[GibsonGM]

Yeah, got it figured, I believe  (with some more prodding above ) 

1) no DC measurable at output. Got 200mV with the dead spkr attached, 0.00 with new one.  Maybe the old one has a rectifier built in, LOLOL.   So that is no issue.
I tested the amp out completely with a 12" guitar speaker to ascertain that there were no electronic issues before ordering new spkr., and everything was/is cool.....and is cool w/ new spk. as long as it's not mounted.   The trouble is occurring when mounting only. 

2)  The FREAKING HOLE is 1/8" too small, leading to an approx. 1/8" gap around said speaker, giving it ample ability to vibe around in there since it's not seating.  So much for a "drop in replacement"!!!!   Now thinking I needs to get me a new gasket for the rear of the speaker, about 1/4" thick, and longer mounting bolts to make it thru. 
Does that seem a logical solution to you speaker pros?  I just don't wanna pork the new $120 speaker, would rather spend $10 on another gasket.   Thanks!!

BTW******  The gasket that comes with a new spkr. on the front, the nice thick soft rubber one -- if you peel that off and place it on the back, then nothing holds the edge of the cone ().  This seems like it'll cause the cone to self-destruct.   Which is why I say I have to buy a new one.  Right?? 

[Nasse]

While I was working for the industry we made gaskets for big pipes at the smithery. We had crude beam compass like thing, made of small length of wood 1x2 inches or whatever, there was a sharp razor blade or stanley knife blade attached at 90 degree, and small nail or the stem of big blind rivet was the axle put in a small hole drilled in the middle of a piece of scrap chipboard.

I have made plywood rings of thin sheet to use under speaker frame but you need a router for that

with thick gasket ya should watch not bend out of shape your speaker frame

[Rob Strand]

> The FREAKING HOLE is 1/8" too small,

Hole and screw pitch differences on speakers are a PITA!

There's a few options I've used at various times:

- File a chamfer around the edge such that it clears the speaker frame.
  If the speaker frame doesn't angle in around the edge this method
  isn't so great, the chamfer needs to be so deep that you might as well
  use this second method.

- Given the amp will probably never get another speaker put in it,
  just jigsaw a tad away from the edge.  (Make sure you don't
  hit the steel T-Nuts, if they are close do skip over them and
  do that part with a file.

- The non intrusive way is to make an adaptor ring to space the
   speaker a little off the front panel. Use some 6 to 10mm MDF
   Again, how thick and how successful depends on the how quickly
   speaker frame angles away.  You have to make sure the speaker
   won't hit the grill when fully extended.

[GibsonGM]

Yes, that's about where I'm at....blah.  I am going to pull the gasket from the blown spkr and see if I can space it out with that.  The new spkr does angle inwards on the back side, but not fast enough to clear the hole. I think the gap when it is centered is less than 1/8", so its not TOO bad.
I suppose I could do any # of things if spacing doesn't work, like taking a bit more off the hole or cutting some 1/4" baltic birch underlay ply I have.  I'd rather not though, LOL.  Now I'm into having to get longer screws for the sucker. 
Just like stomp boxes, you have to learn about speakers, mounting, cabinet building and all that.....AAAAAHHH!!    He he he.   Good info here, though.

[GibsonGM]

Just in case someone else has this problem (replacement speaker is just a tad too big for the hole):
I drew a line and then cut about 1/8" around the hole with a scrolling jigsaw.  Still not enough, so I took a very sharp chisel and beveled all around the hole.  This let the angled back of the new speaker sit down in the hole.  A quick 80 grit sanding, vacuum out the inside of the cab...

Success!!!   Knocked a few pictures off the wall testing it out )