Small Syns

[bwaslo]

Originally Posted by AllenB

Spider maybe? If I understand you are not measuring distortion per se, but the level of the harmonic? I was also thinking something to do with the cabinet?

Yeah, it's a Farina method swept sine, which in effect tracks the harmonics over the sweep. The graph shows the harmonic levels, fundamental level is the black curve at the top. In operation moving the cursor calculates the percent distortion where it points.

Cabinet is still a possibility, but I braced, tightened, stuck microphones around it, couldn't locate anything there related to the peak. The peak seems to drop down when I put some stuffing in the woofer slots, so it does seem to be radiating from there. Putting padding behind the woofers inside the cabinet didn't seem to have much effect.

[wesayso]

How are the woofers mounted to the box? If stuffing behind the woofer doesn't help, how about a bit of decoupling of the woofer mount? Sorbothane or Neoprane gasket between woofers and box and preferably also decoupling the mounting points/screws? Maybe leave out the screws and mount it using a ring with another decoupling gasket clamping the driver. Just an idea...

[bwaslo]

That might help (and probably isn't a bad thing to do -- where do you get sorbothane gasketing?). But the distortion component does seem to be getting radiated from the driver itself rather than off the box -- if I put some cloth in the woofer's output aperture, the distortion drops fast, covering the cabinet doesn't do much of anything.

[wesayso]

Does the woofer have a similar blip in the impedance curve without being in the box?

[bwaslo]

Have to check. There is a small blip visible there on the published data sheet for the driver (FaitalPro 6FE100), but hard to see on the printed graph.

[wesayso]

It would be interesting to know... is it a box/speaker thing, a speaker thing or a crossover artifact, as you said, the mids are out of the list of suspects.

[xrk971]

It might be turbulent wind noise from the ports. Do a RTA and look at distortion spectrum in real time. If wind noise it will be broad band (but picked up by the peak tracker). Bundled soda straws can act as laminar flow elements and reduce noise while keeping CSA about the same.

Another culprit is sometimes an overtight driver mounting screw. It distorts the frame and HD results. Try loosening the screws holding the mids and woofer in place a bit. A sorbothane gasket on driver bezel can only help.

[bwaslo]

Does the woofer have a similar blip in the impedance curve without being in the box?

Yes, it does! On both drivers --


But the 450Hz distortion doesn't happen outside of the box



Time to get out the caulk and put it on all the panel braces and seams, see if that does anything.

The port idea might be something to check (how?), but it doesn't seem like there'd be enough air pressure or velocity happening at that frequency to cause nonlinearity? (the port is all of 1 inch long, too)

Keep the suggestions coming (and thanks)

[AllenB]

Originally Posted by bwaslo

Yes, it does! On both drivers --

How do you test a spider? I suppose I'd be adding a little mass at a couple of symmetric locations part way out or damping it with my fingers. Surround maybe the same.

[bwaslo]

What an afternoon this has been, even worse than yesterday evening. But I FINALLY figured out what it is. I'll let you wait in suspense while I whine a bit, though.

As mentioned above, the woofers were fine when driven by themselves. So, I was about to coat the inside of the cabinet with glue or caulk, but thought it wise to first double check the midrange/tweeter horn. And... (damn, you already guessed...) there was now a 2nd harmonic distortion peak in the lower midrange (350Hz this time). I could even hear it in the sweep, so no "sweeping it under the rug".

Throughout the afternoon, I convinced myself (and then proved myself wrong each time) that it was:

• The amplifier (which I managed to blow up when some wires shorted... no big loss, it wasn't a very good amp other than having a handy stepped attenuator)
• Something buzzing in the horn
• The soundcard
• The measurement microphone
• A bad solder connection on a wire (the distortion went away briefly, though it came back
• The tweeter (which did start distorting until I reseated its back cap)
• The midrange driver (replaced with new.... same thing)
• A 100uF NP electrolytic cap, peak seemed to move when I fooled with it. It is in the woofer circuit, but that section of the crossover was still hanging on the amp..

I strapped together enough film caps to replace the 100uF NP cap... same thing! But that led to finally identifying the culprit... I hope. I'm too tired to rebuild everything just now to make sure all plays happy again.

The solution is coming in the next post! (edit: well, maybe the post after that)