I just refoamed the woofers on a pair of Dahlquist DQM-9 that I recently picked up with rotted rubber surrounds. This is my third refoam job and I think all went well -- the cone moves up and down by hand smoothly. When playing, one speaker is less full & rich, and a tiniest bit fuzzy. Perhaps the refoam has revealed additional issues?
I plan to swap speakers, triple check polarity then (if nessecary) swap the woofers with cabs. I think these speakers have great potential and will make a nice addition in my garage system, bit I just can't work on cars under these audible conditions.
Any other thoughts on possible cause / remedies?
I plan to swap speakers, triple check polarity then (if nessecary) swap the woofers with cabs. I think these speakers have great potential and will make a nice addition in my garage system, bit I just can't work on cars under these audible conditions.
Any other thoughts on possible cause / remedies?
Hi Ohming,
This is just speculation, so take it with a grain of salt! If I had to make a guess, I would say the driver that sounds 'less full' has had its usable cone travel reduced, probably by some change in geometry (between the moving and fixed components).
You mentioned the previous surround was rubber and this new one is foam? The material change will account for some change in driver character, but how about the shape: is the new surround of the same dimensions as the old?
One thought I had was if the cone's 'neutral' position was pulled forward for some reason (if the surround was a different depth than the original for example), this could pull the voice coil forward in the magnetic gap, limiting its travel. The result would be limited bass and distortion when the coil moves out of the magnetic gap sufficiently.
Another possibility is the surround shifted during setting, and there's a strange pre-load on the spider. That would cause non-linear behavior for sure. Did you use paper shims in the voice coil gap when setting the glue for the new surround? An off-center voice coil could rub also, as if it is not centered it will tend to move at a slight angle with respect to a straight up-and-down ideal trajectory.
I'd suggest starting by looking at how centered the voice coil is in the gap. All the best, let us know how it goes.
Jim
edit: I should add - my above comments were based on an assumption (woops!) that you have heard these speakers play correctly before the re-foam job. If that's not the case, your re-foam work might be perfectly fine, but there is another problem elsewhere. To begin with, I'd suggest play each woofer individually, on a known-good amplifier, directly, one at a time. It could be a cross-over issue, or even internal OEM wiring! So make sure you prove there is something wrong with the 'fuzzy driver' over the not fuzzy one first
This is just speculation, so take it with a grain of salt! If I had to make a guess, I would say the driver that sounds 'less full' has had its usable cone travel reduced, probably by some change in geometry (between the moving and fixed components).
You mentioned the previous surround was rubber and this new one is foam? The material change will account for some change in driver character, but how about the shape: is the new surround of the same dimensions as the old?
One thought I had was if the cone's 'neutral' position was pulled forward for some reason (if the surround was a different depth than the original for example), this could pull the voice coil forward in the magnetic gap, limiting its travel. The result would be limited bass and distortion when the coil moves out of the magnetic gap sufficiently.
Another possibility is the surround shifted during setting, and there's a strange pre-load on the spider. That would cause non-linear behavior for sure. Did you use paper shims in the voice coil gap when setting the glue for the new surround? An off-center voice coil could rub also, as if it is not centered it will tend to move at a slight angle with respect to a straight up-and-down ideal trajectory.
I'd suggest starting by looking at how centered the voice coil is in the gap. All the best, let us know how it goes.
Jim
edit: I should add - my above comments were based on an assumption (woops!) that you have heard these speakers play correctly before the re-foam job. If that's not the case, your re-foam work might be perfectly fine, but there is another problem elsewhere. To begin with, I'd suggest play each woofer individually, on a known-good amplifier, directly, one at a time. It could be a cross-over issue, or even internal OEM wiring! So make sure you prove there is something wrong with the 'fuzzy driver' over the not fuzzy one first
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Thanks J.R., bought them with sverve surround rot, so never heard them in playable shape. The replacements appear to be the same shape and fit perfect (old rubber, new foam - couldn't source rubber). Plan on swaping drivers with cabs and elimanting everything else b4 re-refoaming -- didnt shim the voice coil. The dealer who sold me the foam assured me it was not nessecary.
Measure the dcr of both drivers when you have them out, this might indicate whether or not the worse performing of the two might have a damaged voice coil.. You can also check for VC rubs (scraping noises when you push evenly on the dust cap) these can be caused by off center suspension or a burned VC..
The VC rub was a non-issue. I applied enough even pressure to travel the cone 3/8" or so up and down, and it felt velvety smooth.
Whay is the DCR? I'd love to measure it.
Actually, if I throw more juice at them, they "wake up" and the symptom is not noticable as much when approching frat party levels.
And whose idea was it to market a speaker with Olive Drab Suede for a covering. That has to be the wierdest cab material I have ever seen.
Whay is the DCR? I'd love to measure it.
Actually, if I throw more juice at them, they "wake up" and the symptom is not noticable as much when approching frat party levels.
And whose idea was it to market a speaker with Olive Drab Suede for a covering. That has to be the wierdest cab material I have ever seen.
Hey Ohming,
If they 'wake up' with more power applied, this might be a connection issue. A poor connection sometimes has a diode-like threshold effect. Make sure you have a good solid connection to the terminals of the woofer. Unfortunately, a broken winding in the voice coil might do the same thing. But I *think* a broken VC is more likely to fail open than partial.
Oh, about the shims: when replacing a surround, the shims might not be required really, but I think they're a good idea. That's strange that your parts dealer assured you they were not needed, as it is pretty standard practice for replacing a surround. It's easy to pull the voice coil to one side otherwise.
Jim
Time to measure resonance of the drivers - free air if you like but also in the cabinet. All you need to do this is a series resistor (100 ohms is fine but can be nearly any value), a variable frequency audio source (with amplifier if a sound card) and a measuring device. Your audio source can be your computer sound card and while I haven't tried it, the measuring device could be the sound card as well.
Simply put, you add a series resistor in the ground return side of the speaker and measure the voltage across it with different frequencies. Resonance will be the peak and I suspect you'll find significant differences between the channels.
G²
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