I have a pair of Beyma CP755Nd. I was not really happy with them (which was a bit strange, since I really liked the 12AX30's HF, which uses it for the tweeter part) so I wanted to sell them. I have found out that one of the drivers impedance is really strange.
Green and yellow is one driver free air and with a horn. Red and purple is the second one free air and with the same horn. I am really puzzled what this could be.
This is the FR on the horn, blue is the one I think is OK and green is the suspicious one. I did repeat the measurements both FR and impedance a few times and the results were still the same.
What could the cause be? I could try to swap the diaphragms to see if the problem moves or not. Any input is welcome.
Green and yellow is one driver free air and with a horn. Red and purple is the second one free air and with the same horn. I am really puzzled what this could be.
This is the FR on the horn, blue is the one I think is OK and green is the suspicious one. I did repeat the measurements both FR and impedance a few times and the results were still the same.
What could the cause be? I could try to swap the diaphragms to see if the problem moves or not. Any input is welcome.
You could first check if the diaphragm is properly aligned before you swap it.
I took the driver apart, cleaned it as good as I could and now it is much better. The resonance peak is now visible and around 800 Hz. The impedance peak is only 25 ohm versus 37 ohm on the healthy one.
I had the same problem recently.
The new Dayton MB1025 had a strange measurement.
As though something was preventing the cone from moving.
I was about to return the item but decided to try it one more time.
I used my fingers to move the cone in and out.
After a few times, I measured it again and measurement was normal.
I didn't feel the voice coil rubbing against anything when I pushed the cone.
Maybe there was a grit inside.
Regards
Mike
The new Dayton MB1025 had a strange measurement.
As though something was preventing the cone from moving.
I was about to return the item but decided to try it one more time.
I used my fingers to move the cone in and out.
After a few times, I measured it again and measurement was normal.
I didn't feel the voice coil rubbing against anything when I pushed the cone.
Maybe there was a grit inside.
Regards
Mike
Any time the impedance peak is reduced and not well defined with alot of small noisy ripples, there is a mechanical problem with the VC alignment, mostly a rubbing VC. Sometimes the contact isn't constant or static ie. It only occurs with a drive signal around resonance and creates an uneven or non-symetric diaphragm movement, sometimes wobbling and rocking. It can also temperature related, only happening when the VC expands enough to contact the upper / outer pole plate. Very rarely it can be loose windings rattling around or in the case of older drivers with paper VC formers (if you're lucky enough to catch it early) a partially detached VC former.
Most of the time drivers have left over debris that gets churned up during shipping after the driver was tested and packaged. This is where it pays off to have a driver with a removable VC and magnet. Some drivers have their VCs not so perfectly centered, and even though you won't hear any obviously bad things with music alone, the drivers performance can be tweaked a bit by just loosening the motor screws, applying a small sine wave close to Fs and wiggling the VC asy around until you're satisfied its centered as well as it can be. Distortion measurements can confirm this to be successful, as THD can decrease significantly as well.
Some drivers don't have self centering VCs, so you'd have to go through the above procedure anyways. I usually do this anyways as precaution, mainly because there's always some little bit of debris in there and it just gives you piece of mind knowing you're gettingthe best performance the driver is capable of, especially on expensive drivers with large diaphragms. At the very least it pays to check the VC and rear chamber mounting screws for even torque and to do a quick impedance sweep before running the driver for the first time with a higher level input signal. You can catch an issue this way before it becomes a bigger one and potentially avoid an expensive situation.
I can't even imagine how fussy the new Scanspeak Ellipticore drivers are in this regard. What a potential nightmare aligning the VCs in these drivers.
Most of the time drivers have left over debris that gets churned up during shipping after the driver was tested and packaged. This is where it pays off to have a driver with a removable VC and magnet. Some drivers have their VCs not so perfectly centered, and even though you won't hear any obviously bad things with music alone, the drivers performance can be tweaked a bit by just loosening the motor screws, applying a small sine wave close to Fs and wiggling the VC asy around until you're satisfied its centered as well as it can be. Distortion measurements can confirm this to be successful, as THD can decrease significantly as well.
Some drivers don't have self centering VCs, so you'd have to go through the above procedure anyways. I usually do this anyways as precaution, mainly because there's always some little bit of debris in there and it just gives you piece of mind knowing you're gettingthe best performance the driver is capable of, especially on expensive drivers with large diaphragms. At the very least it pays to check the VC and rear chamber mounting screws for even torque and to do a quick impedance sweep before running the driver for the first time with a higher level input signal. You can catch an issue this way before it becomes a bigger one and potentially avoid an expensive situation.
I can't even imagine how fussy the new Scanspeak Ellipticore drivers are in this regard. What a potential nightmare aligning the VCs in these drivers.
I forgot to mention, some woofers have such tight VC gaps that just tightening the chassis down onto an uneven mounting surface will cause it to distort a little. This can create a VC misalignment in the magnet gap, which results in a rubbing VC.
Dynaudio, Beyma, Cerwin Vega and ScanSpeak (magnesium frame) drivers were very susceptible to this.
I had purchased a set of Aerial 10Ts for a good deal, because the previous owner claimed one of the woofers would start to buzz when it was played for a while. I discovered the problem got worse when I tried to tighten up the driver mounting screws. It turned out to be a folded up piece of sealing gasket doubled up on the mounting flange, twisting the driver slightly when torqued down. I just cleaned up the mounting flange, sealed it back up with thin gasket tape and re-installed the woofer (Vifa M26WR09-08), tightening it very evenly, which fixed the problem. It pays to be methodical with these things.
Dynaudio, Beyma, Cerwin Vega and ScanSpeak (magnesium frame) drivers were very susceptible to this.
I had purchased a set of Aerial 10Ts for a good deal, because the previous owner claimed one of the woofers would start to buzz when it was played for a while. I discovered the problem got worse when I tried to tighten up the driver mounting screws. It turned out to be a folded up piece of sealing gasket doubled up on the mounting flange, twisting the driver slightly when torqued down. I just cleaned up the mounting flange, sealed it back up with thin gasket tape and re-installed the woofer (Vifa M26WR09-08), tightening it very evenly, which fixed the problem. It pays to be methodical with these things.
That woofer cost almost $1,000. The buyer should not have to do anything to correct the manufacturer's flaws.
@Michael Chua I mean from the factory, not the end user. I'd expect perfection straight out of the box for that kind of money.
What I forgot to mention is also to check/clean the gap. So remove the diaphragm, clean everything carefully and re-align like here:
Furthermore I think it is important to play a low frequency test tone or sweep still with rear cap removed and check if there is any buzz and fi so try to find another position while the screws of the diaphragm are only loosely in place. Continue to play the test tone while fixing the screws. The tone must remain clear without buzz when the screws are fixed.
Furthermore I think it is important to play a low frequency test tone or sweep still with rear cap removed and check if there is any buzz and fi so try to find another position while the screws of the diaphragm are only loosely in place. Continue to play the test tone while fixing the screws. The tone must remain clear without buzz when the screws are fixed.
And gradually/evenly fix the screws. I had to do a compression driver few times before distortion matched the other unit.
CLIO Pocket is a nice tool to do that procedure as you can easily switch between test tone and impedance shot each time you think the test tone is clear.
But what is essential you need to have some clearance between driver exit and table surface. For this purpose I use very long screws in the mounting holes and additionally put a piece of paper between the table and head of the screws to protect the table.
But what is essential you need to have some clearance between driver exit and table surface. For this purpose I use very long screws in the mounting holes and additionally put a piece of paper between the table and head of the screws to protect the table.
If you don't get it right with the hints already given, change the diaphragma from one driver to the other. If the fault stays with the driver, there is something wrong with the damping or magnetism. If the fault moves to the second driver, something with the diapragma is wrong.
If you measure them with the horn attached, be sure to use the same horn for testing, as there may be a problem with it too.
It is all about eliminating possible causes...
If you measure them with the horn attached, be sure to use the same horn for testing, as there may be a problem with it too.
It is all about eliminating possible causes...
All the described methodology is mainly necessary with the more budget lines of drivers under say $50 - $75, which are manufactured to rather loose tolerances. The ones coming to mind here are the cheaper Eminence, Pyle, Gemini, Peavey, etc usually made in China. The VC gap on these drivers is usually very sloppy and uneven, often contaminated with left over small metal particles. The dyes used to punch out the motor parts isn't monitored closely for wear, making the gap inconsistent, contaminated and out of round.
The other problem area is the VC asy, which is often wound out of round with varying tension. The way all tolerances end stacking up with some of these drivers in question, it makes you have to go through an extra replacement diaphragm or two until you end up with a good pair that performs reasonably identical.
The last issue is how consistent the motor is magnetized. I've personally come across several inexpensive drivers with greatly different output sensitivities among the same part number, sometimes as much as a few dB. After verifying the diaphragm isnt responsible for this, measuring the gap flux shows the culprit. Before coming to this conclusion, I learned the hard way after buying a large case of 1" screw on drivers in attempts to build very reasonably priced 2 way PA speakers. The tolerances were so bad I had to warranty about 15% of them, mostly for varying degrees of motor flux.
The cheaper drivers I've had the best luck with are PRV, Timpano, SB acoustics, Eminence (US made) and Lavoce.
The other problem area is the VC asy, which is often wound out of round with varying tension. The way all tolerances end stacking up with some of these drivers in question, it makes you have to go through an extra replacement diaphragm or two until you end up with a good pair that performs reasonably identical.
The last issue is how consistent the motor is magnetized. I've personally come across several inexpensive drivers with greatly different output sensitivities among the same part number, sometimes as much as a few dB. After verifying the diaphragm isnt responsible for this, measuring the gap flux shows the culprit. Before coming to this conclusion, I learned the hard way after buying a large case of 1" screw on drivers in attempts to build very reasonably priced 2 way PA speakers. The tolerances were so bad I had to warranty about 15% of them, mostly for varying degrees of motor flux.
The cheaper drivers I've had the best luck with are PRV, Timpano, SB acoustics, Eminence (US made) and Lavoce.
The DATS V3 setup has a "go / no go" rub and buzz test feature, allowing you to test for various VC faults related to tolerances and mechanical defects among the same model drivers. You simply measure the impedance sweep of a known good reference driver and input your tolerance allowances. The system will give you a simple pass or fail result based on this. You can also measure driver impedance linearity at various drive levels.
Clio Pocket has a similar facility, just as good and easy to use.
Thanks for all the input. I did swap the diaphragms only to find out that that was not the cause. Both diaphragms also measured a slightly different inductance. However after swapping the diaphragms and wiggling them and the covers while measuring and tightening the bolts resulted in a much better impedance curve match - now only an ohm or so off at the main resonance frequency. I can now try them again or sell them knowing that they are both all right.
They have had consistency issues with VC centering in they're new D8404 domes, that costs 2500$ per. unit🙄
Also heard of instances on the 18w.
Your absolutely right, can't imagine it being fun trying to center them in the gap for a recone.