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Rebuilding Beveridge Circuit Board Stators
Rebuilding Beveridge Circuit Board Stators
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Old 11th August 2019, 10:38 PM   #31
DrJJ is offline DrJJ
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Join Date: Jan 2015
It has been a while since I updated this thread, so here it is!

Some progress has been made, but not as much as I'd hoped. Real life got in the way. I had my amplifier refurbished -- an old Mac MC2105, which is not great, but stable and good enough for now. And before I started, I wanted to measure what I have now, and see if any changes made could be measured.

I've not done audio measurements before, and with Sheldon's advice, I now have a calibrated Dayton EMM-6, a Focusrite Solo, a microphone stand, and yards of cable. I had to get a few additional adapters to get everything to work together. For software I started with REW, which is easy to use and free. It seems to be limited to swept sine waves (is that right?), but it is very powerful from those basic data. It really is a pretty slick program.

The measurements only partially explain what I hear. The bass indeed is awful. Mid/Upper bass (say, 80 to about 300 Hz) is down about 10 dB in level from the 300 Hz and up levels. Low bass is flatulent (by ear), and is lumpy. I'd bet that the replacement woofer used was not the right one, since this sort of response never would have been released. The bass of Bevs always has been bad, but not *this* bad.

From 300 Hz and up, the measurements are not too bad, giving about +- 3 dB from there to about 10 kHz, where response falls rapidly (the thick membrane?). Impulse response is pretty clean, with some ringing that dies down after about 1.5 msec to background levels. They invert phase.

Waterfall plots are interesting. They are pretty uniform with frequency over the entire frequency range, though they wobble a bit. But the decay time is long, at about 50 msec.

Harmonic distortion is below 1%, which is the limit of my (noisy!) room. REW seems to have no good way to measure intermodular distortion.

So what's wrong? Bevs always have had an ability to float a stereo image that is unique and wonderful. These don't do that -- there's no image at all. They also sound painfully bright, and other than the bass and maybe the ringing problems, there is not really anything else to suggest why that is so.

My best guess is that the diaphragm is not uniform, and that there are some measurement challenges. The most common way to measure is to do so in the very nearfield, so that the response one measures is unencumbered by the room. Here it is very difficult to get a true nearfield measurement, since the electrostatic element is buried behind an acoustic lens. The room is excluded, but what exactly one is measuring isn't clear. I'm reluctant to pull out the elements and measure those independently because of domestic concerns. I found the expected difference in nearfield and farfield frequency response slopes. (I note that "farfield" here is not far for a line source -- it was about 8 feet.)

Also, I only checked one portion of the lens, not along its radius or height. It certainly is possible (or likely!) that other portions would measure differently.

I guess the best path forward is to replace the bias/crossover elements that degrade with age (electrolytic capacitors, resistors and diodes), find a better woofer, and replace the electrostatic element diaphragm and repair any other damage.

Stay tuned!
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Old 12th August 2019, 11:54 AM   #32
stokessd is offline stokessd  United States
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This is a super quick thing to try, and I suspect you already did it. But try swapping the speaker wires on one speaker to see if they are out of phase. Your stereo image may snap into focus.

Sheldon
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Old 12th August 2019, 03:51 PM   #33
DrJJ is offline DrJJ
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That's a reasonable thing to try, Sheldon, and I've not done so.
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Old 12th August 2019, 04:06 PM   #34
mattstat is offline mattstat
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I donít have direct experience with Beveridge units, but a few random thoughts:

1) You may be able to get more information about coating condition by measuring capacitance from diaphragm to stator for each panel.

2) Your idea about measuring up/down and side-to-side on the lens seems like a good one. In theory the path through the lens isnít going to affect things much, so measuring the lens output is likely similar to measuring at the panel. Near the panel edges, youíd expect some drop-off in output/bass, but if you have any other major oddities in your measurements, it might point you toward panel problems. Checking the left versus right speaker might point you to something also. Overall, youíre looking for things that donít fit the trend; you wouldnít expect exactly the same nearfield measurements everywhere though.

3) You can also look at impulse response to check polarity of the individual panels and left/right speakers.

4) Iíd try to measure bias before assuming itís the source of your troubles, unless you have a symptom (like left or right speaker having significantly more output). BK Precisionís PR 28A is a 1000x attenuator probe that comes in handy when working with ESL bias supplies.
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Old 12th August 2019, 04:41 PM   #35
DrJJ is offline DrJJ
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Thanks, Matt!

Quote:
1) You may be able to get more information about coating condition by measuring capacitance from diaphragm to stator for each panel.
I've done so, and the capacitance is pretty close to what is written on the panels.

Quote:
3) You can also look at impulse response to check polarity of the individual panels and left/right speakers.
I did, and the polarity is the same (and negative) for both speakers. Still, it is easy enough to swap leads on one speaker and try it. It sure sounds like it is running out-of-phase.

Quote:
4) Iíd try to measure bias before assuming itís the source of your troubles, unless you have a symptom (like left or right speaker having significantly more output). BK Precisionís PR 28A is a 1000x attenuator probe that comes in handy when working with ESL bias supplies.
It is on the list to do. I have a high-voltage probe and decent meters (an old BK and an Agilent).
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Old 12th August 2019, 07:40 PM   #36
mattstat is offline mattstat
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Sorry, I misinterpreted your front/back capacitance measurement sentence earlier in the thread.
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Old 13th August 2019, 08:16 PM   #37
DaveG is offline DaveG  United States
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Hi DrJJ,

If I have understood this correctly the worst initial problem is the level difference between the woofer and panel.

If yours are similar to mine there are no level adjustments.
I guess mine are the original woofers since they blend nicely.

I would guess it will be mostly impossible to find a woofer that will exactly match without padding down the panel and bi-amping will be the only practical method.

Do yours have the 8 banana jacks for the speaker cables?
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Old 13th August 2019, 08:22 PM   #38
DrJJ is offline DrJJ
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DaveG,

While the woofer level is annoying, it is the lack of imaging and brightness that are the primary problems. The imaging is what makes the Bev special, and without that they are not very interesting.

I figured I'd have to biamplify anyway, since the Bev woofers always are an issue, and they set the entire "case" into resonance. So yes, I'll make a separate woofer arrangement, biamplify, and not use the one in the Bev cabinet.

If by your banana jack question you whether it has separate connections for biamplifying or for using the crossover, the answer is "yes." One thing to try is to use my amp with one channel and a digital crossover to see if it helps.
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Old 13th August 2019, 09:37 PM   #39
DaveG is offline DaveG  United States
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My guess would be at the moment the woofer is the problem with the imaging and brightness. Mine is a 250 hz xover.

If you have a spare amp to use or can borrow you can rig up a simple level control and bi amp with the internal crossover woofer xover. You could even do it for one channel with your existing amp.

I use an ALPS 250K in a little plastic box with some old cheap RCA cables that came with a dvd or tv or something as a simple stereo level contol.

I use it as a safety valve when playing with stuff, sound cards and testing, so one click of the mouse does not go to ear splitting full output.

Use it to drop the panel level the 10 dB
Parts Express has them for about $15

Then you are using the original crossover points so when you get the levels correct hopefully you will hear the correct original fullness.
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Old 13th August 2019, 10:23 PM   #40
DrJJ is offline DrJJ
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If I read you right, you are suggesting bi-wiring so that the existing crossover can be used. I'll have to check that this will work with the connections. If that is possible, then I can do one side with my existing amp -- it has output potentiometers.

I don't have a spare amp at the moment, but an inexpensive plate amp may be good enough for the woofer(s). I suspect that quality here is not really limiting.

To touch back, a lot of this diagnostic work isn't really critical. The speaker is 35 years old, and most parts need to be replaced or rebuilt. The costs are modest, and other than the stators, it is not much work once you get into the thing. Also, I only want to do this once, so it makes sense to do as much as is reasonable.

I was curious to see if there were major anomalies that could be measured that correlated with what I hear. It is no surprise that the woofer level is way down. But that nothing else seemed terribly amiss was a surprise.

I have to admit that the measuring is very cool! Remarkable powerful, though it is hard to do unambiguously.
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