Hey folks - first topic post here after lurking for some time. I've searched the forum for this topic and came up empty. Looking for some help from speaker experts.
Leslie speakers typically have a 15" woofer and a Jensen V21 driver in them. The crossover in the cabinet is 12 dB/octave at 800 Hz. The V21 drivers are good for about 100 - 4000 Hz and 40 Watts RMS.
I've got about 10 of these on hand and I'm servicing them by cleaning them up in general, cleaning the gap in particular, and replacing the diaphragms. One thing that is mystifying me is the way Jensen set the voice coil height. For typical speakers, the voice coil height is set by centering it vertically with the top plate thickness in the gap. So at rest you have an even amount of the magnetic field in play - sort of.
But all these V21s are set so that the voice coil is a little more than halfway down into the gap. With the OEM diaphragms, you had to set them at least .03" proud of the top plate with some gasket shims above and below the diaphragm to have some room for it to flex. But there is typically 1-2 more shims above the top plate.
See the attached drawings for a cutaway view.
Is there are reason that these voice coils are not centered in the gap? Is this typical for smaller drivers vs. larger speakers?
With the replacement diaphragms, the geometry is slightly different. You could actually install them so the voice coil is centered. I'm curious is if that would be a good idea.
I'm also curious if the increased distance between the diaphragm and the phase plug will have an audible change in frequency response.
Comments appreciated!
Leslie speakers typically have a 15" woofer and a Jensen V21 driver in them. The crossover in the cabinet is 12 dB/octave at 800 Hz. The V21 drivers are good for about 100 - 4000 Hz and 40 Watts RMS.
I've got about 10 of these on hand and I'm servicing them by cleaning them up in general, cleaning the gap in particular, and replacing the diaphragms. One thing that is mystifying me is the way Jensen set the voice coil height. For typical speakers, the voice coil height is set by centering it vertically with the top plate thickness in the gap. So at rest you have an even amount of the magnetic field in play - sort of.
But all these V21s are set so that the voice coil is a little more than halfway down into the gap. With the OEM diaphragms, you had to set them at least .03" proud of the top plate with some gasket shims above and below the diaphragm to have some room for it to flex. But there is typically 1-2 more shims above the top plate.
See the attached drawings for a cutaway view.
Is there are reason that these voice coils are not centered in the gap? Is this typical for smaller drivers vs. larger speakers?
With the replacement diaphragms, the geometry is slightly different. You could actually install them so the voice coil is centered. I'm curious is if that would be a good idea.
I'm also curious if the increased distance between the diaphragm and the phase plug will have an audible change in frequency response.
Comments appreciated!
Yeah, I’ve got a ton of these currently. They are all .033” thick. The originals range from .011” to .066”, give or take. I have to make thinner ones out of gasket paper.
Drivers that old may have been re-diaphragmed, shims and alignment set incorrectly for the available parts used.
Little or no cleaning may have been done, rather than using folded over masking tape and multiple passes until the tape shows no debris or oxides.
Hard to say whether the reason was intentional, or due to incompetence, stupidity, lack of training, or ignorance without knowing their provenance 😉
No, but high frequency driver's coils are often completely in the gap (underhung) while your diagram shows an overhung coil, normally centered in the magnetic gap.
The Jensen V21 is a midrange driver, it's magnetic gap flux patterns are probably not symmetrical, so a slight offset one way or the other may actually provide the best response in terms of least harmonic distortion when run near the mechanical excursion limits.
That said, the Leslie speaker is a musical sound producer, rather than a reproducer, so various approaches have been used by different techs that have worked on them, depending on their version of what is "desirable", and their ability to make it happen.
The "goodness" of the idea would depend on what "sound" you want at what SPL (sound pressure level).
A coil partially leaving the magnetic gap increases harmonic distortion.
An increased diaphragm to phase plug distance lowers the acoustic band pass frequency, lowering the high frequency content available, and reduces harmonic distortion created by the driver. It also increases the excursion available before the diaphragm contacts the phase plug, so increases low frequency SPL.
The Leslie tube amp adds lots of harmonic distortion when cranked up, whether you like it or not is a matter of taste. Adjusting the driver to add distortion could allow playing at a lower SPL while still sounding similar to the amp at "full tilt boogie".
The drawbars on a Hammond organ can be adjusted for adding harmonics to the fundamental, so different instruments can be emulated.
The upper harmonics can sound downright nasty (or lovely...) with an extended high frequency driver.
Some (deaf..) rock organists would use drivers like the 4" aluminum diaphragm JBL 2440 to get another octave of high frequency from their Leslies.
Some would remove the horn diffusers and back panel, turn the cabinet around to hear that extra high frequency spinning by.
https://electricdruid.net/technical-aspects-of-the-hammond-organ/
The tone wheel "clicks", a big part of the "Hammond sound" are also in that upper "love it or leave it" high frequency range.
Also, the Leslie crossover was designed around a "16 ohm" diaphragm, depending on the impedance of your current diaphragms, the crossover frequency and slope could be far different than intended, even if the crossover capacitors have held their original values.
Art
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First off, I appreciate the detailed response. Many thanks!
I've taken a lot of these apart and the shims are pretty standard as to thickness and placement. .033 and .066 below the diaphragm and .033 above it. I think you need the .033s for diaphragm flex clearance on excursion. It's tight as you can see in the drawing. The .066 sets the height in the gap. Earlier drivers (like from the 50's and early 60's) seem to have more variability and you find .011 and .022 shims in the stackup. Maybe the parts were more variable or maybe the factory took more care in assembly and testing. Those early diaphragms are also usually flat with no ribs vs. the later ones but the overall geometry is the same. You can usually tell if a driver has been taken apart by the condition of the seal between the phase plug below the horn plate, and especially the factory soldering. Not centering the coil was by design or a product of the parts they had.
The replacements are all 16 Ohms. And my crossover I use for final testing is spot on at 800 Hz.
Yeah, with the geometry of the new diaphragm there is no fixing this. I have some high frequency hearing loss so I can't really tell just by listening. I need to build a couple and check the frequency response on a computer.
I test all these on a sig gen through an amp on the bench for sanity, but the real test is they go in my Leslie and then I drive them at max from my A100 Hammond. At full expression and a lot of drawbars out and the Leslie on 10, you should not hear any buzzing or clacking from the driver in the lower frequencies with specific chord voicings.
The diaphragm surround appears to be a half roll on the currently available replacements.
Are you saying the older diaphragm surrounds were more flat?
Interesting that the ones marketed specifically as V21 replacements have two different asymmetrically placed lead arrangements relative to the dowel pins. I'd think that imbalance would tend to cause rocking and buzzing.
Were the older diaphragm lead positions asymmetrically placed, or symmetrical like the right hand Atlas PD-60?
The DC resistance should be around 13 ohms, nominal impedance ~16, but the actual impedance will vary with diaphragm to phase plug distance, diaphragm weight, surround compliance, back chamber volume, et cetera.
The original Jensen V21 had a back chamber, it's impedance curve would be different than the Leslie driver which is run "open back".
The impedance changes when the driver is mounted on a horn, each different horn/driver combination has an individual, distinct curve, each requiring different passive crossover components to result in similar acoustic crossover slopes and frequencies.
The "16 ohm" Atlas PD-60 driver (no horn, sealed back chamber) impedance curve might be something like the V21 with the back cover:
A crossover that is "spot on at 800 Hz" won't be spot on if the impedance curve is not what to what it was designed for, apparent in the different values of C1 and L1 in the following 12dB/Butterworth examples.
None of those values are what Leslie uses..
Checking the impedance curve of the drivers with different diaphragms would also be informative.
Art
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The very early diaphragms were flat. By 1961 they had the ripples on the surround. The replacements have one less "rib" if you will than the OEM Jensens and the ribs are slightly taller. All of them had the asymmetric leads, which are flat and extremely thin - maybe .002". I think they added the ribs by 1960 to help reduce warping and buzzing. In the image below a 1957 diaphragm is on the left, and the new replacement is on the right. The other image is one from 1961 which has the ribs on the surround. Ignore the cutout - I did that to use my calipers to measure it.
There are two big differences in the replacements as detailed in the drawing in my original post. Where the surround is attached is about .09" higher on the replacements, and the voice coil is taller. You can see that here:
So this changes the amount of voice coil that can be in the gap, and it reduces the distance from the phase plug to the dome by a lot. I'm not sure what the main difference would be with the extra voice coil, but I'm pretty sure the bigger gap between the dome and the plug will roll off the high end some.
In addition to removing the bell, Leslie had Jensen drill out the center of the phase plug and they added cork and some fiber material in there. This was in an attempt to "tame" the drivers at the higher end. You can really hear the difference in the "Top Hat" drivers the used in the 1990s. Very shrill.
It may be total BS, but plenty of Hammond/Leslie players claim that the Leslie V21s and the woofers are perfectly matched.
What I meant by that is that both the inductors and the caps are within 1% tolerance in my crossovers. Which is overkill for sure. The standard inductors are ferrite core, and I use a laminated steel inductor on the low-pass side and an upgraded Jensen ferrite core unit on the treble. These have been unwound to be exact values. The inductor placement has less cross-talk and the laminated steel is "supposed" to improve base response. Leslie used laminated inductors for crossovers in early cabinets. I packed my layout in so that it fits in the standard ring Leslie used for the cabinets. I also use non-magnetic hardware. This is all overkill in a system that is a far cry from hi-fi. But it's fun to experiment.
Don Leslie used m-derived half section equations from Otto Zobel for his crossovers, not Butterworth - which came later. 12dB/octave. If you plug in the Leslie values into standard equations the math doesn't work.
Those are relatively huge differences, probably could amount to as much as 2000Hz in the acoustic band pass reduction, unless the different phase plugs used end up with similar diaphragm to phase plug chamber volume.There are two big differences in the replacements as detailed in the drawing in my original post. Where the surround is attached is about .09" higher on the replacements, and the voice coil is taller. You can see that here:
So this changes the amount of voice coil that can be in the gap, and it reduces the distance from the phase plug to the dome by a lot. I'm not sure what the main difference would be with the extra voice coil, but I'm pretty sure the bigger gap between the dome and the plug will roll off the high end some.
The extra voice coil length would allow more excursion, useful for a PA speech driver, not so much for an 800 Hz crossover.
I'd think the longer voice coil would add a fair amount of weight, probably lower Fs unless the suspension is stiffer.
An electrical filter's slope and the acoustic crossover slope are two different things.
The horn/driver raw response (no crossover) that may be near 24dB/octave towards it's bottom end, probably in the 500-700 Hz range.
Unlikely the impedance curves of the different diaphragms/coils/phase plugs would match those of the old, so whatever the crossover frequency and slope was set originally won't be the same with the changes.
That said, you can play with the drawbars to get the timbre desired 😉
Art
Unfortunately, the replacement diaphragms are all you can get, and the geometry is wrong due to the flange location. See the updated diagram with the phase plug details.