From bruce@ashbysolutions.com Sun Oct 05 08:44:39 2003
Subject:Leslie Tweeter Drivers -- LONG POST

All,

WHY 16 OHMS?

In the early 1940's, when the Leslie Vibratone was developed, 16 ohms was becoming the industry standard for speakers. 16 ohms remained the standard all through the '50's (when the Leslie 21H and 22H came about), and even into the mid-'60's, when improvements in transformer technology finally allowed amps to drive 8- and 4-ohm loads. Most '50s vintage guitar amps also use 16-ohm speakers, as did many hi-fi systems of the same era.

Why was it 16 ohms, rather than the 8- or 4-ohm values used today? Power is defined as the voltage across something times the current flowing through that item (power = volts x amps). In order to increase the power, one must either increase the voltage or increase the current, or both. Simply put, it's easier to develop voltage in a tube circuit than current, and transformer technology was such (at the time) that low-resistance loads resulted in very little output power. 16 ohms was the practical minimum impedance to work with in speakers; there were actually some 32- and 50-ohm devices at the time. Since the 122 and 147A are really evolutions of the 22H design, it's natural that the 16-ohm speaker system remains part of the mix.

Solid-state amps, OTOH, are generally better at current than voltage, so the move to 8-ohm speakers was even further cemented in place. Add in today's high-power automotive sound -- where the maximum voltage is pretty much fixed, so all one can do is boost the current -- and it pushes the use of 4- and even 2-ohm speakers. Unfortunately, since the Leslie design is quite old, it's not able to take advantage of these changes -- at least, without a major redesign.

CHOOSING A TWEETER

The balance between woofer and tweeter in a Leslie is a tricky thing to set. If the tweeter isn't efficient enough, the sound will be muted and flat. I the tweeter is TOO efficient, the sound will be nasal and shrill. (Users of the Motion Sound Pro-3T deal with this issue the first time they plug and adjust their setups.) In past years, Ashby Solutions used to sell a high-power Leslie upgrade kit for use with a 200-300W amplifier. I tested -- and rejected -- a lot of tweeters during the development of the product.

The Jensen V21 compression driver has a great, smooth sound. It's weak point is that it's only rated at about 35W -- fine for "clean" Leslie use, but not up to adding a little growl to the mix. As the organ sound becomes more distorted, more of the 40-50W that the tube amp produces consists of high frequencies, so as we move from Don Leslie's original intent (adding pipe-organ-like vibrato) into the realms of Jon Lord and Ken Hensley, the woofer actually gets to slack off, while the tweeter works overtime! The poor Jensen tweeter hasn't got a chance!

There are many PHYSICAL replacements for the V21 that use 1-3/8" threads (called 1" in many places). Some of them have phenolic diaphragms, which make them better suited to Leslie use than titanium ones. Some of them have a frequency response that rolls off somewhere between 6-7KHz, which is also helpful. Very few of them actually sound like a V21 -- or are even CLOSE to a V21's tone. Most of them are 8 ohms, rather than 16, which will present a bit of a problem to the stock Leslie amplifier and crossover, as it changes the crossover point of the tweeter. Here is my $0.02 on the available replacements:

- The best two replacements are the Electro-Voice/Community EV1829, which has been out of production for many years, and the BT-75 that BT Productions sells. Both of these tweeters have "the V21 sound," both of them are 16 ohms, and both of them are more robust than the V21. Both of them require a 1" threaded adapter to fit, and may require a new horn spindle (or a modification) because the mounting hole locations on the adapter are slightly different than the original Jensen bracket.

- The black 16-ohm tweeter that H-S used to sell is also good, particularly if a 15-ohm resistor is placed across the input terminals. This change reduces the shrillness of the driver. I don't know if these tweeters are still available; Goff Professional's website still shows one in their photo. I know H-S no longer sells them as replacements. They also have different mounting hole locations than the V21 (but the same as the 1" threaded adapter plate).

- I've heard good things about the 12-ohm Ruby driver, although I have no personal experience with it. The 12-ohm impedance may or may not affect the crossover point enough to matter. (Does it need an adapter?)

- H-S now uses a ferro-fluid cooled tweeter that is supposed to be rated at 100W. I've never heard it. People who have seem to either love it or hate it. It uses the adapter plate.

- Pro Systems (Gollehon) makes a good high-power (120W) tweeter, but it a) is an 8-ohm part, and b) requires attenuation to match the woofer. This is what I used in my high-power upgrades, with a custom crossover of my own design. Again, it needs the adapter plate.

- The QSL tweeter doesn't look promising, IMHO. The frequency response of many raw speakers is measured at -10dB, which is not the same as saying a speaker "has smooth response" over the same range. Take a look at the JBL 2485J, which was considered a good -- albeit, horrendously expensive -- Leslie upgrade: It has a 5dB change from 500-5,000Hz, which means that the human ear will hear a mostly constant level in this band. The QSL tweeter, OTOH, has a 10dB change over the 500-5,000Hz range. Again, it's 8 ohms, and needs the adapter plate.

- I've tried the Selenium D250, and I thought it was WAY too shrill. (This was my original candidate for the high-power driver, and it went back to the vendor within minutes of trying it!) The D250 has a 10dB range from 400-3,000Hz -- totally unsuitable to replace the V21. It's also 8 ohms.

- I've also tried the Selenium D210Ti, with its titanium diaphragm which is much smoother, but goes TOO high -- its 5dB point is around 15KHz. I wish they made a phenolic version; it might be just the ticket.

NOTE: All of the above comments assume that one wants to replace the V21 with an EXACT tonal equivalent, only more robust. Some players may actually like the sound of many of the units that I cast out as unacceptable, particularly if used for loud music. If you're looking for more edge to your sound, all I can suggest is to experiment with the available candidates, and pick what sounds best to you.

Regards,

-BW
--
Bruce Wahler
Ashby Solutions™ http://music.ashbysolutions.com
978.386.7389 voice/fax
bruce@ashbysolutions.com