[Roger A. Modjeski]

I have been getting many inquiries about the Direct Drive amplifier that was a necessary part of the Beveridge System 2 and 2 SW.

Firstly, many people think I designed that amplifier, I did not. The amplifier was in its final design several years before I met Bev. He claimed a peak power of 1500 Volt Amps (VA). It really does that, I measured it some years later when I was working with Bev here in Santa Barbara. (More about my career path at http://www.ramlabs-musicreference.com/bio.html)

Bev saw the amp only as a means to run the speaker since no other amp we knew could provide 1500 peak Volt Amps (VA). For those not familiar with VA they are volts times amps but include the phase component. A capacitor can draw lots of volt amps and no watts. If you plug a 20 uf film capacitor into the wall it will draw 1 amp but won't influence your electric meter even though the electric company will be footing the bill for the extra losses. Industry actually pays extra for VA usage. The speaker was close to a 20 uF load if it were transformed to appear as a 4 ohm load. Very few amplifiers could drive this load. The marketing fellow was twisting my arm to make the 2SW drivable by an amp of the owner's choice. I designed a suitable (big) transformer but it was not practical. Capacitive loads are difficult for any amp to drive and load-line limiters often kick-in before full power is realized. That sounds like severe clipping.

The marketing fellow pointed out several things that he thought were lacking in the amp and I came up with several mods. I reduced the hum coming from the speaker, quieted the fan and designed a new woofer amp and crossover system. The new designation was 2SW-1.  I later added the "air" and "more air" toggle switch on the back over the input jack making it the 2SW-2. I was so busy developing the RM1, 2 and 3 that I wasn't looking for anything else to do. I was also helping Bev with the system 3 which would have a step up transformer. I would have liked to design a new amp from the ground up but Bev was wanting to get away from making amps. Acoustat already had abandoned direct drive amps due to their greater numbers and reliability issues. For those who have never heard the Acoustats on the direct drive amp it is something to behold. They had to go to a transformer to make things simple for them and the owner. Unfortunately, the sound suffered.

I have received a firm order for 2 pairs of all tube amps at $10,000/pair. If anyone is interested let me know.  Several owners are now running their speakers with the amps on top. What I plan to make can sit on top or off to the side.  Like all Music Reference products it is reliable. I have been using one for several years and a friend in Virginia has been using my modified Acoustat amps for over 15 years.

A discussion here is welcome with any suggestions. I can't take any orders till after Labor Day and we are all on vacation anyway, right? This is just something to think about.

[Baranyi]

Roger,
        I just saw this topic regarding the Beveridge 2SW amps. I happen to own one of these great speakers.  In your post you mention that some owners of this speaker put the amps on top of the speakers.  I was just told this by someone in Sweden who runs this speaker that they run the speaker upside down.  He claims that by doing this he can disconnect the fan.  Is it safe to do this?  Lastly, I have been told by Audiophiles that the earlier original model 2 was even more impressive in many ways than the 2SW speaker as far as imaging and clarity.  I have also been told that all the employees of the Beveridge company actually preferred this speaker.  Could you please comment on the differences with the speakers.  I wish I would have known about high end when I lived in Richmond Va in the early 80's.  We might have crossed paths.  All the Best, Bob

[Roger A. Modjeski]

If you put the amps on top and remove the bottom (which is now on top) you can disconnect the fan.

I have a Fan Servo circuit that can be easily installed on any Bev amp. It's totally quiet and modulates the fan speed to keep the internal temp constant. Sounds better too because of that. The boards are $400 a pair.

[Baranyi]

Roger,
        Thanks for the previous answer about the amps on top of the speaker.  Could you comment on the differences between the Bev 2 and the 2SW?  Your comments can leave out such things as higher SPL and the greater bass extension of the 2SW's.  I have been told that the original model 2 actually imaged better and gave a more palpable image than the later moder 2sw.  I also have been told that the employees of Beveridge actually had a preference toward the original model 2 over the modified 2sw.  Since you were there at the time I would love you to set history straight.  All the best,  Bob

[Baranyi]

Roger,
         Your thoughts about the differences regarding these two speakers would be most appreciated!  Bob

[Baranyi]

Roger,
        As a new years present could you respond to my question of the differences between the Beveridge Model 2 and 2sw?  I already own the Model 2sw and am about to also purchase a pair of the original model 2's and want to know if the 2's were superior in imaging over the 2sw.  I obviously know that the 2sw  added bass and spl's but many people say that the original 2 was really superior in many ways.  I don't have the luxury of listening for myself before making the purchase so your input would be very valuable.  All the best,  Bob

[Baranyi]

Roger,
        Is it impossible for you to give an answer to my question regarding the differences between the original Beveridge 2 and the later 2sw? I don't mean to step on your toes but I have posted a number of times asking this question. 

[Roger A. Modjeski]

This all goes back a long way and there are many improvements and increased expectations in current speakers and amplifiers. That said, here is what we did.

Bev increased the SPL by 10 dB by modifying the lens only. He did this on the demand of the dealers that the speaker play louder. If 90 dB is loud enough for you then you can use the 2. The Helmholtz resonance of the system went from 40 Hz to 80 Hz in the 2SW and the amp was equalized differently. I designed a 100 Hz crossover and solid state amp to drive the sub. The driver in the sub and the cabinet were not nearly as good as the ESL but better for my revision which had to use the same $13 woofer and cabinet. The resistance-loaded woofer I use with my ESL is far superior and was made available to Bev but not explored due to time and money constraints.

If you have a long narrow room, set them up facing each other with their backs against the side walls, 6-10 feet from the rear wall and twice that distance to the listener they present a pleasing image. This is the only way Bev would play them and I must agree. This is why I developed my own ESL which is very versatile in set-up and has a high WAF.

[rmfurzeland]

Roger,
Could you explain a bit more about what would make a good direct drive amplifier for an ESL with its high capacitance.    Assume we are not using a transformer.  If one has say a push pull 845 tube amplifier with sufficiently large voltage swing, what more is needed?  I guess adequate current across the frequency range?  What should I be measuring to determine this?

I just happened to have built such a push pull amp. and it was pretty good at driving a 11" by 40" home spun ESL panel, not as big as the Beveridge though.  Just wondering if I can improve on it through some mods. to the amp.

Thanks,
Ron Furzeland

[rmfurzeland]

PS I am new to the forum so I get graded as a "newbie", can I get an upgrade in status based on having built my own ESL speaker and direct drive amp ! 

[JohnR]

PS I am new to the forum so I get graded as a "newbie", can I get an upgrade in status based on having built my own ESL speaker and direct drive amp ! 

Sure, can you post some pics? 

[Roger A. Modjeski]

Roger,
Could you explain a bit more about what would make a good direct drive amplifier for an ESL with its high capacitance.    Assume we are not using a transformer.  If one has say a push pull 845 tube amplifier with sufficiently large voltage swing, what more is needed?  I guess adequate current across the frequency range?  What should I be measuring to determine this?

I just happened to have built such a push pull amp. and it was pretty good at driving a 11" by 40" home spun ESL panel, not as big as the Beveridge though.  Just wondering if I can improve on it through some mods. to the amp.

Thanks,
Ron Furzeland

Hi Ron,

Sorry for the delay in responding, I have been busy building direct drive amplifiers. To answer your questions we should start with what you capacitance is. From that we can calculate the current required. Most direct drive amplifier designs I have seen are very deficient in current. My amplifier can provide about 1/2 amp of current at 4000 volts making the output 2000 Volt Amps. This may appear to be a large number but it is quite realistic and also explains why large amplifiers are needed for large area ESL speakers. Roger Sanders is also aware of this and makes some very large solid state amps for his Innersound speakers.

As to your 845 push-pull: Is that the one published in Speaker Builder in the late 1970's? If so that design cannot provide any significant current as it uses resistive pull-ups. I have considered the 845 tube but it is not the right thing for direct drive.

Can you give a brief description of the circuit or a diagram for me to comment? It would also be helpful to know your electrode spacing and polarizing voltage.

[airhead]

.5 amp at 2000 watts sounds quite dangerous.  Am I wrong about this?

[Roger A. Modjeski]

.5 amp at 2000 watts sounds quite dangerous.  Am I wrong about this?

That is what is going on in all ESL speakers, that's what it takes. Its only there at max levels and high frequencies. What comes out your wall outlet at 120 Volts is far more dangerous. It's there all the time.

[rbbert]

.5 amp at 2000 watts sounds quite dangerous.  Am I wrong about this?

As the former long-time owner of some stock and then modified Acoustat Servo-Drive amps I can say that tube failure was quite an exciting event!

[Roger A. Modjeski]

As the former long-time owner of some stock and then modified Acoustat Servo-Drive amps I can say that tube failure was quite an exciting event!

Yes, I have observed the Acoustat Servo meltdown. In all kindness to the designer, he knew more than you might imagine about the potential problems. For instance there were several resistors in the path of danger that were in plug-in sockets so as to be readily replaced when a tube failed.  In addition the tubes and their sockets were not well chosen and the  B+ was too high.

In my amplifiers the B+ is 20% lower (only giving up 2 db of peak level), the tubes are octal based in good handwired sockets and the resistors that typically failed in the Acoustat amp are protected against surges. Its an entirely new amplifier designed for best sound and high reliability. It is just as reliable as the tube amps I have been building for years.

The amplifier is all tube and all hand wired.

[rmfurzeland]

Dear Roger,
Thanks for your interest, I will send you details soon.  The design is my own and any resemblance to the Speaker Builder one will be coincidental.   My S/D spacing is 1/8" and bias V is in the 3-4000 range.
My 845 tube supply lines are "only" 1250 Volts and tube current is of the order of 65 ma.  The ESL design is a DIY constant charge design (Lucas, Sanders, etc)  not the Beveridge style - so capacitance I am estimating at 2 not 20 microfarads.
Obviously limited power but boy, does it sound good!  No stepdown (7k:8 ohms) then V stepup transformers to interfere with the sound.
Regards,
Ron