Poll

Direct Drive ESL Amplifiers for all

I have ESLs and I would like to explore direct drive
10 (16.9%)
I can do basic re-wiring myself
11 (18.6%)
I need a technician to adapt my speakers
2 (3.4%)
I want to send my speakers for Roger to do it
1 (1.7%)
I want to make my own ESLs
6 (10.2%)
I have made ESLs and need a good way to drive them
1 (1.7%)
I find ESLs fascinating and want to know more about them
15 (25.4%)
I have read DIY articles and toyed with the idea of making some
6 (10.2%)
I think a DIY ESL can achieve excellent sound and is a rewarding project
6 (10.2%)
I have read DIY articles and find them confusing or unclear as to what to do
1 (1.7%)
The drive systems seem poorly conceived in the DYI articles
0 (0%)

Total Members Voted: 26

ESL Independence Day

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Roger A. Modjeski

ESL Independence Day
« on: 4 Jul 2012, 07:55 pm »
Happy 4th. Sparks and Sparklers but no sparks from my ESL speakers... :nono:

Inspired by reading the recent coffee-table book on QUAD and after a year of thinking about it I figured out how to drive the QUAD 57 from my direct drive amplifer! As QUADAPHILES know that speaker has rather special requirements to both protect it from over voltage (sparking) and is somewhat amplifier fussy. The RM-10 has been popular among QUAD owners and it is widely known that I was using that speaker when I designed the RM-10. Some years later I got a pair of 63's and added some banana jacks on the back so I could direct drive them from a high voltage OTL amp. It's a very easy thing to do and I am willing to post a how to if there are interested parties who have the skills to solder and play safe around high voltages. Actually as long as any ESL is not playing music it is quite safe. The only high voltage that is present at that time is the polarizing and getting shocked by that is only an annoyance, not a danger as it is DC and of low current. It's the high voltage AC from the music that one must not come in contact with. For my pair I installed 4 jacks so I could jumper back in the standard drive electronics easily.

My speaker, the Acoustats and most DIY ESLs are simple 3 wire systems. Those being the front, back and diaphragm wires. There are dozens of articles on how to build your own panels. In fact ESL panels are the easiest speaker to build needing only moderate woodworking skills and easily obtainable materials. By contrast, a magnetic planar or cone driver is almost impossible to build in one's garage shop. The difficulty the DIYer encounters is the drive electronics. The often given solution, reverse connected output transformers being the most popular, is just horrible. Those transformers never have high enough ratio or peak voltage rating. Usually the DIYer finds he has to get a several hundred watt amplifier to get the voltage and then the transformer can't take it. The best solution is the direct-drive amplifier, but those are not for the unskilled to design or make, evidenced that only Acoustat and Beveridge made them and Acoustat quickly converted to a poor sounding substitute transformer box (I worked on one, it's really horrible and doesn't sound nearly as good as a direct drive amp). Acoustat quit making their amp because they wanted wide appeal and their amp was giving them a lot of trouble. Beveridge's amp was better but sat under the speaker making tube replacement difficult. Both were build on circuit boards, which was a bad idea. Both were built at the lowest possible cost as both designers were more concerned with the speaker, not the amplifier. They both had solid state front ends, the Acoustat having an OP AMP. I on the other hand have a lot of concern about amplifier sound and reliability. My amplifier is all tube and has balanced and unbalanced inputs.

The world of ESLs is largely divided into two categories. Here I am just discussing the ESL itself, not considering any cone drivers that may be employed.

1. Full range drivers that have large area and that area produces all frequencies. These tend to be very directional (the Innersound is a prime example) unless they are curved (Martin Logan) or approximated into a curve (Acoustat, Sound Labs). Most DIY designs are full range and either curved or flat, the latter being much easier to build. All are less efficient than multi way as they have high capacitance and will sound very, very bright unless EQ is applied. They all have EQ and this is best applied in DIY situation by a separate graphic EQ which can also correct for the room. This what my customer with the Acoustat Eight (8 panel speaker) does with great success. At 2 KHZ, the response is pulled down 10 dB and the curve is a gentle smile on both ends. This results in flat response in the room. Beveridge and Acoustat both employed EQ in their amps and Beveridge was very particular about his. Many DIY builders do nothing about EQ and have some very strange sounding speakers that could be easily corrected with a descent graphic EQ that can be had for $500. Yamaha and DBX make these both having good sound, balanced XLR output and can drive long lines to the amps that typically sit close to the speakers.

2. This second class of ESL speakers is multi way using panels of different dimensions and often different voltages to obtain line source dispersion. By adjusting the panel size and shape flat response can be obtained without the need for EQ. All the QUAD speakers do this having much larger area for the bass than the treble. The 57 is a 3 way line source where the 63 is a two way point source. I prefer the 57.

The 63 is an easy drive as it is low capacitance (much lower than the Beveridge and Acoustat) and has a single drive voltage. Only 3 wires are needed from the amp to the speaker and test lead wire is quite suitable and safe. The 57 is quite an different issue as it uses two drive and two polarizing voltages which are 1500V for the midrange/tweeter and 6,000 V for the woofer. These are provided in the speaker by the polarizing supply and the audio input (step-up) transformer which is quite complex and contains some crossover components which often go bad. The polarizing supply almost always needs to be replaced if it is original. With my direct drive amp these components can be removed (and sold for good money) as all you need are working panels.

My direct drive system provides both polarizing and audio voltage that will never overdrive the speaker. A common problem with the 57 is that it was intended to be used with the QUAD 22 amplifier which was 15 watts at 16 ohms which turns out to be 30 watts at 8 ohms or a peak of 30 volts which is just what the RM-10 provides, making it the safe choice for standard 57s. People get in trouble with bigger amps as they have higher output voltages. Clamping devices have been created for the 57 but many think they impair the sound of this very clean speaker.

Here is the run-down on what my direct drive amp can do. It can be made at any output voltage and I have a high and low current version because some ESLs draw very high currents due to high capacitance and some don't. This is just the same as the fact that speakers can be 2,4,8,16 ohms and anywhere in between. ESL speakers can be low capacitance, mine are just 100 pf. Beveridge model 2's are 4500 pF. That's a 45 to 1 difference, a much larger range than we see in cone speakers. In addition being capacitive the impedance varies inversely with frequency being lower at higher frequencies. When Beveridge went to transformer drive the result was a speaker that went from 100 ohms at 100 HZ to 1 ohm at 16 KHz. We had to find amplifiers that would deliver over 40 amps of current. These speakers were not suitable for most conventional amps tube or transistor. Roger Sanders makes a solid state mono amp that delivers 2000 watts at a cost of $8000 per pair.

Acoustats are about 300 pF per panel but the good ones have at least 4 panels per side making them 1200 pF. That still puts them in the high range. However if you want to make a small rather directional ESL you can lower that capacitance but a woofer will be required. If there is interest I will direct DIYers to the construction designs I find most appropriate or provide some easy to build suggestions. Acoustat transducers appear to be somewhat available from people who have scrapped the speakers. The panels are almost always good. The problem has always been driving them.

The Quad 57s are only 200 pF and the 63's are low also. In general the ESLs that use full range panels have the highest capacitance and are therefore least efficient. The multi-way ESLs (that means mulit-way in the electrostatics themselves) tend to be low capacitance and much more efficient. The extremes are the QUAD 57 needing 15 watts of drive and the Beveridge needing 1,500. Trumpet music is most demanding. Being a little in disbelief about the 1500 watt number I measured a half amp at 3000 volts. No wonder these speakers can't be driven any other way. The QUAD on the other hand needs the voltage of a 15 watt amplifier (at 16 ohms) but from a 4 ohm tap. In my experience that although the QUAD 22 amp was made to drive the 57 speaker, it was rather rolled off at the top due to the fact that the speaker impedance did fall to about 4 ohms. Not everything QUAD did was exactly as they said, contrary to the rather perfectionist philosophy proposed in the QUAD book.

One nice thing about my direct drive system is that by changing just 2 capacitors in my crossover one can set the brightness of the speaker to his desires either more or less than the standard by as much as 12 dB in either direction. Far more than one would need.

Please respond to the poll so I can get an idea of your interest and have a happy 4th.


OzarkTom

Re: ESL Independence Day
« Reply #1 on: 5 Jul 2012, 10:40 am »
 I see plenty of Acoustats selling in the neighborhood of $500.

Roger A. Modjeski

Re: ESL Independence Day
« Reply #2 on: 5 Jul 2012, 04:13 pm »
please let us know when you do by posting a link.

Roger A. Modjeski

Re: ESL Independence Day
« Reply #3 on: 5 Jul 2012, 04:15 pm »
I see plenty of Acoustats selling in the neighborhood of $500.

I've been looking on ebay. Where do you see them? Perhaps you could post a link when you seen some.

Clio09

Re: ESL Independence Day
« Reply #4 on: 5 Jul 2012, 08:42 pm »
You will see them on eBay and AudiogoN with some consistency. I had been looking for a set of ESL or planar speakers for awhile and had a high interest in Acoustat due to low prices. Problem is nobody wants to ship them (somewhat understandable) and most of what I saw was on the East coast. if I see any in the future I will post here.

BTW - I ended up with a pair of Magnepan 1.6QR speakers and may have the Peter Gunn mods done to them. I would still like a set of ESL-57's to pair with my RM-10 though.

Clio09

Re: ESL Independence Day
« Reply #5 on: 5 Jul 2012, 09:13 pm »

OzarkTom

Re: ESL Independence Day
« Reply #6 on: 5 Jul 2012, 10:48 pm »
Sometimes I look at the Electronic Shopper.

http://www.theelectronicsshopper.com/

the best place is Craigslist but use the Searth Tempest. Try this url. If it works, you will see a lot.

http://www.searchtempest.com/results.php?location=65622&maxDist=50000&region_us=1&search_string=acoustat&keytype=adv&Region=na&cityselect=zip&page=0&showeb=1&category=8&subcat=sss&minAsk=min&maxAsk=max

Here is a pair of Model 3's for $600 in Fontana Ca.

http://orangecounty.craigslist.org/ele/3073540209.html

It looks like many are in Ca.

Also, I signed up for Oodle Alert, that helps.

I also look here sometimes. They use to have Craiglook.
http://claz.org/

Here is a pair of 4's in Aurora Co. for $800 including a Yamaha reciever.

http://www.reachoo.com/ads/56569135#denver-Nice%20Acoustat%20speakers%20and%20receiver%20(aurora)

OzarkTom

Re: ESL Independence Day
« Reply #7 on: 5 Jul 2012, 10:55 pm »
Here is a pair of 4's listed in Memphis for $500.
http://memphis.craigslist.org/ele/3099574973.html

What is funny about this ad, they list the transformers as mono amps.


OzarkTom

Re: ESL Independence Day
« Reply #8 on: 5 Jul 2012, 11:09 pm »

airhead

Re: ESL Independence Day
« Reply #9 on: 6 Jul 2012, 06:12 am »
As I wrote on a different thread, I have Stax F83's driven by NYAL OTL3's.  I think the sound is glorious (and yes, including, maybe
especially, trumpets, despite the drop in impedance of these speakers at high frequencies).  I only buy equipment when necessary,
eg I bought Roger's great subwoofers because I was having serious bass distortion, and I recently replaced by 30 year old van den Hul cartridge with another van den Hul (just one step up).  If the transformers of the Stax's die (not unheard of, unfortunately), or if the amps die and can't be repaired, and if the DD amps aren't too pricy, then I would certainly consider them.  I wonder though if it is okay to have them at the other end of the room, with a  long speaker cable?  Also, it seems to me that it would be a good idea to have some sort of box to cover the speaker terminals.  Even with my Futterman's I once got a painful shock from the amps while music is playing when I touched the leads.  Couldn't the DD amp be fatal under such circumstances?   

OzarkTom

Re: ESL Independence Day
« Reply #10 on: 6 Jul 2012, 11:01 am »
I sold Acoustat and NYAL back in the 80's. I received a pair of the Acoustat Minitors with DD amps on trade. It was a night and day difference between the DD amps and the NYAL 3's, no contest. The DD amps blew the Futterman's away.

airhead

Re: ESL Independence Day
« Reply #11 on: 6 Jul 2012, 02:37 pm »
I wouldn't be surprised if the DD is much better.  Didn't Roger say above in particular that the transformers Accoustat used were not good?

Roger A. Modjeski

Re: ESL Independence Day
« Reply #12 on: 6 Jul 2012, 03:15 pm »
As I wrote on a different thread, I have Stax F83's driven by NYAL OTL3's.  I think the sound is glorious (and yes, including, maybe
especially, trumpets, despite the drop in impedance of these speakers at high frequencies).  I only buy equipment when necessary,
eg I bought Roger's great subwoofers because I was having serious bass distortion, and I recently replaced by 30 year old van den Hul cartridge with another van den Hul (just one step up).  If the transformers of the Stax's die (not unheard of, unfortunately), or if the amps die and can't be repaired, and if the DD amps aren't too pricy, then I would certainly consider them.  I wonder though if it is okay to have them at the other end of the room, with a  long speaker cable?  Also, it seems to me that it would be a good idea to have some sort of box to cover the speaker terminals.  Even with my Futterman's I once got a painful shock from the amps while music is playing when I touched the leads.  Couldn't the DD amp be fatal under such circumstances?   

The Direct drive amps should be relatively near the speakers as long cables will add capacitance, which is the one thing we want to minimize. To keep cost down I am planning a stereo version with 8 output tubes and 6 drive tubes on an RM-200 chassis. I plan to set mine between the speakers with a 6 foot run to each. The connectors are in-line and shock proof. The cables are covered with the typical plastic braid we see so often on speaker cables. Tentative price for a stereo amp is $4500. The monoblocs $8000 per pair.

I am also making monoblocks which I have set behind my speakers but it seems a shame to have them hidden when they could look so nice in front. No matter where my equipment rack sits, I always like to put the amp on a small stand between the speakers.

Roger A. Modjeski

Re: ESL Independence Day
« Reply #13 on: 6 Jul 2012, 04:00 pm »
I sold Acoustat and NYAL back in the 80's. I received a pair of the Acoustat Minitors with DD amps on trade. It was a night and day difference between the DD amps and the NYAL 3's, no contest. The DD amps blew the Futterman's away.

Thanks for report on an excellent comparison. Here are a few contributing factors. If played loud the Futtermans have not the current needed to drive Acoustats, especially ones 3 panels or larger. The impedance of any large, full range panel speaker is going to approach 1 or 2 ohms at high frequencies and Futterman amps put out nothing into 1 ohm nor do the tubes appreciate the task.

It is a long standing myth that OTL amps and ESL speakers were made for each other. This is true in one combination, the Futterman OTL and the KLH-9 which was a 16 ohm speaker that stayed pretty constant over the range. I have a copy of Julius Futterman's impedance measurements in my "Futterman File". He was certainly interested in driving this speaker as he measured its impedance at over 20 different frequencies. The advantage of his amp over others was it had lots of voltage which the KLH-9 needed. I am told that one of the classic listening tests was to compare the Futterman vs the Marantz 9 driving the KLH, particularly on Saturdays at Lyric Hi FI, NYC.

A Futterman can drive the QUADs or the Stax that airhead has but one has to be very careful not to exceed the maximum voltage which is 35 volts peak for the QUAD 57's and about 40 volts peak (100 watts from a 8 ohm amp) for the 63's. I do not know what the peak is for the Stax. I do know that most Futterman amps can put out 150 volts peak and therein lies the danger.

When Acoustat gave up on making their "Servo Amplifier" they went to a two transformer system, a system that is flawed in its very nature,. There is really no way to drive a single panel ESL with two transformers one being for the lows and one for the highs. One can split the band on the input side but they have to re-combine the bands on the output side and that causes the high frequency transformer to "see" enough of the low frequency information to cause saturation at higher levels and significant 3rd harmonic distortion at moderate levels. Although I don't have my measurements of the Acoustat Magnetic Interface handy, I do recall it is not easy to drive. It needs lots of voltage and lots of current.

Although rarely mentioned, the transformers in ESL speakers often eat up 25 to 50% of the drive energy due to their capacitance at the high end and saturation at the low end. When we eliminate them and the output transformer in a traditional amp and connect the tubes directly to the panels there is a great relief of work that the tube have to perform.

As you can see, from both a safety and sonic perspective, direct drive makes a lot of sense. When you can go directly to the panels, an ESL is actually easier to drive than a magnetic speaker.

OzarkTom

Re: ESL Independence Day
« Reply #14 on: 7 Jul 2012, 03:05 am »
I tried many amps back then, since I sold Acoustat. I even made it to the top 5 in sales in 1982-83. I was very proud of that.

The Acoustat twin Nova amp was actually one of the best to drive them, although it did not sound the best. I tried NYAL Futtermans, David Berning, Conrad Johnson, Qucksilver, Rowland, Threshold, Belles, Sumo...... the list was very long. The worst combination was the tube amps with output transformers. Just way to many transformers in the system, it sounded very sluggish and veiled.

But when I hooked those Monitor 3's with DD amps up, voila! About 3-4 layers of veil was lifted. And I was very surpised of the bass response, much deeper and faster.

With the $4500 price tag for the stereo amp and a used pair of Acoustats at about $500, this is a steal in today's market of ultra high-end audio. $8500 isn't too bad either.

steve f

  • Full Member
  • Posts: 682
Re: ESL Independence Day
« Reply #15 on: 8 Jul 2012, 12:41 pm »
What we need is a kit to accompany the new amp. Building ESL panels can be relatively easy. Sourcing parts definitely isn't. Instead of buying used antique Accoustat panels, with their rather thick and heavy panels & membranes, a simple panel can be designed with precut insulated perf-metal sheets, some copper foil, a suitable Mylar (the tricky part) diaphragm, and quality double stick tape. With the right supplies this should be a one day project. Especially if flat panels are desired. Roger's, amp would make all of the related transformers and bias supply, and their related headaches, unnecessary. If you desire lots of volume, a woofer and crossover would be required. Done right, it could be easier than assembling a backyard swing set.

steve

OzarkTom

Re: ESL Independence Day
« Reply #16 on: 8 Jul 2012, 01:33 pm »
What we need is a kit to accompany the new amp. Building ESL panels can be relatively easy. Sourcing parts definitely isn't. Instead of buying used antique Accoustat panels, with their rather thick and heavy panels & membranes, a simple panel can be designed with precut insulated perf-metal sheets, some copper foil, a suitable Mylar (the tricky part) diaphragm, and quality double stick tape. With the right supplies this should be a one day project. Especially if flat panels are desired. Roger's, amp would make all of the related transformers and bias supply, and their related headaches, unnecessary. If you desire lots of volume, a woofer and crossover would be required. Done right, it could be easier than assembling a backyard swing set.

steve

The Acoustat Monitor 3's and 4's played with a lot of bass and volume without any subs. As long as you used Acoustat's DD OTL tube amps and not the transformers.

steve f

  • Full Member
  • Posts: 682
Re: ESL Independence Day
« Reply #17 on: 8 Jul 2012, 02:48 pm »
Tom,
I remember them well. I recently, well last year anyway, heard a pair of "X's" that were still working after all these years.  I just wonder if rehab jobs of their other speakers are worth the effort. Materials today are better.
I'm also a member of the " hybrid is preferred" club.
Steve

OzarkTom

Re: ESL Independence Day
« Reply #18 on: 8 Jul 2012, 03:08 pm »

I'm also a member of the " hybrid is preferred" club.
Steve

Are you taling about hybrid amps, Steve? Of the Acoustats with transformers, the Moscode 300 and 600 amps sounded the best back then.

If I would run ESL's today that had transformers, the Moscode 401HR by George Kaye would probably be at the top of my list. I would still prefer DD amps no matter what ESL it was.

But you are probably talking about hybrid speaker system. I have always preferred full range. I have never heard a sub as quick as ESL's.

airhead

Re: ESL Independence Day
« Reply #19 on: 8 Jul 2012, 04:17 pm »
It took me a long time to accept the idea that I should use subwoofers with my Stax F83's.  They did have satisfying fast, tight, dry bass which could be impressive.  But after getting them fixed, I noticed there was some distortion with ultra low (not necessarily loud) notes, and decided probably the Futtermans weren't  deliver the current the speakers needed.  Therefore I got a pair of Roger's subs.  This solved the problem, but they sounded so different that I wasn't sure at first I liked them.  I eventually realized that, unlike the stax's one-note resonance induced bass, they were actually playing tunefully in the low registers, with a sort of bloom and musicality that had been lacking.   This is not like the big thumps that make people go "wow" when they hear most dynamic woofers, and, although not super loud, quite fast and realistic.  Nothing is perfect, but I think that, even apart from the distortion issue I was occasionally hearing, there is a big improvement.  Plus I feel much safer at higher volumes. 

By the way, the impedance of the Stax's ranges from 4.5 ohms at the frequency extremes to about 160 ohms in the midrange.  I don't think the Futtermans have any trouble at all with the 4.5 ohms at the highest frequencies.  They are very inefficient, and need lots of voltage, hence the Futtermans.  Probably its true that the AR D250  made  was the worst sounding combination among those I tried (years ago).