2 plus 2, Various, some questions

Read more on the Acoustat 2 plus 2 in our home audio section

Sam Kurutz2004-02-23 22:21

I\'ve had a few questions about my Acoustats that maybe y\'all can help me with.

1. My MK units don\'t have tubes in them, rather just two large Xformers (one for step up the other for power). Are these Medallion transformers? How can I tell.

2. I\'m embarrassed to say that my Cat used one of my 2+2s for a scratching post. I\'ve got some (not alot) of grille damage. What are my choices; Buying some grille yardage and sewing it myself, or does anyone have any recommendations??

3. Any good modifications I can make to get just a little more top end extension > 10Khz would be much appreciated. I guess I got spoiled with the Apogee Ribbon.

4. My Acoustats boast that they are internally wired with Monster Cable. At the time it was the \'ticket\'. Nowadays... there is alot better stuff. I cracked the MK units open and removed the 9” of MC (which was pretty badly oxidized - green). I replaced the wires with some Modern Audio Design 10 Gauge silver speaker wire run in parallel with some Audioquest F-14. This gave me a minute improvement in high frequency extension, but I\'m hoping for more. M.A.D. has some real nice High Voltage Teflon silver wire that would work real well wiring the panels. I\'m hoping to get some opinions on rewiring these guys. Have anyone in the ESL forum done this type of mod?? Thanks in advance for all your help.


Sean Bowes2004-02-23 22:21

Sean Bowes provided the following answer (July 2000)

1. Might be if the unit is a late model.


3. You need the late model 7 wire panels to get better highs. See the older model Acoustats used a 5 wire panel. But in later years they changed to a 7 wire panel. Meaning 7 wires on the front stator and 7 on the back. What this did was allow them to isolate the sound better for bi-amping. My 3+3 are like this. Although I have ripped out the medallian transformers and put the Servo Amps on them. If you want a pair of OTL or Servo amps built from the ground up. Give Danny Fanny a call from American HyBrid Technologies. 609-599-3828 He is a known figure on the East and West Coast for high end Acoustat system, and a good friend of mine. He is the one who showed me the Cap Pack design too, and a heap of other designs he has. He runs a set of 6\'s which are about 15 feet high and 4 feet wide, on custom tube amps which toss out 7-8kv and a Goldmund reference. I am amazed at the system, which cost heaps and sounds better then a million dollar system

4. The panels you have are 2+2 and I am assuming that they are the late models which had the medallian transformers. Although some say that these are the finest setups out - I do not agree. The Servo Amp or OTL, was the best setup. The problem was that the tubes used came from a Black and White Television, they used the horizontal output stage tube. Why in hell would you do that is something I often wonder. The medallians have limited range and some medallians like those on the 3+3 or X\'s could be biamped. I have a set of these actually.
I have seen some Acoustat that look like Acoustats, that is the panels, but the frame looks like someone made them in their garage. So look closely, if they look unprofessional, then they probably were re-built at some point. People do this since the panels sometimes blow - although this has never happened to me. In regards to your question. Yes and no. I use a Silver Micro wire. It is flat ribbon cable wire that has silver coated on the wire. That wire is used in high tolerance systems, like the Space Shuttle in NASA. I do not think you are going to get much out. See you are using the old poor quality caps and parts in your output stage. So start there. You can get more sound out of those panels if you start at the panel and work back.

Here are some ideas.

1. Replace all the old caps with some high end Vishay caps. Get the low tolerance caps, the king they use in Space equipment. It will pump, you will see the difference. Do not let anyone kid you, when you use high end parts - it sounds much better.

2. Consider using a \'Cap pack\' with high end parts. What I mean is build a set of 20 caps to hold massive amounts of joules of power. This will boost the panel when it needs power. Also since the medallians or any transformer system for that matter, the power is cut in the wave cycle. I hear alot of guys who love transformers. But what happens is when you start to drive, the power cycle swings and you do not get continuous power like a tube amp. One way to get around that is with a power Cap Pack. I have a set here that I use to give the panel more power to push bass when it needs it. That transformer system is not going to be able to recover fast enough and the action on a Cap Pack will show you that.

3. Consider re-designing the board. The old boards leaked badly, the traces where too tight. You might consider building a new board which covers the traces and spaces them out better. An example of this is in the AudioStatic ESL 100 board. The traces are too damn close. The power leaks across the board and you loose power.

John2004-02-23 22:21

Where is the ”cap pack located? On the secondary side of transformers? Voltage rating?

Andy Szabo2004-02-23 22:21

Servocharge Amplifier vs. Transformer Interface

Only early model Acoustats used a vacuum-tube amplifier. You have what Acoustat called a ”transformer interface”, allowing the use of any high quality amplifier. To determine if you have the upgraded Medallion Transformers, please see my Technical Bulletin ”Acoustat Medallion Transformer Modification”.

Yes, there are some audiophiles who still swear that nothing drives the Acoustat panel better than the Servocharge Amplifier. This is matter of opinion, of course, and the amplifier and transformer do sound different. What is a fact, though, is that the transformer interface is much more reliable, and offers greater flexibility in choice of amplification.

Grille Cloth Damage

The grille cloth can be removed and cleaned or replaced. See my Technical Bulletin Grille Cloth Removal, Cleaning and Replacement for further information. The section Available Parts also lists a source for new grille cloths.

Top End Response/Wires per Panel

Acoustat never had any panels with seven wires per panel. Very early Acoustats had three wires leading from each individual panel: 1 bias wire, 1 wire connected to one end the front stator grid, and 1 wire connected to one end the rear stator grid.

Later models had five wires per individual panel, since both ends of the front and rear stator grids were connected: 1 bias wire, 2 wires connected to the front stator grid, and two wires connected to the rear stator grid. As far as I know, the number of wires had little or nothing to do with high frequency response.

Even for the Spectra Series, where the stator grid was electrically divided, only five wires per individual panel were used: 1 bias wire, 1 front left stator, 1 front right stator, 1 rear left stator, and 1 rear right stator.

Admittedly, the Acoustat could be somewhat limited at the very top end. Have you tried adjusting the High Frequency balance control on the interface?


It is not clear where in the circuit the ”Cap Pack” capacitors are to be added. What I am sure of is this: if such capacitors are added anywhere in the audio circuit, only bad sound and/or damage will result.

I think the intention is for these capacitors to be added to the bias power supply. I have heard many such theories, and after numerous discussions with Jim Strickland (The Father of Acoustat), he and I both agreed that all such bias supply modifications are a bad idea.

Most of these suggested modifications indicate a misunderstanding of the function of the bias power supply. (See ”Technology of Full Range Electrostatic Loudspeakers by James Strickland” posted elsewhere on this site.) The bias power supply creates a constant charge (not a constant voltage) on the diaphragm: large musical peaks do not require a sudden rush of current from the bias supply.

If capacitors are added to the output (after the 500 megohm resistor) the constant-charge principal will be violated, and distortion will result. If they are added before the 500 megohm resistor, it is true that the supply will theoretically have more ”muscle”. However, that extra ”horsepower” never gets put to use because of the long time constant of the 500 megohm resistor and the panels\' capacitance.

Another bad thing about these modifications is that they can kill you. The Acoustat power supply puts out 5000 volts, but at a very low current. If you touch it, it will hurt, but you\'ll live. If you connect massive amounts of capacitance, that extra current reserve is enough to kill you. Just don\'t do it.

I have also seen suggestions to completely bypass the 500 megohm resistor. This is also a bad idea, as it violates the constant charge principal, and results in distortion.

I would suggest NO modifications be made to the bias power supply. I think Jim Strickland did it right the first time. His supply creates the constant charge required by the theory, and it does it in an effective and safe manner.

Some sonic improvement may be had by replacing the capacitors in the audio circuit. Replacing the electrolytic capacitors in the primary (low voltage) side with polypropylene or other film types would be the easiest and most rewarding change possible. Replacing the high voltage capacitors with premium types is also a possibility, although they can be difficult to obtain (6000 volt minimum rating). If your interfaces are Medallion (either modified or original), these high voltage capacitors are already a high quality polypropylene type.

I would suggest against any attempts at changing the wires leading to the panels. This is a special high voltage wire that has been 100% tested for 10,000 volt breakdown. Changing the wires inside the panels, is, of course, impossible, since the panel is a sealed assembly. Any attempts at opening it would ruin the panel.

Post a reply

Your name will appear on the website next to your contribution. Your email address will only be used to contact you if something is wrong with your contribution. It will not be shared with others.