Acoustat servo amps ??

I'm looking for any info on acoustat servo amps for monitor III speakers. right now i have the transformers (not the medallion) and love them w/ my arc vt100mkII. however i keep hearing that the servos will bring these to a new level.

i'm debating on whether or not to go for the servos and sell my arc.

mostly i listen to jazz 50% lp, 50% cd and am using the speakers with a vandersteen 2wq sub.

anyone have thoughts? is it worth it? how much can i expect to pay for the servos? where should i look? should they be upgraded and how? who works on them...etc.

how about a passive pre w/ these? recommended?

im also interested in thoughts on the model IIIs compared to the other models... 4s, the 2+2s, X, spectra 1100, 22, etc.

any info is appreciated.

thanks in advance
I owned the "monitor III" speakers with their "servo-charge" amps for 18 years, having purchased them in 1979. The "model III" succeeded Acoustat's attempts at selling a tube-powered speaker. Acoustat offered a couple upgrades over the years to the amps that involved replacing capacitors, I believe. They definitely helped on the low end and with clarity. Joe Curcio offered a $2000 mod to the amps that involved replacing part of the diode circuit with tubes. The power tubes were 6hb5's, which are unique to this amp, so may be getting a bit hard to find. I sold my speakers a few years back for about $650. If you're lucky you might find someone selling them locally. They were a great pair of speakers, but had their own issues and quirks.
Kubla, if you send me your phone number I'll give you a call and share my 23 years of experience with Acoustats. I am a former dealer who sold monitor IIIs and have had 2+2s for the past ten years. Just bought another pair of 2+2s this week (these have the Medallion interfaces) and haven't even uncrated them as yet. Don
If you want to hear Accoustats at their best, the servo amps are the way. One of the best stat systems I have heard over the years was a pair of Accoustat 2+2's with servos. Unfortunately, the servos are hard to find and expensive when you do find them these days. There is a cult for these amps, so most of them have been hunted down and are in the hands of people who know their worth. I have a friend with a pair but he won't part with 'em...just in case he gets "Accoustat Fever" again. Happy Hunting!
Well, yes, the servos can be great, but there are MANY different mods and levels of servos -- the best (and most lethal, literally) in my experience, are the AHT/Dan Fanny modified units that use the 6LB6 tubes and dual 7308 input drivers. Please note that high-voltage parts are sometimes hard to locate and these amps do require regular periodic maintenance to ensure top performance and to replace certain components BEFORE they disintegrate.

One advantage to the 6LB6 servos is that they can drive up to 8 panels per amp -- so if you own Acoustat Sixes or Eights, you don't need 2 amplifiers per speaker; one will do nicely.

Of the Acoustats I have owned the ones that I liked the best were 2+2s and IIIs. The Sixes are great with the AHT-modified servos if your listening room is VERY large (mine wans't big enough). They are capable of playing unbelieveably loud, with tremendous bass. A few of my friends own these systems/amps and are quite happy. :)

I have since moved on to the InnerSound Eros because it plays loudly and precisely, is smaller, and I no longer have to worry about dealing with the attention the servos require.
thanks for the posts everyone.

plato et al: i have found a pair of monitor 4s w/ the servo amps ($1600) and was wondering if anyone was able to compare what these might sound like (i'll drive to hear them next week if it seems promising) compared to my model 3s with the transformers or model 3s potentially 'upgraded' with separate servo amps i might find.

If the servos on the IVs are stock and the ESL panels are early generation vintage, then factor in the operational condition of the amps and tubes and I'd say you have a heck of a lot of variables and a wide performance span. Heck, the only way to sort that out is to go have a listen.

What vintage are your IIIs, anyway? --original coarse 3-wire, OFC-copper 3-wire with improved coating, or 5-wire version?

I will say that if the IVs are early vintage and the amps are stock then $1600 is probably top-dollar -- and that provided they are in good working order. You may be much better off picking up a good pair of updated/modified servos and using your present IIIs if they are at least the OFC-version panels.
BTW, Kubla, I'm not certain that you can use the servos correctly with Spectra-series panels, plus I'm not sure that Spectra panels are actually as good as the late 5-wire Acoustat panels.

Additionally, the servos come with different EQ curves depending on whether you are using stacked panels (e.g., 2+2s, 1+1s, or 6s) or non-stacked (1s, 2s, 3s). The bass EQ is increased to drive the taller models, I believe.

A passive preamp will drive stock servos OK, but then, the stock servos have adjustable input pots. Some modified servos have an input driver stage (and volume pot) removed and need a lot of input voltage -- these would not work well with passives but would sound better because the suboptimal active gain stage is deleted.
hey plato thanks again for all the great info.

i'm not sure the vintage of my model IIIs. they were bought new by my father in '81 or '82 i believe but they may have been store demos and older than that however. receipts are long gone unfortunately (i hooked up with a guy at rockford corp who could have replaced the panels for free if i had it. crap.) how can i check to find out which version i have? -these aren't the medallion btw and have the 'internally wired by monster cable' sticker on the back if that provides a clue to their year.

that same guy at rockford can do the medallion upgrades ($700 including shipping) and upgrade the caps and wire if i supply him with the parts (i figure about $3-400 including cardas posts, etc. that option is what im debating against the servos. he recommended infinicaps and solens (47uf) but i've been told by an acoustat freak NOT to go with the infinicaps for this application because they sound thin. someone once told me they personally only use black gate in their acoustats but i cant seem to find any w/ sufficient voltage.

thanks again.
My luck has been with the model X. I like the sound and value so much, I bought a second pair. Besides upgrading the tube recipticals, I have left both pair in original condition. Some time back, returning home, one amp was making a terible noise. The Antique Radio Store fixed me up. But now, I feel better killing the power when they are not being used. Also I found out that they must be grounded. I was using one of those three to two plug adapters. Doing this lets the groung charge build up. My cloth was worn on one pair of the X's. No luck finding the same cloth until I bought a pair of Klipschorns. Noticed the cloth is a match and still avalible from Klipsch.
waveriding, thanks for the klipschorn idea. i'll probably do that. there is a company still making acoustat socks for virtually any model but they dont have a great selection of colors. For your records just in case you ever get a cat keep this handy: "The Upholstery Co" in mesa arizona 480-898-9521
I think Plato may be a little confused, or I do not understand what he is saying. Spectra's include 33's and 3300's, Mdl. 3's are not spectras. I have Mdl. 3's and 3300's. I am using a slightly modified Moscode 600 amp to drive them. I intend to upgrade it to a 500 watt a side amp, and believe that will finally satisfy the Acoustats. If you have a contact at Rockford who can get transformers, I am considering bi-amping, but would need 4 more Spectra transformers to do so....


Dennis, a mint pair of Monitor 4's sell for $550.00 to $650.00, look elegant, and will sound similar to your 3's. That means the servo amps are going for about $1000. Not bad if they are also mint, but not a bargain. If you can pry the amps away without the panels for about $700-$800, that's a bargain. Obtaining 6HB5's is not a problem, Michael Percy and Caddock will have the other parts you'll need.Ed

To determine which panels you have you need to look at the wires coming off the panels. If they are the very early panels, the stranded wire is relatively coarse and silver in color. If you have the OFC panels the wire will be fine copper stranded. The 5-wire panels use the same type of OFC stranded copper wire, but obviously there are 5 wires per panel instead of 3.

Lou, I never said that the model 3s were Spectras, so perhaps you are confused. BTW, The Spectra panels are the same as the late 5-wire non-spectra panels except that they have high-voltage resistors wired in to limit the high frequency reproduction to a more narrow section of the panel. In theory this seems like a plausible approach but I'm not sure how great it worked in actual practice. Some say the late non-Spectra panels are more transparent because the signal isn't being routed through resistors. So it would seem there is a trade off between transparency and dispersion at play.
guys thanks for all your help.

for the record and future searches on acoustat here are some good contacts:

for upgrades to the acoustat servo amps email mike at analogue associates: upgrades are all they do! i spoke to mike today and he seems like a great guy and i can tell this is a labor of love for him. he's an accountant by day.

for repairs on stock servos try 'sounds like new' at 561-736-0041

I'm thoe echthited!
If anyone needs it, the former owner of Acoustat (Jim Strickland) works at the Rockford tech. department in Phoenix, AZ. The phone number I have on file is (800)366-3565. By the way, the lady at "The Upholstery Company" in Mesa, AZ. used to make the socks for Acoustat back when they were in business in Florida. She just made me a new set of black ones for my 1+1's.