diyAudio (
-   Planars & Exotics (
-   -   Acoustat Servo rebuild/mod questions (

hella356 2nd February 2013 02:34 AM

Acoustat Servo rebuild/mod questions
Iím planning to set up an eight-panel (4+4) Acoustat speaker with two amps per channel in balanced mode (amp #1 driving front stators, amp #2 driving rear) with a lot of Mike Savutoís parts. Iím thinking of ditching the Acoustat casing and separating the bias from the audio. Ideally, with a three box solution per channel: front amp, rear amp, bias supply. I'd really rather do away with the replacement costs and heat of vacuum tubes, if this is possible. Iím no expert on schematics, so I may be off base about whether the tubes are integral to the audio circuit or could be replaced with SS gear. Iím wondering if the amp portion of the Servos could be retained, while using the basic concept of the bias supply from the Acoustat transformer interfaces. Not sure if the bias supply would need to be beefed up to charge eight panels; the interfaces donít seem to mind if thereís 2, 3, or 4 panels connected, maybe I could just use the existing parts from my interfaces. The other option would be to go with Mike Savutoís God Mod, which, despite the replacement costs and heat of tubes, is a well-established way to go. But, it would be great if the great qualities of the Servos could be had without the tubes.

I have also been mulling the thought of an easy way to dial in the EQ and wall effect portions of the amp to suit my room. Maybe install switches connecting different values of caps, resistors, etc., to lock in the best values. I think if I can nail that down, I could eliminate the high frequency control from the circuit, as well. Iím also planning on eliminating the turn-on circuitry. Which leads to...this may all be better facilitated by new circuit boards with these changes (and any others that Savutoís mods could drive) accounted for, or point-to-point wiring.

I admit I'm probably misunderstanding the design, but hopefully you guys have some thoughts. Thanks!

djmiddelkoop 6th February 2013 11:53 AM

First of all, I do recommend to use another separate bias supply iso the standard 500Mohm resistor coming from the HT+ inside the Servo.
Why don’t you keep it simple and for a start just use each Servo to drive 2 panels, in the way it was designed.
The Servo amps are balanced already.
If you are no expert, be carefull, the voltages are very deadly !
Depending on placement in your room, you can try to eliminate the low frequency EQ, but the high frequency control is needed.
There are other tubes that can be used, but SS is very difficult to get reliable, tubes are easier for this job.
It is true that using a tube circuit to drive the output tubes iso the SS circuit inside the servo will sound a lot better.

Where can I find the Mike Savuto mods ? I googled, but nothing found.


hella356 7th February 2013 12:42 AM

1 Attachment(s)
Thanks for the input, Dick. My ultimate goal is to utilize the "balanced" Servo method Kevin Ferry came up with and has worked with Mike Savuto to develop. This features common-mode noise rejection by separating the signals to the front stator amp & a rear stator amp. Since everyone I know of who's tried Servo amps with more than four panels per side thinks it's essentially mandatory to use two Servos per channel to drive eight panels per side, I figure I should take it all the way to this "balanced" setup. This has led me to rethink the physical layout of the amps and the possibility of separating the bias. After more perusal of the schematic, it does seem to me that the tubes really are part of the audio portion, and wouldn't be easy to replace with a complete SS solution. That's not the end of the world for me; if the heat & cost of tubes are what it takes, I can live with it.

My thinking regarding the turn-on, EQ, HF, etc. portions of the amp is (if at all possible) to remove parts of the circuit which might not be needed in a one-off design, and to alter the circuit (where appropriate to my needs) as opposed to the more "one-size-fits-all" approach needed for a commercial design. I know Acoustat included the HF & Gain controls to help dial them in to whatever room the speakers may be installed in, but my thinking is to mod the amps (along with new speaker frames) to work in my specific room, obviating the need for such controls. Again, not sure if this is a realistic (or even worthwhile) goal, but it seems to make sense to me. :) Basically, the full-range, Servo-driven Acoustats sing to me in a way no other speaker does, so I'm attempting to take the concept as far as I can, trying to improve upon it wherever possible.

Attached is the most recent list of mods that I have from Analogue Associates, Mike Savuto's company.

djmiddelkoop 7th February 2013 07:35 AM

Thanks for the list from AA.
Iím very familiar with these amps, these mods are recommended indeed, however I have upgraded several Servo Charged amps with a tube circuit on a separate PCB which bypasses all the SS and other mediocre parts.
You are right in driving not more than 4 panels with one Servo charge amp, this is already a tough load for the amp.
Any good designed and built balanced amp has common-mode noise rejection. Separating the balanced signal in 2 separate amps to only drive 1 stator each doesnít make any sense to me, beside that you loose the properties of the balanced amp. Or am I misunderstanding the method of Kevin Ferry ?


hella356 10th February 2013 04:46 PM

I'm certainly no expert, but my understanding is that having the + and - audio signals going directly to front & back stators more or less yields a balanced signal at the speaker level, as opposed to the amp level. I've never heard a system set up this way, just going by what I've been told; that it makes a worthwhile positive improvement in performance.

I'm intrigued by the idea of a full tube solution, although that takes the heat/tube replacement issues even further. My thinking has been to go with the tried & true AA way of doing things, with the option of trying a tube circuit after getting to that level. How would you describe the difference in sound between the SS & tube circuits? Do you have a schematic? Thanks!

djmiddelkoop 28th February 2013 11:27 AM

Most ESLs are driven with opposing signals on their stators, ie balanced. This can be done with a special step-up transformer or with an amplifier.
Such an amplifier must be outputting a balanced signal driving the front and back stators, just like the Acoustat servo amp does.
Hope this explanation helps you, as what you describe, if I understand correctly, is just the way these amps work already.
There are more amps out there doing the same thing, like the Sanders ESL amp (Audio Amateur 1/1976) so no rocket science here.

I cannot disclose schematics, sorry.

gorko1 28th February 2013 12:59 PM


Originally Posted by djmiddelkoop (
Iím very familiar with these amps, these mods are recommended indeed, however I have upgraded several Servo Charged amps with a tube circuit on a separate PCB which bypasses all the SS and other mediocre parts.

Something like this? :) Acoustat OTL Amp - Tube Upgrade


djmiddelkoop 1st March 2013 07:46 AM

exactly !

djmiddelkoop 1st March 2013 01:27 PM

And of course I forgot to mention the Shackman OTL amp :
ESL Tube Amps

pfl1946 23rd March 2014 05:25 PM

The best way to get in touch with Mike Savuto is to email him at

All times are GMT. The time now is 02:20 AM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 18.75%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright ©1999-2017 diyAudio