I don't see any major problems in running the two transformers in series, except perhaps its effect on damping factor. That is, each transformer is normally directly connected to the amplifier, but in series mode, you have another transformer in series which will effectively halve the damping factor. But, since you are using an OTL amplifier, which typically have a high output impedance, your damping factor is zilch already, so perhaps this is not an issue for you. Me, I'm a card-carrying member of the High Damping Factor Club.
I would not expect any changes to be required of the RC input network. The 1-ohm resistor in series is there to prevent the impedance from going below 1-ohm at very low frequencies. The capacitors in parallel with the resistor bypass the resistor at higher frequencies.
The Acoustat C-Mod was NOT just a change in capacitor value. The high-pass filter feeding the HF transformer changes from a series network to a series/parallel network, resulting in a smoother roll-in and less chance of core saturation. Merely changing the capacitor value will NOT give you the C-Mod, and doing so is not recommended. The resistor values and network configuration must also be changed.
While it is true that the LF transformer does not have a low-pass filter associated with it, it does not really operate full-range due to its natural roll-off.
Schematics and instructions for the C-Mod are available from www.audiocircuit.com. Note that doing the modification is beneficial for both original and Medallion transformers: they are independent changes. The parts required are commonly available, and the instructions provide several options for creating the new HF Balance Control. The 16-ohm slider resistor required for interfaces so equipped may still be available from eBay seller Soundvalves, who bought most of the obsolete Acoustat parts inventory from Rockford. If not still available, the instructions provide a way around that.
One nice thing about the C-Mod is that the 47-uF capacitor is much easier to upgrade to a film-type, as compared to the original 220-uF (huge AND expensive!)
If you can't find the above referenced documents, I can post them directly here.
I might suggest two options, either or both of which may ease the difficulty of mounting the 47-uF film capacitor.
The first would be to find a capacitor with a lower voltage rating, which would likely be smaller in size. You could also use two smaller values in parallel, which might open-up further mounting possibilities.
The second would be to use self-adhesive pads designed for securing cable-ties. Find a spot on the rear surface of the chassis, mount the tab, run the cable-tie through it, and secure the capacitor to that. Using the self-adhesive pad avoids any need for drilling holes in the chassis. I have used a similar technique in my Spectra's (which admittedly have a little more room available).
That is a great idea Andy about the cable tie mounts. There are mounts like that that also only need one screw if adhesive was not enough.
Here is a photo of my interface that someone else had done.
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When I made my Spandex socks I ended up with the fabric being half the circumference of the speaker, so it ended up being stretched to twice its size. This yields a semi-transparent look where you can just make out the grids of the panels if you look closely. Under normal viewing they just look like blue monoliths. They are far more acoustically open than the stock socks, though; the highs are much clearer this way. I'd never consider going back to the old socks, both in looks and sound.
Can't find actual C-Mod on The Audio Circuit
I am almost never able to find *anything* on AudioCircuit because there are no cross-links. If I pull up the Acoustat page, there is no "modification" tab on the right (which seems implied by a number of posts, I know TAC itself has gone through many changes...not long ago it wouldn't even work on Macs). If I search for "C-Mod" I never find the actual C-Mod, I simply find endless comments like this:
"The C modification is easy to do... Most of the information is posted on this site... [Useless or outdated instructions for finding C-Mod information under "Modifications" (and where is "Modifications", you can't navigate to that from forum, etc.)]
**** Update. I think I found it here:
AudioCircuit http://www.audiocircuit.com/A-PDF/AA-Brands/A/Acoustat-ACO/MK-121-2_MED________-M-EN2-941-ACO.pdf
I am almost never able to find *anything* on AudioCircuit because there are no cross-links. If I pull up the Acoustat page, there is no "modification" tab on the right (which seems implied by a number of posts, I know TAC itself has gone through many changes...not long ago it wouldn't even work on Macs). If I search for "C-Mod" I never find the actual C-Mod, I simply find endless comments like this:
"The C modification is easy to do... Most of the information is posted on this site... [Useless or outdated instructions for finding C-Mod information under "Modifications" (and where is "Modifications", you can't navigate to that from forum, etc.)]
**** Update. I think I found it here:
AudioCircuit http://www.audiocircuit.com/A-PDF/AA-Brands/A/Acoustat-ACO/MK-121-2_MED________-M-EN2-941-ACO.pdf
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Navigating The Audio Circuit
don't start with SEARCH. Instead, starting at TAC home page, click on "Home Audio Section" which shows list of manufacturers, then click on Acoustat, then scroll down and down on page to "Refurbishing and Modifications", then subheading Andy Szabo's Technical Bulletins, then subsubheading modifications, and the last one is MK-121-2(A) Medallion "C" Modification.
don't start with SEARCH. Instead, starting at TAC home page, click on "Home Audio Section" which shows list of manufacturers, then click on Acoustat, then scroll down and down on page to "Refurbishing and Modifications", then subheading Andy Szabo's Technical Bulletins, then subsubheading modifications, and the last one is MK-121-2(A) Medallion "C" Modification.
For those who read schematics better than they read words, here is another set of great circuit diagrams from Andy depicting what he just posted in words.
Attachments
Low Power Amps on Acoustat
I have not tried First Watt amps. I notice that they are spec'd at 25W at 8 ohms and 40W at 4 ohms. That doesn't sound adequate except for chamber music, etc.
When I first got 1+1's, I borrowed a Parasound HCA-1000A from my bedroom system. That amp got incredibly hot and even shut down while playing orchestral music. That's 125W/8 ohms and 200W/4 ohms.
The Acoustat amp has 200W/8 ohms and 300W/4 ohms. That's a sort of benchmark for what the speakers require, of course, though it's also said that beyond mere power ratings, it's designed to be well suited to the electrostatic load (which tends to reflect energy back into the amp).
I've had no trouble with Parasound HCA-1500A which has 205W/8 and 300/4, similar ratings to Acoustat amp.
There are a number of Pass Labs amps which look suitable...
I have not tried First Watt amps. I notice that they are spec'd at 25W at 8 ohms and 40W at 4 ohms. That doesn't sound adequate except for chamber music, etc.
When I first got 1+1's, I borrowed a Parasound HCA-1000A from my bedroom system. That amp got incredibly hot and even shut down while playing orchestral music. That's 125W/8 ohms and 200W/4 ohms.
The Acoustat amp has 200W/8 ohms and 300W/4 ohms. That's a sort of benchmark for what the speakers require, of course, though it's also said that beyond mere power ratings, it's designed to be well suited to the electrostatic load (which tends to reflect energy back into the amp).
I've had no trouble with Parasound HCA-1500A which has 205W/8 and 300/4, similar ratings to Acoustat amp.
There are a number of Pass Labs amps which look suitable...
It was always Acoustat's official stance (as well as my own personal opinion) to recommend AGAINST using ANY brand of amplifier in bridged-mode for Acoustat speakers. When you bridge an amplifier, each half of the amplifier 'sees' only half the load impedance. Since the Acoustat is already a tough load, you do not want to make it twice as bad.
In the case of driving Acoustats (or other difficult-load speakers), if you need more power, a larger amplifier is the only solution.
Hi my name is Andre MrAcoustat Sevigny i have been with Acoustat's since the early 80s i have owned 2+2s - Spectra 2200s - Spectra 3300s - and three pairs of my favorites 1+1s the one's i own now where modified by Jocelyn Jeanson from Quebec Canada he is very well known in the Acoustat circles they can be seen on Flickr MrAcoustat Photostream they are all metal frame with modifications to the panels and interfaces they weigh 200 pounds each the only thing left to change is the 4 resistors i am waiting for my MS322 50K 22W 1% Caddocks resistors then it should be over ( for now ) but with Jocelyn you NEVER know he's our Canadian ( Roy Esposito ) when he tells me to jump i ask him HOW HIGH.
Thanks for the reply to the bridged amp question. The F-4 I asked because I understood that they can be built to produce up to 150 watts/channel when you bias them high enough. I had figured the standard amp at <50 watts was not a good candidate. But my technical knowledge is very lacking, so I may have misunderstood what I read at the Pass threads about the F-4 amp.
I had used a classdaudio sds-254 and had thought of buying another so I could bridge each for more power, but after the recommendation above I will abandon that route.
Thanks again for the very helpful advice/comments.
I had used a classdaudio sds-254 and had thought of buying another so I could bridge each for more power, but after the recommendation above I will abandon that route.
Thanks again for the very helpful advice/comments.
HELP ANSWERMAN
I have monitor 3's with servo amps-one of my kids kicked the cable and it wont work- I changed tubes and still doesnt work--where can I get the amps updated and repaired--Thanks
ahhkundun@hotmail.com
Matthew
I have monitor 3's with servo amps-one of my kids kicked the cable and it wont work- I changed tubes and still doesnt work--where can I get the amps updated and repaired--Thanks
ahhkundun@hotmail.com
Matthew
I agree, Mike Savuto is THE man to repair/upgrade your servocharge amplfiers.