Hiya everyone.
Fascinating thread here. Says a lot about the product itself.
A short intro: I'm a former EE. Cut my teeth in the audio electronics world, and eventually ended up in aerospace electronics. I own audio gear because I like to listen to music, not because I like to fool around incessantly with audio gear. I laugh my butt off at people who think power cords make any difference. I am in 100% agreement with Ken Rockwell's hilarious essay on audiophiles.
That said, I'm the proud owner of a pair of Monitor 3 speakers, purchased around 1980 -- long enough ago that I no longer remember exactly when it was. They came with the Servo Charge amps. After living with them for a year or so, I decided that I couldn't live with the lack of low bass (64' and 32' pedal ranks, ya know?) and added a pair of Velodyne ULD-15 subs, which were recent arrivals to the marketplace at the time.
So time marches onward and I get increasingly dissatisfied with the SC amps, for the incessant tube problems (arcing, leaking, loss of gain...) and, frankly, the suboptimal sound of the amps themselves. So as luck had it, I managed to unload those miserable amps to some poor ("tubes are always better......") sucker and managed to get a set of the MK-121-C interfaces, which turned out to be among the last manufactured. At the same time, I bought a pair of B&K Sonata M-200 monoblocks to drive them. The amps, as far as I'm concerned were a perfect choice as they are capable of reaching nearly deafening SPLs without a hint of complaint, but without signal are dead quiet. Yes, I know that they're not Krell Bazooka-47000s et.al., but I'm totally okay with that. The proof is in the pudding, and the results are eminently satisfying. The MK-121-C and M-200 sounds a LOT better than the SC amp ever did on its best day.
At the same time, I decided that placing those interface units (and, actually the same thing applies to the SC amps) on the base of the speakers was a Really Bad Idea. While the panels themselves are made to live with broad-spectrum vibrations from audio reproduction, those boxes of electronics clearly were NOT. And, after reading this huge thread, I'm more convinced than ever that remoting the interfaces was a Really Good Idea, since there has never once been any kind of hiccup in this system's 33+ years of operation. (Actually, the more I thought about it, locating those SC amps on the base of the panels was a really poor design choice, because of the vibration environment. Tubes like the 6HB5 were NEVER intended to operate while being shaken. You have to wonder how much of the poor performance and reliability of those amps could have been avoided simply by locating them elsewhere.)
Anyway, I decided to locate the M-200s and Interfaces on a shelf in the basement directly under the speakers. It's cool and dry there all year round. And since they're mounted on a shelf attached to the house foundation, there's absolutely no chance for vibration. Given that I've occasionally abused the daylights out of those things (there's a small bag of spare 5A Slo-Blos taped to one of them) and, with the possible exception of the high-pass caps, they're still performing like new. I made a minor mod to the Interfaces by putting a clear plexiglass panel on the back to hold three banana receptacles. Wires to the upstairs panels simply plug into those. I used 18ga copper-conductor neoprene-insulated high-performance spark plug wire to carry the bias and two phases through holes in the floor to the panels. Have never had a single issue with this!
Then in 2005 the first ULD-15 speaker blew out due to rotten foam. Not being able to find anyone who was willing to take on the refoaming job, I gave it a shot (nothing to lose!) myself and got the expected mediocre results. So I got rid of those and replaced them with a pair of HGS-15X that cross over at 100Hz.
Finally, my most recent change is to go all-digital in the front end, by converting my entire library to flacs and using a Cambridge Audio DacMagic+ as the only source. And, in acknowledgement of the declining sensitivity of my hearing above 12KHz, I've added a pair of tweeters crossed-over at 9.5KHz.
Anyway, it's really great to see the activity around these incredible speakers. At the time I bought them, I probably didn't know that I was buying a legend-to-be. I picked them because they just sounded better than anything else in the showroom, including stuff that cost a lot more.
I was pleased to see earlier in this thread that someone else has undertaken to replace the old non-polar electrolytic caps in the interfaces with a nice fat film cap. My caps for that purpose are sitting in a box in the basement awaiting installation. Not surprisingly, the previous poster and I ran into the same issue -- where to put those huge things. In retrospect, it probably would have been wiser to split the 56uF into three or four smaller caps, which I may still do. Electrically, it would probably be a better choice because of dividing the current among more caps with lower net ESR and lower self-inductance. Not to mention that smaller caps would probably be a lot easier to mount.
Finally, I have a question that maybe Andy can shed some light on: The SC amps had a "wall-effect" phase altering all-pass filter in them which was active by default. The Interfaces, of course, do not. That seems a little logically inconsistent. If the wall-effect was important enough to include compensation circuitry in the amps, how was eliminating it in the Interfaces justified? I realize that as a practical matter, an equivalent filter would have been difficult and expensive to implement in the Interfaces. There's no doubt that the compensation had some effect, as I noticed immediately on firing up the Interfaces the first time that there was a definite difference in sound in the midrange. So what was the thought process around this issue? BTW, as an experiment, which I'm seriously considering making permanent, I put 8 sq ft of Sonex behind one speaker to reduce the "wall effect" and have concluded that the net result is a tangible improvement in the mids, perhaps similar to the wall-effect filter.
So that's about it for the moment. I'd appreciate hearing any comments or questions.
Fascinating thread here. Says a lot about the product itself.
A short intro: I'm a former EE. Cut my teeth in the audio electronics world, and eventually ended up in aerospace electronics. I own audio gear because I like to listen to music, not because I like to fool around incessantly with audio gear. I laugh my butt off at people who think power cords make any difference. I am in 100% agreement with Ken Rockwell's hilarious essay on audiophiles.
That said, I'm the proud owner of a pair of Monitor 3 speakers, purchased around 1980 -- long enough ago that I no longer remember exactly when it was. They came with the Servo Charge amps. After living with them for a year or so, I decided that I couldn't live with the lack of low bass (64' and 32' pedal ranks, ya know?) and added a pair of Velodyne ULD-15 subs, which were recent arrivals to the marketplace at the time.
So time marches onward and I get increasingly dissatisfied with the SC amps, for the incessant tube problems (arcing, leaking, loss of gain...) and, frankly, the suboptimal sound of the amps themselves. So as luck had it, I managed to unload those miserable amps to some poor ("tubes are always better......") sucker and managed to get a set of the MK-121-C interfaces, which turned out to be among the last manufactured. At the same time, I bought a pair of B&K Sonata M-200 monoblocks to drive them. The amps, as far as I'm concerned were a perfect choice as they are capable of reaching nearly deafening SPLs without a hint of complaint, but without signal are dead quiet. Yes, I know that they're not Krell Bazooka-47000s et.al., but I'm totally okay with that. The proof is in the pudding, and the results are eminently satisfying. The MK-121-C and M-200 sounds a LOT better than the SC amp ever did on its best day.
At the same time, I decided that placing those interface units (and, actually the same thing applies to the SC amps) on the base of the speakers was a Really Bad Idea. While the panels themselves are made to live with broad-spectrum vibrations from audio reproduction, those boxes of electronics clearly were NOT. And, after reading this huge thread, I'm more convinced than ever that remoting the interfaces was a Really Good Idea, since there has never once been any kind of hiccup in this system's 33+ years of operation. (Actually, the more I thought about it, locating those SC amps on the base of the panels was a really poor design choice, because of the vibration environment. Tubes like the 6HB5 were NEVER intended to operate while being shaken. You have to wonder how much of the poor performance and reliability of those amps could have been avoided simply by locating them elsewhere.)
Anyway, I decided to locate the M-200s and Interfaces on a shelf in the basement directly under the speakers. It's cool and dry there all year round. And since they're mounted on a shelf attached to the house foundation, there's absolutely no chance for vibration. Given that I've occasionally abused the daylights out of those things (there's a small bag of spare 5A Slo-Blos taped to one of them) and, with the possible exception of the high-pass caps, they're still performing like new. I made a minor mod to the Interfaces by putting a clear plexiglass panel on the back to hold three banana receptacles. Wires to the upstairs panels simply plug into those. I used 18ga copper-conductor neoprene-insulated high-performance spark plug wire to carry the bias and two phases through holes in the floor to the panels. Have never had a single issue with this!
Then in 2005 the first ULD-15 speaker blew out due to rotten foam. Not being able to find anyone who was willing to take on the refoaming job, I gave it a shot (nothing to lose!) myself and got the expected mediocre results. So I got rid of those and replaced them with a pair of HGS-15X that cross over at 100Hz.
Finally, my most recent change is to go all-digital in the front end, by converting my entire library to flacs and using a Cambridge Audio DacMagic+ as the only source. And, in acknowledgement of the declining sensitivity of my hearing above 12KHz, I've added a pair of tweeters crossed-over at 9.5KHz.
Anyway, it's really great to see the activity around these incredible speakers. At the time I bought them, I probably didn't know that I was buying a legend-to-be. I picked them because they just sounded better than anything else in the showroom, including stuff that cost a lot more.
I was pleased to see earlier in this thread that someone else has undertaken to replace the old non-polar electrolytic caps in the interfaces with a nice fat film cap. My caps for that purpose are sitting in a box in the basement awaiting installation. Not surprisingly, the previous poster and I ran into the same issue -- where to put those huge things. In retrospect, it probably would have been wiser to split the 56uF into three or four smaller caps, which I may still do. Electrically, it would probably be a better choice because of dividing the current among more caps with lower net ESR and lower self-inductance. Not to mention that smaller caps would probably be a lot easier to mount.
Finally, I have a question that maybe Andy can shed some light on: The SC amps had a "wall-effect" phase altering all-pass filter in them which was active by default. The Interfaces, of course, do not. That seems a little logically inconsistent. If the wall-effect was important enough to include compensation circuitry in the amps, how was eliminating it in the Interfaces justified? I realize that as a practical matter, an equivalent filter would have been difficult and expensive to implement in the Interfaces. There's no doubt that the compensation had some effect, as I noticed immediately on firing up the Interfaces the first time that there was a definite difference in sound in the midrange. So what was the thought process around this issue? BTW, as an experiment, which I'm seriously considering making permanent, I put 8 sq ft of Sonex behind one speaker to reduce the "wall effect" and have concluded that the net result is a tangible improvement in the mids, perhaps similar to the wall-effect filter.
So that's about it for the moment. I'd appreciate hearing any comments or questions.
This is one of the few "Acoustat Servo Amps are worst than Step Up Transformer Interface" opinions I have heard. I now feel better that I did not spring $1500 for a pair of good condition servo amps!!
I did buy a Shackman Tube amp specifically designed for ESLs, with stacked EL84s, (for quite a bit less $$ than the Servo amps) that I will eventually finish and decide for myself which (tube vs interface) is better on my three Acoustat 9" panels and ML CLS panels...but the Shackman is only good for 300hz and up, while original Acoustat Servo Amp is full range - not exactly a fair comparison....
I did buy a Shackman Tube amp specifically designed for ESLs, with stacked EL84s, (for quite a bit less $$ than the Servo amps) that I will eventually finish and decide for myself which (tube vs interface) is better on my three Acoustat 9" panels and ML CLS panels...but the Shackman is only good for 300hz and up, while original Acoustat Servo Amp is full range - not exactly a fair comparison....
Lawnmower-
The original servocharge amps DID have some reliability problems that gave them a bad reputation among some owners. The modifications currently being done by Mike Savuto largely correct the amp's shortcomings, so the idea of using servocharge amps should not be 'thrown out with the bath water', so to speak. There are still plenty of the amps running, and some people will swear that they are the greatest.
I don't know much about the 'wall effect' circuit, as production of the sevo amps was long gone by the time I joined Acoustat. I believe it was done to allow the speakers to be placed close to the rear wall. Since the speaker was first introduced in the 70's, it was uncommon then for audiophiles to consider placing speakers anywhere BUT against the wall. In later years, the wisdom of giving the speakers some 'breathing room' became more prevalent, and therefore no attempt was made to include that feature in the transformer interfaces. And as you correctly point out, doing so in a high-powered passive circuit would have been very difficult.
The original servocharge amps DID have some reliability problems that gave them a bad reputation among some owners. The modifications currently being done by Mike Savuto largely correct the amp's shortcomings, so the idea of using servocharge amps should not be 'thrown out with the bath water', so to speak. There are still plenty of the amps running, and some people will swear that they are the greatest.
I don't know much about the 'wall effect' circuit, as production of the sevo amps was long gone by the time I joined Acoustat. I believe it was done to allow the speakers to be placed close to the rear wall. Since the speaker was first introduced in the 70's, it was uncommon then for audiophiles to consider placing speakers anywhere BUT against the wall. In later years, the wisdom of giving the speakers some 'breathing room' became more prevalent, and therefore no attempt was made to include that feature in the transformer interfaces. And as you correctly point out, doing so in a high-powered passive circuit would have been very difficult.
Regarding the issues around the SC amps -- certainly vibration of the entire thing was an issue. It was rather crudely built with no attention paid to securing the tubes other than their wire-pin sockets. Plus, those 6HB5s were run smack up against (or over) their absolute max rating (I can post the spec sheet from the original 1968 RCA tube manual on request). I literally bought them by the case when they were readily available and cheap. You could hear and see the famous bright purple glow when they reached the end of their (short) life. That's if they didn't just arc-over and take out their mate.
The "wall-effect"..... Again, from memory, the wall effect filter (enabled by default) was claimed to provide correction for rear-wave cancellation when the speaker was installed as recommended. That is, something like 3.5' from the wall. If the speaker was to operate freestanding, you could defeat the wall-effect filter by adding a resistor across it. A note for that is on the amp schematic.
When I have a chance, I'll scan what documentation I have for the Monitor3 and post it up here. That would include the original brochure, installation instructions (which don't seem to mention specifics about location or use of the wall-effect eq) and the amp schematic. I also have information on the MRP-1, including the schematic, if anyone's interested.
The "wall-effect"..... Again, from memory, the wall effect filter (enabled by default) was claimed to provide correction for rear-wave cancellation when the speaker was installed as recommended. That is, something like 3.5' from the wall. If the speaker was to operate freestanding, you could defeat the wall-effect filter by adding a resistor across it. A note for that is on the amp schematic.
When I have a chance, I'll scan what documentation I have for the Monitor3 and post it up here. That would include the original brochure, installation instructions (which don't seem to mention specifics about location or use of the wall-effect eq) and the amp schematic. I also have information on the MRP-1, including the schematic, if anyone's interested.
I don't remember what I sold the Problem Children for to the guy, but I did just run across the invoice for the pair of MK-121-Cs that I got directly from Rockford.... $1000. Plus shipping! And the pair of B&K M-200s was $1600. So this was not a financial coup by any means. However, you couldn't GIVE me a pair of those tube monstrosities. With the price of tubes these days, if you can find them at all, and at the rate those amps ate them, that proposition would be similar to the definition of a boat: "a hole in the water into which you throw money".
As a side note: In retrospect, I suspect that the bias current specification may have been too high (I kept them adjusted to spec) and that was what was causing the rapid deterioration. They ran really plate-glowing hot. Plus, tubes have a failure mode analogous to second-breakdown that's well-known in power transistors. So when you combine the very high plate voltage with high bias current, you have a prescription for short lifetime.
All I can say is don't obsess over "tubes". A competent engineer can create an excellent amplifier with any technology. Similarly, an incompetent engineer (and believe me, there are plenty of those around) can make a mess out of any technology. Simply because something has "tubes" in it, doesn't make it good, much less great. I think a lot of the issues with semiconductors v. tubes came up a long time ago when people didn't understand the ramifications of certain design topologies. Early transistor amps were designed with the philosophy that high open-loop gain with loads of feedback was a good thing. Easy to do with solid state amps, but a rotten idea. With tube amps, that was never an issue because they have inherently lower gain and require (and can't tolerate) a lot of distortion-reducing feedback. From that point onward, this became a cultural norm and "tribal knowledge" among those who really didn't understand the underlying principles. And it persists to this day.
BTW, the EL84 is only a 500V tube. When you say "stacked", you'd have to stack a LOT of them in order to achieve 5KV p-p direct drive like the two 6HB5s did. And the reliability of that..... you don't even want to think about it. OTOH, if they're only driving a transformer with those, you'd be just as well off with a good solid state amp.
Correction, and a Q for Andy
I omitted words in the preceding post that confuses the whole sentence: "With tube amps, that was never an issue because they have inherently lower gain and ideally do not require (and can't tolerate) a lot of distortion-reducing feedback."
And the question for Andy:
You mentioned previously that it takes some amount of time (more than a minute or two) for the panels to stabilize their 5KV bias. Could you please explain that?
I can't imagine any issue with sheet resistivity of the panel material that could cause anywhere NEAR that kind of time constant. If the sheet resistivity is that high that it truly takes hours to stabilize, then it's already on the verge of not working, which I can't believe is the case, ever. And for that matter, the SC amps applied the bias at normal turn-on and no one even flirted with the idea of having always-on bias like someone (was it you?) earlier suggested with the Interfaces. The speakers were up and running as soon as the tubes woke up, far as I can recall.
While it's just my opinion, I've never heard any hint of a "startup" phenomenon associated with speaker panel bias, and certainly not enough to justify another, however small, 24/7 parasitic power consumer. If that's not already enough, then consider how much dirt that a constantly-energized panel is going to attract! It's like one of those ridiculous Orek air cleaners. No wonder people sometimes have to run these through a wash cycle if they're leaving them sitting at 5KV constantly.
I omitted words in the preceding post that confuses the whole sentence: "With tube amps, that was never an issue because they have inherently lower gain and ideally do not require (and can't tolerate) a lot of distortion-reducing feedback."
And the question for Andy:
You mentioned previously that it takes some amount of time (more than a minute or two) for the panels to stabilize their 5KV bias. Could you please explain that?
I can't imagine any issue with sheet resistivity of the panel material that could cause anywhere NEAR that kind of time constant. If the sheet resistivity is that high that it truly takes hours to stabilize, then it's already on the verge of not working, which I can't believe is the case, ever. And for that matter, the SC amps applied the bias at normal turn-on and no one even flirted with the idea of having always-on bias like someone (was it you?) earlier suggested with the Interfaces. The speakers were up and running as soon as the tubes woke up, far as I can recall.
While it's just my opinion, I've never heard any hint of a "startup" phenomenon associated with speaker panel bias, and certainly not enough to justify another, however small, 24/7 parasitic power consumer. If that's not already enough, then consider how much dirt that a constantly-energized panel is going to attract! It's like one of those ridiculous Orek air cleaners. No wonder people sometimes have to run these through a wash cycle if they're leaving them sitting at 5KV constantly.
Every ESL manufacturer I know recommend that the panels are always powered on to avoid dust and dirt sticking to the panel. I'm pretty sure there is a reason why.
Think about what you're saying.. Take a big sheet of metallized mylar and hang it up somewhere busy. Take another sheet and charge it up to 5KV and hang it next to it. Come back a year later. Which do you think will have more dirt on it? The answer is obvious. Equally obvious is that anyone claiming that the charged sheet will be cleaner is trying to put one over on you.
They're not really "stacked". If you re-draw it, that's basically the same topology as the SC amps, only at 900VDC instead of 5KVDC. So you'll give up a few dB of SPL. Even so, at swings to the rails, the tubes will have more plate voltage than they're rated for. As long as they're biased into class AB2, they'll probably be okay, but there's no getting around the fact that they are specified at a lower plate voltage.
Reflecting on this very long thread, if someone wanted to do this right they'd design something like the MK-121-x inside a suitable solid state amp with appropriate feedback to mitigate the transformer anomalies, small as they might be. You'd end up with the benefits of solid state reliability with full drive voltages, and without the issues that open loop transformer operation causes. Hmmmm....... sounds like a fun project.......
Reflecting on this very long thread, if someone wanted to do this right they'd design something like the MK-121-x inside a suitable solid state amp with appropriate feedback to mitigate the transformer anomalies, small as they might be. You'd end up with the benefits of solid state reliability with full drive voltages, and without the issues that open loop transformer operation causes. Hmmmm....... sounds like a fun project.......
I just completed a two-week+ renewal project for my aging audio equipment. This included replacement of every electrolytic capacitor in the B&K M-200 monoblocks, the non-polar 47uF high-pass filter cap in the Medallions, and nearly every CapXon (ick) electrolytic cap in the Velodyne subs. I also set the bias current a hair higher in the M-200s. While that looks like a short list, it was not.
While I can't say exactly what did what, the results made my jaw drop. I have never heard this stuff sound this good.
I think the major contributor to the improvement was the surgery on the Medallions. I cut out the non-polar electrolytic and installed in its place 4x12uF Jantzen 400v polypropylene "CrossCaps" in parallel connected by 12ga copper busbar. Those were mounted on a 1/4" polycarbonate backplate I made for the interface boxes that included mounting for the new caps and banana jacks for the three HV outputs. The four caps were connected with 14ga hookup wire to the terminal strip where the original 47uF np cap was. I also inspected every connection for cold solder joints (none in the Medallions), and cleaned/reseated all non-soldered connections. I also cleaned and re-seated the slider on the wirewounds. The high voltage boards were problematic. The solder workmanship was okay, but they didn't clean the flux spatter off the boards! Needless to say there was plenty of dirt collected on it and some minor corrosion in spots. I inspected all the solder joints under magnification and then THOROUGHLY cleaned off the boards with isopropyl to remove all the crud and flux residue. I have to wonder how much leakage there was across that junk that may have, as a minimum, reduced the panel bias voltage on the speaker side of the isolation resistor.
(In case anyone's wondering, I elected to use 4x12uF instead of a single 56uF cap (and removing the existing 10uF film cap) to minimize the ESR of the capacitor circuit.)
After all this, I just can't believe the difference. I've owned these things for 30+ years, through the pain of the SC amps and then with the Medallions since '93, and I have NEVER heard anything like this coming from these speakers. If I had to point to just one thing out of the two+ weeks of work it has to be those new 12uF caps. I think the Acoustat/Hafler people didn't do themselves or their customers any favors by using a nonpolarized electrolytic in such a critical location. I know the excuse is "we paralleled it with a 10uF film cap", but based on what I'm hearing now, that simply doesn't hold water.
In summary, I would encourage anyone with unmodified Medallions to get to work and replace that cap. (And inspect/clean the HV board) You won't be disappointed with the expense and effort.
While I can't say exactly what did what, the results made my jaw drop. I have never heard this stuff sound this good.
I think the major contributor to the improvement was the surgery on the Medallions. I cut out the non-polar electrolytic and installed in its place 4x12uF Jantzen 400v polypropylene "CrossCaps" in parallel connected by 12ga copper busbar. Those were mounted on a 1/4" polycarbonate backplate I made for the interface boxes that included mounting for the new caps and banana jacks for the three HV outputs. The four caps were connected with 14ga hookup wire to the terminal strip where the original 47uF np cap was. I also inspected every connection for cold solder joints (none in the Medallions), and cleaned/reseated all non-soldered connections. I also cleaned and re-seated the slider on the wirewounds. The high voltage boards were problematic. The solder workmanship was okay, but they didn't clean the flux spatter off the boards! Needless to say there was plenty of dirt collected on it and some minor corrosion in spots. I inspected all the solder joints under magnification and then THOROUGHLY cleaned off the boards with isopropyl to remove all the crud and flux residue. I have to wonder how much leakage there was across that junk that may have, as a minimum, reduced the panel bias voltage on the speaker side of the isolation resistor.
(In case anyone's wondering, I elected to use 4x12uF instead of a single 56uF cap (and removing the existing 10uF film cap) to minimize the ESR of the capacitor circuit.)
After all this, I just can't believe the difference. I've owned these things for 30+ years, through the pain of the SC amps and then with the Medallions since '93, and I have NEVER heard anything like this coming from these speakers. If I had to point to just one thing out of the two+ weeks of work it has to be those new 12uF caps. I think the Acoustat/Hafler people didn't do themselves or their customers any favors by using a nonpolarized electrolytic in such a critical location. I know the excuse is "we paralleled it with a 10uF film cap", but based on what I'm hearing now, that simply doesn't hold water.
In summary, I would encourage anyone with unmodified Medallions to get to work and replace that cap. (And inspect/clean the HV board) You won't be disappointed with the expense and effort.
Sounds like you made the Bias work better,Most just think,oh there only 25+ years old There just geting old thay still sound good but??
In ESLs 80% of the sound comes form geting the bias up.To were it should be i have had 4-6 pr of 121 interfaces, an all the bias were in the 3k if i was lucky.
The Best sound i got is with the bias setup in a box that i can set from 4-7kV away from the hi-low board,
An yes the input crossover caps can go one way are the other,better sound are just diff.
But the bias is the Blood of the ESL it well make are brake the sound,
Thanks for your time an info......Long Live Acoustat..& Apogees
In ESLs 80% of the sound comes form geting the bias up.To were it should be i have had 4-6 pr of 121 interfaces, an all the bias were in the 3k if i was lucky.
The Best sound i got is with the bias setup in a box that i can set from 4-7kV away from the hi-low board,
An yes the input crossover caps can go one way are the other,better sound are just diff.
But the bias is the Blood of the ESL it well make are brake the sound,
Thanks for your time an info......Long Live Acoustat..& Apogees
Servo Amps Tube Front End
Yes, Dan Fanny of AHT knew more about the servo amps than anyone else on the planet. He modified the amps to use a 7308/6922/6DJ8 front end and changed the original 6HB5 to use a 6LB6 cap tube. We also had full wave transformers built to supplant the half-wave originals. Wow, what an amp!
If anyone on this forum knows what happened to Dan, please contact me. TIA, John
Yes, Dan Fanny of AHT knew more about the servo amps than anyone else on the planet. He modified the amps to use a 7308/6922/6DJ8 front end and changed the original 6HB5 to use a 6LB6 cap tube. We also had full wave transformers built to supplant the half-wave originals. Wow, what an amp!
If anyone on this forum knows what happened to Dan, please contact me. TIA, John
I've heard great things about Mr. Fanny's work, but he seems to have vanished off the Earth. I would love to hear a set of his Servos.
Dan Fanny, whereabouts unknown
Yes, IMHO Danny did the best job of modifying the servo amps. I used to drive my amps from MN to his house in NJ for him to work on them. To this day, I've never heard a pair of 2+2s do what they did at his house. His whole house would disappear on large scale orchestral. You thought you were sitting a third of the way back in Orchestra Hall in Chicago. Once he got the circuit down, he actually had new boards made; I think about 20 amp pairs. I would LOVE to locate a pair. Both my pair had all the mods done to original boards and the one pair became inoperable when the power bounced in my house one night and took out all the high voltage parts. I was 'trained' in well enough to almost get the amps working again but in the end I needed his expertise and he had, as you wrote, vanished... I hope he's living his dream of some day living in the Caribbean. I've contacted all of our common friends and they all say he just disappeared. Hopefully he's not nestled next to Hoffa...
Yes, IMHO Danny did the best job of modifying the servo amps. I used to drive my amps from MN to his house in NJ for him to work on them. To this day, I've never heard a pair of 2+2s do what they did at his house. His whole house would disappear on large scale orchestral. You thought you were sitting a third of the way back in Orchestra Hall in Chicago. Once he got the circuit down, he actually had new boards made; I think about 20 amp pairs. I would LOVE to locate a pair. Both my pair had all the mods done to original boards and the one pair became inoperable when the power bounced in my house one night and took out all the high voltage parts. I was 'trained' in well enough to almost get the amps working again but in the end I needed his expertise and he had, as you wrote, vanished... I hope he's living his dream of some day living in the Caribbean. I've contacted all of our common friends and they all say he just disappeared. Hopefully he's not nestled next to Hoffa...
I haven't had any luck finding Fanny either. Does anyone have a schematic or pic's of his board? I think i contacted you a couple years ago about an ad you had elsewhere Allvinyl. I live in NC Iowa. I never had such a short lifespan or any problems with my SC amps. Just the misfortune of having one stolen. 
Servo Amps Schematic
As I remember, Dan told me he didn't have a schematic. But I have always been curious as to how he drew and had made those new boards that he used in his statement version of the servo amps. I only ever saw his own pair as the remainder of those 20 pairs he had made had already gone out.
Of course, one of the first mods was to remove the protection circuit. My pairs both had 2 switches you had to flip in order. First, for the tube heaters and then once they'd been on for a couple of minutes the second switch to give 'em voltage. You had to reverse that on shut-down. But there's the rub. Though sonic-ally superior, it opened up the amps to damage if the power was suddenly removed and then quickly applied again. That's exactly what toasted my first pair and without Dan I couldn't quite revive them. The second pair didn't have all the mods so I decided to sell.
I haven't had any luck finding Fanny either. Does anyone have a schematic or pic's of his board? I think i contacted you a couple years ago about an ad you had elsewhere Allvinyl. I live in NC Iowa. I never had such a short lifespan or any problems with my SC amps. Just the misfortune of having one stolen.
As I remember, Dan told me he didn't have a schematic. But I have always been curious as to how he drew and had made those new boards that he used in his statement version of the servo amps. I only ever saw his own pair as the remainder of those 20 pairs he had made had already gone out.
Of course, one of the first mods was to remove the protection circuit. My pairs both had 2 switches you had to flip in order. First, for the tube heaters and then once they'd been on for a couple of minutes the second switch to give 'em voltage. You had to reverse that on shut-down. But there's the rub. Though sonic-ally superior, it opened up the amps to damage if the power was suddenly removed and then quickly applied again. That's exactly what toasted my first pair and without Dan I couldn't quite revive them. The second pair didn't have all the mods so I decided to sell.