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Scotty, I need more power!

TSmith8605

TSmith8605

Senile Member
Subscriber
I am finding that both my MC 2505 and MC 275 are (slightly) underpowered when driving my Acoustat panels (one panel per for everything above 100 Hz). My room is smallish so the current setup can average about 90 dB at the couch with either amp with clear, clean amazing (to me) sound. But about 5% of the time I want to get a little louder and both amps get a little congested or less amazing - never really hard clipping - just not as clear when I ask for another 3 dB or so. My prior ARC D-115 Mk II, rated only at 100 Watts per could get there somehow, although a lower levels was not nearly as good sounding as either of these Mc amps I have.

So, I have been thinking about a larger, modern McIntosh amp. You guys have sold me on the MC 452 but two things are holding me back. One is my inability to physically deal with a 110 lb package and the other is my worry that a MC 452 (and a couple tastes of Corzo Blanco) might cause over-power damage to the speaker panels. The the total speakers (including the satellite acoustic suspension woofer that operates below 100 Hz and is powered by a different amplifier) are rated at 120 Watts. The largest amps I've used over the last 30 years are about 120 Watts per on the panels with absolutely no sign of problems. BTW, I bypassed the speakers' protection fuses 30 years ago based on the theory that the fuses would degrade the sound but the fuse could go back in if I got an MC 452 or pair of MC 301s.

But I am worried that an MC 452 could damage the panels even before triggering power guard. I suspect the failure mechanism would be either over voltage on the stators causing arcing to punch holes in the diaphragms (although I've read that the Acoustats' stators are well insulated and not prone to this issue), or possibly melting the coils in the speakers' step-up transformers.

So, I am also considering an MC 152 (which would be much cheaper) or a pair of MC 301s (which are more expensive but I could move them around myself). I prefer the looks of the MC 452.

MC 152 is about half the price of the other alternatives I list and would probably do the trick.

I also thought about a second MC 275 but I'm ready to not have to deal with that many tubes. 2 MC 275s would have about the same power as a single MC 152 - which would sound better?

It would be nice to have a home trial of a MC 152 and MC 452 but I can find only two McIntosh dealers remaining in the Bay Area and neither have a MC 152 demonstrator sitting around. One has an open MC 452 listed at full retail, but I don't want to ask for an in-home trial since I don't know how to get that amp into my house (rent a truck and some help from the Home Depot parking lot), and I don't want to pay full retail. I would never take home a trial amp from a dealer and then buy the item elsewhere.

Anyone have any experience, advice, etc.? Would a simple in-line fuse protect the speakers? I would be replacing the fuse every ten minutes?
 
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I have a friend that is a knowledge base on the Acoustats. Let me pick his brain on the power handling question. I'll get back to you with his reply.

Goes without saying that you'll be putting the fuses back in . . .
 
damacman said:
I have a friend that is a knowledge base on the Acoustats. Let me pick his brain on the power handling question. I'll get back to you with his reply.

Goes without saying that you'll be putting the fuses back in . . .
Click to expand...

Cool! Thanks!
 
I think you'd be very surprised with how good the MC152 is, and it is right in your power and weight ball park. It can be ordered from a number of online retailers, some that offer up to 60 day satisfaction guaranties. If you don't like it, you can ship it back. You'd just be out the return shipping cost of course. If you'd like to stick with tubes, another option would be the new MC2152 which has the power you're looking for. The problem with it would be that it's a heavy bugger at 125 pounds.
 
My buddy Garry's response . . .

Jim [Strickland] used to tell us that we should use at least 200 watts per channel…

This guy has a separate amp on the LF drivers so there really is no limit on the power handling of the panels, as the amplifier is simply driving the transformer, not the panels themselves.

I use a 9505 rated at 250 watts/ch (8 ohms) with no issues. The internal fuses can (should) be replaced, they simply prevent overcurrent to the transformer primary.

One thing… Acoustats (all electrostats actually) will reach a point where additional power has no real effect on volume. This typically occurs somewhere between 200 and 300 watts depending on the model of speaker. These speakers just won’t play much louder than about 95dB.
 
Wow! When I asked a question about 30-year-old speakers I expected silence - crickets. I did not expect a complete, authoritative answer in, like, an hour. I will copy post #5 and file it with notes I have collected about the speakers. Finding info on these speakers is difficult.

Anyway, thank you and Garry!

I do find these speakers can get pretty loud and remain clear and that's why I like them for rock and roll especially, in my small room. I know that with the ARC D-115, it got loud as I wanted to go which is not much more than I get now. I assume these large drivers don't have to move very far to move a lot of air and so driver compression comes on at higher volumes than one might expect. I also assume there is a limit and I have not hit that limit so far with amps I have used.

These speakers, which have almost zero resale value, do a lot of things right and a few things not so well. I'm not sure what I would replace them with. I have heard plenty of speakers that sound better and also costs tens of thousands of dollars. I've also heard many speakers in that price range that I did not like or who's sound did not seem to justify their price. So, the stats remain.

Sounds like a MC 452 would be about perfect by providing about 3 dB headroom above the recommended power for the speakers. And when I do blow up the speakers, the MC 452 will be able to drive just about any replacements I find. Just got to figure out how to get one into my living room. . .

Thanks.
 
Which Acoustats?

I've driven them with 200wpc and didn't need it all. Depending on the condition of the panels, you might get diaphragm slap before you get any louder, especially on the LF.

Removing the fuse? They were designed with the fuses, you'll be fine with them in. If they're early versions (before David Hafler) the interfaces can certainly be upgraded, and be sure that your HV supply is up to the task or you're just putting power into the transformers and causing heat. If you toast a transformer (the LF ones are the most vulnerable) you'll have a very hard time sourcing another, pretty much not worth saving at that point. IIRC the biggest re-design from David was moving the crossover to be before the transformers where it was originally running full-input to both transformers and crossing after. There are two transformers, one for HF and the other for LF, blended together again at the panel input.

There is a method of re-tensioning the (heat-shrink) diaphragm also, will help its response greatly if the diaphragm has started to get sloppy.

These love tubes, present a difficult load, there are definately good and bad amplifier choices. My preference is (McIntosh) output transformers, an inductive drive to an inductive load.
 
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TSmith8605 said:
Wow! When I asked a question about 30-year-old speakers I expected silence - crickets. I did not expect a complete, authoritative answer in, like, an hour. I will copy post #5 and file it with notes I have collected about the speakers. Finding info on these speakers is difficult.

Anyway, thank you and Garry!

I do find these speakers can get pretty loud and remain clear and that's why I like them for rock and roll especially, in my small room. I know that with the ARC D-115, it got loud as I wanted to go which is not much more than I get now. I assume these large drivers don't have to move very far to move a lot of air and so driver compression comes on at higher volumes than one might expect. I also assume there is a limit and I have not hit that limit so far with amps I have used.

These speakers, which have almost zero resale value, do a lot of things right and a few things not so well. I'm not sure what I would replace them with. I have heard plenty of speakers that sound better and also costs tens of thousands of dollars. I've also heard many speakers in that price range that I did not like or who's sound did not seem to justify their price. So, the stats remain.

Sounds like a MC 452 would be about perfect by providing about 3 dB headroom above the recommended power for the speakers. And when I do blow up the speakers, the MC 452 will be able to drive just about any replacements I find. Just got to figure out how to get one into my living room. . .

Thanks.
Click to expand...
You're welcome. Garry is a long time friend whom I worked with at Rockford. Garry was also close to Jim Strickland - the designer. Garry still has big Acoustats at his facility. Garry's company (Cogent Audio Labs) does product evaluations and competitive analysis on consumer electronics and he's one of the sharpest cats I know.
 
So I'm tempted to not get greedy here, but I do have a couple more questions for Garry if he'a amiable. No problem if he's too busy. If he happens to be in the Bay Area, I'd be happy to buy him a beer or two.

Mine are Acoustat model 1.

1. Is any maintenance of the panels ever needed? I have recapped the crossover twice over the years and reformed the woofer once but do I need to clean and / or tighten down the stators or anything such (would be PITA to get at them)?
2. Is an impedance curve for the model 1s available somewhere? I understand they drop to 3 ohms at the high end.
3. What is the sensitivity for 1 Watt at 1 meter (the manual lists it in some other format)?

BTW, the fuse is 5 A, slow blow, which roughly correlates to 200 W at 8 Ohm and 75 Watts at 3 Ohms.

I'll need to wire the fuses back in before I buy another bottle of Corzo Blanco because I just got off the phone with Ernie at Audio Classics and placed my order for a B1 MC 452. Turns out that two used MC 452s are currently on the local Craigs List at two different prices and AC's price was exactly half-way between that of the two used units but with the AC 90-day warranty and 10-day return if the sound sucks. A new MC 452 is also on display at World of Sound, a dealer about 40 miles north and over the GG bridge in Mill Valley, at full retail.

My MC 452 is on the way. Hope it beats my MC 2505.
 
I think it'll beat the 2505.

Model 1? I had the 1+1, which is two panels stacked for each channel and it eliminates the subwoofer. The interface box is different for the 1 and I don't know if they have two transformers per speaker (three including the HV power supply transformer) like the 121 interfaces or only one audio transformer.

I can't imagine you need more power for these, they are the smallest Acoustat and can only handle so much, after I fried one of my transformers I gave up on more power.

Yes the panels do need to be vacuumed or gently blown out periodically. Dust build-up can cause bridges that will carry the HV bias voltage and potentially burn pinholes in the membrane. You remove the top panel and unbolt from the stand/interface, pull the staples, remove (and wash) the socks, then you can vacuum the panels carefully and some use compressed air, I only used gentle air from a blow-dryer. Be sure that the panels have been powered down and ground the panel HF leads so that it discharges the HV from the panels first, otherwise the (5kV) static charge will continue to attract dust with the socks off.

If they've been smoked around (especially heavily) then you're looking at a whole different cleaning, probably leave alone if there's not a problem.

Look at the membranes at this point, clearly visible. The stators look like flourescent light "egg-crate" diffusers, the membrane behind it should look smooth and taut. Some people will re-tension with a heat-gun, I decided to be more gentle with mine and used a hair-dryer, same as tensioning the heat-shrink film that people used to put over windows in the winter, start around the perimeter and work inward. Lots of information on this over at Planar Assylum, lots of fans.

They're great sounding speakers. I think that the 1s pre-date the Hafler years but the same panel technology. The "common wisdom" is that all of the panels lose tension from the day they're delivered and re-tensioning with a heat gun isn't an exact science, but does restore much of the original sound quality and helps to prevent the snapping sound of the membrane hitting the stators from over-excursion.

The usual problem that will cause diminished audio output is a weak bias supply. The bias supply is what draws the membrane fore and aft to make sound, the audio signal just gives the static charge a forward or rearward direction.

As mentioned the 1+1s require 5,000VDC to operate, not sure what the 1s require but likely the same as it's the same panel spacing etc. (my Quads require 6,800VDC!). If (and usually when) this bias supply has gotten weak, the static charge is weak and the sound output drops dramatically. This usually leads to the owners using more and more power, until an audio transformer goes bye-bye. The supply is a fairly simple ladder of HV diodes and capacitors, can be replaced quickly and easily by anyone with good circuit-board soldering skills. I suggest doing this before you start to get aggressive with the new amplifier, I went down the same road with a new amplifier and it didn't end well for the Acoustats. David Hafler is now gone as are all of the replacement transformers. Plan to throw them in the trash if you toast a transformer.

I no longer have the links for the Acoustat brains out there, there are a couple, but I'm thinking you can get there through the planar assylum pages.

Enjoy the new power.
 
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jayvis said:
If you'd like to stick with tubes, another option would be the new MC2152 which has the power you're looking for. The problem with it would be that it's a heavy bugger at 125 pounds.
Click to expand...

Wow, I had no idea it weighed that much. Almost seems like McIntosh is doubling down on the weight thing.
 
SaturationPt said:
I think it'll beat the 2505.

Model 1? I had the 1+1, which is two panels stacked for each channel and it eliminates the subwoofer. The interface box is different for the 1 and I don't know if they have two transformers per speaker (three including the HV power supply transformer) like the 121 interfaces or only one audio transformer.

I can't imagine you need more power for these, they are the smallest Acoustat and can only handle so much, after I fried one of my transformers I gave up on more power.

Yes the panels do need to be vacuumed or gently blown out periodically. Dust build-up can cause bridges that will carry the HV bias voltage and potentially burn pinholes in the membrane. You remove the top panel and unbolt from the stand/interface, pull the staples, remove (and wash) the socks, then you can vacuum the panels carefully and some use compressed air, I only used gentle air from a blow-dryer. Be sure that the panels have been powered down and ground the panel HF leads so that it discharges the HV from the panels first, otherwise the (5kV) static charge will continue to attract dust with the socks off.

If they've been smoked around (especially heavily) then you're looking at a whole different cleaning, probably leave alone if there's not a problem.

Look at the membranes at this point, clearly visible. The stators look like flourescent light "egg-crate" diffusers, the membrane behind it should look smooth and taut. Some people will re-tension with a heat-gun, I decided to be more gentle with mine and used a hair-dryer, same as tensioning the heat-shrink film that people used to put over windows in the winter, start around the perimeter and work inward. Lots of information on this over at Planar Assylum, lots of fans.

They're great sounding speakers. I think that the 1s pre-date the Hafler years but the same panel technology. The "common wisdom" is that all of the panels lose tension from the day they're delivered and re-tensioning with a heat gun isn't an exact science, but does restore much of the original sound quality and helps to prevent the snapping sound of the membrane hitting the stators from over-excursion.

The usual problem that will cause diminished audio output is a weak bias supply. The bias supply is what draws the membrane fore and aft to make sound, the audio signal just gives the static charge a forward or rearward direction.

As mentioned the 1+1s require 5,000VDC to operate, not sure what the 1s require but likely the same as it's the same panel spacing etc. (my Quads require 6,800VDC!). If (and usually when) this bias supply has gotten weak, the static charge is weak and the sound output drops dramatically. This usually leads to the owners using more and more power, until an audio transformer goes bye-bye. The supply is a fairly simple ladder of HV diodes and capacitors, can be replaced quickly and easily by anyone with good circuit-board soldering skills. I suggest doing this before you start to get aggressive with the new amplifier, I went down the same road with a new amplifier and it didn't end well for the Acoustats. David Hafler is now gone as are all of the replacement transformers. Plan to throw them in the trash if you toast a transformer.

I no longer have the links for the Acoustat brains out there, there are a couple, but I'm thinking you can get there through the planar assylum pages.

Enjoy the new power.
Click to expand...

This is great info. Thanks! Looks like another project, though.

Yes, these are pre-Hafler with 121 interfaces. Both speakers still have the same output level (centered image) so if the bias supplies have deteriorated they have done so identically in both speakers - which may or may not make sense. Still, I will look into rebuilding them.

No smokers and the covers are still look clean. I've always worried that if I removed the covers, I'd never get them back on correctly but I do have a staple gun around here somewhere. Sounds like not that big of a deal.

Anyway, thanks.
 
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kevzep said:
Good Grief!! 108db!! Hahahaha.......might just get the meters moving on the 452.....but in any case I think sound of the 452 will deliver the goods...
Click to expand...

Yep. In fact, that number makes no sense to me so I read it again and it says program material peaks. Right now, according to my iPhone SPL app, I get music averaging around 90 dB at 14 feet with maybe 70 Watts per. My math says that to hit that number, 108 dB at 15', I need about 4,480 Watts per channel for peaks.

I don't think the MC 452 will do that but it will be enough amp.
 
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TSmith8605 said:
This is great info. Thanks! Looks like another project, though.

Yes, these are pre-Hafler with 121 interfaces. Both speakers still have the same output level (centered image) so if the bias supplies have deteriorated they have done so identically in both speakers - which may or may not make sense. Still, I will look into rebuilding them.

No smokers and the covers are still look clean. I've always worried that if I removed the covers, I'd never get them back on correctly but I do have a staple gun around here somewhere. Sounds like not that big of a deal.

Anyway, thanks.
Click to expand...

The HV/Bias supply rebuild is a cinch, IIRC it's a dozen diodes and six capacitors, about $20/speaker plus an hour to remove and replace the supplies. Might bring them new life. If they've not been rebuilt, they are likely weak.

Or, ... if you have a friend with a HV probe you can check the voltage.
 
SaturationPt said:
Which Acoustats?

I've driven them with 200wpc and didn't need it all. Depending on the condition of the panels, you might get diaphragm slap before you get any louder, especially on the LF.

Removing the fuse? They were designed with the fuses, you'll be fine with them in. If they're early versions (before David Hafler) the interfaces can certainly be upgraded, and be sure that your HV supply is up to the task or you're just putting power into the transformers and causing heat. If you toast a transformer (the LF ones are the most vulnerable) you'll have a very hard time sourcing another, pretty much not worth saving at that point. IIRC the biggest re-design from David was moving the crossover to be before the transformers where it was originally running full-input to both transformers and crossing after. There are two transformers, one for HF and the other for LF, blended together again at the panel input.

There is a method of re-tensioning the (heat-shrink) diaphragm also, will help its response greatly if the diaphragm has started to get sloppy.

These love tubes, present a difficult load, there are definately good and bad amplifier choices. My preference is (McIntosh) output transformers, an inductive drive to an inductive load.
Click to expand...

I originally bought these because I wanted tube amps as my prior Tympani speakers seemed too mushy for tubes - especially the tweeters. I used and ARC D115 Mk II for maybe 20 years with these until I discovered that both my lowly MC 2505 and MC 275 IV both sound much better.

The Model 1 has only 1 interface transformer that does not have to deal with low bass below 100 Hz, so hopefully that will help with its preservation. I just read the link you provided detailing upgrade mods for these speakers and they said they also removed the audio-line fuses, rewired internally with Kimber cable (by coincidence, the same wire I used) and added quality caps in the crossover. I did those same things years ago (on my own) and I also wired the leveling resistor out of the circuit as I can control balance with an attenuator on the bass amp. I will check the bias supplies and take the covers off to clean things. I will put the fuse back for the MC 452.

Thanks again.
 
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