Driving 1 ohm


I'm actually driving my recently refurbished Acoustat 2+2 electrostatic speakers with a Conrad Johnson MF 2500A. My Acoustats have been completely modernized with new more rigid frame, new electronics in the interface, Medallion transformers and other tweaks.They really get down low with a lot more dynamics than before.

A lot of electrostatics owners will often chose pure Class A amplifiers to drive the load these speakers command. The 2500A plays beautifully and doesn't get very hot at the task.

My question is : am I slowly damaging the amp without noticing it ?
" am I slowly damaging the amp without noticing it ?"

Not really - you do put some additional stress on this amp but if it did not failed during initial period then it was designed properly to deal with the stress so enjoy and don't think about it. If you amplifier will ever fail it will be due to lightening or other 10,000 reasons why amplifier cam fail.

Good Luck
It depends on how loudly you play the amp.
Thanks Simon !

Rwwear, not very loud, we can still speak at normal level while playing music. Most of the time, below half the volume dial on the preamp. But.......even at a moderate volume level, on good recordings, the bass level will drop at a point that a vibration sometimes continuous, can be felt. Those speakers are really something else now. And it is specifically that kind of effect that makes me cautious about the amp.
Well, depends.
Ask CJ if your amp is rated down to 1 Ohm &/or meant to drive 1 Ohms loads. Most amps in the market are not & even if they are, they sound like crap!
If it's rated down to 1 Ohm & meant to drive 1 Ohm then CJ should have provided adequate heat-sinking & an able power supply.
Otherwise, the amp would clip & the power transistors would saturate thereby degrading them over time.
Further, there are 1 Ohm loads & then there are 1 Ohm loads. How benign is your 1 Ohm load? Is there lots of phase shift in the 200Hz - 8KHz region? If yes, then your amp is being pushed pretty damn hard. If there is minimal phase shift, say, 10 degrees, then your amp is not being pushed hard. It's still driving a very low impedance but it does not have to provide too much power/current to drive the reactive load in the speaker x-over.
The amp-speaker interface is dynamic over 20Hz-20KHz so there is no fixed answer - all depends on how "nice" your speaker is being to the amp. Plus, as Rwwear pointed out, how loud you are playing your music.
Check out FM Acoustics line of power amps.
Thanks Bombay,

Actually, I didn't ask CJ, but I saw a reply they gave a few years back to a same inquiry about an Evolution 2000. CJ answered it wasn't designed for 1 Ohm without beeing specific if it was damageable either. That is why I am first asking here.

Your giving me figures, I appreciate. I'll have to check it out with one of my buddies who is familiar with such specifications. But it doesn't clip, that is for sure. And it doesn't get very hot, but does this also means that I'm not overriding the power transistors.

I feel reassured about the answers, thanks to all !
The amps that state stable at 1 ohm advertise this fact. You will damage the speakers if phase shifts the cover off and you see flame. Or you buy the Organ and drum disc to see how your bass sounds. All of a sudden the amp become a source of red hot ballistic bits.
Get a Spectron Musician IV that is supposed to work well, so I am told, when presented with low impedance . Another choice maybe a refurbished, no plateau biasing, monster Krell.
I think the Sanders designed Coda style, OEMed Innersound (RIP) type amps all make this claim.
I had no idea they all sounded like crap. I live in happy never lower than 4 ohm speaker land which lets me use the tube amps of my choice.
Heck I have 2 pairs of vintage speakers 16 ohm nominal impedance. My modern tube monoblocks don't have a 16 Ohm tap. That's Okay My DA-60 doesn't either nor two other integrated tube amps . Don't fret my 1959, 1960, 1961 Sherwoods do. Now that I think about it since I don't own planars I doubt I have a speaker lower than 4 ohms .
I do know that only a couple of the SS amps sound like crap. That's because they lost a channel or two or are 90s stop gap crap.
You might ask Acoustat what they recommend. You may not be hurting your amp but you may not be getting everything your speakers are capable of delivering. Amps that can deliver on 1 ohm loads are rare, but they can be found.
I have an amp you'd like I think that fits perfect for your speakers,Appogee's et al.1991 Krell KSA 300S.Only model Krell TRULY stable into 1 ohm they ever made (and you do the math that's doubling power for each load halving from 8 ohm nominal for a huge 2,400 watts!!).Class A of course and went for $9500 back in '91.Now you can get them for $3K to $3500.Others might prefer some subtleties of latter amps but signature Krell sound is same.Funny though they don't sound good at all with Maggies so being the ex Krell salesman that my friend turned to for his underpowered 3.6's I hooked him up with the classic Bryston/Maggie match.Ask Acoustat what Krell sound like with and if it's a go remember KSL was only true 1 ohm amp they made.I am using he Krell KRC2/KPE combo in one system but the amp I have has just sat in basement for 6 years.Maybe time to sell it all or separate.But there are others out there.
A true high current amp will double it's output as speaker impedence halves. Some of the early Krells would double their load all the way to .5ohms. Some of the Apoggees were .5ohms.
Ha .5ohm you say?--bring back the oldies but goodies-- my Electro Research A75s would drive 0.25 Ohms load no problem all day!

The Primary of the Dayton Wright XG8 ELS transformers.

The c-j ss amps are lovely, but not the right tool for the job at hand.
Rwwear, ESLs do not respond well if the amplifier can double its power as impedance is cut in half. It will result in no bass and too much highs. This is why tube amplifiers are often preferred for ESLs, despite the misguided attempts of several ESL manufacturers of making the speaker's impedance really low in an attempt to make them work better with transistors. The result is often a speaker that neither tubes nor transistors can actually make sound right.

For speakers like this tubes and a set of ZEROs are usually the best solution. See

for more information.
I wasn't recommending anything just defining a high current amp.
The Zero Impedance device looks like an autoformer or a similar device. It doesn't seem like something an OTL amplifier design would promote.
Best amp to drive ESL?
Roger Sanders (http://www.enjoythemusic.com/magazine/manufacture/0702/index.html) of course suggest to use solid state amplifiers, specifically his own design.

I believe that Andr did not strated the tread asking advise to help him to find different amp then one he uses and enjoys - rather what harm it can endur by driving 1 Ohm load.

...of course when I owned Quad ESL 63 I used only tube amps (heh, heh, heh...)

Simontju is right on that one. The 2500A inherited the Acoustats. Long story short : I had Acoustat model 3 for a year and decided to have them rebuilt in 2+2 s. What I got back is something supposedly easier to drive but tremendously upgraded on the full range. And that brought me the concern. The rest of the electronics is probably forever : Simaudio Eclipse, CJ Premier 16,cables. The 2500A is here to stay until I get upgraditis again but for the time beeing, the WAF has been tested enough with the 2+2 s in the living room. I would not dare testing it again with large monoblocks,class A or tube amplifiers, at least for awhile. Class D maybe a solution but did not have the chance to audition some. In my vicinity(Canada), they remain quite scarce.
My Acoustat 2+2's do amazingly well driven by a PS Audio HCA-2 amp. Grab one here on Agon for $600 used.. you won't be sorry, and your back will thank you!
If they were Apogees or Kappa9's, a rugged SS amp would be the way to go but for ESL's, Ralph is right.

Another option is "monostrapping" or parallelling an identical pair of stereo amps. That would double the current and wattage and halve the output impedance. Some manufacturers don't approve of this method.
Rwwear, in the real world, people want to use OTLs with low impedance speakers. If you use an autoformer to do this, you often meet with success. The fact of the matter is that the autoformer has bandwidth that is wider than most amps, tube or solid state, which is only possible because its turns ratio is so low.

IOW, using the ZERO would not be practical if you did not already have a low output impedance and no DC at the output. So that could be an OTL, but it could also be a transistor amp. Its already been shown that a transistor amp will sound better driving 4 ohms or less if it does so through a set of ZEROs rather than direct.

So the argument holds regardless of the amplifier technology, which inherently includes OTLs.
For $2000, the H2O Audio Signature 100 can drive a 1 ohm load with no sweat. Hell, they can handle the vaunted Apogee Scintilla with absolute ease. There aren't too many (great sounding) and truly affordable power amp's that can make such a claim.
If I take the aforementioned Apogee Scintilla as a reference (straight 1 Ohm throughout the frequency spectrum) these are some of the amps that will drive them:

- Classé 400
- Krell 250
- Hafler XL600
- BAT VK-60 triode tube amps
- Sphinx Project 16
- Sphinx Project 26 mono amps

There are probably many more amps that will drive these kinds of loads, like the big Thresholds/Pass amps etc.
The Scintilla does not need a lot of power. We drove it very easily with a set of MA-1s and a set of our Z Music autoformers (we no longer make that product, but something similar is sold under the name ZERO). Its not normally a load that you would put on a tube amp but it worked great!
I have been using a Hafler 220/ bone stock built from a kit in 1979 to drive my Acoustat 2s.
It has never been touched and sounds fantastic.
That said I would like to go the tube route and I think that it could really reveal some magic,,,,with the right amp.
I have heard and read folks talking about low impedances but I know the "nominal" load for this speaker is 6 ohms.I understand that the lowest impedances occur in the extreme high frequencies, not the bass.
Also I read that current is the key to these speakers. So can someone explain to me why a tube amp is not what I want? Also, the original Acoustats had tube amps for transformers, so there must be something there.
thanks in advance

The Acoustat 2s aren't supposed to drop below 3 ohms and only require over 50 watts. Maybe a fairly powerful tube amp will work fine with them. Hopefully you can try one before buying.