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Old 6th December 2010, 01:44 PM   #1
Faust3D is offline Faust3D  United States
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Question Stax ESL-F81 - need a good amp

Got a good deal on Stax ESL-F81 electrostatic speakers. They are in good working condition.

I am trying to figure out what amps would drive F81 best. I have a Class A 20W custom made ECC88/MOSFET Amp: works great with high sensitivity speakers but fails badly with Stax, McCormack DNA-1 Deluxe: drives Stax fine but is not as fluid as Aleph 3, Pass Aleph 3: works fine but clips at higher SPL and Symphonic Line RG11 that works nicely but does not bring anything special to the table. As you can see these are very demanding speakers.

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With impedance all over the place from 2Ω to 500Ω depending on the frequency what would be an ideal amp? Please provide commercial models as well as DIY choices and general ideas.
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Old 11th December 2010, 04:38 PM   #2
cfb is offline cfb  United States
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Default Stax F81 amplifier recommendations;

The STAX esl seems to have a synergistic match with OTL amps like the Futterman, Counterpoint SA4 or AtmaSphere, much like the SoundLab esls. A second choice would be a high power PP tube amp of at least 120 watts like the Jadis, VTL, Tube Research Labs or other heavy iron known for a superior midrange performance. You can forget about any flea power SET amps and low power class A amps, start with at least 100 watts output. One of the problems with the STAX F81 and even its big brother the F83 which is just a stacked pair of 81s, is that they are basically a midrange and up speaker, no real bass to them at all. Oh they played games with the tuning of the diaphragm but that just produces a sort of one note bass performance even worse than the original Martin Logan CLS had. The STAX is a polite speaker best suited for small ensemble works and especially vocals at which it possibly excels the vaulted QUAD 57 one of the best midrange speakers ever. The character of the midrange is such that it is not easy to match any dynamic cone subwoofer to the system. What is needed is a midbass driver covering the range of 60hz to 150hz, maybe a pair of Magnepan bass panels perhaps. That will necessitate an electronic crossover and separate bass amp. Marchand offers the XM26 and Bryston and FM acoustics both have high-end and costly crossovers; on the budget side look for a used Audio Research EC2 crossover but update its capacitors and diodes. Some suggest that the bass amp can be different than the high pass amp but that transition is always noticeable on highly transparent speaker systems. I recommend using the same model for both the bass and midrange amplifiers. One could build a pair of DIY OTL amps as the schematics are out there, either the totem pole Futterman circuit or the circlotron based AtmaSphere; although the 60 may not be enough power you really want at least 100 watts or more from the tube amp. The second major caveat of the STAX is its astoundingly low efficiency, the two worst offenders I've run across in this regard were the original 3 way Apogee Full Range ribbon and the STAX esl, both need a high power amp to achieve even normal listening levels while at the same time will overload and compress if driven too hard. There is a narrow volume range at which the speaker will perform at its best.
You might ask over on the AudioAsylum planar forum.
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Old 11th December 2010, 05:27 PM   #3
jcx is offline jcx  United States
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by "synergy" do you mean 3-5 Ohms series output R between amp and ESL-F81's xmfr?

it certainly looks like that could help flatten the on axis frequency response in the top octave: http://www.audioasylum.com/images/staxELS.pdf

but resistors could be added to any SS amp output too

ridiculous conservatism would suggest resistor power rating similar to amp power - 200 W can be had for ~$20-30 ea

but realistic music 3 KHz power bandwidth and peak overload ratings of big reisitors mean you could proably get away with much lower rating - just put them in a flame proof box with enough surface area to get rid of the heat

Last edited by jcx; 11th December 2010 at 05:33 PM.
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Old 11th December 2010, 06:05 PM   #4
jcx is offline jcx  United States
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bridged & paralleled chip amps could be OK, wrap in a quality op amp overall feedback loop, other ideas from A chip-amp to rival Hi-End - design advice

extrapolating from Stereophile's Golden Eared reviewer's experience you couldn't expect anyone to do better: Blind Listening Tests & Amplifiers

Last edited by jcx; 11th December 2010 at 06:17 PM.
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Old 11th December 2010, 08:39 PM   #5
cfb is offline cfb  United States
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Default Stax F81 and OTL amps

No, I wasn't referring to transfer characteristics, I meant the sound quality or character of the two products compliment each other. Furthermore, a resistor between a solid state amp and the step-up transformer of the Stax is not going too improve matters any. As far as I remember from the 80s when the STAX F81 was manufactured I do not recall any solid state amp working very well with it, strictly a vacuum tube amp type of speaker.
In regards to vacuum tube amps they should have an output impedance of less than 5 ohms. Interestingly, Roger A. Modjeski of RAM tubes fame designed the SA 4 for Counterpoint and I recall Counterpoint quoting a damping factor of 4, I could be mistaken on that. Damping factor is not as important for electrostatic speakers as it is for conventional dynamic cone speaker systems. OTL amps will deliver more power into a high impedance load ( by that I mean above 32 ohms and certainly above 100 ohms) than a solid state amp will. Now solid state is great for driving a sub one ohm ribbon speaker but not the best choice for driving a one hundred ohm electrostatic. I sense a reluctance to try out an OTL amp, why not at the very least audition an AtmaSphere amp at a dealer as it seems to be the most commonly available OTL on the market. And, while OTL amps can be problematic with conventional speaker systems they really do work quite well with electrostatic speakers both Beveridge and Acoustat had direct drive tube amps for their electrostatic speakers. Or, if you already have a collection of electronic parts why not build a Futterman OTL, if not to your liking you could always sell the power tubes? 6AS7 and 6LF6 tubes always have a ready buyer..
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Old 11th December 2010, 08:55 PM   #6
Faust3D is offline Faust3D  United States
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Thanks for all the info. OTL sounds like an interesting idea. I will research that. McCormack DNA-1 Deluxe does a good job driving them to good volume levels and sounds very nice, but I bet good OTL will sound better.
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Old 11th December 2010, 09:26 PM   #7
jcx is offline jcx  United States
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Quote:
Originally Posted by Faust3D View Post
... but I bet good OTL will sound better.
the point of the Carver Challenge is that the "best ears in the industry" Did Lose that bet - when the midprice SS amp was tweaked to match in frequency response, output impedance to a "5 figure SOTA Tube amp"

I agree that you don't need continuous power drive capability at 20 KHz - but you might want the current output capability to be there for transients like snare drum hits - so you do need an amp capable of pushing current into the lowest input impedance of the speaker

That's where a custom chip amp solution can save some bucks - TAD7293/4 handle high ps V but you wouldn't have to size the heatsink, ps for continuous power into the 4 Ohms at 20 KHz unless you just want to watch the 'scope trace (I wouldn't want to be in the room with the speakers!)

if you need the reassurance of audiophile snob appeal - the chips have been used in Kilobuck "audiophile" amps from Linn and others

Last edited by jcx; 11th December 2010 at 09:33 PM.
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Old 12th December 2010, 07:12 AM   #8
Calvin is offline Calvin  Germany
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Hi,

when You want to use chips like the TDAs with ESLs then buy them in Kilos and make sure You can easily replace them.
That such chips are used in kilobuck amps (no audiophile ones for sure) just shows how expensive You can sell cheap stuff when You´ve learned Your marketing lessons.

jauu
Calvin
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Old 12th December 2010, 06:55 PM   #9
jcx is offline jcx  United States
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just use enough in the 1st place and they won't be over stressed - 4x parallel with the "eye-ball" amp output series R target of 4 Ohms for the Stax would give 8 Ohm resistors in series with each chip amp in bridged-parallel

I certainly hope its pretty hard breaking a chip amp with 8 Ohm real resistance in series with any passive RLC combo on the output and "music" signals

they're certainly cheap enough that you can even double those numbers and together the chips still cost less than tubes in OTL capable of 10% of the output current of just one of the chips
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Old 13th December 2010, 07:32 AM   #10
Calvin is offline Calvin  Germany
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Hi,

Quote:
4x parallel with the "eye-ball" amp output series R target of 4 Ohms for the Stax would give 8 Ohm resistors in series with each chip amp in bridged-parallel
could You explain what that means? It sounds like wasting terrible amounts of power in series resistors.

jauu
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