Stax stax ESL-F81 - please help

The Stax ELS F-81 is one of the most difficult loads one can find. From 40Hz-800Hz the impedance is well above 100 ohms. Around 180Hz the impedance peaks around 512 ohms! Amazing.

Solid state does not like this sort of load. A Zoebel network is highly recommended if using solid state. OTL tube amps are ideal but can be pricey. Transformer coupled tube amps will handle this kind of load.

Power output will depend on required SPL in the listening room. A smallish room with a nearfield set-up will yield SPL in the 90-92 range with a mere 60 watts. I use an ARC D-70, 65 watts, with good results except for clipping if I want to drive things. Or I have an ARC Ref300, 280 watts, that will drive them louder than I like without clipping issues. Well I can clip the Ref300 but then I am pushing the speaker close to failure, probably the transformer.

Any power amp will need to have a large, robust and stable power supply. An ARC VT130, 110 watts, would be ideal with these speakers and is within your budget. The AtmaSphere M-60 would be a great match if maybe just a little under powered.

No matter what you decide in terms of power amps there are a couple of key items that need to be checked in the speaker to keep them healthy.

Check the input RC network resistors. They will have failed. A failed RC network resistor and a large output amp will cause the diaphragm to hit the plates and cause damage. Replace the failed resistors with a good quality wirewound rated for at least 10 watts.

Replace the input fuse. It is worthless. Actually worse than worthless since one assumes they have some protection with the fuse in place. You don't. The stock 2 amp slow-blow will never fail and you will loose a transformer in the process. Replace the fuse with a standard 2 amp normal blow.

Good luck. You have a rare masterpiece. One of the finest speakers I have ever heard. Above 500Hz nearly perfect. Everyone that has heard them, no matter their exposure to high-end, has been floored.

Hey thanks for all the info. How do I get to the fuse and RC network resistor? Do I need to take off the top plate of the box? I cleaned the beck from the deteriorated foam and never replaced it. Do they sound best with or without this foam?

I listen only near-field and at low volumes, they are my late night speakers, so I am looking for something to drive them to SPL 90 or so, but with warmth and clarity.

Yes remove the top wood plate. All the electronics are bolted to the bottom side of the plate.

The input fuse is located on the RC network board. Replace with a 2A normal blow fuse. Check the resistance on the RC network resistors, (2) 12 ohm/2 watts. They may be visibly burned or not. Likely these have failed. If they have not failed I would still highly recommend they be replaced with (2) 12 ohm/10 watt or higher. Fly these large resistors off the board to allow for cooling. Make sure they do not touch anything. The resistors get extremely hot.

The foam on the rear protective cover is always rotted. If you have the speakers out from the back wall at least 3-4' the missing foam will have no impact. If they are close to the back wall some rear wave damping is helpful. A light weight foam or fabric will do.

A good tube amp in the 60-100 watt range should work well with your requirements. The ARC D70 works nicely with these speakers. Articulate, open, warm, it mates well with the seductive, soothing quality of the Stax. $700-$900 on the used market in good shape. With any TC tube amp use the highest output tap possible.

If I decide to go with OTL tube amp what should be ideal specs? Any amp suggestions? I just want to see what the market is.

I have F-81's and Stax DA100M Amps which work very well together. The DA100M's were designed and built to drive the Stax F-81's. However, I have found that my Radford TT100 Solid State/KT88 Valve Hybrid actually drives the F-81's better than the DA100M's! In fact the Radford is one of the most amazingly dynamic and staggeringly powerful 100W amplifiers I have ever come across. it's a true audio work of art. The Stax amps can run out of power at higher volumes causing significant distortion on more complex programming which is both annoying and potentially damaging. I have never had this problem with the Radford driving the Stax. My advice would be to truely make the Stax sing you need a powerful Valve amp. OTL would probably work well. Good luck and have fun experimenting.