InnerSound has long been known for manufacturing very high quality Electrostatic Speakers (ESLs). Because ESLs tend to have low impedance (2 Ohms) at some frequencies, they are considered difficult to drive. InnerSound decided to make their own amplifiers, optimized for their own speakers, and the ESL-800 was the result. Since then, they have expanded their amplifier line to include the RS-1000, a 1000 watt monoblock, and the ESL-300, a 300 watt x 2 stereo power amplifier. Any of these products will drive their ESLs, or other manufacturer's ESLs for that matter, as they are rated into 2 Ohms.
The ESL-800 is an 800 watt (RMS into 8 Ohms) monoblock that will deliver 1200 watts into 4 Ohms. It uses 36 bipolar output devices and has only three gain stages. The input stage is differential, followed by a single-ended second stage, and complimentary output stage (bridged). It is a very clean design, using precision metal film 1% tolerance resistors, no capacitors in the signal path, and minimum use of wires.
Its minimalist appearance belies the massive raw power that it will deliver. Two small green LEDs on the front indicate power on and overload. I would like to see a row of LEDs to indicate relative power being delivered, such as 1 watt, 10 watts, 100 watts, 800 watts, and 1,200 watts.
The rear panel has the on/off toggle, and it is assumed you will leave it powered on all the time due to its high efficiency. There is an XLR input along with an RCA unbalanced input. The ESL-800 is fully differential (balanced), so connecting the XLR input makes use of this feature.
Speaker binding posts are large and gold plated.
The inside of the chassis has plenty of space for air flow. The output devices (bipolar transistors) are along the sides next to the heat sinks, while the rest of the circuitry is at the rear. The toroidal transformer is very large, at 2,000 VA. There are 80,000 µF of power supply capacitance, with ± 92 Volts on the caps, giving 338 Joules of energy storage. That is a big power supply!
I tested the ESL-800 in a two-channel setup, with a McCormack Transport and DAC, BAT VK-5i Tube Preamplifier, Carver Amazing Ribbon Speakers, and Nordost Cables (XLR balanced between preamp and power amp).
My Carver Amazings (ribbon speakers) are very demanding speakers, just like ESLs. I threw some very difficult material at it, including my amp-buster CD, Copland's "Fanfare for the Common Man" on Telarc. But, I think this CD is an amp-buster no more, as I could detect no clipping regardless of the volume, and I had it cranked. Usually, when an amplifier has a problem with this CD, the bass in the drum thuds does not go as deep as it should. Well, it went deep alright. No subwoofer was necessary to take up the slack, because there wasn't any slack.
The sound quality was in between my reference BAT VK-500 and McIntosh MC-602. The BAT is very smooth, while the Mac has some snap to the edges. The ESL-800 was somewhere in the middle.
Dukas' "La Peri" does not have the bass drums of Copland's Fanfare, but it has the intensity, and the ESL-800s delivered it completely. With a lesser product, the brass would have been "brash" instead, but not so with the InnerSound. All instruments, including trumpets and trombones, were distinct. There really is no substitute for raw power. I could not find the limits of this power amplifier within loudness levels that I could tolerate, and I decided not to push my luck any further than that.
With more delicate music, such as Bach's Brandenburg Concertos, the ESL-800 did just as well even when only using a small fraction of its power. Violins were clear and concise, and cellos kept their poise. Noise levels were at the minimum. I had to put my ear right up to the ribbon tweeters to hear any hiss at all, which was so low, it might very well just have been my imagination.
(You can click on the photos of the CD cover art to see more details of the albums.)
All in all, I was very surprised and satisfied with the ESL-800's performance. The InnerSound amplifiers could very well be a big undiscovered secret.
On the Bench
With a 1 kHz sine wave input, THD+N was within spec. I could not measure distortion at full output due to its balanced design.
IMD was very low, using 1 kHz and 1.5 kHz input sine waves.
With 5 kHz and 6 kHz signals, IMD was, again, very low.
At 10 kHz, this was the only time I found THD to be higher than spec.
With 10 kHz and 11 kHz input signals, IMD was creeping up, but still manageable.
The measured frequency response was within ± 0.5 dB, 10 Hz to 96 kHz.
The InnerSound ESL-800 Monoblock Power Amplifier is superb. Its massive raw power capability made my jaw drop on a continuous basis. For anyone who wants an amplifier worthy of being called a cornerstone for his/her audio/home theater system, the ESL-800 is certainly ready to play the part.