Home OTL Asylum

OTL, Output Transformerless Amplifier User Group.

[ Follow Ups ] [ Post Followup ] Thread: [ Display  All  Email ] [ OTL Asylum ]
[ Alert Moderator ]

Re: Atma-sphere with Martin Logan

ALL ESLs by the very nature of the design are essentially gigantic capacitors. (Think about it: a charged diaphragm enclosed between two grids carrying an AC voltage.) So ALL ESLs will have a falling impedance as frequency rises, just like the reactance of a capacitor. Yet, IMHO there is no better match for an ESL than an OTL tube amp, in terms of maximizing sonic performance of the ESL.
The "problem" with Martin Logan products in the past (I have no knowledge of their current products) is that they have designed their spkrs to mate as well as possible with transistor amps. Thus, they manipulate the impedance to be on the low side (ca 4 to 6 ohms), even at low audio frequencies, and as a consequence the impedance at higher frequencies can become unacceptably low for ANY tube amp, due to the nature of the ESL as a capacitor. (One iteration of the CLS that I owned dipped down to 0.5 ohms at it's impedance minimum in the upper frequencies.) This spkr (CLS IIA, I think) really didn't mate well with the Futterman-type amps that I then owned. When I phoned M-L to discuss the problem, they informed me that I was using the "wrong" type of amp. The CLS IIA was unuseable, but I have to admit that other M-L products I owned worked quite well with Futtermans and would probably mate even better with Atma amps.
It is true also that all ESLs impose a transformer between the output of any amp and the grid, in order to raise the voltage level of the audio signal. But this is always true no matter what type of amp is used. The solution is to build an amp that can directly output an audio signal riding on a high voltage. Such an amp can truly direct couple to the spkr with no transformer at all. Such an amp is also quite dangerous in the home. It's been done a few times commercially; I think the Acoustat company marketed a large ESL with a built-in and truly direct-coupled amp. To my knowledge, there is no product on the market now that can direct-drive an ESL with no interposed transformer at all.
Finally, how big a problem is this unavoidable reactance of an ESL? I think it is mitigated by the fact that the power required to generate a given sound pressure from a given spkr falls as frequency rises. Thus, although an amp with high Zout can deliver less power into an ESL as frequency rises, it may not pose much of a problem, and if one hears a problem in the highs with an OTL/ESL combo it's not for that reason, IMO. I do think from listening experiences that transistor amps with vanishingly low output Z often sound dreadful with ESLs, because they become unstable into the reactive load. So, I will take a tube OTL over a transistor amp any time for driving ESLs. And as long as we usually HAVE to have one transformer at the spkr, why use a transformer-coupled tube amp, which interposes yet another one?

This post is made possible by the generous support of people like you and our sponsors:
  VH Audio  

Follow Ups Full Thread
Follow Ups

[ Top Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2025, Audio Asylum, all rights reserved ]
[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]