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ESLs and OTLs

I don't doubt the truth of what you say,...


Posted by Lew (A) on January 30, 2014 at 06:55:45

In Reply to: RE: I'm thinking that this amp was not class A or anywhere near. posted by morricab on January 30, 2014 at 01:04:19:

but those two instances do sound a bit odd. I wonder why that would happen; I know of no special problem with the two speakers you mention. But I don't know whether each of the two amplifiers is a totally kosher Futterman type, either. I think Bruce Rozenbitt adds some tricks to the standard formula for a Futterman, and Mr Park and his colleagues were a bit unorthodox in their Silvaweld designs, if my SWH550 phono stage is any criterion. (Not even Mr Park has a SWH550 schematic; I traced it painfully with my meter.) Maybe Ralph can comment. Even with loads that were very unsuitable for it, my old Futtermans never malfunctioned. It could sound bad into low Z loads, but it tolerated them in terms of stability. Circlotron will drive anything, but again low Z speakers are not the best match. However, the circlotron just seems to lose power into low Z, whereas the Futterman loses sound quality as well, suggesting distortion goes up. Did you consult with Rozenblitt, re your experience with his amplifier?



ESLs can be tricky due to the change in impedance from low frequencies to high, which can be by as much as 10:1.

If the amplifier has feedback and it is not handled correctly, this can cause stability problems. Futterman knew that ESLs were a likely load and it seems that he knew to be careful about how he set up his feedback to prevent oscillation. But it does not seem that this knowledge transferred to later amplifiers made after his death.

Another problem that I have seen in a variety of OTLs is the interaction between the grid capacitances of the power tubes, the coupling caps used to drive them, and the bias network to control them. In a nutshell, there is a tension between the value of the coupling caps and the values used in the bias network. The problem is you make the caps too large and they affect the sound at all frequencies- a problem anytime large film caps are used (regardless of their construction).

This has to be weighed by the designer against the values used in the bias network. The more power tubes you have the lower the values need to be in the bias network to maintain control of the tubes. If the values are too high, there will be voltage drops across the grid resistors, which can create instability.

So a lot of designs fudge the in-between area, trying to put the smallest coupling cap in the circuit they can get away with, while simultaneously trying to put in the largest values in the bias network they can get away with. This can result in instability, particularly if the amp is overloaded!

Between these two factors, the amp may wind up being unsuited for use on ESLs, even though if things are done properly the result is a match made in heaven (been that way since the 1950s...).

To complicate things, some more modern ESLs have very low impedances, in an attempt to make them compatible with transistor amps. What this really does is make for a very difficult to drive speaker (Martin Logans for example are frequently found with impedances that are half an Ohm or less at 20KHz)!

In such cases a set of ZEROs will often set things right. It does not matter the tube amp- the load has to be within the limits of the tubes. ESLs can often present problems to transformer-coupled amps because an output transformer transforms impedance in both directions- it translates the impedance of the tubes to the speaker and that of the speaker to the tubes at the same time. So a really low impedance is always a problem- one that is often solved by a set of ZEROs.

Anyway- if the amp is struggling with the load and also has some of the other issues I have outlined, it may well behave in an unstable manner. But that is more the fault of the design rather than if it is or is not an OTL- that really has nothing to do with it.


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Topic - ESLs and OTLs - Ralph 12:27:38 01/30/14 (20)

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