Tube Amp for Martin Logan Speakers

Unsound, you state above "that [if] one can get greater sound pressure into a 4 Ohm load with a ss amp, then we can use that extrapolation to suggest that as frequency response is measured in dBs, one can extend bass response and therefore extend frequency response." With respect, IMO, your statement may be a non sequitor.

Here's my thought process. A speaker's SPL is a function of its sensitivity and the amount of watts the amp is driving into the load. While it is true that a 4 ohm load will draw twice as much current as an 8 ohm load, I fail to see why that necessarily means the speaker's SPL is necessarily twice as high at 4 ohms.

Consider my speaker, the Paradigm 8s. These beasts have a impedance curve that looks like a roller coaster, ranging from a low of 4 ohms in the bass/low midrange FR range and a high of 20+ ohms at the midrange/tweeter x-over point of 2K Hz. Yet ... the S8s have a very flat FR.

How can that be, I rhetorically ask? The reason is that these speakers were designed to be driven by a low impedance/high current SS amp. The fact that impedance rises to 20+ ohms at 2K Hz, thereby resulting in the speakers drawing LESS current and power from the amp, only means that the drivers are very sensitive at that frequency. Otherwise, the speaker's measured FR would be grossly skewed.

Consider Maggies. I recall reading that the 1.7s have a flat impedance function of 4 ohms or thereabouts, yet are not sensitive. As a consequence, one needs a beast of a SS amp to drive these pups to decent SPLs. In fact, I also recall reading a review of my Ref 150 that mentions the reviewer hooked up my Ref 150 amp to Maggie 1.7s and found the SPL just adequate.

As far as extended FR is concerned, I surmise that has more to do with the speaker design than it's impedance spec as a function of FR. I suspect what may have a more perceived impact on low end bass response is the amp's DF and the speaker's impedance spec. As Ralph, Al and others have mentioned elsewhere, DF is a factor equal to the quotient of a speaker's impedance at a particular frequency, divided by the amp's output impedance. The output impedance of most SS amps is so low that the amp's DF is not really a relevant stat.

But, if the amp's output impedance is "low'ish," say .5 ohms, and the speaker's impedance is "only" 4 ohms, ironically the DF will only be 8. By contrast, if the speaker's impedance is 8 ohms, then the DF jumps to 16. I surmise that the doubling of the DF may (??) have the effect if making bass sound tighter and perhaps more extended sounding.

I think Ralph was probably on target when he said that if a speaker presented a flat impedance load as a function of FR, say 8 ohms or higher, and zero phase angle over the speaker's full FR bandwidth, I think pretty much any SS or tube amp would be a very happy camper.

But as far as a speaker's FR being flat, extended or whatever, I think that has more to do with the speaker's (i) design objectives and build, (ii) impedance variations, (iii) phase angle stats, and (iv) SPL sensitivity. Of course, also critical is whether the designer intended that the speaker be driven by a SS or tube amp.

Really sorry for the long post. I hope it's cogent and the reasoning linear.

I'm sure I mixed and matched concepts, but hopefully our tech members will correct my meandering musings.

Regards,

Bruce

Bruce, thank you for your thoughtful response. It's late for me now, I'll try to find time in the near future for a more deserving answer. In the mean time, you might want to reread the chronology of the posts above. I think you'll understand why I ended that post to Atmasphere with a :-).

Unsound, I don't see the comments you found doing anything but supporting my position, and also this:

http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

The bit about Jim Theil using drivers in parallel to get more current out of the amp at low frequencies is a classic Voltage Paradigm move. The problem is it also increases the distortion of the amplifier.

As the other quotes you found also state (paraphrasing) - if sound **quality** is your goal then there is no argument for 4 ohms. If sound **pressure** is the goal then there is a weak argument for 4 ohms if you have a solid state amplifier that supports that operation.

Now in the case of Jim Theil, by using two woofers in parallel, and assuming an amplifier that can produce constant voltage into most loads, the simple result will be that it makes twice as much power into the woofer array than it would if there was only one woofer. Its also likely that the woofers chosen may well be 3 db less efficient than the midrange and tweeter, if by any chance the latter are 8 ohms.

Now in the case of a Martin Logan ESL panel, the panel impedance is not the result of the behavior of a driver in a box. It is the result of a capacitor, whose range of impedance is set by a matching transformer. As a result, its not to your advantage to see an increase in power at low impedances- the panel is as efficient at 10KHz as it is at 1KHz, even though the 10KHz impedance is much lower.

It is for this reason that it is perfectly reasonable for one to use a set of ZEROs on the speaker, even though there is a matching transformer in the speaker, and perhaps another one in the tube amplifier employed. IOW, the 4 ohm tap on many tube amplifiers may not be enough to allow the amp to deal with the 0.5 ohm impedance at 20KHz that is common with many ML ESLs. Heck, a lot of transistor amps have trouble with that too- especially if a speaker cable is being used between the amp and speaker.

Atmasphere, My disagreement was and continues to be with your earlier post(s):
"Now it is a simple fact that there is no good argument for lower impedance (4 Ohms or less) in high end audio; that is to say if **sound quality** is your goal."

^^ OK. Its my contention that reducing distortion in the reproduction chain leads to better fidelity/better sound, especially if the types of distortion we are reducing are the types to which the ear is particularly sensitive.

In this regard I favor sound quality over sound pressure (volume); I think you will find that most audiophiles do.

You can see in the specs of any amplifier (solid state, class D or tube) that the distortion is indeed higher driving a lower impedance load. If you think the distortion involved is negligible, its not, it usually is of the types that the ear cares more about.

This can be so significant that the use of an autoformer to raise the load impedance to an otherwise very capable solid state amplifier can result in improved sound. Were this not the case, the insertion of the autoformer would have adverse effects.

Here is the text of some communications that Paul Speltz (designer of the ZERO) has received from a well-known solid stage amplifier designer:

Hi Paul -

I hope you had a nice weekend out in the woods - it's cold out there!
I'm still enjoying the autoformers very much, and they are continuing
to improve as they break-in. I wanted to comment on why I finally
decided to try them (and wish I had a long time ago!). In the past, I
had always thought of autoformers as a "speaker tweak" or a kind of
crutch for amplifiers that couldn't handle difficult loads. I figured
that my amps could drive anything, so why worry about adding extra
boxes that I didn't need. Over the past several years, though, I have
been working more and more with line-level coupling transformers (part
of my new VRE-1 preamp design) and I have seen some evidence that
suggested to me that something in the nature of the load with the
transformers made the source driving circuits "happy," to use the
scientific term ;-) Thus I came to be interested in the autoformers as
a possible *amplifier tweak.* (I guess maybe you see them from the
speaker's point of view, while I tend to see things from the amp's
perspective.) Whatever the reason, they certainly work well, and I do
feel that the amp is "happier" with the conjugate load. Maybe it's
nothing more than a more benign impedance, but I suspect there is
something more at work. I suppose it might be possible to try them
with a high-efficiency,16 Ohm speaker as a test of some sort, but I
suspect that the sound would still improve with the autoformers.
Anyway, whatever the truth of the matter is, I am a convert.

Thanks again, Paul.

Happy holidays,

Steve McCormack
SMc Audio