Tube Amp for Martin Logan Speakers

The trick with Martin Logans is that quite often if you have a tube amplifier, you will want something like a set of ZEROs (http://www.zeroimpedance.com) to help the amp drive the really low impedance of the speaker, particularly at high frequencies.

ML has been trying to get their speakers to work with transistors; to this end they have kept their impedances low. The problem with this approach is that ESLs and tubes have been the match made in heaven since the 1950s- you put transistors on the speaker and brightness is the name of the game. So it you want to use tubes, you have to deal with the impedance and that is what the ZEROs are for.

Atmasphere,

What's the difference between an output transformer and an autoformer? Also, as most tube amps are not OTL's, is there a drawback for having both in the chain? I always thought autoformers did pretty much the same thing as OT's.

Zd542, I was tempted to take a shot at your question but decided not to. Ralph (Atmasphere), Al (Almarg) and several other members are EEs and will do a much better job responding.

Having said that, I will just put out there this simplistic explanation. An autoformer raises the speaker impedance presented to the amp, or perhaps it might be better said, that the device makes the amp think the speaker impedance is higher than it actually is. There's a white paper on the Autoformer web site. Just do a search.

I'll anticipate you next question which is why can't one simply insert a resister in series with the speaker. I think the answer is that it will draw off a lot of watts as heat and will make the output impedance of the amp look higher, thereby screwing up the DF. It may also affect bandwidth ... and there I stop.

Looking forward to reading a better explanation from the EEs.

Bifwynne, if you installed a resistor to raise the impedance of the load it would certainly absorb power. That is why you use a transformer, in this case an autoformer.

The reason the ZERO is an autoformer is that it does not have to block DC, which makes it simpler and better performance (wider bandwidth). The ZERO has wider bandwidth than almost any amplifier made, tube or transistor.

The second thing about the ZERO is that the amplifier already has a lower output impedance, its not like 3000 ohms or something, instead we are simply going from 16 ohms down to 4, 3 or 2 ohms. With such a low resulting turns ratio again greater bandwidth is possible.

Now its a simple fact that there is no good argument for lower impedance (4 ohms or less) loudspeakers in high end audio; that is to say if sound **quality** is your goal. If sound **pressure** is your goal then you have a weak argument for lower impedances, if you have a solid state amp capable of supporting that lower impedance properly; not all do.

This is where the ZERO comes in- for those speakers that unfortunately are lower impedance due to poor design choices on the part of the designer (I added that last caveat because that is precisely the problem), the ZERO is a problem solver. Certainly it will help a smaller OTL drive such loads, but it will also help transistor amps that might seem perfectly comfortable on 4 ohms to actually sound better (although with less power). IOW, a transistor amp will sound smoother and more detailed when driving a higher impedance (due to making less distortion)- enough so that there can be a benefit to using the ZERO for that reason.

Paul Speltz, who makes the ZEROs, has a letter from Steve McCormick describing this.

I don't see the ZEROs as obviating the 'OTL sound' either. As I mentioned earlier, the ZERO has very wide bandwidth, wider than any amplifier; it would not be possible if it had to block DC or work with a high impedance input!

If speaker designers ever sort out how the load affects the distortion produced by the amplifier then we will see higher impedance speakers and no need for the ZERO. But I suspect that day will never come, so the ZERO will continue to be quite useful as a problem solver for those prefer sound quality over distortion.

Ralph, ... if a speaker has a rock and roll impedance curve that varies from 4 ohms to 20 ohms as a function of FR, would using a ZERO to double the impedance, say ranging from 8 ohms to 40 ohms, cause a Voltage Paradigm amp (i.e., a SS amp or low output impedance tube amp) to choke driving such a speaker?