Tube Amp for Martin Logan Speakers

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?

Once again I disagree with Atmasphere's oft-repeated assertion.

03-17-14: Unsound: Once again I disagree with Atmasphere's oft-repeated assertion

Agreed, I have yet to listen to a Zero Autoformer that has worked without causing other problems.

A simple way to hear what they do wrong, is to put one between an amp that has no problem driving the speakers/load with out it.
Then you will hear what they do wrong with it in.

In my view they are a bandaid fix to allow an amp that has no chance of driving the speaker by it's self, to make it usable with that speaker. And I use the word usable lightly.
Far better off getting the right amp for the said speaker to start with.

Cheers George