Would switching amps make a big difference?

First i ordered a pair of the Zero Autoformers.
Just put them in a few days ago and the results were HUGE with my tube amps.

The trick with getting the highs right is something called gain bandwidth product. With most amps this is pretty poor. This can result in a harsh high end and is why a lot of solid state amps have troubles in this regard, especially when confronted with a low impedance load at the same time. The ZEROs at least help out with the latter. Making the amp's job easier is usually audible :)

Of the list above I would do the Sachs or deHavilland. The Backert Labs is an excellent preamp; IIRC it employs an output transformer. Since your amp is single-ended input I don't see an advantage with that. I also have a preference for 6SN7s as a line stage tube- to me they are more robust and musical than 6922s/6DJ8s or any of the 12A*7 series (we use the 6SN7 in the line section of all our preamps); that is how I arrived at this opinion.

The performance of a solid state amp might sound subjectively brighter but that’s not because it’s tilting up the highs, but because it’s not rolling them off. A speaker designer who designed a speaker with a highly variable impedance AND tuned it for a flat power response would be an idiot.

@rwortman:
On an ESL sold state is going to sound bright on two accounts- first it is indeed tilting up the highs because of simple physics: as a voltage source it will attempt to double power as impedance is halved, and since ESLs have decreasing impedance as frequency goes up, there are your tilted highs. This simple fact is inescapable. Google the impedance curve of any ESL and you will see what I mean.

The second reason is low level higher ordered harmonic distortion, which causes solid state amps to be bright to begin with and this is why tubes are still around 60 years after being declared ’obsolete’.


I would be hesitant to call speaker designers who

designed a speaker with a highly variable impedance AND tuned it for a flat power response would be an idiot.

an idiot.

Some of them are quite brilliant, and like the way tubes sound. You might also consider that before about 1956 or so, all speakers were driven by flat power response as that was the only game in town.

@mijostyn Don't include the parentheses at the end of the link.

A resistor will simply absorb power. The ZEROs transform the impedance so you get more power and less FR error.


@viber6  This statement is incorrect:

Actually, I think that although the SS amp has the capability of providing 2x the power as impedance is halved, the SS amp is still flat in freq response at any given moderate power demand for higher impedance.  

If the amp is behaving as a voltage source, it will double power as impedance is halved. This also means it will cut power in half as impedance is doubled. The solid state amp will only have flat response if the speaker its driving is intended to be 'Voltage driven'. ESLs are a technology that isn't based on the Voltage Paradigm (see http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php for more on that). Under the voltage rules, the impedance curve of the speaker is also effectively a map of its efficiency: peaks in the curve represent resonance, dips represent a loss of efficiency (such as at crossover points). For example if you have a woofer in a box it has a resonance in that box. To control that, the amp has to put out *less* energy (power) into that resonance, which is also seen as a peak in the impedance curve in the bass.

ESLs are not based on a driver in a box. Their impedance curve is essentially based on a capacitance. So with a typical ESL where the impedance varies over about a 10:1 range (the Martin Logans are no exception; 4 ohms in the bass and 0.5 ohms at 20KHz) you can see that to make 92 dB at 50Hz a Voltage Paradigm amplifier (voltage source) will make X amount of power; to do the same thing at 10KHz it will have to also make X amount of power, but in reality is will be about 4X the power because its voltage output is constant with respect to impedance and at 10Khz, the MLs are about 1 ohm. The only thing that prevents this from happening is that these HF impedances are so low that the speaker cable itself has a DCR that becomes significant, and a good number of solid state amps can't double power into such low impedances (IOW they are not perfect voltage sources). IMO Martin Logan is trying to make an ESL that works with solid state rather than tubes but to this end (again IMO) they are only partially successful since brightness is part of the result; inevitable when you mix transistors (Voltage Paradigm) with ESLs (Power Paradigm).

Any thoughts on which tube amps work best with Martin Logan speakers?

The problem you are up against is that Martin Logan is trying to make an ESL (which is normally a pretty good combination with tubes) to work with solid state amps. The issue is that ESLs have an impedance curve that typically varies by about 9 or 10:1. Whatever impedance it has in the bass, it will be about 1/9th or 1/10th of that at 20KHz.


Since solid state amps tend to double power as impedance is halved, right away you can see that the MLs will be a bit bright if the amp actually got away with that. So they make the speaker really low impedance and that prevents the amp from being as bright because even solid state amps have troubles driving 0.5 ohms. But fortunately there isn’t a lot of energy up there so the amp isn't required to make a whole lot of power at that frequency. But the flip side is no tube amp can drive such a low impedance and so makes no power at all.


If you find out that you prefer tubes over solid state (maybe because its not as bright) you have a problem- how to drive such a low impedance? That is what the ZEROs are for. IMO/IME you are better off putting out the match rather than putting out the fire- get a set of ZEROs and then your amp will drive the MLs pretty well. I know it works- we’ve had customers with MLs for nearly 30 years. You’ll get your highs back (and likely the amp will sound better overall) without it being bright.

I've heard our amps in several customer's homes using our amps. But all of them were also using the ZEROs. No problem playing the highs at all.

A. If Martin Logan speakers is 4ohms, .8 ohms at 20kHZ - ideal amps would have what output impedance?

B. Tube amps you might suggest?

There is a device meant for this sort of thing called the ZERO (www.zeroimpedance.com). Essentially it allows the amp to operate at a much higher load impedance while driving lower impedance loads. Its an autoformer, which is like a transformer but only one winding.

Because it only has to deal with a fairly low impedance translation, it actually has wider bandwidth than any tube amp made, even ours (which are full power from 2Hz to 100KHz or more) as it can go from about 2Hz to over 1MHz!

So if you seem to have enough power, you might try a pair of these between your amps and the speakers.