Tube Amp for Martin Logan Speakers

With a measured damping factor of 4.7 and an output impedance of 1.5ohms it's closer to an 8ohm tap than a 4ohm tap.
But all this can change with a bit more global feed back to raise the damping factor and lower the output impedance.

George, this may come as a surprise but increasing feedback does not affect output impedance. It stays the same, and the proof is simple.

Your assertion that they "solve a problem" is correct, technically. But at the cost of sound quality. If sound quality is not a priority, then sure, buyers have indeed got their mismatched components to "work" together. But only that. The degradation of sound quality is obviously secondary to them. As a manufacturer, I would have thought your emphasis would be on making sure your amplifiers were matched with speakers that show them at their best ? Surely you're doing your products a disservice, by suggesting the Zero's as a bandaid panacea for buyers ?

Tradeontheweb, sounds like you and I are more on the same page than different. Whenever someone calls me and is interested in one of our amps I always ask about their loudspeakers to see what I am up against. Sometimes they have speakers that they don't want to sell that are IMO not all that compatible. Sometimes they won't work even **with** the ZEROs. But if they will work, I suggest them. Quite often they ask 'doesn't that defeat the OTL aspect of the amplifier?' to which the answer is 'No.'

That is because the ZERO has such a low turns ratio due to its only going from 16 ohms down to 4 ohms or so. It also does not have to block DC; between the two that gives it more bandwidth than most amps. You have to start with an amplifier that already has a low-ish output impedance and puts out no DC...

People also ask if it degrades things as you allude to, the answer to that is "The ZERO is a problem solver and there will be no tradeoff whatsoever, if not it is either not being used properly or does not need to be used at all."

Now its the former in that last statement that needs some looking into. For example, you can set the ZERO up to run only a certain portion of the loudspeaker. An example might be a speaker that has a 4 ohm woofer array but is 8 ohms in the mids and highs, and the speaker can be bi-wired. In that case you can recover some of the lost impact in the bass by using the ZERO only on the woofers.

There are those cases where a person has a speaker that is so inefficient and/or with an adverse impedance curve that there is nothing for it; then I let them know that:

If you want a tube amplifier, your tube investment dollar in whatever amplifier that is will be best served by a speaker that is higher impedance and/or higher efficiency.

The ZERO only comes in when it is obvious that a person is simply not going to budge on their loudspeakers and that is that.

Again, the ML ESLs have a very low impedance as they want more of the solid state market; to use them successfully with tubes the ZERO is an excellent option. The reason for this is simple- a tube amp using a set of ZEROs still has an excellent chance of sounding better than transistors for all the reasons that have been debated about tubes and transistors in the last 50 years. The ZEROs don't change that.

If one looks at the Zeros as a type of adapter, its not that complicated to determine the value case by case, which will vary widely.

Some cases they will add value, but only when an adapter is what is needed to make two devices not designed to work well together work well together.

But clearly if starting from scratch, in most cases, one will be better off NOT using an adapter, especially one that costs big bucks in order to bridge a gap.

FRom the perspective of an amplifier vendor whose product plays by a different set of rules than most, there are surely many good applications of the adapter.

However, if speaker maker and amp maker play by the same set of rules, there should be no need for an adapter if they dd their job well. IF they didn't, they will probably go out of business and no adapter will help save them.

04-04-14: Atmasphere : George, this may come as a surprise but increasing feedback does not affect output impedance. It stays the same, and the proof is simple.

Oh yes it does Ralph it may supprise you even more, as I can show from many many others even smarter that all here put togther also on the net, as this one sample below, from your OTL opposition, Bruce Rozenblit of Transcendent Sound.

Just read at the bottom of the page under, Single Ended, Push Pull, OTL and the conlusion all highlighted for your easy reading. The same also applies to SS amps.

http://cc.bingj.com/cache.aspx?q=negative+feedback+transcendentsound&d=4590165989524871&mkt=en-AU&setlang=en-AU&w=VZvSRMth7T4nXfW-P63Cwy4N-Vhpuv2m

Cheers George

Ralph and I and others discussed his views on the relation between feedback and output impedance in this thread. It essentially comes down to a matter of terminology.

Regards,
-- Al

Al and Ralph, I read the article that George kindly provided. I also re-read the posts at the link Al provided. I think Al correctly states that there is a bit of friction caused by the use of terminology.

I was looking for, but didn't find the post where I coined the terms "synthetic" or "apparent" constant voltage source where negative feedback (NF) is used. I think that using such terminology is useful insofar as it helps one to better appreciate whether an amp becomes a true constant voltage source if NF is used or just acts like a constant voltage source to some extent.

Let me explain. My take-away from George's article is that if an amp has an output impedance that is greater than Zero, some part of its power output is being consumed internally, resulting in a voltage drop at the output taps when driving an external load.

The author illustrated this effect with some simple examples using an external speaker load of 8 ohms and two hypothetical amps -- one having an output impedance of: (a) .1 ohms; and (2) the other having an output impedance of 2 ohms. The math then follows illustrating how the amp's output voltage and corresponding power output changes under each scenario.

I surmise that the terminology confusion comes into play because of what NF does ... and does NOT do. That is, if an amp has an output impedance of say 2 ohms, then as Ralph says, that is its output impedance ... period. But if NF is used, the amp compensates for the voltage drop by increasing output voltage to comepensate, thereby causing the amp to perform like, but not actually be, a constant voltage source amp.

As such, with output voltage adjusted to offset the voltage drop at the output taps resulting from the interaction of the amp's own internal impedance and the speaker's impedance, the amp functions as though it was a constant voltage source amp, or like one to a greater extent.

But there is always a BUTT. This picks up on what Al, Ralph and the author of the EPDR article have been saying over and over again. Using NF cannot create energy. That is ... an amp using NF will be able to generate power only to the extent it is capable of doing so based upon its internal architecture. Or, as the EPDR article said, within its SOA (safe operatwing area). If the amp operates outside its SOA, it will cut back its power output, clip, become unstable, produce distortion, etc.

I said this at the beginning of this thread and I'll repeat it again. I think we are all agreeing with each other much more than not. Butt, I think we are talking past each other.

IMO, given that **most** (not all) speaker manufacturers seem to be making speakers with roller coaster impedance and phase angle plots, erring on the side of am amp having lots of SOA headroom and low output impedance is a safe play. That generally equates to a SS beast that can deliver lots of current/power.

OTOH, as I said here and in other threads, to illustrate my understand, my ARC tube amp functions within a pretty wide operating corridor like a SS amp, but to do so, requires NF, and a massive power supply. Or using my own terminology, my amp performs like a "synthetic" SS amp, to some extent within a large "SOA."

Hope this helps ... butt it probably doesn't.