Tube Amp for Martin Logan Speakers

03-27-14: Atmasphere: do so despite George's remonstrations

Are you kidding, far from pleading with you Ralph.

I agree that an Autoformer is a great (pseudo) bandaid fix for whatever amp that cannot drive a speaker that is not a good match, because of it's inability to deliver current/wattage or is working out of it's SOA (safe operating area) A FAR BETTER FIX IS TO CHANGE THE AMP/S OR SPEAKERS!

You state that even a good solid state amp will measure less distortion on it's output from using a Zero Autoformer into a 4ohm load compared to 8ohm.

Yes I agree that with the Autoformer even a good solid state amp will measure slightly less distortion at it's output terminals at 4ohms (this is not to say it's out of it's SOA)
But it's what happens after the Autoformer (things like phase shifts, damping factor etc) that is far more detrimental to the sound than the slight increase in distortion before it without the Autoformer.

EG: What one would basically do to a good amp say like a Pass Labs, Krell, Gryphone or similar amp by sticking an Autoformer on it, is to turn it into the solid state series MacIntosh's that also use a type of Autoformer on their outputs, but without the benifits of using them within the amps feedback loop, to keep the (damping factor, phase shifts, etc) in check.

Cheers George

There are many, many speaker cables to choose from. I'm confident that one would find many (if not most) will work just fine with a 4 Ohm load speaker.

This is for Ralph, since I'm quoting him. But any others are welcome to jump on it as well.

"I am not stating that 4 ohm speakers are bad speakers. I am stating that any amplifier driving them will sound harsher and less detailed as opposed to the same amplifier driving the same speaker that was 8 ohms, were all other matters to be equal."

I admit my knowledge of electronics is very limited, but this raises a question for me. Is the singular impedance rating of any speaker worthwhile or even all that important? Or is a better question what is the impedance plot across a speaker's response range?

I ask this because for many years I owned Duntech speakers which were rated at 4 ohms. However, reviewing Duntech's impedance plot, it dipped close to 2.5 ohms at two points, the most troublesome being between 60 and 80 Hz as I recall. Considering the power called for at that frequency range and the dip below 3 ohms, well, not ever amp rated to drive a 4 ohm load got off easy!

So my sense is to consider the full impedance curve, not just a static number. Maybe that is where "all other matters" are not equal.

I do not manufacture amps that may or may not need the Zero in circuit, so there are no ulterior motives for me to hide or protect.
I just don't like to see blanket statements like the Zero will improve all amplifiers regardless of topology.
They have their place in that they make an amp that has trouble to drive certain loads with relative ease, able to do so.
But it is not the definitive answer, it is a bandaid fix, changing the amp/s or speakers is the way to go in these cases.

Here is another point made from Dick Osher's review of the Zero Autoformer that was conveniently left out of the link to the excerpt of the review.

"T-Rex 300B SET, a DIY design project in the works (still unpublished) using Plitron output transformers with only 8-ohm taps, and the Lowther DX4 BassZilla - a 97dB.
At his point, it became clear that although bass lines still benefited from the ZERO, overall the T-Rex - BassZilla interface sounded better with the ZERO out of the way."
"Small, but noticeable errors of commission crept in. Soundstage transparency diminished, which reduced the music's intimacy. Microdynamics now sounded slightly over damped, releasing some of the music's tension. The midrange, which for me paints a window onto the music's soul, became slightly cloudier. On balance, I would rather give up 20% of bass definition for a 10% increase in midrange clarity.":

As you can read the 300B set amp (and others eg OTL's) not known for good drive current, was improved in the bass area with the Zero, but the midrange diminished by 10%
The same 10% detriment will happen to the midrange using a Zero with a good solid state amp or powerful tube amp that have no problem in delivering good bass without the Zero.
The word Dick Olsher used "cloudy" in reference to the midrange is also what I heard in my system with the Zero's attached to a good SS amp that could drive the speakers with no problems, and as well as the bass tightness and drive diminished just as much with it in.

Cheers George

George, not having ever used the ZEROS, I can't speak to their pros or cons. Having said that, as I mentioned in one of my posts above, I tried a couple of gizmo tweaks in the signal path in various places and was disappointed. So, I suppose based on those experiences, I would be biased (pun) against inserting **any** artifact in the signal path .... unless my back was up against the wall.

For that reason, I would shy away from using ESL speakers with wacko impedance and phase angle curves, even if ZEROS would enable my amp to drive them. At one point, I was wondering whether picking up a pair of QUAD 2805s would be worthwhile. After doing some reading about their impedance curve, I decided against it.

Hey ... there's a lot of amp ball-busting speakers out there. The impedance and phase angle specs of some "fav" dynamic speakers are killers. So I happen to concur with the parts of your last post that counsels caution when matching amps and speakers.

As I also said above, it is only through dumb luck that my ARC Ref 150 seems (??) able to do a good job with my Paradigm S8s. Talk about amp ball-busters. I think the reason the combo works is because (i) I use a self powered sub woofer to pick up some of the low end load, (ii) the S8s are spec'ed at 92 db sensitivity, (iii) the Ref 150 is rated at 150 wpc, and (iv) the Ref 150 can presumably deliver serious current because it has a beefed up capacitive power supply of 1040 joules.

Btw, I currently use my amp's 4 ohm output taps. Output impedance is probably on the order of .5 ohms or so. If correct, the "actual" damping factor is probably 8 or 10'ish in the S8's bass region (60 Hz to 500+ Hz) because the S8's spec'ed impedance is about 4 or 5 ohms in that frequency corridor. That is clearly better than if my amp had an output impedance of 4 ohms, in which case the DF would be about ONE (1) .... not so good.

Finally, I think there is more we agree about than not. Hopefully, we'll all read each other's comments more carefully to avoid talking past each other.

Cheers,

Bruce