While I enjoy and value Atmasphere's posts on the subject, I will take issue with the major point in the paper he presented. I don't see that these two paradigms exist at all . . . except in a hypothetical world where there is a simple, binary choice in available loudspeakers: Apogees and Lowthers.
If you look at the symbiotic evolution of amplifier and speaker designs over the past eighty years or so, it's commonly accepted that an increasing abundance of amplifier power enabled loudspeaker designers to trade efficiency for other factors, such as smaller cabinet size and improved linearity. But it has been the loudspeaker designers that have, in turn, been consistently demanding more "current impervious" performance from the amplifiers. This is why the hallowed amplifier designs of the pre-war era were triode designs: yes, for linearity, but just as importantly, for lower output impedance. Even an Altec VOT system and an Altec 604 duplex monitor would have presented very different impedance curves to the amplifier. And in either case, a flat frequency response from a linear amplifier was highly desired.
Even seventy years ago, loudspeaker designers were working with a voltage-source model, not a current-source model. While the reasons for it are my own speculation, they seem pretty obvious. First, high-frequency transducers almost always have a huge efficiency advantage over low-frequency ones. Second, advances in transducer technology are mostly advances in materials (diaphragm materials and suspensions, magnetic materials), and mathematical modeling (horns and lenses). Designing loudspeakers and crossovers to effectively take advantage of what the transducers have to offer is extraordinarily easier, and achieves better results, when working from a voltage-source model.
The presence/absence of multiple impedance taps on amplifiers, for this discussion, is a non-sequitur. If one wanted to design a conventional transformer-coupled tube amp that put out 50 watts into 16 ohms, 100 watts into 8 ohms, 200 watts into 4 ohms, etc. from a single output tap, it could be done . . . there would simply be huge tradeoffs in terms of efficiency and performance into a given impedance. Very similar tradeoffs also exist in solid-state amplifier design . . . the difference is one of cost and benefit. If you already have an output transformer, then adding additional taps usually makes sense. If you don't . . . then it's of course bit harder and costlier.
My point is that there really is no "Current Paradigm". The interface between high-fidelity amplifiers and their respective speaker systems have ALWAYS been based on a voltage model. (The term "high-fidelity" is meant to simplify the discussion by excluding things such as field-coil speakers and 70V distribution systems, not a snub to anybody's amplifier design.) And high-fidelity amplifiers have always been expected to have reasonably "current impervious" operation. What "reasonably" means in absolute terms is a debate that has been around many years longer than solid-state amplifiers . . . but if an amplifier's output is intended for a "4-ohm" load, then I would expect it to be fairly "current impervious" over the range of current that a "nominal 4-ohm" loudspeaker would require, plus some extra for good measure. Most good conventional tube amps achieve this.
I maintain that a high output impedance, for a high-fidelity audio power amplifier, is ALWAYS a liability, period. Now it may be that some of these amplifiers have other performance aspects that outweigh it, and some speakers are tolerant of it (and a few even subjectively improved). But this idea that there's one branch of the speaker-design profession that optimizes their products to work with amplifiers that have high output impedances? I don't buy it. If there is, then exactly what is the output impedance that they're expecting? |
Well, Cyclonicman, legend has it that James Lansing, immediately prior to his untimely death, wrapped a piece of Alnico V in a largish bath towel and "went postal" on the electronics staff at Altec . . .
But seriously, they did it by designing speakers that people wanted to buy, and that were more demanding loads for the amplifier. 40 years ago, virtually all amplifiers had 16-ohm output taps, and today, an amplifier's performance into a 16-ohm load isn't even a footnote. I guessing this is because, er, how many modern 16-ohm hi-fi speakers can you think of?
A great example is the Apogee full-range ribbons I alluded to. The two things that people remember about them are that they sounded amazing, and that they blew up amps. I have heard from a few sources about how these loudspeakers influenced Mark Levinson's amplifier designs . . . I'm not so sure that the timeline works out for that to be true, but the Apogees definately had a huge influence on the current output capability of "flagship" solid-state amps of the 1980s and 1990s. |
Hi Atmasphere . . . my main point is that hi-fi speaker designers simply do not consider an amplifier to be anything other than a voltage source, and that they never have. Further, it seems obvious to me that amplifiers have historically been intended to operate as voltage sources. And please believe that I'm not categorically critizing amplifiers that deviate from this practice, but I believe that a high output impedance, as an intentional, acceptable goal, is a completely modern phenomonon that is unrelated to what all but a very few speaker designs are anticipating.
The impedance at which an amplifier produces maximum power output, again, is completely non-sequitur. When I completed the restoration on the Marantz Model 2s currently in my system, I measured the output impedance at about 0.18 ohms from the 4-ohm taps - for all intents and purposes, a voltage source. This was the only tap I measured, but let's say that the 8-ohm taps have about 0.4 ohm output impedance. I would guess that my "4-ohm" Mezzo Utopias (typical reflex cabinet) would range from about 4-15 ohms. The modification of the speaker impedance on the voltage response of the amplifier would thus be about 0.3dB from the 4 ohm taps, and about 0.6dB from the 8-ohm taps . . . very little difference between the two. My point is that even if the load is mismatched and grossly affects the maximum power output, these 1950s-era amplifiers behave overwhelmingly as a voltage source, NOT a power source or a current source - if they're operating below clipping.
If I was to look for evidence that loudspeaker designers viewed an amplifier as a current source, here's what I would expect to find: Filter values and woofer conjugates in crossover networks that are calculated with the expectation of a high source impedance. Parallel resonant networks inside crossovers to dampen the impedance peak(s) from the cabinet/port. Standard models for calculating woofer responses from Thiele/Small parameters, that include a high source impedance. A specification from a speaker manufacturer that reads something like "recommended amp output impedance: 2-6 ohms". If I've been living under a rock, please tell me, but I've NEVER seen any of the above.
I chose the Apogee as an example of voltage-source thinking because I remember it being a very capacitive load, not simply low-impedance; maybe my memory fails me. But it doesn't surprise me that a capacitive speaker could sound nice from a high output impedance SET amp, for a couple of reasons. First, there's nothing like a high output impedance to keep an amplifier within its optimum current range . . . in the same way as a series resistor! Ditto for avoiding stability issues that many amps exhibit into capacitive loads. And third, I could easily see a capacitive load causing a resonant peak in the output transformer that might kinda offset the Ohm's-law HF rolloff. But again, I don't think the Apogee designers were anticipating these conditions.
Anyway, I find this interesting because there are so many "high-end" speakers out there that leave me scratching my head as to why they don't sound good to me at all, and I wonder if this is the way they're "supposed" to sound. |
Atmasphere, I owned AR-3s (identical to AR-1 except for mid & tweet) for many years, in fact I have some of the dog-eared original documentation right here . . . the only thing I see about a recommendation for the amplifier is "25 watts minimum per channel". In addition, for the frequency-response graphs, the Y-axis is labelled "OUTPUT IN DB (INPUT 6.3v)". Voltage source. QED.
I'm not familiar with the details on the Sound Labs, but sure, let's look at ESLs . . . how was the Quad II amplifier designed? Similar (low) output impedance to my Marantzes, and I think it's a pretty safe bet that they were originally designed with ESLs in mind.
And I totally lost you on the back-EMF from horns thing. Are you really suggesting that the inertia from, say, even a JBL 375 compression driver (huge diaphragm) could possibly generate any measureable back EMF? And then make it back through a couple of crossovers (N7000 and N500 in the case of Hartsfield & Paragon) to the amplifier? Ludicrous. Look at those crossover schematics and reverse the math, and it's pretty plain that they assume a constant input-voltage vs. frequency relationship.
I am in absolute agreement with you that there exist a great many bright-sounding solid-state amps with thin-sounding bass - and omigod, one of these on a pair of Klipshorns is seriously painful. And we're probably in agreement that simply raising the output impedance by sticking a resistor in series won't really help one bit. So okay, the sound is still bad because of transistors, feedback, the devil, etc . . . quite possibly. All of those to me are completely separate issues, each that deserves careful, systematic analysis.
The association of characteristics such as high output impedance, zero loop feedback, DHTs, single-ended output stages, single-driver full-range, L/C phono equalization, etc. etc. with each other is artificial . . . it stems from modern audio credo, not history or engineering. After all, the people who designed the classic audio gear were NOT triode purists, no-feedback believers, horn affectioniados, single-ended snobs, or whatever. They were simply using the resources they had to address what they felt were the biggest weaknesses of the audio chain.
We're lucky that so much of what they accomplished is applicable in a modern hi-fi context . . . but I think it's a bit of an insult to their work to assume that their philosophy fits neatly into one side or the other of a 21st-century audiophile belief paradigm. |
Okay, so I am a little jealous of your AR-3s. Mine went away during one of the audio-gear purgings that accompanied a cross-country move. I do have fond memories of the way they sounded in a bedroom system running off of a cheap Knight 6-watt tube amp, which uses 6GW8s in P-P and no NFB (with tone controls set flat). But they really came alive when I moved them to the main room and ran them with Mac MC75s . . . it's in this setting that I felt I had an idea how they were "supposed" to sound.
But FWIW, it's interesting that both the ARs and the Macs are gone, but I still have the Knight . . . it's running a pair of B&O CX100s in an office system. When I told this to the man who designed the CX100s, he of course looked at me like I had five heads . . .
Again, it's not that wonderful sound can't be obtained from amps with high output impedances, I just feel that it greatly increases the chances that when paired with loudspeaker X or Y, the sound will be less a "realization" of the loudspeaker's sound, and more of an "interpretation".
An analogy would be a performance of solo Bach . . . there are many shades of grey between a fresh, modern performance and one fraught with tacky rubato. And there is indeed so much room for opinion . . . but to dislike a "deviant" approach (i.e. Glenn Gould, Modern Jazz Quartet) is in my book a fundamentally more defensible position than to dislike a highly compentent scholarly approach (i.e. John Holloway). Ah, but what determines what's "deviant"? It's not simply the approach that's less in vogue, it's the performance that deviates more from what is found in the written score.
And I think that our point of fundamental disagreement is this: I feel that in defining the amp/speaker relationship, "the written score" is the voltage at the speaker terminals. And just like Bach, to deviate from "the score" isn't fundamentally bad (I like MJQ but don't like Glenn Gould), it just puts the amplifier on shakier ground.
Nelson Pass is one who has stood on this shakier ground for many years . . . but he manages to stay there because of the fundamental competency of his designs. There exist far more designs that have ventured onto the same shaky ground, and without a level of design competence to hold them up . . . and those amplifiers sink right through to join the Phase Linear 400s in the landfills, which is where they belong. I also have the impression that many owners of Pass' amplifier designs are willing to choose their speakers to make the amplifiers perform at their best, which is consistent with the traditional view of a amplifier with a high output impedance.
But I ramble. What I'd really like to do is conduct some measurements to determine how much back EMF comes from some 1950s loudspeaker drivers. And I just happen to have some prime specimens lying around waiting for installation - a pair of JBL 375 compression drivers, and four 15" JBL D130s - all just expertly rebuilt. As far as high sensitivity, small magnetic gap designs go, it doesn't get much better than this.
So I'd like your input on the test methodology. The 375s are easy - I'll feed it with a square wave (maybe 2KC) from a very high source impedance, like 600 ohms ;). If there is significant back EMF, it should manifest itself as ringing when viewing the voltage at the speaker terminals on a 'scope. I even have a N7000 and a N500 crossover networks to see their effect when placed in series. Sound good?
The D130s will be a bit harder, I'm thinking that I can set a pair of them face to face, and couple their dust caps together with a piece of memory foam (low time constant). I can then drive one and measure the back EMF from the other. I can flip the around the driving/driven connections to roughly calibrate the amount of input voltage that corresponds to a given cone velocity (null out the foam coupling), and then calculate the ratio of input voltage to back-EMF voltage in dB. I would do this at the hypothetical port-tuning frequency for a D130 in a reflex cabinet, where the effect should be the most pronounced in a real speaker. I would also use a couple of different loading resistors, to simulate the amplifier output impedance. What do you think? |
Hi Atmasphere . . . I'll mock up the speakers this afternoon and see what happens. It has since occured to me that I will need to in both cases null out the effect of voice coil inductance(s) on the measurements. I think that by calculating the EMF as a power ratio rather than a voltage ratio, it will remove the inductive kick from the voice coil from the equation.
Also, I don't think I have a suitable piece of foam that would couple the D130s together without introducing a lot of extra mass . . . so I think I'll just tape the edges and have them couple with air pressure. I'll make the measurements at the free air resonance frequency, which should be at the lowest point of modulation of the air pressure between the two cones.
For the 375s, I'm going to start by measuring the difference in power input, and change in input waveform, between having a lens on the driver, and having the throat plugged. That should easily separate the effects of the air loading from the diaphragm mechanical damping.
The fact that I'm using power calculations rather than voltage calculations is interesting per our previous conversation. I'll have to chew on that . . . actually, I might start a new thread for the results.
There are three reasons why I've kept the little Knight KG-240. First, it's really useful for a secondary system - it's very small and compact, doesn't put out too much heat, reliable, and sounds fairly decent. Second, I have really come to appreciate the engineering behind it - it's definately a flawed piece, but it was sold for $30 in kit form, and it uses every cent of that in a well-balanced manner to perform as well as it can. Third (and most importantly), it was my father's . . . he bought it at a time when he could afford very little, and soldered it together himself on the kitchen table. He used it for over ten years as the only stereo in the house . . . and it's been used on and off for another three decades. I'd say he got his $30 worth. |
