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. |
Kirkus, the Acoustic Research AR-1 is a good example of a speaker that was designed with intention to be used by a 'current source' amplifier. They recommended an amplifier with an output impedance of 7 ohms. Sure enough, it actually does sound better with such a thing.
The AR-1 was the first production acoustic suspension loudspeaker.
When speakers were first created, so you didn't have to wear headphones, there was nothing out there that was practical except horns. This was a long time ago- 1910s and 1920s. The only amplifiers around were triode class A zero feedback. They were the only game in town. Around WW2 the idea of negative feedback was developed, and the debate around it at that time was the 'listener fatigue' that often resulted.
The effects of odd-ordered harmonics were not understood but the effects of them were.
After the war the feedback debate continued. In the meantime, loudspeakers continued to be built that expected a fairly high output impedance out of the amplifier. Many of those speakers (Altec, JBL, Klipsch, EV, Lowther, Quad) are collectable and sought after today.
Feedback began gaining ground in the 1950s with the main proponents being Marantz, McIntosh, Fisher and Electro-Voice. EV and Fisher in particular were cognizant of some of the underlying issues and often recommended variable current feedback as opposed to voltage feedback. Variable, on account of it does not work the same way, depending on the intention of the speaker designer.
In 50-60 years since, we are still facing the same issues. How a voltage source does not work with some speakers is a transistor amplifier on an ESL- Sound Lab for example. Sound Labs have an impedance curve based on a capacitive nature. When a transistor amplifier with high feedback (voltage source) is put on a speaker like this, the highs are too pronounced and there is no bass. The speaker has an impedance over 50 ohms in the bass. Put a tube amp (which tries to make constant power) on this load and all of a sudden the speaker is making bass.
The highly reactive nature of horns is another technology that does not work so well with 'voltage source' amplifiers. Often the back EMF produced by the speaker gets into the feedback loop of the amp, causing excess harmonic generation- certainly not a lot, but enough so that horns get the reputation of being harsh and honky. Anyone who is running a 'current source' (zero feedback) amplifier on horns knows this reputation is ill-deserved.
Another sign that the 2 paradigms exist is amplifier specification. How often have audiophiles experienced the phenomena of the specs saying nothing about how the amp sounds? In fact, sometimes a negative correlation is perceived (higher distortion on paper--> better sound).
Speaker designers have been designing for 'current source' amplifiers for a long time. If anything, there are more of them now then there were 50 years ago (there are more tube amplifier manufacturers in the US now then there were in 1958...). So this issue is very much with us.
At the crux of the paradigm debate are the rules of human hearing. On the one hand (Voltage Paradigm), the only rules respected are human limits of hearing (20Hz-20Khz) and decibels, primarily resulting in a set of inaudible benchmarks that have little to do with how we hear. OTOH (Power Paradigm) the RHH (Rules of Human Hearing) are the *only* thing that matters, eschewing the bench measurements as having no meaning if they mean nothing to the human ear.
This is at the root of the tube/transistor debate and the objectivist/subjectivist debate. Its not that it does not exist- it **is** that it won't go away quietly, no pun intended... :) |
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. |
Hi Kirkus, I have a set of AR-3s myself- I use them for monitors. They are power hungry but they like low feedback amps just fine.
Actually, the idea of putting a resistor in series with a transistor amplifier is a good one. Nelson Pass suggests that in an article he wrote about a year ago. This simulates a high output impedance amplifier quite nicely, and mellows out a lot of horn systems when used with transistors.
If you think about high efficiency horns, one thing that should be obvious is how much tighter the voice coil gaps are. Take a look next chance you get. Apparently you don't believe it but yes, the back EMF they produce by their very nature **has** to be higher- they have greater efficiency, going the other way they will have more output. Any voltage that is not part of the output of the amplifier is something that the amplifier is supposed to correct if it has feedback.
And yes, you are correct, in the old days designers were simply working with what they had. What they had were amplifiers with high output impedance. Amps like that are still around today. Sure you can build a tube amplifier with a lot of feedback, but then again that amplifier will likely sound harsh. This is all about the difference between designing to meet the rules of human hearing as opposed to designing for arbitrary rules that exist only on paper.
Again, look to Nelson Pass- read his articles- as one who began wondering over ten years ago why people would not give up their tube amps. He started building zero feedback transistor amplifiers and given the right (power paradigm) speaker they are some of the best-sounding transistor amps around.
I've heard many Quad systems in my day from the 57 and 63 on. So long as the amplifier can deal with the low impedance at high frequencies, and amplifier that otherwise plays constant power on the speaker will also be the one that makes it play bass.
In recent years Quad has followed Martin Logan in trying to develop low impedance ESLs so transistor amplifiers will work better with them, but in order to get the speakers to not be too bright, the amplifier driving them is usually tube-based.
Its important to understand that this is not a tube/transistor conversation, and also in the intervening 50 some-odd years that the 'prior art' has continued to advance. So think about a designer that worked with what was available 70 years ago, then think about the raft of modern designers that have looked back at that earlier art to see what there was that might have been lost.
We started making triode zero feedback amplifiers in the 1970s and 80s, and Cary Audio began in earnest about 1990. Today zero feedback amps are prolific. What happened? There was an acknowledgment amongst designers that a measurement is not important if you can't hear it, that that if you can hear it maybe we should find a way to measure it.
As I pointed out in the article, for one sufficiently grounded in a paradigm, anything outside that paradigm is either hearsay or does not exist. So I expect challenge on this issue- its part of the definition! It also points to some of the fundamental and longest-lasting debates that have existed in audio over the last 20 years. |
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? |
