Why do amps sound different?

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?

Kirkus, Your technique for the D130s should do the job. I'm not so sure about the other but why not give it a try?

I had one of those Knight 6 watt amplifiers too. FWIW the AR-3s really like power; I have a zero-feedback Dyna ST-70 that barely has enough power to make them go. They work OK in a smaller room though.

There are a number of tube amplifiers that have held on to their position like Nelson Pass' amplifiers. Western Electric 211 SETs for example- still worth a pile of cash after 6-7 decades!

I think the thing to get about this is that there has been an evolution. In the 1950s and 1960s, it appeared that the Voltage Paradigm was the way to go (certainly it made a good story for selling transistor amplifiers and cheaper speakers), but evolution has continued, especially tube research has continued. Tubes are not capable of the 'constant voltage' ideal- by rights they should not sound so good, but in fact they do. That does suggest that maybe the constant voltage model might have some holes. In fact the holes are the rules of human hearing: for the most part tubes adhere more closely to those rules than transistors.

Why did you keep your Knight?

Atma, why do you say tubes obey the rules of human hearing more than ss? I've heard that said a couple of times recently but I'm not sure why that would be. If a SS amp outputs the same waveform as is input how is that not obeying the rules of human hearing? I agree what's done in the recording studio often doesn't obey the natural laws of sound though!

regards, David

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.

Kirkus, sounds like he did!

Since a feedback signal is one of voltage, to satisfy the test might be easier than you think. Just place a speaker with a test tone coming out of it about 1 foot in front of the speaker under test and measure the AC voltage that results at the speaker terminals.

Wireless200, Tubes (triodes in particular) are the most linear amplification known to man. There are some semiconductors that are as linear in some portions of their curve, but not overall. Tubes also have a 'space charge' effect, again particularly noticeable with triodes, that prevents immediate saturation at full output. This limits the production of odd-ordered harmonics.

Anyone with an oscilloscope can view the clipping characteristic of any tube amp and see that the clipped waveform has rounded rather than sharp corners- this is a lack of odd-ordered content at clipping.

Due to the linear characteristics, its possible to build tube amplifiers that employ no negative feedback. Global feedback enhances the loudness cues (5th, 7th and 9th harmonics) that the human ear uses- in effect adding 'harshness'. The addition is slight, but our ear/brain system is such that even hundredths of a percent are detectable. Audiophiles use words like 'hard', 'harsh', 'brittle', 'clinical', 'chalky' and others to describe this effect.

So the trick is to avoid techniques that increase distortion, and to do so while avoiding global feedback. That results in an amplifier that can be very detailed while also being very relaxed.