Why do amps sound different?

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.