Why do amps sound different?

@roberjerman,
The first 5 watts or so it what most amps don't get right. With many push-pull amps, the distortion below 5 watts is rising as power is decreased! Not all amps do this though- a lot depends on the topology of the amplifier.

Tvad, it can be (usually not clipping though), but there are other things that can do that that I would think would be more likely. Resonance excited by volume in the system is where I usually start when looking to kill sibilance. Cartridge setup, driver resonance, odd microphonics and cables are a few of the things that I have found to be more common.

Amplifiers and preamplifiers be guilty too so you have to be suspicious of everything.

Although alluded to before, it seems that the amount of negative feedback is a big issue.

2 amplifiers can easily have the same bandwidth, but one running with feedback and one without. The one without will likely sound more relaxed, since it lacks the global feedback which enhances odd-ordered harmonics which in turn behave as loudness cues. We are not talking a lot, like Kurt says- hundredths of a percent is all it takes to make the difference.

A further complication is the idea of 'constant voltage' output, which is the same as doubling the output power as the load impedance is cut in half. Some speakers are designed to expect this (B&W 802). Other amplifiers are designed with the idea of "constant power" in mind- that is that power does not change regardless of the load (tube amps are good examples of this). Such amplifiers, sometimes referred to as 'current source' amplifiers, have a higher output impedance and an entirely different class of speakers exist to accommodate them (Sound Labs and horns for example).

In fact two paradigms of design and measurement exist in audio today:

http://www.atma-sphere.com/papers/paradigm_paper2.html

This is about more than just simply matching components, but that is what you have to do. Normally one paradigm will take over in a field of endeavor but that did not happen in audio because the 'prior art' (tubes) did not go away like they were supposed to- too many people like them.

Amfibius, take a look:

http://www.atma-sphere.com/papers/paradigm_paper2.html

'Current source' is a Voltage paradigm term for amplifiers that have a high enough output impedance such that they exhibit (or at least try to exhibit), constant power with respect to load. Usually this refers to a tube amplifier, but not universally.

"Voltage source' is again a Voltage paradigm term for an amplifier that can express a constant voltage regardless of load. Another way to put this is that power is doubled as the load impedance is halved (or halved as the load impedance doubles). Usually this refers to a transistor amplifier, but again, not universally- there are always exceptions because this is not about tube/solid state.

Phd, one of the distinctions of the Power paradigm vs the Voltage paradigm is under the Voltage paradigm, the specs often have no meaning as you already know. Under the Power paradigm, the specs correlate with what you hear.

Hi Kirkus, I'm not the one who has created these paradigms; they simply are what is. And for the record, you would be hard pressed to build a tube amp of the type you describe! Even if you ignore the taps of the output transformer, most tube amps will exhibit the constant power quality anyway. The taps are there to allow optimized loading on the tubes- its not the other way around.

You are correct in that the Voltage Paradigm was being developed about 60 years ago- during the 50s and 60s... **almost** 60 years ago. That bit of history probably needs to be in the paper so thanks for pointing that out. I don't like to think that the 1960s are that distant yet :)

FWIW the Apogees and Lowthers are both Power paradigm technology. If you want a better comparison, compare the B&W 802s (needs a 'voltage source' amplifier) to the Lowther (needs constant power).

Apogees are in the Power paradigm as their impedance curve has very little to do with resonance in a box and so does not exhibit the classic impedance curve of such a device. Being a nearly resistive load, zero feedback tube amps work great with them if they can deal with the impedance (some Apogees are a simple 4 ohm load, others as you know are quite a bit lower, but other than that they are easy to drive)- a set of ZEROs provide the access for that.

Paul Bolin (at the time with TAS) reviewed a set of zero feedback triode amplifiers (and gave them a Golden Ear Award) using the Apogees for his speakers. Prior to that another TAS reviewer ran his 1 ohm Apogee Full Ranges with a zero feedback triode amplifier (which made 100 watts) and gave good marks to it as well. I had the opportunity to hear that setup, and the 100 watts seemed to be plenty of power- they were at once very relaxed, detailed and with plenty of authority on the bottom end. A fabulous speaker!

I've tried to school myself as best I can about this subject, and I appreciate your input- the more this issue gets airtime I think the better for the art.

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... :)

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.

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?

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.