Cross breeding
Join Our Community Subscribe to Paul's PostsPurity is appreciated when it comes to purpose, water, and immorality. It’s not so great when it comes to a power amplifier’s architecture, where hybrids rule.
For many years, amplifier manufacturers were determined to keep their designs pure: 100% solid-state, all vacuum tubes, nothing but FETs, class D from input to output. Over time we’ve come to grips with why this commitment to design purity is not such a great idea.
Power amplifiers are misnamed and therein lies the problem.
On the surface, they seem simple enough: little signal in, big and powerful signal out.
What’s missing is the recognition that inside a power amplifier we have two completely distinct systems each with very different amplification duties: voltage and power.
The input voltage gain stage takes a small voltage and amplifies it into a big voltage. From beginning to end there is only voltage and no power. If you were to take the output of a power amplifier’s first stage and attempt to drive a loudspeaker you’d be met with silence.
To produce watts we need the second system, the actual power amplifier (where it got its name).
The fact that each of these two systems has such very different functions should be clue enough to understand why a purebred power amplifier’s a bad idea.
The smart designer recognizes the difference between the two systems and applies the best technologies for the job: vacuum tubes and FETs are much better at delivering voltage while bipolars, power MOSFETS, and Class D stages are best at delivering power.
Purity benefits us most when we apply it to where it matters.
Hybrids have been around a long time. The Quad 405 was a hybrid and it was launched in 1975. Analogue Class A voltage and Class B current. They sold almost 200,000 of that model and they have been making them ever since and the current version (Artera Power) continues to be very popular. It’s much the same as in the 1970s with a prettier face. The purpose of the design was plenty of very clean power with ultra-low distortion and cheap to make not needing expensive components. The 405 was 100w, since around 1990 they have been 140w. The current version costs £1,500 ($2,000).
“Vacuum tubes and FETs are much better at delivering voltage”. Another generalisation. Not everyone will agree.
For a second I thought PSAudio was out of business. Only error messages when I tried to enter the website for my daily portion of Paultalk.
But fortunately that was not the case.
Anyway, I don’t know enough (understatement) about today’s topic to take part in a discussion at a high level. (not even at a low level).
What I can say is that it made me think about yesterday’s post.
“…For many years, …..not such a great idea”
What is considered the best way/idea to do something today, may not be considered the best tomorrow. (progressive insight).
Everything is relative.
For years I thought power is THE most important feature of an amp. Now I know that is not necessarily the case.
Low power, tube amps can sound wonderful, especially, of course, with high-sensitivity speakers.
And there were times I thought specs were very important when I compared amps. Now I know better.
And speaking of Quad : in the early 1970’s I was the owner of the famous 33/303 amp. I bought it for around 600 USD.
Today a lot of (more or less) refurbished Quad’s are on the market.
I think the purest amp concept ever was realized in my “pure digital” TacT Millenium amp featuring only digital inputs and an integrated digital equalizer and a “quasi analog” PWM output to the loudspeakers. Hey Paul, why not designing a TacT successor with DSD input? TacT Audio no longer exists!
Back on the “grid” 🙂
Book / CD sales must have been brisk enough to crash things … Both Good and bad? 🙂 High product interest, not high enough server capacity?
I always considered an amp as one piece from the consumer point of view. Not giving much thought to topology, but rather to sound quality, and the ability to drive my speakers of choice correctly.
Sorry Paul but this is junk science. It is captive to a false notion of system design. We come back to the basic engineering concept of transfer function. Two systems will perform identically if they have the same transfer function. They are considered black boxes. It doesn’t matter what is inside them. There are multiple ways to alter transfer function to get one you desire. The ideal way is to produce one that is stable over time and repeatable. Vacuum tubes fail on both counts. They change their characteristics with time and different tubes of the same generic type perform differently from one manufacturer to another. The fact that solid state input stages of electronic circuits make poor voltage amplifiers is due to the fact that the output stage of the device prior to them is poor and creates a mismatch. It’s so poor that even changing the connecting wires alters the system transfer function when it shouldn’t.
The concept of transfer function in its rigorous mathematical presentation is introduced to electrical engineering students in their sophomore year. By that time they should know enough calculus to understand it. (This doesn’t come from hobbyist magazines written by know nothing reviewers and glorified bench technicians, this comes from engineering textbooks. )
https://en.wikipedia.org/wiki/Transfer_function#:~:text=In%20engineering%2C%20a%20transfer%20function,output%20for%20each%20possible%20input.
The ideal voltage source as expressed in Thevennin’s theorem has zero series output impedance and infinite shunt admittance. Cathode follower and emitter follower output stages fail on both accounts. Class AB amplifiers come much closer. The system transfer function can be altered using an equalizer circuit. It simple terms this means that the sound of any solid state amplifier can be matched by any other solid state amplifier by duplicating its frequency response not on a test bench with a resistor for a load but with the same speaker connected to it. The reason a solid state power amplifier cannot duplicate the sound of all but OTL tube amplifiers is that unless they use an output transformer they cannot duplicate the hysteresis distortion the transformer creates. This is why a solid state amplifier with an output transformer, even an autotransformer like McIntosh uses panders to market preferences for second harmonic distortion but from an engineering point of view it’s a dumb idea.
Geeeeeeeez man… 2nd order harmonic distortion is god in a jump suit – just enjoy the music, take off the lab coat and chill a little!
“Purity is appreciated when it comes to … immorality.” Now there’s a thought of the day!
This is why I oft say, some of the Japanese designs of the late 70’s early 80’s were the epoch of the pinnacle of audio amps. Whether Class A, AB or G or H or even today’s D Class, <0.005% is still <0.005% no matter what the year. And that is where the real winners come forth…
How an amp sounds matters. An amp has to be able to provide power to properly drive speakers and has to sound right. If a violin sounds as if it is made from steel and not wood, then something’s wrong, no matter how correct the theory or how low the measured distortion is.
It’s all in how an amplifier is designed not so much what type parts are used. Like a great cook.
Joe,
I have to agree with you on that one.
My wife is a fabulous cook & she is also extremely well designed 🙂
Ahhh yes, topology…’concerned with the properties of a geometric object that are preserved under continuous deformations, such as stretching, twisting, crumpling and bending, but not tearing or gluing.’
The last probably does not apply to your good lady wife, but obviously cooking should be included in that def. 🙂
A truly great amp will have both, a great design from a great designer AND great quality parts that the design is built around.
I didn’t say to use inferior parts, I said what type of parts used. FET, Tube, MOSFET, Bipolar, etc. The type parts used are not so relevant though I agree you might prefer MOSFET over Bipolar or vice versa. It’s all in how the amplifier is designed that make it a great amplifier or an average one and even a well designed amp with lesser quality parts can sound better than a poorly designed one with the best quality parts. Just as a bad cook can use the highest quality ingredients and whip up a bad meal.
Many, many tube fans would tell you that the TYPE of parts do matter. You take away EL34’s or KT88 or KT120 from a tube lover and they’ll tell you that all the emotion in music just got taken away.
Purity is still the way to go if one wants the best sound. In the hierarchy of sound reproduction pure tubes are still at the top with pure solid state at the bottom and hybrids in the middle. Has one ever wondered why hybrids use tubes in the input stage ? Its because of the tube’s superior sound and sense of involvement. This of course is watered down by the use of all kinds of solid state output devices which have a rather anaemic mid range and thin sounding highs. Still the sound is better than all solid state. The advantages of hybrids are 1. very cheap to produce. One can get a handful of transistors for the price of one good tube. 2. Doing away with output transformers which if of really good quality can be quite expensive specially if a wide frequency response and really good bass is the goal. 3. very good bass but with a sound not cut from the same cloth because of the anomalies of the mid range and highs mentioned above. Hybrid technology is very old popularized by a New York OTL amplifier maker under the name of MOSCODE. Nothing new here. As for class D. The power supply is cheap to produce , does not take up much space, has good bass but the mid range and highs are not in the same league as the bass. Just read the reviews of class D amps. They never make it to the top. There may be exceptions as is always the case but probably cost a fortune. Regards.
I don’t know what the first hybrid amp was, but I’m pretty sure the Quad 405 was an all transistor amp, using typical BJT output. One of the earliest tube/transistor hybrids, and the first I owned (back in the early 80s) was a Moscode made by New York Audio Labs. Very musical sounding amp. A few years later I had a Counterpoint SA-12, also a tube/transistor hybrid, utilizing 6922s in the front end and Mosfet output. Nice sounding but incredibly unstable design. In spite of the musicality inherent in many hybrid designs, I’m really looking forward to the unveiling of Soundmind’s solid state transfer function amp though.