It does not only sound so great, the schematic looks great too. It is just the 3 terminal active device and one resistor above and one resistor below. The Pleiades electron tube version uses just one resistor on top from anode to grid to make operation possible at a few volts. Simplicity at its best.
It is found on the best equipment on the planet. Examples are. The Neumann U47 microphone internal (front end) amplifier. The Fender guitar amp (first stage) preamplifiers. Triode single ended class power amplifiers. It is also used on the Pleiades pre preamplifiers and power amplifiers.
As mentioned again this circuit being inside a Neumann microphone U47 has been the closest in the signal path of singers like Leonaed Cohen, Madonna, the Beatles...The following example is a use of the Neumann U47 on the final version of the song Take on Me - a-ha doing justice to the angelic voice of Morten Harket. It is the electron tube U47 version, the internal amplifier using the VF14 tube at deliberately lower heater voltage as designer Georg Neumann had chosen. Earlier versions of the song were done with a U47 FET having many transistors in the signal path. The electron tube version uses just one tube from the output of the voice capsule to the output cable of the mic. Warner executives were never happy with the older versions of the song and re rehired a 3rd producer. There are countless changes the remaking of a song can have. God as J.S. Bach used to say is in the details.
https://m.youtube.com/watch?v=djV11Xbc914
The more than a hundred years old circuit we are talking about is called anode follower or common cathode, collector follower or common emitter, drain follower or common source. The name depends on whether the active device is an electron tube, a bipolar transistor or a field effect transistor respectively
It is the bread and butter of electronics. And it can be built on a breadboard in a few seconds. Just an active 3 terminal device, one or two resistors and a battery.
Class A operation means by definition that electrons flow all the time through the active device. At small signals the active devices are extremely linear just like the earth around small distances. At large distances ie signals, for example at peaks they are non linear ie curved which is a great thing for audio. They artfully instantaneously limit nasty peaks and therefore keep the average signal high. And this gradual distortion increase as the input signal increases and euphoric harmonics are produced. Just like our ear do. Just like trumpets do. The louder we play them the more overtones they produce. This is a reason these amplifiers sound so big and dynamic yet gentle and buttery smooth near the zero crossing point of the waveform where the cream of the music signal exists.
It is ironic in our times that we spend so much energy and processing power to increase the average signal level and suppress the peaks with software. Class A circuits do this at the speed of light. And at the lower levels where the music juice is they are the smoothest and distortionless amplifiers on the planet.
It is interesting that some men, women are not afraid to keep things simple. Less is more. The circuit we are talking about is the first circuit one learns on good textbooks of electronics and it could well be the best on the planet.
It is nice that there are designers, electronic engineers, producers, musicians and listeners who trust the Beauty of such circuits. They have a-phobia and courage to trust the best judge. The human ear and brain perception. Our fantastic mind. And all this great as it is perceived sound amplified with just 2 or 3 components.
After all, the most healthy and best tasting croissants are made with just top quality flour, butter, salt, sugar and water. Not with vegetable oil, palm oil or fat which has a reputation of clogging the arteries and other unpronounceable ingredients on the small letter ingredients list of a typical packaging.
The same happens with great music composing. It is just 3 or 4 notes sounding together that make the miracle as can be heared and seen in the 4 Seasons - Vivaldi full score.
Does this principle hold in electronics too? If you need more than 4 components to make a circuit is it out?
It is great that Great inventors like Lee de Forest, H. D. Arnold, Edwin Armstrong, Allan Blumlein and others did so much for us. Conceived more than a 100 years ago their simple circuit is The circuit. This beautiful circuit can be seen for example inside the Neumann U47 electron tube microphone. And we can hear it on Leonard Cohen, Frank Sinatra, the Beatles, Madonna?
And when were in the nineties and everything around is op amps, one needs to have even more courage to say the truth. To say how nice the class A, simplest on the planet topology, sounds .
Such a man is J. Donald Tillman.
He proposed a simple use as a guitar preamplifier using just one JFET .
And not only this, he incorporated the JFET amplifier in the cable itself by building it inside the 1/4in jack that is connected to the electric guitar.
The description of this application is here:
http://www.till.com/articles/PreampCable/
And there, there is also a reference to US Patent 5,585,767 by Thomas W. Wright, Jr. Was Thomas W. Wright the first who had the great idea of building a preamp inside the cable connector?
The Pleiades K117 uses a similar circuit. Just a K117 JFET and a low source resistor. Originaly designed for moving coil or ribbon microphones it can be housed inside an XLR or Neutrik module. Or inside a mini Tuchel or Amphenol microphone connector. This makes possible direct amplification of an MD421 HN, MD21 HL or Unidyne III microphones exploiting fully their internal high impedance step up signal transformer. The signal to noise ratio achieved in this way is hard to believe.
As the Tillmen implementation can be phantom powered by a console so should the Pleiades K117 be.
The Pleiades K177 was designed to be directly powered by the mic input connector used for electret microphones.
Specifically the Pleiades K117 has its component values optimized for being connected directly to iPad. The iPad internal resistor becomes the load resistor .
For connecting an electric guitar or bass guitar the Pleiades K117 can be made inside a 1/4in jack and the other side connected to the iPad through the headphone - mic connector. A short cable with female headphone jack can be added for monitoring purposes.
It could be better if the iPad GarageBand has a low cut filter that can be disabled.
We could then set it to flat frequency response. And if we wish to cut the excessive bass frequencies perceived by out brain the best way on the planet is possibly an inductor in parallel with the signal source. This is the Pleiades filter used on microphones too to cut the excessive perceived bass. For example for an electric bass an inductor of 5 Henries across the bass guitar pickup removes all the confusion and brings out a deep as needed bass full with every high frequency mid and transient detail.
If wanting to use electron tubes a similar circuit can be made. If a 4 Megohm resistor is connected between anode (plate) and grid electrons become free. This makes operation of the tube possible at even 3 Volts. This is the Pleiades preamplifier. Very similar to a Neumann amplifier with much lower anode and heater voltage. In fact the electron tube is underheated deliberately as is done on the VF14 tube of the Neumann U47.
There is no reason why not having these ideas expanded and making a power JFET drive a directly connected speaker. The speaker being the load resistor itself. More information on this on the Pleiades power amplifier at other euroelectron posts.
To better understand these amazing and simple class A amplifier circuits more details can be found at the references.
The amplifier miracle.
References:
Tubes vs transistors is there an audible difference? - Russel O' Hamm - JAES
Electronics, a systems approach - Neil Storey
A practical introduction to electronic circuits - Martin Hartley Jones
Operating features of the Audion - Edwin Armstrong
Enchiridion (manual of life)- Epictetous
Tao te Ching - Lao Tzu
How to win friends - Dale Carnegie
Fringe knowledge for Beginers - Montalk
Applied Ekectronics - T.S.Gray - MIT
Audio Amplifiers - J.R. Davis - page 41 - 2 transistor miniature A.F. Amplifier by K. Jones
The 4 Seasons - Antonio Vivaldi - full score
http://www.soundonsound.com/people/ha-take-me
Neumann schematic:
http://www.davegroupjapan.com/ekoukoku2.html
Pleiades preamplifier schematic:
Pleiades V6 schematic:
https://euroelectron.blogspot.gr/2017/07/pleiades-v6-schematic.html
US patent 1,349,252 - H. D. Arnold
No comments:
Post a Comment