Q#3B: Is this a unique design? it would great to understand the innovation. Is it something similar to the XXXXX gain stage (conceptually)?
The circuitry in the iCAN Pro is quite unique. It is not a cookie cutter, cut & paste from some textbook or a manufacturer’s design note. With the greatest respect to others, conceptually it has no parallels – except in classic professional studio equipment using tubes, like for example from the legendary R.E.D.D.37 & 51 tube recording consoles.
REDD.51 tube recording console, Studio Two/Abbey Road
However, the circuitry is not a mere rehash of the best classic British or German studio gear either.
All tube circuits have their own challenges, so we use the tube where it makes the
greatest difference, in the first stage, though it may be switched out for a J-Fet instead for a completely solid-state circuit path.
The tube or J-Fet is followed by a second fully discrete gain stage using a combination of bipolar transistors and mosfet selected and operated to achieve very specific goals in terms of linearity, gain etc.
The result is a circuit that very much behaves like a modernised, transformer-less and direct-coupled version of classic recording gear. So much of the common sonic colouration traits found elsewhere from coupling capacitors and transformers do not exist in the Pro iCAN.
Q#6: I noticed you spec JFets rather than the more commonly used Mosfets. What is the difference?
As for tube vs. J-Fet vs. mosfet vs. bipolar transistor, these are all different amplification devices. They have all different parameters and all are subject to limitations and have undesirable traits. No device is ideal for all applications or uses. One of the key skills of a good design is to select the right device for the right application.
A Short History of Amplification Euphoria
1. The tube
Of these four active devices we have available, the tube is the oldest and is now over a century old.
Lee de Forest, the father of the radio, invented the ‘Audion’ tube – the first electronic amplifier device in 1906
Among the very desirable traits of tubes are a very high input impedance, a high level of inherent linearity for a given gain, the fact that the tube is a depletion device (it produces the highest current it can with 0V between input and common terminal) which allows simpler circuits and the fact that all parasitic (unwanted) effect are essentially constant with signal level and frequency.
The downside is relatively low absolute gain, a high internal impedance combined with a low current capacity and the need for very high voltages to operate, as well as the need to provide extra power to heat up the cathode to a dull red glow temperature. As a result, tube equipment has mostly disappeared from common use, yet especially for input stages they are often still without equal.
The iCAN Pro uses genuine USA made General Electric New Old Stock Tubes
2. The J-Fet
Perhaps surprisingly the J-Fet is the second oldest amplification device and the first solid-state amplification device. It was patented in 1926 by Julius Lilienfeld and he put a working theory of operation in place by the mid-1930’s.
The 1926 J-Fet Patent
While not actually a tube, interestingly, the J-Fet shares many traits with it.
It has a very high input impedance, a moderate level of inherent linearity for a given gain, the fact that the J-Fet is a depletion device (it produces the highest current it can with 0V between input and common terminal) which allows simpler circuits and the fact that if enough voltage is applied all parasitic (unwanted) effect are essentially constant with signal level and frequency.
The downside is merely moderate gain, a high internal impedance combined with a low current capacity and the need for relatively high voltages to operate. Like tubes, J-Fets are often used in Input stages and while they do not equal tubes, if applied correctly they can come quite close. Compared to generic transistors (Bipolar and Mosfet) J-Fets are difficult to make, have a process that is variable and requires selection and cost is around 100 times that of Mosfets and Bipolar Transistors.
3. The Bipolar Junction Transistor
In 1948 William Shockley invented the Bipolar Junction Transistor which rapidly became the device of choice for modern electronics and for three decades remained the device of choice in the design of discrete and integrated circuits.
Electronics rapidly became miniaturised and energy efficient. Instead of the huge, hot running wooden tube radios there were battery powered pocket radios that could drive headphones and could be certainly carried everywhere. Portable audio became possible with the bipolar transistor. Nowadays, the use of the BJT has declined in favour of MOS(fet) technology in the design of most circuits and IC's.
Replica of the first functional bipolar transistor from 1947
Compared to tube and J-Fet, the Bipolar Transistor was a radical departure and practical electronics had to be nearly re-invented to suit it. The bipolar transistor is actually a current driven device, meaning it has a low input impedance and it is an enhancement device (meaning it passes no current with 0V between input and common terminal, requiring more complex circuitry. Parasitic effects are wide and varied and all are highly signal level and frequency dependent and linearity is quite poor.
However bipolar transistors can handle much higher currents than Tubes or J-Fets, work well with very low voltages, are easy to manufacture with good consistency and at very low cost, so they displaced tubes and J-Fets could barely carve out a niche where bipolar transistors could just not work well.
4. The Mosfet
The newest amplification device is the Metal Oxide Semiconductor Field Effect Transistor a.k.a. Mosfet. Invented in 1960 it was meant to displace the bipolar transistor by giving a device that worked much like a bipolar transistor but with high input impedance.
The 1963 Mosfet Patent
Problems in manufacturing initially caused higher cost and it was a long time before Mosfets became viable alternatives as linear (audio etc.) amplifiers to the J-Fet and Bipolar transistor. It was however discovered that Mosfets could be made very small and that they made good switches, which drove the revolution in digital integrated circuits and in the end led to modern computers which use chips that combine literally millions of tiny, tiny Mosfets to form the logic.
As a linear amplifier the Mosfet does less well, even in the 21
st century, very few types are particularly suitable for the job. Operated at low voltages, the Mosfet has high input resistance but very high and signal variable input capacitance which causes a very specific type frequency dependent distortion that rises with frequency a lot more than bipolar transistors. Linearity is also quite poor, but the more modern types have huge levels of gain allowing very large amounts of negative feedback to be used to overcome the non-linearity.
There you have it, a brief rundown of each amplification device and the pros and cons – as you can gather, in the end no device is perfect and using the right device for the right job in the right circuit can give an excellent outcome. Get it wrong and the results are dire
Stay tuned for more!
