Does MOSFET have a signature sound?

phipiper10 · March 2009

Would anyone care to indulge me in giving a shot at what you would consider the sonic signature of mosfet type amp?

Signature sound for other "SET" types?

ShinAce · March 2009

Did you mean 'FET' types? Field-effect transistor.

In theory, mosfet is better than normal transistor(BJT). It has an almost infinite input impedance, and produces even harmonic distortion(tube like sound). Not to mention they withstand tons of abuse and keep on trucking. Don't take the tube like sound too seriously. It's a meager detail that gets trumped by circuit desing.

In reality, there are trade-offs.

We are obviously dealing with low bandwidth and fairly low power applications. There's no reason to use one type over another with this info.

If I wanted to build an active device that doesn't amplify(pre-amp, active crossover, EQ), I'd go with JFET. They are very low noise and do not drain much current from the source to work.

If I wanted to build a high speed switch that could handle current, I'd be on the fence about MOSFET or BJT, likely going MOSFET.

If I wanted to build an amp, I'd use BJT, since it's easy enough to find matched pairs for cheap.

Systems · March 2009

phipiper10 wrote: »

Would anyone care to indulge me in giving a shot at what you would consider the sonic signature of mosfet type amp?

While MOSFET's are transconductance devices similar to tubes,the actual topology of the circuit they are used in will have a big influence on how it will sound.

lakesailor · March 2009

Many years back I; like most everyone at the time; had an Adcom GFA 555; basically an old bi-polar based amplifier that Stereophile couldn’t say enough good things about. Years later when the mosfet based GFA 5500 came out to replace it; I sold my old GFA 555 and brought home a shiny new GFA 5500

The sound of the 5500 was definitely different. I didn’t enjoy it as much but figured it was a break in thing (even though my understanding of mosfets said otherwise) after a year I was able to trade it straight across for a GFA 555 Mark II. Best deal I ever made and the guy who got my 5500 said he would take the 555 back in a minute

Mind you he now uses the 5500 in his HT system and say it work great for HT. Who knew.

heiney9 · March 2009

lakesailor wrote: »

Many years back I; like most everyone at the time; had an Adcom GFA 555; basically an old bi-polar based amplifier that Stereophile couldn’t say enough good things about. Years later when the mosfet based GFA 5500 came out to replace it; I sold my old GFA 555 and brought home a shiny new GFA 5500

The sound of the 5500 was definitely different. I didn’t enjoy it as much but figured it was a break in thing (even though my understanding of mosfets said otherwise) after a year I was able to trade it straight across for a GFA 555 Mark II. Best deal I ever made and the guy who got my 5500 said he would take the 555 back in a minute

Mind you he now uses the 5500 in his HT system and say it work great for HT. Who knew.

I used to be the same way and for these particular Adcom models, I agree. I currently have a Pass Aleph 30 which is a Mosfet pure Class A design with ony 2 gain stages and I must say it has alleviated all my Mosfet misconceptions. By far one of the best sounding most musical amps I've heard.

It's all in the design and Mosfets have many of the same electrical characteristics of tubes..........and they do have a more tube-like sound compared to bi-polar transistors. To the OP, this might help.

A very important consideration in attempting to create an amplifier with a natural characteristic is the selection of the gain devices. A single ended Class A topology is appropriate, and we want a characteristic where the positive amplitude is very, very slightly greater than the negative. For a current gain device, that would mean gain that smoothly increases with
current, and for a tube or field effect device a transconductance that smoothly increases with current.

Triodes and Mosfets share a useful characteristic: their transconductance tends to increase with current. Bipolar power devices have a slight gain increase until they hit about an amp or so, and then they decline at higher currents. In general the use of bipolar in a single ended Class A circuit is a poor fit. Another performance advantage shared by Tubes and Fets is the high performance they deliver in simple Class A circuits. Bipolar designs on the market have between five and seven gain stages associated with the signal path, but with tubes and Mosfets good objective specifications are achievable with only 2 or 3 gain devices in the signal path.

Readers of The Audio Amateur Magazine will be familiar with my “Zen” design, which uses a Mosfet in a power amplifier which has only one gain stage, and only one gain transistor. Research continues at Pass Labs in improving the performance of very simple gain circuits. Yet a third advantage tubes and Mosfets have over bipolar devices is their greater reliability at higher temperatures. As noted, single ended power amplifiers dissipate comparatively high wattages and run hot. Bipolar devices are much more prone to failure at high temperatures.

In a decision between Triodes and Mosfets, the Mosfet's advantage is in naturally operating at the voltages and currents we want to deliver to a loudspeaker. Efforts to create a direct coupled single ended triode power amplifier have been severely limited by the high voltages
and low plate currents that are the province of tubes. The commercial offerings have not exceeded 8 watts or so, in spite of hundreds of dissipated watts. Transformer coupled single ended triode amplifiers are the alternative, using very large gapped-core transformers to avoid core saturation from the high DC current, but they suffer the characteristic of such a loosely coupled transformer as well.

The promise of the transconductance characteristic in power amplifiers in providing the most realistic amplified representation of music is best fulfilled in Mosfet single ended Class A circuitry where it can be used very simply and biased very high. The Pass Aleph 30 uses International Rectifier Hexfet Power Mosfets exclusively for all gain stages. These Mosfets were chosen because they have the most ideal transfer curve for an asymmetric Class A design. Made in the United States, they have the highest quality of power Mosfets we have tested to date. We match output devices to within 2%.

The input devices are matched in circuit for lowest noise and distortion. The smallest of these, the input devices, are capable of peak currents of 5 amps. The largest are capable of peaks of 25 amps each, and are run in parallel pairs. The power Mosfets in the Pass Aleph 30 have chip temperatures ratings to 150 degrees Centigrade, and we operate them at small fractions, typically 20% of their ratings. For extended life, we do not allow chip temperatures to exceed 85 degrees C.

Regardless of the type of gain device, in systems where the utmost in natural reproduction is the goal, simple single ended Class A circuits are the topologies of choice. It is a very simple topology, which is a key part of the sound quality. Other solid state amplifier designs have five to seven gain stages in the signal path in order to get enough gain to use feedback to provide adequate performance. In this amplifier, we get greater linearity by providing much more bias through two gain stages: a differential input stage, and the output transistors.

Mosfets provide the widest bandwidth of solid state power devices, however they were not chosen for this reason. The design of the Aleph 30 does not seek to maximize the amplifier bandwidth as such. The capacitances of the Mosfets provide a natural rolloff in conjunction with the resistive impedances found in the circuit, and the simplicity of the circuit allows for what is largely a single pole rolloff characteristic.

The slew rate of the amplifier is about 40 Volts/uS load , which is about 10 times faster than the fastest signal you will ever see, and about 100 times faster than what you will be listening to. In and of itself, the slew rate is an unimportant factor when evaluating tube and simple Mosfet designs. It becomes more important with complex circuit topologies where there is heavy dependence on feedback correction, but even then its importance has been overstated

Long read, I know and perhaps some terms not understood, but you get the general idea.

H9

p.s. I highlighted some of the important points.............this is not to say Mosfets are the best for every topology known.

Does MOSFET have a signature sound?

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