Transistorlyd

[shras]

Egenlyden kan vel bedst beskrives som værende den forvrægning der tilføres signalet. Netop på dette punkt er der forskel imellem eksempelvis rørgrej og transistorgrej, da de to komponenter bidrager med forskellig harmonisk forvrængning.

Hvis jeg husker rigtigt (og her må i meget gerne rette mig), så består den harmoniske forvrængning fra rørgrej primært af "lige" harmonisk forvrængning (2., 4., 6. harmoniske osv), hvorimod transistorgrej primært bidrager med "ulige" harmonisk forvræning (1., 3., 5. harmoniske osv). De mest ørevenlige skulle i denne sammenhæng være de "lige" harmoniske.

Alt afhænger dog at apparatets konstruktion, og man finder som sagt både skidt og kanel ligegyldigt hvilken teknologi man sværger til.

[Groove]

Det er nærmere kredsløbsopbygningen der bestemmer det harmoniske indhold.

Single-ended design giver stort set kun 2. harmonisk (lige), den er i hvert fald dominerende. Push-pull/symetrisk design give undertrykning af lige harmoniske, så de ulige bliver dominerende (men ikke højere end i single-ended versionen).

Derudover vil frere trin gøre tingene mere komplekse og tilførsel af modkobling ligeså, så der kan komme flere højere ordens komponenter med...

Da rørgrej typisk er med få trin og lav modkobling, er de typisk med overvejende 2. harmonisk. Men transer kan nemt laves på samme måde....Nelson Pass har faktisk lavet en forstærker hvor man ved hjælp af et potmeter på fronten kan ændre forstærkerens karakter  

[shras]

Takker, Groove... Jeg blev klogere.  

[Live24]

faldte lige over denne tekst. synes ellers alle steder man kigger rundt finder man mos eller j fets

Amped Up: MOSFET or Bipolar?
Which are better and why?

By Jeff Kuells



Do you really care if the amplifier you are using is a MOSFET (Metal Oxide Semiconductor Field Effect Transistors) or Bipolar design? Probably not, at least as long as the amplifier continues to perform. With modern amplifiers, few people notice the difference between the designs until the amp failed and they receive the repair bill, then they will take a special notice. What follows are the advantages or disadvantages and myths of these two designs.

BACK ON THE LOUD FRONTIER

For many years now, designers and users of professional amplifiers have had the same discussions (arguments) regarding which output devices sound or perform better. Unfortunately, these discussions have created more myths than factual statistics.

When talking about Bipolar or MOSFET designs, we are usually talking about the output stage of an amplifier. The output stage can be compared to the engine of a car. The output stage provides the horsepower to the speaker.

The differences between the two design approaches for this topic have very little to do with front end designs or power supplies. To understand how an amplifier works, please read the LSMAG! May/June 2001 Amped Up, titled “Amplifier Classifications”.

Engineers of the 1960s and 1970s were frontiersmen of high power silicon for audio. Most of their success in amplifier design was based on the guinea pig method. Stand back and watch for smoke!

They didn’t have many textbooks for amp designs or the Internet to print off the latest findings. Their designs were not always successful due to unproven engineering and inadvertantly created modern misunderstandings about MOSFET and Bipolar transistors.

Bipolar designs have been around since the 1960s, when silicon evolution led us astray from the common tube designs. The majority of professional amplifiers in the market, past and present, are of the bipolar topology. The sound quality of a bipolar design is what we have become acquainted with. Unfortunately, this does not mean that all bipolar amplifiers sound good.

CONVENTIONAL AMPLIFIER WISDOM

MOSFET amplifier designs are known to be more musical than Bipolar designs, especially in the mid to hi frequencies. Bipolar designs are known for their ability to deliver high current into various loads. This is good for low frequencies (LF).

As such, it was once believed that the ultimate sound system would be to use MOSFET amplifiers for the MF/ HF and Bipolar amplifiers for the LF. Why is this? Is it still true today?

To understand the differences between the two topologies one must have an understanding of the history of theses devices and their designers. Have these devices improved through the years? Of course they have! Have the engineers improved their designs through amplifier failure? Certainly!

The greatest amplifier design advancements have come from designers and not necessarily with the components. Sure, many more manufacturers are supplying us with a better selection of these devices. However, the real change for today’s engineer is proven circuitry available for their reference.

MOSFETs are a relativity modern addition to the family of power transistors. They were introduced by Hitachi for audio applications in 1977. A MOSFET transistor consists of three elements, Gate, Source and Drain.

A MOSFET transistor in its simplest terms works like a water valve. When voltage is applied to the Gate, the valve opens and lets current flow from the Source to the Drain.

The early problem of these new devices was the Drain to Source internal impedances were high resulting in low Damping factors. In those early days of the Hitachi device, the internal resistance of MOSFETs was greatly reduced to yield LF performance that rivaled Bipolar LF performance.

This gave us the impression that MOSFETs cannot produce good LF. As the old saying goes, “it only takes a minute to create a good impression but a lifetime to overcome a bad one”.

BIG DRIVERS REQUIRED

Bipolar designs have the ability to deliver enormous amounts of current to a load. A Bipolar transistor consists of three elements as well, Base, Collector and Emitter. The current path is from the Collector to the Emitter. There is also a significant amount of current flow from the Base to the Emitter. Bipolar amp designs require a hefty drive stage.

These devices require additional heat-sinking because the Base of each output transistor is current driven not voltage driven, causing them to heat up. MOSFETs also require a drive stage, but have much less of a load requirement compared to a Bipolar driver stage because they are voltage driven, not current driven.

Bipolar transistors are positive temperature coefficient. When a transistor passes power, it heats up. When a Bipolar transistor gets hotter its internal resistance decreases and tries to pass more power making it even hotter.

Because no two individual devices are identical, a positive coefficient means that one transistor tries to “hog” the load and tries to do all of the work.

To prevent this, large Emitter resistors are placed in the current path to help equal the sharing of the load. If the heat is not controlled by temperature sensing devices thus reducing the drive of the output stage, thermal runaway will occur, thus blowing up the device.

The real “shocker” comes with failure. When a Bipolar fails, it tends to short, connecting the supply rail directly to the output. When this happens you will experience the catastrophic effect causing all devices in parallel with the bad device also to short.

Have you ever experienced the “your amp went DC” diagnosis? Lets hope not! If you haven’t, your lucky, because when this happens whatever is connected to the output (i.e. loudspeakers) is also history.

In most designs there are DC sensing circuits incorporated to shut the channel or amp off. In most cases, however, these circuits aren’t fast enough to prevent serious speaker damage caused by thermal runaway.

HOW CAN NEGATIVE BE POSITIVE

MOSFET Transistors are negative temperature coefficient. When a MOSFET heats up the internal resistance increases causing it to pass less power. MOSFET amplifier designs are inherently thermally stable without additional circuitry and by nature has no chance of thermal runaway.

Each output device in effect self regulates to carry the load equally with all of the other output devices. When a MOSFET fails it usually opens, so there is rarely a chance of damaging a speaker from output stage failure. This means there is no real need for additional thermal tracking and DC sensing circuitry.

Many designers that feel MOSFETs, if not matched properly, may be unnecessarily paralleled without the use of large (physical size) source resistors to equalize the current draw between devices. As such, MOSFETs have no advantage over Bipolars in relation to space and cost reduction of expensive Emitter resistors for Bipolar circuits and Source resistors of MOSFET circuits.

MOSFET amplifiers have a much higher slew rate than Bipolar designs. MOSFET devices are very fast and can switch several amps in nanoseconds. This speed makes them thirty to one hundred times faster than equivalent Bipolar devices.

If you are accustomed to listening to traditional Bipolar designs and compare them to the speed of MOSFET design, you will notice a significant, almost unnatural audible difference in the HF. Such clarity is a pleasant added experience.

Engineer Wayne Colburn of PASS Labs, a world leader in MOSFET amplifier design, stated,” there really are no major disadvantages of MOSFET devices”. When compared to Bipolar devices, there are only two disadvantages for concern, the first being cost.

MOSFETs can cost up to two-three times more than an equivalent Bipolar device. This is a huge concern, especially in high-powered amps where it takes several devices to achieve the desired power.

To achieve high output levels, a tiered power supply may be required to feed the front end several more volts than the output stage. This translates to additional cost for everyone.

WHY AREN'T MOSFET'S USED MORE

If MOSFETs have more advantages than disadvantages over Bipolars, why are there so many more Bipolar amplifier designs in use? Cost is the biggest factor, but pricing is rapidly improving.

There is also a greater variety of Bipolar transistor manufactures to choose from, as well as packaging types, voltage and current selections. These areas are finally improving for MOSFETs as well.

Expect many more MOSFET designs to reach the professional market in the future.