I asked myself this question recently. Primarily because it just so happens I was searching out another amplifier. My previous exp with SS provided me a great exp with a BAT amp which had MOS FET in the design. Currently however, Ive been well pleased with another amp of substantially lower cost in my Odyssey SS amp which uses bipolar instead of MOSFET.
What to do? For me it came down to this
cost. I went with a Butler TDB amp using bipolar instead of waiting and seeking out some other MOSFET arranged amp. Another factor was the Butler was a hybrid design involving the use of a tube in each of its amps. Hybrids come in both versions BTW. Bi-polar & M-FETs. See Dodd & Moscode.
But the TDB multi amp was to me. Additionally so many owners of the thing were gag a about it I felt Id throw the switch and see for myself.
Ill put things this way
of all the SS amps Ive owned the BAT VK500 BP, was and is still my fav 100% SS amp, despite the xlr only aspect. I ran mine both ways, and with the right pre, going out SE instead, was as good if not better IMHO. But then for contrast I went from a $4K orig price preamp to a $9K+ orig price preamp, which probably had something to do with things too.
The Butler TDB 5150, which I posted a review just the other day here, is as close to that BAT amp as Ive found while still providing a taste of the tube harmonic, and control of the squeakers themselves. For an amp delivering well less wattage than the VK500, its remarkably adept at handling the big bass drivers on both my tower speakers
Phase tech PC 10.5 (87db, 4ohm), and Silverline Sonata IIIs (93db uh, reportedly 8ohm).
During that idea tossing about marathon I ran across this article which for me, gave sufficient illumination on the subject matter. Perhaps itll help you too
. However, the question for me isnt so much a whats in the box sort as it is whats it sound like tuype
. And if I want it, can I afford it?
I think thats what it always comes down to as well. Sound good? OK How much?
Ive yet to say, Oh My God that sounds fantastic, BUT theres a bunch of such & such thingys in there, instead of thing a ma jigs, so Ill pass. Thanks anyway.
Ive never done that and likely never will. Its always been and hopefully will remain in the camp of, how good for how much instead. And why not? I aint fixing the damn thing so why should I care what is inside the box? Apart from the fun of the time well wasted discussions on subjects like this
But heres a few portions of the article
Amped Up: MOSFET or Bipolar?
Which are better and why?
By Jeff Kuells July/August 2001 Live Sound International
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
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!
.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!
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 havent, 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 arent 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.
Jeff Kuells is a Performance Audio Engineer and Audio Manufacturing Consultant. Previously Director of Engineering for a major amplifier co.
Given the way the dollar has plummeted this past year, wether or not the prices have actually changed and become more ameneable to a manufacturerrs funding remains to be seen and we won't be seeing it in particular... we only see the end result... the actual price of the whole thing.
both designs from what else I've gleaned once instituted properly are stable, have great performance to their price ratios, and are effectively long lasting and near trouble free.