Pioneer MOSFET AMPS: pros/cons (1 Viewer)

[Kelly Scott Rickards]

I notice Pioneer uses MOSFET amps but I have no experience with these type of amps...

What are the Pros and Cons of MOSFET amps?

 

[RicP]

My Parasound 1500A uses a MOSFET output stage as well. Basically MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is the opposite of bi-polar, which is another very common output stage in power amplifiers.
Bi-polar devices are switched on by current running through them, while MOSFET devices are switched on by voltage -- which is how vacuum tubes operate as well.
MOSFET's have some other technical attributes and they tend to sound a little warmer than bi-polar amps. A smooth treble that can tame bright tweeters and a warm midrange are the characteristics of a MOSFET amp.
The one drawback for the liquidy warm treble is a less than perfectly detailed sound. A bi-polar amp will sound sharper and may have more defined soundstaging, at the expense of MOSFET's warmer tone.
This is sometimes referred to as "MOSFET mist".
I love MOSFET amps, and they are an excellent match with speakers that tend toward the bright side like Paradigm.

 

[Robert George]

What Ric said...
I also fell in love with the sonic characteristics of MOSFET amps when I bought my last Pioneer Elite receiver, the VSX-29TX. That receiver had (has) a warmth and smoothness of sound that I had never heard in a Japanese receiver up to that time.
But, as Ric also notes, there can be some loss in detail. If one is using a MOSFET design, it must be well executed and be coupled with a very good preamp section. It was in this area I found the Pioneer 29TX was lacking. While it has a very pleasant sound in general terms, once one gets used to the basic sound quality, one is subject to begin finding the receiver's shortcomings, which is mainly lack of fine detail and a rather blurred forward soundstage.
I did say "can" be some loss in detail because this is not necessarily so. I have beeen using the B&K AVR307. This piece also uses a MOSFET output stage in its amp section coupled with exceptional processing. Also, the amps in the B&K are a couple of steops up from what was being used in the 29TX. The difference in sound quality between these two pieces is not small and one does not have to strain to hear the differences.
Another factor that has to be considered when chooosing an amp is the speakers it will be driving. I have chosen B&W Nautilus speakers for my front channels. These are wonderful sounding speakers. Very detailed with exceptional imaging. However, with the wrong amp, these can be overly bright, even harsh. A good MOSFET amp is a perfect match for these, or any speaker that tends toward the "bright" side.
BTW, I did not give up on Pioneer. In fact, their new 49TX, with newer MOSFET amps and about the best digital section I have seen (on paper), intrigued me enough to sell my B&K and order a new 49TX. I should have it, perhaps, by the end of the week. I'm sure I'll have something to say about it when I've had a chance to give it a good workout.

 

[Kevin C Brown]

Hmmm. I cannot comment in the use of MOSFET's vs bipolar devices for power amps. (Although the amps that I have looked at in the past for what I would buy, have typically been bipolar device amps.)

But I can at least correct some of the info here.

MOSFET devices aren't the "opposite" of bipolar devices. They are both semiconductor devices, just that they differ in operation.

Both are essentially "switches" controlled by voltage.

MOSFETs are slower, lower power devices. Cheaper to make too. Bipolar devices are faster, and have higher current carrying capability.

MOSFETs use majority carriers for the "switch", bipolars use minority carriers. MOSFETs are typically either on or off (better for digital applications). Bipolar devices are better for analog applications in that the current flowing through the device can be controlled to a much finer degree than MOSFETs.

MOSFETs are typically easier to make though. Much easier to "foundary" out than bipolar devices.

 

[ling_w]

Correction on corrections:

Bipolar are controlled by the amount of current flowing through it. MOSFET is a Field Effect Transistor, therefore, no current flows through, just the voltage which creates the field. Having a current driven gain stage requires the prior stage to have current producing capability, or else non-linear output would occur.

Biploar transistors has a negative temperature coefficient, meaning when it gets hot, there is the possibility of thermal runaway. MOSFET is the opposite, preventing thermal runaways.

There is another type of transistor, IGBT (Insulated Gate Bipolar Transistor) which is a cross between the above two. It is a bipolar transistor that is voltage driven.

 

[RicP]

Both are essentially "switches" controlled by voltage.

Incorrect. See Ling's post. But thank you for incorrectly attempting to correct me.

 

[Kevin C Brown]

Dang, I *knew* this was going to happen...
Ling- Incorrect. A MOSFET works by the voltage over the gate creating a channel *for current conduction* under the gate.
When you have current from the source to the drain, the "switch" is on. No current means the switch is off. The voltage on the gate determines whether a channel exists under the gate (under the gate oxide) for current flow or not.
For a bipolar device, the emitter-base voltage (Veb) determines whether the transistor is on or off. Period. But you are somewhat correct in that the current *gain* of the device is controlled by the current injected into the base (Ib). That's what determines the Beta, or efficiency or gain of the device.
If this gets more technical than this, I'm going to have to raid my garage for the solid state device physics books I used in college.

 

[ling_w]

Kevin,
You seem to have misunderstood me. I don't mean there is no current flowing through a MOSFET device at all, just that the gain is not controlled by a current flow into it like a Bipolar transistor.
YOu must be reading some IC application book, MOSFET being good only for on/off device. Your source must also be telling you vacuum tubes are only good for electron guns.

 

[Tony Lai]

>Ric, Parasound amps only use MOSFETS in the driver stages and use bi-polars on the output stages (like Classe amps). Most of the distortion takes place in the driver stages and thats where MOSFETS are best used. MOSFETS do sound warmer than bi-polar amps.

All Parasounds are like this - IMO you should stick to bipolar poweramps. There's a reason why they're the most popular for mid to high end SS electronics.

T.

 

[RicP]

Ric, Parasound amps only use MOSFETS in the driver stages

You are right. I meant to type driver stage but I was thinking about something else. Driver stage is correct. Or to be more exact...
Hand matched complementary JFET input stage and MOSFET driver stage
16 beta-matched 15 amp, 60 MHz bipolar output transistors

 

[Kevin C Brown]

Ling- Kind of hard to misunderstand...

MOSFET is a Field Effect Transistor, therefore, no current flows through, just the voltage which creates the field.

But yes, I think we are on the same page now.
And I wasn't meaning to imply that MOS devices could *only* be used for on/off, just that that's what they typically are used for.
Obviously, either MOSFETs or bipolars can be used to make great sounding amps. But, I wanted to correct some of the info regarding the operation of the devices themselves.
(Like the voltage/current question for bipolar transistors...)
But Tony basically verifies what I've informally thought about bipolar based amps being preferred (by *some* people, anyway!)