BiPolar vs. MOSFET Designs?

Archived from groups: rec.audio.high-end (More info?)

Seadweller wrote:
> Greetings,
>
> I've always felt that if audio manufacturers truly aspired to achieve
> 100% accuracy in electronic components, everything would slowly begin
> to sound the same. In my quest for a new amplifier, it seems the use
> of varying technologies flys in the face of accuracy, and creates
> certain sonic "signatures".
>

This is what some quacks are claiming, the reality is not so.

> I've read that the matching of an amplifier to a set of speakers is
> critical, and that some speaker designs lend themselves to BiPolar
> amplifier designs, while others match better with MOSFET amplifier
> designs. It is said that BiPolar amplifiers sound "faster" and
> "punchier," and MOSFET designs are "softer".
>
> Based on some general research, MOSFET's devices are technically
> superior to BiPolar devices in just about every parameter:
>

This is not so, inherently BJT have a much higher transconductance and thus
are more linear.

> Mosfet devices are inherently self regulating. Since they exhibit a
> positive thermal co-efficient, they don’t suffer from the dreaded
> BiPolar thermal runaway.
>

This is only true for very high currents, which are only used in digital
amplifiers. Normally the FETs are used a way below the max current, because
they are operated in the linear region and would dissipate too much heat
with the max. currents. For low currents the FETs have also the tendency to
thermally run away. So unless it is a class A amplifier there is not much
difference.

> Mosfet devices are very fast and switch several Amperes in nano
> seconds, which is 30 to 100 times faster than equivalent BiPolar
> devices.
>

You do not need nanoseconds risetime in linear amplifiers. We have a maximum
slew rate, which can be calculated by looking at a sinewave of max.
Frequency and max. voltage.
dVout/dt = 2pi*f*Vpeak
for a 20kHz 60Vpeak we get 7.54V/us (this means 200Wrms into the tweeter!).
Much faster risetime is not needed. Nanoseconds are only useful in digital
amps, where MOSFETs are used exclusively.

> Unlike BiPolar devices, Mosfets will handle short overvoltage and
> overcurrent operating conditions without terminal failure.
>

That would be nice, but unfortunatly you cannot use the device above the
max. ratings. Hot spots will destroy the dice as fast as any BJT. It is true
that Mosfets do not exhibit second breakdown, but if you stay in the SOA
(safe operating area) the BJT is as reliable as a Mosfet.

> In practice the reliability of Mosfets in surviving real world abuse,
> usually overheating and driving output shorts, is vastly higher than
> that of comparably rated Bi Polar output stages, even with their usual
> extensive protection.
>
no, just the same

> Mosfet devices require very small amounts of current to operate
> correctly, allowing a much simpler drive circuit to be used,
> maximising reliability and performance.
>

They do not require a continuous base current, but they want a much higher
gate voltage for the same output current. The complexity of the driving
stages is comparable, as the big gate capacity has to be charged/discharged
to much higher voltages.

> Mosfets exhibit a "soft clip" characteristic when driven into clipping
> levels. This is predominantly made up of even order harmonics rather
> like that of Valve Amplifiers, not the harsh odd order harmonic
> clipping produced by BiPolar Amplifiers.
>

You are spreading another urban myth, the required amount of feedback to
linearize the quadratic Fet characteristic will create the same limiting or
clipping. Put an oscilloscop on the output and then speak out again!

> The misconception that Mosfet Amplifiers have by definition low
> damping factors was primarily due to early low output Hitachi device
> based amplifiers with marginal Power Supplies. This is not a factor
> with today’s Third generation high output Mosfet devices that are used
> throughout the ARX range.
>

Maybe another misconception of yours? Just those Hitachi devices were
optimized for linear use with the zero temp coefficient in a much lower
current range. People complain they are no more available.

> If MOSFET designs are technically superior, why do companies like
> Levinson and Krell use BiPolar designs?
>

Seatweller, please get informed rightly. Your claims are almost all plainly
wrong or mis-interpreted. Get real data and look yourself instead of
repeating false statements.

> Also, is there truth to the contention that BiPolar designs are
> "faster" sounding?
>

Ridiculous. This is a laymans word, meaning nothing.

> Thanks!

Sorry to blast your baloon, but wouldn't your question above (why they use
BJTs?) indicate your erroneous position?
--
ciao Ban
Bordighera, Italy

Archived from groups: rec.audio.high-end (More info?)

Seadweller wrote:

> Greetings,
>
> I've always felt that if audio manufacturers truly aspired to achieve
> 100% accuracy in electronic components, everything would slowly begin
> to sound the same. In my quest for a new amplifier, it seems the use
> of varying technologies flys in the face of accuracy, and creates
> certain sonic "signatures".
>

Only if errors are audible.

> I've read that the matching of an amplifier to a set of speakers is
> critical, and that some speaker designs lend themselves to BiPolar
> amplifier designs, while others match better with MOSFET amplifier
> designs. It is said that BiPolar amplifiers sound "faster" and
> "punchier," and MOSFET designs are "softer".

Many things have been said in audio, particulat in high-end audio, that
are wrong.

>
> Based on some general research, MOSFET's devices are technically
> superior to BiPolar devices in just about every parameter:
>
> Mosfet devices are inherently self regulating. Since they exhibit a
> positive thermal co-efficient, they don’t suffer from the dreaded
> BiPolar thermal runaway.

This is a real advantage for MOSFET's, but we understand how to deal
with this problem in bipolar designs.

>
> Mosfet devices are very fast and switch several Amperes in nano
> seconds, which is 30 to 100 times faster than equivalent BiPolar
> devices.

Not true. Certainly you do not need to switch transistors off and on
that fast in linear audio amps. We can also easily turn on and off
bipolar devices in nanoseconds if we need to.

>
> Unlike BiPolar devices, Mosfets will handle short overvoltage and
> overcurrent operating conditions without terminal failure.

I don't believe that's true. You need to design in current limit
circuitry to protect against output shorts.

>
> In practice the reliability of Mosfets in surviving real world abuse,
> usually overheating and driving output shorts, is vastly higher than
> that of comparably rated Bi Polar output stages, even with their usual
> extensive protection.

Any evidence here?

>
> Mosfet devices require very small amounts of current to operate
> correctly, allowing a much simpler drive circuit to be used,
> maximising reliability and performance.

MOSFETS have much lower transconductance, which is the ratio of output
current to input voltage. This is one area where Bipolars enjoy a great
advantage.

This means you need a much larger input at the gate of the MOSFET to
cause the same output current to flow.

>
> Mosfets exhibit a "soft clip" characteristic when driven into clipping
> levels. This is predominantly made up of even order harmonics rather
> like that of Valve Amplifiers, not the harsh odd order harmonic
> clipping produced by BiPolar Amplifiers.

Not true, because any truly symmetrical designs will have very low even
harmonics, MOSFET's or Bipolars. The higher harmonics are reduced by
careful design and by using feedback judiciously.

>
> The misconception that Mosfet Amplifiers have by definition low
> damping factors was primarily due to early low output Hitachi device
> based amplifiers with marginal Power Supplies. This is not a factor
> with today’s Third generation high output Mosfet devices that are used
> throughout the ARX range.

What is ARX?

Can you get MOSFETS with the same output current capability as Bipolars
at equivalent price points? Can you get complementary devices as easily?

I have not looked at the latest power MOSFET's so I don't really know
how far power MOSFET's have developed.

>
> If MOSFET designs are technically superior, why do companies like
> Levinson and Krell use BiPolar designs?

Because they work well, meaning outstanding performance?

>
> Also, is there truth to the contention that BiPolar designs are
> "faster" sounding?

You first have to define what "faster" sounding means. Is that something
that is tied to a measurement?

>
> Thanks!

Archived from groups: rec.audio.high-end (More info?)

Your conjecture about converging sound amps is testable, how would you go
about doing such a test? Are you aware of any testing in the audio
community that already has addressed this question?


>I've always felt that if audio manufacturers truly aspired to achieve
>100% accuracy in electronic components, everything would slowly begin
>to sound the same. In my quest for a new amplifier, it seems the use
>of varying technologies flys in the face of accuracy, and creates
>certain sonic "signatures".
>
>I've read that the matching of an amplifier to a set of speakers is
>critical, and that some speaker designs lend themselves to BiPolar
>amplifier designs, while others match better with MOSFET amplifier
>designs. It is said that BiPolar amplifiers sound "faster" and
>"punchier," and MOSFET designs are "softer".
>
>Based on some general research, MOSFET's devices are technically
>superior to BiPolar devices in just about every parameter:
>
>Mosfet devices are inherently self regulating. Since they exhibit a
>positive thermal co-efficient, they don=92t suffer from the dreaded
>BiPolar thermal runaway.=20
>
>Mosfet devices are very fast and switch several Amperes in nano
>seconds, which is 30 to 100 times faster than equivalent BiPolar
>devices.=20
>
>Unlike BiPolar devices, Mosfets will handle short overvoltage and
>overcurrent operating conditions without terminal failure.=20
>
>In practice the reliability of Mosfets in surviving real world abuse,
>usually overheating and driving output shorts, is vastly higher than
>that of comparably rated Bi Polar output stages, even with their usual
>extensive protection.=20
>
>Mosfet devices require very small amounts of current to operate
>correctly, allowing a much simpler drive circuit to be used,
>maximising reliability and performance.=20
>
>Mosfets exhibit a "soft clip" characteristic when driven into clipping
>levels. This is predominantly made up of even order harmonics rather
>like that of Valve Amplifiers, not the harsh odd order harmonic
>clipping produced by BiPolar Amplifiers.
>
>The misconception that Mosfet Amplifiers have by definition low
>damping factors was primarily due to early low output Hitachi device
>based amplifiers with marginal Power Supplies. This is not a factor
>with today=92s Third generation high output Mosfet devices that are used
>throughout the ARX range.
>
>If MOSFET designs are technically superior, why do companies like
>Levinson and Krell use BiPolar designs?
>
>Also, is there truth to the contention that BiPolar designs are
>"faster" sounding?
>
>Thanks!

Archived from groups: rec.audio.high-end (More info?)

On 26 May 2005 00:33:13 GMT, rc620@hotmail.com (Seadweller) wrote:

>Greetings,
>
>I've always felt that if audio manufacturers truly aspired to achieve
>100% accuracy in electronic components, everything would slowly begin
>to sound the same.

It does, if you avoid deliberately degraded 'audiophile' labels.

> In my quest for a new amplifier, it seems the use
>of varying technologies flys in the face of accuracy, and creates
>certain sonic "signatures".

Really? And what is your evidence in suppport of this claim?

>I've read that the matching of an amplifier to a set of speakers is
>critical, and that some speaker designs lend themselves to BiPolar
>amplifier designs, while others match better with MOSFET amplifier
>designs. It is said that BiPolar amplifiers sound "faster" and
>"punchier," and MOSFET designs are "softer".

I can't imagine where you read that, but it's rubbish. All *good*
amplifiers sound the same.

>Based on some general research, MOSFET's devices are technically
>superior to BiPolar devices in just about every parameter:

Er, no, they're not.

>Mosfet devices are inherently self regulating. Since they exhibit a
>positive thermal co-efficient, they don’t suffer from the dreaded
>BiPolar thermal runaway.

Correct, but no big deal given decent design.

>Mosfet devices are very fast and switch several Amperes in nano
>seconds, which is 30 to 100 times faster than equivalent BiPolar
>devices.

Untrue, modern power BJTs can have equally fast switching speeds.

>Unlike BiPolar devices, Mosfets will handle short overvoltage and
>overcurrent operating conditions without terminal failure.

So will many BJTs.

>In practice the reliability of Mosfets in surviving real world abuse,
>usually overheating and driving output shorts, is vastly higher than
>that of comparably rated Bi Polar output stages, even with their usual
>extensive protection.

No, it isn't, as any repair tech can tell you.

>Mosfet devices require very small amounts of current to operate
>correctly, allowing a much simpler drive circuit to be used,
>maximising reliability and performance.

No, they don't, as MOSFETs have very high gate capacitances, which
require large slugs of current to drive them if good slew rates are to
be achieved.

>Mosfets exhibit a "soft clip" characteristic when driven into clipping
>levels. This is predominantly made up of even order harmonics rather
>like that of Valve Amplifiers, not the harsh odd order harmonic
>clipping produced by BiPolar Amplifiers.

This is utter nonsense, and a persistent 'urban myth'.

>The misconception that Mosfet Amplifiers have by definition low
>damping factors was primarily due to early low output Hitachi device
>based amplifiers with marginal Power Supplies. This is not a factor
>with today’s Third generation high output Mosfet devices that are used
>throughout the ARX range.

Output impedance has very little to do with the power supply.

>If MOSFET designs are technically superior, why do companies like
>Levinson and Krell use BiPolar designs?

Because MOSFET designs are *not* technically superior. Or inferior.

>Also, is there truth to the contention that BiPolar designs are
>"faster" sounding?

No.
--

Stewart Pinkerton | Music is Art - Audio is Engineering

Archived from groups: rec.audio.high-end (More info?)

On 27 May 2005 00:10:59 GMT, Stewart Pinkerton <patent3@dircon.co.uk>
wrote:


>> In my quest for a new amplifier, it seems the use
>>of varying technologies flys in the face of accuracy, and creates
>>certain sonic "signatures".
>
>Really? And what is your evidence in suppport of this claim?

Thanks everyone........Just to be clear, I read the BiPolar vs MOSFET
information elsewhere, so I'm not supporting, nor can I take credit
for the data.....

I guess as with any technology, there's always contradictory data at
your fingertips (on the internet)!!!

In reply to Chung who wondered like me
> What is ARX ?

It is the name of a MOS-FET devices company as visible here

http://www.arx.com.au/Australia/Mosfets.htm

One can recognise the list of assertions from the original post.