MOSFET output stage

[Jan Panteltje]

I think it should be possible [I could] design powered speakers with a WiFi
interface.

Yuck, wireless...

Around the house these days Homeplug is far better than WiFi.

d

I have looked a bit at homeplug, because the new TVs seem to support it,
but really, I am a step ahead, I use the laptop as portable TV monitor to
watch satellite TV, and any file on the server I like.
Using WiFi ! so wireless.
This is an old blog entry, have added much cool stuff since:
http://panteltje.com/panteltje/blog/index.html
Playing nice slide shows now too.
http://panteltje.com/panteltje/jpg_to_mjpegtools_yuv/

It is in this that it shows, and is proof of,
that WiFi is not in any way a real bandwidth problem
This is all Linux soft of course, so highly efficient.

The worlds is just spitting out one marketing term after the other, now
it is 'home plug'.
Really, as far as network is concerned, ssh, netcat, scp, transport stream,
etc... rules it all.

They (Philips, others), want to sell you an 'internet radio', a 'digital picture frame',
a this, a that, while any laptop running Linux with a 5 line script does it all.
With more possibilities and more flexible, and future proof, and wireless too.
Often for less money.

 

[Jan Panteltje]

Jan Panteltje write:
I mean I try to avoid wireless whenever possible. So when I'm at home, I
watch cable TV and listen cable radio and use my landline phone...

OK, whatever you like best.
See also my other posting.

 

[Chronic Philharmonic]

Who do MOSFET sound better than bipolar, as an audio amp output
driver?

A lot of bits have been spilled on this thread so far, but I think it is the
same question as "Why do tubes sound better than transistors?" or "How many
angels can dance on the head of a pin?"

First of all, it implies a truth that may not objectively be the case. But
tubes, BJTs and MOSFETs all have very different characteristics and
engineering trade-offs. I think it has been reasonably well established that
it is possible to make excellent sounding amplifiers with any of those
technologies.

In the end, I think it boils down to the experience and skill of the
engineer in working with a given technology, and how that impacts the
economics of producing the end product using said technology.

For example, I think we no longer use vacuum tubes for mainstream audio
power amplifiers because they:

1. Require an additional power supply and heat to operate.
2. Are bulky compared to their solid-state alternatives.
3. Are not available in complimentary pairs (e.g., P-channel/N-channel)
4. Have output impedance that usually requires an output transformer.

None of these are insurmountable obstacles, but they do increase the size,
cost, power consumption and heat load for a given output power.

Similar, but less obvious considerations affect the choice of BJTs and
MOSFETs. They have different characteristics, and the skilled engineer will
exploit them for optimal effect -- sonic quality, economy, reliability and
so on. The marketing department will define the requirements for the product
based on the target market, and the engineer will attempt to design a
product that fits within those constraints. In all that I have read here, I
have not seen anything that would consistently make me select MOSFETs over
BJTs. But I might have a preference based on a *given set* of product
requirements.

 

[Vladimir Vassilevsky]

Jan Panteltje wrote:

I think it should be possible [I could] design powered speakers with a WiFi interface.

How would you synchronize the different channels?

Yes, good point, timestamp would be one way, but that does not solve the delay.
the delay would be fatal in a live application.

Here is the idea: using the power frequency as the common timing
reference. In the local WiFi network, the ping time would be at the
order of 1ms, so all channels could be PLLed to the same half period of
the AC power without an ambiguity. With the sufficient amount of
buffering, that should allow streaming multiple synchronized channels.
Sooo simple... I bet somebody already got a patent on that.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[Jan Panteltje]

Jan Panteltje wrote:


I think it should be possible [I could] design powered speakers with a WiFi interface.

How would you synchronize the different channels?

Yes, good point, timestamp would be one way, but that does not solve the delay.
the delay would be fatal in a live application.

Here is the idea: using the power frequency as the common timing
reference. In the local WiFi network, the ping time would be at the
order of 1ms, so all channels could be PLLed to the same half period of
the AC power without an ambiguity. With the sufficient amount of
buffering, that should allow streaming multiple synchronized channels.
Sooo simple... I bet somebody already got a patent on that.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

Clever!
How about this: we give each speaker a GPS.
It will also send back its position, and the 'mixer' will
then calculate the optimum sound pattern for 5.1.
GPS also has a very precise clock.

 

[Eeyore]

Most people are satisfied with AC3 though.

Dumbing down wins again.

Graham

 

[Eeyore]

Yes, that's exactly what I meant.

And a chopper amp has exactly WHAT to do with ultra high quality audio ?

Graham

 

[Eeyore]

Boundary condition.

Boundary condition.

AFAIK, BJTs still hold the world record for cutoff frequency.

In devices suitable for audio amps ?

You are totally incapable of staying on-topic.

Graham

 

[Eeyore]

Offset? Two BJTs of the same part number will have delta-Vbe well
below a tenth of a volt.

IME typically within a 10mV range. That saved a trimpot and the line time to
adjust it on more than one occasion.

A similar pair of mosfets will have Vgs-th
and transfer curves that can differ by a volt or more.

For vertical types, yes. Not for laterals.

And the mosfet thresholds will change with time a lot more then the bjt's.

No comment. Never known it be a problem with laterals.

Mosfets have zero storage/desaturation time,

Well, 60ns on the data sheet was good enough for me.

far better SOAR specs, and are easier to drive.

True.

Graham

 

[Eeyore]

"RichD" wrote


A lot of bits have been spilled on this thread so far, but I think it is the
same question as "Why do tubes sound better than transistors?" or "How many
angels can dance on the head of a pin?"

First of all, it implies a truth that may not objectively be the case. But
tubes, BJTs and MOSFETs all have very different characteristics and
engineering trade-offs.

Actually, certain mosfets are not at all unlike triodes.

More to the point is whether you are anti-science and despise negative feedback.
And / or LIKE your music deliberately distorted as some seem to do.

Graham

 

[Vladimir Vassilevsky]

On a sunny day (Sun, 21 Sep 2008 13:41:43 -0500) it happened Vladimir
Jan Panteltje wrote:
I think it should be possible [I could] design powered speakers with a WiFi interface.
How would you synchronize the different channels?
Yes, good point, timestamp would be one way, but that does not solve the delay.
the delay would be fatal in a live application.

Here is the idea: using the power frequency as the common timing
reference. In the local WiFi network, the ping time would be at the
order of 1ms, so all channels could be PLLed to the same half period of
the AC power without an ambiguity. With the sufficient amount of
buffering, that should allow streaming multiple synchronized channels.
Sooo simple... I bet somebody already got a patent on that.

[/QUOTE]

How about this: we give each speaker a GPS.

Yes, something like GPS but it should be 10 times more precise and be
able to operate indoors. Local positioning system is another perpetual
problem; it would be a good thing to have regardless.

It will also send back its position, and the 'mixer' will
then calculate the optimum sound pattern for 5.1.
GPS also has a very precise clock.

Lots of things can be done; however the consumer audio is very sensitive
to the cost. Many suggestions in this thread, although technically
standing, are totally missing this point.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[Jorden Verwer]

And a chopper amp has exactly WHAT to do with ultra high quality audio ?

Well, it proves my point.

 

[Jamie]

Well, it proves my point.

LOL!
good one.

http://webpages.charter.net/jamie_5"

 

[Phil Allison]

"Chronic Philharmonic"

Similar, but less obvious considerations affect the choice of BJTs and
MOSFETs.

** Which mosfets ?

You must not confuse switching types with laterals.

They have different characteristics,

** Which mosfets ?

You must not confuse switching types with laterals.

and the skilled engineer will exploit them for optimal effect -- sonic
quality, economy, reliability and so on.

** Which mosfets ?

You must not confuse switching types with laterals.

The marketing department will define the requirements for the product
based on the target market, and the engineer will attempt to design a
product that fits within those constraints.

** It don't have to work that idiotic way at all.

In all that I have read here, I have not seen anything that would
consistently make me select MOSFETs over BJTs.

** Which mosfets ?

You must not confuse switching types with laterals.

But I might have a preference based on a *given set* of product
requirements.

** Pigs might fly too.

Wot an posturing old wind bag you are.



...... Phil

 

[Eeyore]

Why? The physics is the same... oxide thickness, trapped charge, all
the things that modulate gate threshold.

I'm telling you that's how laterals are in practice.

Better processing perhaps ?

The datasheets I've seen show a volt or so spread of Vgs-th spec,
plenty enough to defeat simple biasing/balancing schemes for
paralleled devices.

Laterals work beautifully like that.

Graham

 

[Chronic Philharmonic]

Actually, certain mosfets are not at all unlike triodes.

I almost said that, but decided to leave it because many other physical
and/or electrical parameters are distinctly different, and I figured I'd get
flamed for that.

More to the point is whether you are anti-science and despise negative
feedback.
And / or LIKE your music deliberately distorted as some seem to do.

I think negative feedback is an extremely powerful tool in electronic,
economic, social and natural systems. But people don't seem to understand
it, so they don't trust it.

 

[Trevor Wilson]

Actually, certain mosfets are not at all unlike triodes.

More to the point is whether you are anti-science and despise negative
feedback.

**It matters not whether a person despises NFB or not. NFB is present in
every amplifier known to man. There are, however, a variety of ways that NFB
can be applied.

 

[Jorden Verwer]

No. Google "widest bandwidth transistor" or some such.

All that yields is your post.

Okay, sillyness aside, without the quotes it does indeed return some
results. But I couldn't find a definitive answer either way. I do remember
reading about a BJT that had an ft of 500 GHz a couple of years ago, which
was a new record at the time. It's indeed possible that things have changed
since then, although I'd bet that BJTs can't be far behind.

Of course this is all a whole different game than audio, but frankly I think
both BJTs and MOSFETs offer sufficient bandwith in audio applications these
days.

 

[MooseFET]

The idea of the feedback directly from the cone is no new. The main
problem is economical: it is easier to make a reasonable conventional
speaker rather then trying to make a good speaker from the bad one by
the means of feedback and feedforward compensation.

I wasn't trying to suggest that I had invented the idea, only that I
like the idea.

Silk purse != sows ear holds true for cases with feedback.

Once you have a good speaker, adding local feedback, it seems to me
would back a very good speaker. The 3db points could become 1db
points and the distortion of the low frequencies would be reduced by
about what the loop gain is.

 

[Vladimir Vassilevsky]

I wasn't trying to suggest that I had invented the idea, only that I
like the idea.

Silk purse != sows ear holds true for cases with feedback.

Once you have a good speaker, adding local feedback, it seems to me
would back a very good speaker.

Again, it turns out that it is cheaper to make just a very good speaker
rather then making a very good speaker from a good one by means of some
feedback magic. Fixing the origin of a problem is better then finding a
way around a problem.

The 3db points could become 1db
points and the distortion of the low frequencies would be reduced by
about what the loop gain is.

At one time, I was trying to get more usable SPL from subwoofer by
applying the feedforward and feedback corrections by DSP. There was an
improvement of somewhat 2dB. However the result was the rapid mechanical
destruction of the speaker; the heat dissipation was the issue, too.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com