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Class D Audio

J

Joseph Roberts

Jan 1, 1970
0
Has anyone out there ever designed class D amplifiers? I have scrubbed out
a basic design, but have no experience with class D at all. Any advice
would be greatly appreciated. Thanks.
 
R

Rolavine

Jan 1, 1970
0
Subject: Class D Audio
From: "Joseph Roberts" [email protected]
Date: 2/2/2004 12:18 PM Pacific Standard Time
Message-id: <EgyTb.9940$Ii2.2903@lakeread03>

Has anyone out there ever designed class D amplifiers? I have scrubbed out
a basic design, but have no experience with class D at all. Any advice
would be greatly appreciated. Thanks.
You have checked out TI, National Semi, and Zetex? They all make ICs for class
D audio?

Rocky
 
N

N. Thornton

Jan 1, 1970
0
Joseph Roberts said:
Has anyone out there ever designed class D amplifiers? I have scrubbed out
a basic design, but have no experience with class D at all. Any advice
would be greatly appreciated. Thanks.

My only ssuggestion would be to ask less vague qs
 
M

Michael Black

Jan 1, 1970
0
N. Thornton ([email protected]) said:
My only ssuggestion would be to ask less vague qs

I believe Class D amplifiers have actually made it into real applications,
which would suggest that yes, someone "out there" has had experience
designing them.

Who knows if any of them are here.

Yes, the question is silly, because whatever he really wants to know
might easily be answered by someone who doesn't have first hand experience.
Or the question may be more general than he assumes, so someone with
experience in another realm may still have the answer without first
hand experience.

Michael
 
S

Spajky®

Jan 1, 1970
0
You have checked out TI, National Semi, and Zetex? They all make ICs for class
D audio?
There is also a Phillips car amp IC for that ...

-- Regards, SPAJKY ®
& visit my site @ http://www.spajky.vze.com
"Tualatin OC-ed / BX-Slot1 / inaudible setup!"
E-mail AntiSpam: remove ##
 
P

Paul Mathews

Jan 1, 1970
0
Joseph Roberts said:
Has anyone out there ever designed class D amplifiers? I have scrubbed out
a basic design, but have no experience with class D at all. Any advice
would be greatly appreciated. Thanks.

have a look here:


http://www.classd.org

Paul Mathews
 
N

N. Thornton

Jan 1, 1970
0
I believe Class D amplifiers have actually made it into real applications,
which would suggest that yes, someone "out there" has had experience
designing them.

Who knows if any of them are here.

Yes, the question is silly, because whatever he really wants to know
might easily be answered by someone who doesn't have first hand experience.
Or the question may be more general than he assumes, so someone with
experience in another realm may still have the answer without first
hand experience.

Michael

Hi

I'm sure one could write a fair bit about class D, but not many folk
here will write someone a book in response to a vague comment. App
notes might be a place to start.

I assume many folk here have done class D amps, or maybe I should say
Class S. Theyre an effective way to considerably cut power
consumption, and thus dissipation, and those are exactly whats needed
in laptops for example. There are now filterless class D amps as well.

Regards, NT
 
W

Walter Harley

Jan 1, 1970
0
N. Thornton said:
My only ssuggestion would be to ask less vague qs


Okay, let me try, since I have what I think is essentially the same question
as the OP:

I'd like to design class D audio amplifiers, capable of several hundred
watts or possibly up to 1kW, with THD+N less than 0.1% at rated power. Of
course I don't expect to get there in one step, but it's the eventual goal.
Commercial manufacturers have obtained this, so it is possible.

As an initial goal, I'd like to design a decent-sounding and reliable 20W
(avg continuous 1kHz sinewave) audio amp with THD+N < 0.25% (1kHz,
unweighted, 20Hz-80kHz) into 8 ohms. But I'd like to understand what I'm
designing: I don't just want to plug in an eval board and call it done,
because I won't have learned anything.

However, I have no experience with class D at all.

Are there any textbooks and/or good tutorials that cover the topics relevant
to designing class D amps? I am aware that there are some manufacturer app
notes for IC-based solutions, but I would prefer information that was more
thorough in its coverage and less biased toward particular solutions.

If there are not any such texts, can anyone **with practical experience in
this area** recommend a preferred design approach, or some preferred
manufacturers to start with? For instance, will I likely get farthest
working with discrete components, or should I base my designs around one of
the chips by Zetex or TI or such? What might the tradeoffs be, and are
there particular dead-end alleys I should be careful to avoid?

Thanks for any advice you can give me.

-walter
 
F

Frithiof Andreas Jensen

Jan 1, 1970
0
Walter Harley said:
"N. Thornton" <[email protected]> wrote
Commercial manufacturers have obtained this, so it is possible.

Would appear so: http://www.st.com/stonline/press/news/year2004/p1399d.htm
As an initial goal, I'd like to design a decent-sounding and reliable 20W
(avg continuous 1kHz sinewave) audio amp with THD+N < 0.25% (1kHz,
unweighted, 20Hz-80kHz) into 8 ohms.

The ST "DDX" STA304/6/8 + STA505 amplifier chips would appear to meet that
requirement.
Are there any textbooks and/or good tutorials that cover the topics relevant
to designing class D amps? I am aware that there are some manufacturer app
notes for IC-based solutions, but I would prefer information that was more
thorough in its coverage and less biased toward particular solutions.

I would go looking for the patents on Class D amplifiers and related tech.
f.ex. DDX - Apogee Technology holds many of them - that may be a place to
start. http://www.apogeeddx.com/ . Thomcast SA once took out a patent on a
"Pulse-Step Modulator" that may be applicable too.

There is also Bang & Olufson's "Ice Power".
the chips by Zetex or TI or such? What might the tradeoffs be, and are
there particular dead-end alleys I should be careful to avoid?

I think one will get further in less time with the chipsets and here is why:

The digital amplifers achieve their low distortion by applying proprietary
pre-distortion algorithms to the signal; The algorithms are likely to be
described in the patents but - in my experience - algorithms are difficult
to implement; With DSP's there are often hardware tricks/deficiencies on
needs to know and the tools are $$$. Much better to buy the custom
audio-processor chip.

That leave the power stage *but* with the price of components being what it
is it is way cheaper and easier to just buy the matching integrated device -
sorry ;-)

One can then spend time and effort on producing a decent power supply, a
proper layout and adequate cooling - not to mention the software interface
requirted to control these chips. This is a challenge in itself.
Thanks for any advice you can give me.

Home construction is no fun anymore - in the good olde days one could
actually build stuff that:

1: cost a great deal less than the stuff one could buy
2: performed much better
and:
3: where not even available "on the street", thus bringing Hack Value into
the benefits.

Sadly, this is not so. Perhaps within Robotics - but within "straight"
electronics one cannot even obtain the parts at less than four times the
costs of a commercial unit with good performance!
 
R

Rolavine

Jan 1, 1970
0
I think I saw a Sterophile recommended class D audio amp made by PS Audio
within the last year. Was I dreaming?

Rocky
 
B

Ban

Jan 1, 1970
0
Walter said:
Okay, let me try, since I have what I think is essentially the same
question as the OP:

I'd like to design class D audio amplifiers, capable of several
hundred watts or possibly up to 1kW, with THD+N less than 0.1% at
rated power. Of course I don't expect to get there in one step, but
it's the eventual goal. Commercial manufacturers have obtained this,
so it is possible.

As an initial goal, I'd like to design a decent-sounding and reliable
20W (avg continuous 1kHz sinewave) audio amp with THD+N < 0.25% (1kHz,
unweighted, 20Hz-80kHz) into 8 ohms. But I'd like to understand what
I'm designing: I don't just want to plug in an eval board and call it
done, because I won't have learned anything.

However, I have no experience with class D at all.

Are there any textbooks and/or good tutorials that cover the topics
relevant to designing class D amps? I am aware that there are some
manufacturer app notes for IC-based solutions, but I would prefer
information that was more thorough in its coverage and less biased
toward particular solutions.

If there are not any such texts, can anyone **with practical
experience in this area** recommend a preferred design approach, or
some preferred manufacturers to start with? For instance, will I
likely get farthest working with discrete components, or should I
base my designs around one of the chips by Zetex or TI or such? What
might the tradeoffs be, and are there particular dead-end alleys I
should be careful to avoid?

Thanks for any advice you can give me.

-walter

Walter,
you can design and build a good quality PWM-modulator like this. No special
knowledge or patents needed. The principle is very simple and in fact you
will find this incorporated into PICs and other MCUs.


Triangle Generator Comparator Power stage Lowpass Filter
.-------------.
| | +-----+ +-+ +---+ + +5V
| /\ | | | | | | | |
| / \ |--+ --+ +-+ +-----+ +---+ -5V
| \ / | |
| \/ | |
| | | |\ .--------------.
'-------------' +---|+\ | | ___ __ /|
| >----| |----UUU---+--o---| | |
+---|-/ | | | +-|__| |
Analog In | |/ | | --- | \|
o----------------+ '--------------' --- |
| |
=== ===
GND GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Now I can help you with the circuits as well, but if you understand the
principle, you will find out the required frequencies etc. yourself.

ciao Ban
 
W

Walter Harley

Jan 1, 1970
0
Ban said:
you can design and build a good quality PWM-modulator like this. No special
knowledge or patents needed. The principle is very simple and in fact you
will find this incorporated into PICs and other MCUs.
[...]


Thanks, Ban. I understand that is the basis for class D. (I have a little
knowledge, just no *experience*). But I believe that there are practical
issues, such as how to keep the power stage from having both transistors on
at the same time. I am sure the companies who make class-D audio amps have
already discovered them... maybe I will have to rediscover them myself, the
hard way.

An example of the sort of practical issue that I don't understand: I have
read (here on s.e.d) that it is preferable to use a linear power supply for
a class-D amp, because it is hard to stabilize a SMPS in this application (I
don't know why). I have also read the opposite assertion. I wonder what
the people who actually make class-D amps have discovered, and whether and
how the problems have been remedied. But that is just an example - I am
sure there are many such issues.

I suspect many of the issues of class D audio are the same as they are for
switch-mode power supplies, and I have a few textbooks on the theory and
practical design of those. And of course there are quite a few good books
on the theory and practical design of linear audio amps. But it is mildly
surprising that there seems to be nothing at all written on practical
class-D audio.

Anyway, it seems I've gotten as good an answer as is available; thanks to
all.

-walter
 
J

Jan Panteltje

Jan 1, 1970
0
Ban said:
you can design and build a good quality PWM-modulator like this. No special
knowledge or patents needed. The principle is very simple and in fact you
will find this incorporated into PICs and other MCUs.
[...]


Thanks, Ban. I understand that is the basis for class D. (I have a little
knowledge, just no *experience*). But I believe that there are practical
issues, such as how to keep the power stage from having both transistors on
at the same time.
If you take that circuit, and use a diff amp output for the slice voltage, and 2
comparators, you can slice + and - side, and get 2 180 degree apart pulses
that each can drive an output transistor.
Even then it is strongly recommended to have some I measurement in the output
transistors, that will limit the 'on' times.
It is indeed a lot like switch mode design, it IS in fact switcht mode design with
a fast variable reference (the audio).
JP
 
N

N. Thornton

Jan 1, 1970
0
Ban said:
Walter,
you can design and build a good quality PWM-modulator like this. No special
knowledge or patents needed. The principle is very simple and in fact you
will find this incorporated into PICs and other MCUs.


Triangle Generator Comparator Power stage Lowpass Filter
.-------------.
| | +-----+ +-+ +---+ + +5V
| /\ | | | | | | | |
| / \ |--+ --+ +-+ +-----+ +---+ -5V
| \ / | |
| \/ | |
| | | |\ .--------------.
'-------------' +---|+\ | | ___ __ /|
| >----| |----UUU---+--o---| | |
+---|-/ | | | +-|__| |
Analog In | |/ | | --- | \|
o----------------+ '--------------' --- |
| |
=== ===
GND GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Now I can help you with the circuits as well, but if you understand the
principle, you will find out the required frequencies etc. yourself.

ciao Ban


Thats a basic class D amp, and the quality wont be good. Open loop
amps like that are wide open to PSU V variations and PSU hum, and
suffer from distortion caused by imperfect switching, nonlinear output
inductors, and relatively high output impednace. Even the simplest
linear amps OTOH have none of those problems.

Its a good starting point to learn, to make something like the above
with an opamp and a couple of output trs, but dont expect hifi.


Regards, NT
 
N

N. Thornton

Jan 1, 1970
0
Walter Harley said:
Ban said:
you can design and build a good quality PWM-modulator like this. No special
knowledge or patents needed. The principle is very simple and in fact you
will find this incorporated into PICs and other MCUs.
[...]


Thanks, Ban. I understand that is the basis for class D. (I have a little
knowledge, just no *experience*). But I believe that there are practical
issues, such as how to keep the power stage from having both transistors on
at the same time. I am sure the companies who make class-D audio amps have
already discovered them... maybe I will have to rediscover them myself, the
hard way.

That one is quite easy. Arrange your trs so they only switch on when
drive voltage is close to the power rail, ie use pnp on top and npn on
bottom. Now drive them off one common connection: it is now impossible
for both to be on together.

All digital signals slew, so however you do it just picture the drive
signal slewing and ensure each tr is turned off before the other one
starts to conduct.

An example of the sort of practical issue that I don't understand: I have
read (here on s.e.d) that it is preferable to use a linear power supply for
a class-D amp, because it is hard to stabilize a SMPS in this application (I
don't know why). I have also read the opposite assertion. I wonder what
the people who actually make class-D amps have discovered, and whether and
how the problems have been remedied. But that is just an example - I am
sure there are many such issues.

Class D has no inherent PSU rejection, whereas linear amps have. So
you either need a very clean very stable PSU, or else error correction
in the amp to work round the modulation by the vagaries of the PSU.
SMPS will work, linear supplies will also work but with much less
grace, and tend to cause junk modulation in the amps output.

I suspect many of the issues of class D audio are the same as they are for
switch-mode power supplies, and I have a few textbooks on the theory and
practical design of those. And of course there are quite a few good books
on the theory and practical design of linear audio amps. But it is mildly
surprising that there seems to be nothing at all written on practical
class-D audio.

Actually there are a list of issues with Class D that dont really crop
up with SMPS or linear audio. Most of them boil down to distortion,
which is quite tough to minimise with class D. There are also
questions of output filter design, EMI, operating frequency, and
efficiency, but the big one is distortion.


Regards, NT
 
B

Ban

Jan 1, 1970
0
Walter said:
An example of the sort of practical issue that I don't understand: I
have read (here on s.e.d) that it is preferable to use a linear power
supply for a class-D amp, because it is hard to stabilize a SMPS in
this application (I don't know why). I have also read the opposite
assertion. I wonder what the people who actually make class-D amps
have discovered, and whether and how the problems have been remedied.
But that is just an example - I am sure there are many such issues.

I suspect many of the issues of class D audio are the same as they
are for switch-mode power supplies, and I have a few textbooks on the
theory and practical design of those. And of course there are quite
a few good books on the theory and practical design of linear audio
amps. But it is mildly surprising that there seems to be nothing at
all written on practical class-D audio.

Walter,
when you look at commercially made amps they mostly do have conventional
power supplies. And I do not know why, because you throw away form factor
and efficiency. But then a 1kW supply with PFC and wide range input voltage
and synchronized switching frequency is a beast by itself, so maybe with
time this is gonna change.

It is also not possible or too expensive to go beyond 1MHz switching
frequency, hardly any drivers for MosFets available that fast, let alone
BJTs switching times.
I use 768kHz with the HIP4081A H-Bridge Driver and STP19NB20 Fets in a
bridge configuration. The driver takes care of shoot-thru current in this
case.
The triangle generator can be made by charging/discharging a capacitor
with current sources, which are referenced by the supply voltage, this gives
some suppression for this hum-problem that usually comes up because of the
missing PSSR. The varying resulting freq. is not a problem, as long as both
channels use the same DC-supply.
I'm still struggeling with a sample by sample integrator feedback from
the output to the quantisation stage, as I want to drive the amp also
digitally with a toslink cable.

Well, I gave you a few hints, but didn't reveal all the secrets (yet). If
you are a little further in your design, drop me a mail and I try to answer
then.

ciao Ban
Bordighera, Italy
http://www.bansuri.my-page.ms/
 
J

John Woodgate

Jan 1, 1970
0
when you look at commercially made amps they mostly do have conventional
power supplies. And I do not know why, because you throw away form
factor and efficiency.

Audio designers can generally design linear power supplies, but SMPS are
a different art.
But then a 1kW supply with PFC and wide range
input voltage and synchronized switching frequency is a beast by itself,
Indeed.

so maybe with time this is gonna change.

It probably depends on the relative costs of the two technologies. Size,
efficiency and mass are sometimes important, sometimes not.
 
W

Walter Harley

Jan 1, 1970
0
N. Thornton said:
Thats a basic class D amp, and the quality wont be good. Open loop
amps like that are wide open to PSU V variations and PSU hum, and
suffer from distortion caused by imperfect switching, nonlinear output
inductors, and relatively high output impednace. Even the simplest
linear amps OTOH have none of those problems.


In a linear amp, global negative feedback addresses many of those issues.
Is the problem with Ban's picture that the L-C output filter introduces too
much phase shift to be useful as NFB? (I've read that was an issue with
tube hifi amps, trying to use feedback to correct distortion in the output
transformer.)

More broadly, how come you guys know this stuff? I mean that literally, not
rhetorically: that is, for those of you responding with practical specifics,
did you get the answers because you've designed class D amps, or because
you've read about it in industry periodicals, or texts, or because you're
surprisingly good at predicting real-world complications of theoretical
designs, or...? I suspect that I'm missing out on some sources of
information.
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Walter Harley
In a linear amp, global negative feedback addresses many of those
issues. Is the problem with Ban's picture that the L-C output filter
introduces too much phase shift to be useful as NFB? (I've read that
was an issue with tube hifi amps, trying to use feedback to correct
distortion in the output transformer.)

It was an issue, but very much tied up with the details of transformer
construction, the sort of thing than many amplifier designers left to
transformer manufacturers. Basically, the 'too much phase-shift' was
caused by excessive self- and inter-winding capacitances, and could be
substantially eliminated by attention to insulation thicknesses and
winding plans.
 
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