switching(class d?) audio amp schematic

[blisca]

hi to everybody from Diego,Milan,Italy
i'm looking for the schematic of a good switching audio amp
a couple of years ago i built a very simple pwm audio amp ,the quality is
poor,but suitable for electric guitar
I would like to do something hi-fi,witouth using texas,zetex or tripath
ics,with the purpose of making some experiment
thank you
Ciao !

 

[Mochuelo]

hi to everybody from Diego,Milan,Italy
i'm looking for the schematic of a good switching audio amp
a couple of years ago i built a very simple pwm audio amp ,the quality is
poor,but suitable for electric guitar
I would like to do something hi-fi,witouth using texas,zetex or tripath
ics,with the purpose of making some experiment
thank you
Ciao !

If you don't want an all-digital solution, you can use analog feedback
to improve the performance. Sense the output voltage right at the
output of the full-bridge switching stage (so, before the low-pass
passive filter) and use it to correct the input voltage to the analog
PWM generator. If you do it right, the benefit can be impressive.

Also, make sure that the dead times at the switching stage are not too
long. Supplies and PCB should be carefully designed, too. If stereo,
use separate supplies (or at least separate supply filters) for each
channel.

 

[Genome]

hi to everybody from Diego,Milan,Italy
i'm looking for the schematic of a good switching audio amp
a couple of years ago i built a very simple pwm audio amp ,the quality is
poor,but suitable for electric guitar
I would like to do something hi-fi,witouth using texas,zetex or tripath
ics,with the purpose of making some experiment
thank you
Ciao !

You am want look at.....

http://www.diyaudio.com/forums/forumdisplay.php?forumid=49

Read lotz before asking questions.

DNA

 

[blisca]

If you don't want an all-digital solution, you can use analog feedback
to improve the performance. Sense the output voltage right at the
output of the full-bridge switching stage (so, before the low-pass
passive filter) and use it to correct the input voltage to the analog
PWM generator. If you do it right, the benefit can be impressive.

What i don't understand is how can i sense the voltage without integrate the
PWM with another low pass filter
Should i measure it directly as a time?I mean with stuff like a
microcontroller or other digital circuit?
I'm filling the registration module for that forum that Genome suggested
me,looks very interesting!
Thank you again

 

[Mochuelo]

What i don't understand is how can i sense the voltage without integrate the
PWM with another low pass filter

Sense the output voltage between the switching stage and the low-pass
filter, scale it down, convert it to asymmetric (if the input is so,
and you are using a full-bridge), low-pass filter it, subtract it from
the input voltage, and add some amount of the resulting error to your
input voltage, to create the signal that will drive your analog PWM
generator.

Should i measure it directly as a time?I mean with stuff like a
microcontroller or other digital circuit?

No, you don't measure the pulse widths, and you don't need any digital
part. The LPF takes care of going back from PWM to a continuous analog
signal that you can use for the comparison (subtraction).

Best,

 

[Tim Williams]

Sense the output voltage between the switching stage and the low-pass
filter, scale it down, convert it to asymmetric (if the input is so,
and you are using a full-bridge), low-pass filter it, subtract it from ^ ^ ^ ^
the input voltage, and add some amount of the resulting error to your
input voltage, to create the signal that will drive your analog PWM
generator.

So, you originally said to take the signal before the integrator? And you
say to LPF it anyway???

Tim

 

[Mochuelo]

So, you originally said to take the signal before the integrator?

I didn't mention any integrator.

And you say to LPF it anyway???

Don't know if I understood you. The LPF for the feedback path does not
need inductors, and can be more linear and with tighter tolerances
than the high-power, passive LPF at the output of the switching stage.

Best,

 

[Tim Williams]

I didn't mention any integrator.

It seems to me a LPF integrates. Whatever you're doing, if you view it in
the time domain, you're integrating PWM'd up and down pulses into a smooth,
slower waveform. In the frequency domain, you're cutting out the carrier
and sidebands, leaving the low frequency band of interest. Same thing.

Don't know if I understood you. The LPF for the feedback path does not
need inductors, and can be more linear and with tighter tolerances
than the high-power, passive LPF at the output of the switching stage.

Well, the goal is to get excruciatingly linear *output* voltage, isn't it?
So wouldn't the idea be to correct the output voltage itself?

BTW, if you drop inductors, you can't get as sharp a cutoff response in the
loop, correct? So to get the same amount of filtering of the carrier,
you'll really slow down the loop, possible causing oscillation? Fast op-amp
active filters aside.

Tim

 

[Mochuelo]

It seems to me a LPF integrates.

False. Try dc input.

Whatever you're doing, if you view it in
the time domain, you're integrating PWM'd up and down pulses into a smooth,
slower waveform. In the frequency domain, you're cutting out the carrier
and sidebands, leaving the low frequency band of interest. Same thing.

An integrator has infinite dc gain. An LPF does not.

Well, the goal is to get excruciatingly linear *output* voltage, isn't it?
So wouldn't the idea be to correct the output voltage itself?

That can yield better performance, but it is more complex to design
the feedback so that the system is stable, and I don't recommend it to
someone that wants to do it himself. Besides, this is analog. To aim
at the highest performance, I would recommend an all-digital solution
(upsampling, uniform-to-natural sampling conversion, noise shaping,
digital pwm generation, MOSFET driver, full-bridge switching stage,
passive low-pass filter and some elaborate feedback).

BTW, if you drop inductors, you can't get as sharp a cutoff response in the
loop, correct? So to get the same amount of filtering of the carrier,
you'll really slow down the loop, possible causing oscillation? Fast op-amp
active filters aside.

You don't need op-amps, but it is not that you cannot use them. You
can, if you want, and know how to.

Best,

 

[Tim Williams]

False. Try dc input.

Well obviously, it fails for very slow rates. But I was hoping you had the
intelligence to integrate over a period somewhat shorter than the audio
you're trying to amplify in the first place......

An integrator has infinite dc gain. An LPF does not.

Fair enough. How about frequencies of interest? Hmmmm?

Tim

 

[Mochuelo]

Well obviously, it fails for very slow rates. But I was hoping you had the
intelligence

Tim, what's your problem? Tell us.

 

[Mochuelo]

Well obviously, it fails for very slow rates. But I was hoping you had the
intelligence to integrate over a period somewhat shorter than the audio
you're trying to amplify in the first place......


Fair enough. How about frequencies of interest? Hmmmm?

Tim

Integrator : -20 dB/decade everywhere.
LPF : approx. flat in its passband.

Do they still look like the same to you?

Besides, which are the "frequencies of interest" when you work with an
integrator? What's its bandwidth? What's its cutoff frequency?