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Old 20th October 2008, 02:05 PM   #1
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Question Triangular waveform

Hey homies

Im building a class d amplifier, and i need to generate a triangular wave for the PWM/comparator circuit.

Can anyone recommend an excellent IC? ...or a circuit?
A low THD is very important

Im considering the XR2206

Cheers
Me

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Old 20th October 2008, 03:27 PM   #2
tiki is offline tiki  Germany
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Hello,
the THD of a triangle generator cannot be very low.
look here, it's based on a proposal of the good JohnW.
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Old 20th October 2008, 07:25 PM   #3
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But then how are people supposed to achieve good results with class d amplifiers?

Is it because of the high frequency of the triangular waveform?
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Old 20th October 2008, 08:03 PM   #4
tiki is offline tiki  Germany
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The linearity(!) should be excellent.
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Old 21st October 2008, 12:47 AM   #5
bwaslo is offline bwaslo  United States
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Quote:
Originally posted by dead rat
But then how are people supposed to achieve good results with class d amplifiers?

The better ones are self-oscillating and don't use triangle waves. See, for instance, IcePower or UcD.

Check the "Paper 6690, Presented at the AES 120th convention, Paris 2006" in the Applications section at http://www.hypex.nl/
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Old 31st October 2008, 07:26 PM   #6
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Unhappy Improving triangular waveform

hellooooo

I have made a triangular wave using the XR2206.

It works great at 1 kHz.

But when i increase the frequency to 400-500 kHz the triangular wave flattenes out at the top,
this is really not helping my PWM signal.

So first, can i fix this and how?

Recommended replacement waveform generator?
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Old 5th November 2008, 05:45 PM   #7
Corax is offline Corax  Germany
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Well, if you don't mind have a look in various application/design notes from i.e. Linear Technology, Analog Devices, Texas Instruments, ...
In particular check the opamp and comparator sites. At least Linear Technology has some "Circuit Collections" with various stuff in it. The chances are good to find something.

I know there was one particular schematic with an ultra-precise triangle generator going all the way up to 1 or even 2 MHz. But unfortunately I cannot remember from whom it was.

Use the search function on there homepages and you should come up with a few or maybe a bunch of example schematics.
If I find some time I'll look out for it too.
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Old 8th November 2008, 10:31 PM   #8
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Quote:
Originally posted by bwaslo

The better ones are self-oscillating and don't use triangle waves. See, for instance, IcePower or UcD.
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I definitely like Bruno and his designs.
But saying that self oscillating is generally better.. - that's sounding like you never tried both in design and also never tried both in listening.
Couldn't it be that self oscillating can give equivalent good results at lower costs?
And how about the issues with beating in multi chanel systems. Already in stereo systems this effect is existing and if you then add the fact that self oscillating amps change the switching frequency depending on the music signal, then you unavoidingly get a beating interference, which is depending on the music signal.

You can extract this effect for demonstration with the following set up.
Two signal generators , fast stereo amplifier, two piezo tweeters (please put 10 Ohms in series to each piezo tweeter, in order to avoid trouble with the fast amp).
Connect one side to signal generator 1 and the side to signal generator 2. Now adjust gen1 to 49kHz and gen2 to 51kHz. You will hear a 2kHz sound.
Now start modulating the frequency of gen2 between 47kHz and 51kHz (manually is fine).
==> You might feel like sitting in a park and listening to the birds.

Fortunately this effect is not loud at all and the higher you choose the frequencies, the less you might notice it. But it is hard to avoid in self oscillating designs and it is nothing else than what thousands of teachers tell to 16 year old pupils.

Both approaches have their advantages and disadvantages.
I.e. post filter feedback ist definitely an advantage of self osciallting designs.
But this does not mean that post filter feedback is impossible in clocked designs. In clocked designs it is more difficult to implement and is bound to more narrow limitations.
I.e. lock up effects is definitely a struggle that you will not have during designing a clocked amp, but it might 'sweeten' you one or the other evening during designing self oscillating amps.
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Old 9th November 2008, 12:35 AM   #9
fredos is offline fredos  Canada
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I have already implement clocked design with post filter feedback, take a look at my old 1200 amplifier. I agree with chocoHolic, beating is alway there. The stereo amplifier are good exemple, but imagine a pro audio rig multi way, multi amp of self oscilating stuff...I wont be there!

In my opinion, the best way to get clean triangle is to use a simple RC network to get triangle wave, as close as possible to triangle input of comparator, BUT you should design it to be in the linear region of the RC network...Again look at my 1200 schematics! Just dont forget to drive the RC with low impedance source!
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Old 9th November 2008, 12:36 AM   #10
bwaslo is offline bwaslo  United States
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My tweeters don't respond at all at 300kHz+ and I doubt that yours do either!

So mixing would have to occur (maybe) in power supply rails. Has anyone heard or measured this happening with UcD or ICEpower? Bridged UcDs should be doing this problem all the time, with the two amps involved running in opposite phase and sweeping past each other in frequency constantly. ICEpowers often run several off a common supply. Has anyone heard birds chirping in their speakers when running multiple self-oscillating Class D? Anyone report listening to a triangle wave derived Class D and thought it sounded better than ICEpowers or UcD?

Nope, I haven't heard every amplifier in the workd, and who knows maybe there's magic in some triangle wave fixed frequency derived Class D some where. No reason to ignore that technique, if some advantage can be found. I haven't heard every Victrola cylinder phonograph, so I can't say that CDs generally sound better than those either...
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