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Class D Switching Power Amplifiers and Power D/A conversion

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Old 15th October 2012, 08:14 PM   #11
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Default Correction of error

There was an error in the second schematic of posting 7.
Two connections were missing. Here is the corrected version.

P.S.
To all readers, please don't hesitate to point out errors.
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Old 16th October 2012, 08:25 AM   #12
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How does this discrete frontend compare to an ordenary opamp in performance ? It kinda reminds me of the current feedback scheme used in some 70's-80's amplifiers.

Would you say clocked class d should be given more attention than it gets ?
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Old 16th October 2012, 08:45 AM   #13
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Quote:
Originally Posted by Tekko View Post
How does this discrete frontend compare to an ordenary opamp in performance ? It kinda reminds me of the current feedback scheme used in some 70's-80's amplifiers.
According to mine and also others listening experiences of NC400 amps it does make a difference. Discrete circuit can be perfectly adopted to do job correctly as planned.
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Old 16th October 2012, 10:42 AM   #14
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Quote:
Originally Posted by ChocoHolic View Post
To all readers, please don't hesitate to point out errors.
It's not an error, but discrete input buffer works much better if R12, R13 are replaced by CCS.

Otherwise very nice D topology you've presented. Hopefully it will lead to a working amp.
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Old 16th October 2012, 12:42 PM   #15
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Quote:
Originally Posted by ChocoHolic View Post
Now let's talk about the other triangle....

There are many discussions about carrier aliasing and how to avoid or milden it.
Often discussed are gain control ideas, which change the gain of the error amp or the size of the triangle on the fly, in case of high signals.
Typically pretty complicated in real life.
Instead of above ideas we could also change the shape of the triangle.
Of course only at its peak area. The triangle shall have mostly or fully identical shape as an ideal triangle except for very high modulation levels. Around its peaks the triangle can be shaped more steep (==> reduction of gain).

Of course this effective reduction of loop gain close to clipping will affect the loop properties in this range.
In case of the 'barrier triangle' your amp will have a hard change from normal loop gain to no loop gain and the output can show the modes of the output filter, looking like in pre filter feedback systems.
In any case triangle shaping allows tons of desired shapes and makes it easy to settle the right trade off for most amps.

Attached some shapes for inspiration, feel free to create your own shaping ideas.
I've just tried (in sim) to high-pass or rather high-boost the triangle signal to exactly conteract non-ideal diffenciator from your standard configuration.
Traingle of 10x amplitude and a network of 10k||27p and 1k to ground.
Wow! more than 20dB improvement in THD!
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Old 16th October 2012, 07:23 PM   #16
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Quote:
Originally Posted by Lazy Cat View Post
It's not an error, but discrete input buffer works much better if R12, R13 are replaced by CCS.

Otherwise very nice D topology you've presented. Hopefully it will lead to a working amp.
Yup, CCS would be an further improvement, but in simulation this part of the circuit already without CCS was better than all the rest of the circuit.

Edit: Please note that the biasing currents of both stages are fed to the cascode - leaving just the error caused by the difference in base currents of the first stage. ...thinking about this, I am not convinced that a CCS would bring further improvement.

And facing the unsaint component count of my MD approach with the discrete input stage and discrete comparators and discrete servo current sources...., I was glad that this point could be left simple.
It will lead to an working amp. Routing the PCB was a longer game.
The PCB contains also EMI-torroids, fusing, thermal protection, speaker protection with MosFet relay and a high quality output filter consisting of an oversized choke (from commercial view, but running at low flux densities) and parallel array of 8 WIMA MKP2.
Attached a picture of the layout.

@ Tekko: For a design that only needs to measure fine, there is nothing wrong with OP amps. Basically the HF carrier can irritate the OP amp and cause artifacts in the audio band. How much will depend on the OP amp. IMHO a good indicator how much the OP amp would be struggling is to watch the IMD. The listed types in my earlier post will be a good starting point,
and to me there is no scientific evidence that a discrete solution must generally be better. Depending on the know how and experience of the designer a discrete solution even bears more pitfalls (especially in layout), which can completely spoil all efforts.
It is more a question of my personal taste that I target a class D amp with clearly audiophile focus, instead of the common class D focus, which is size, efficiency and cost.
70-80s: Audio amps with diamond buffer input and splitted current feedback?
I don't think so, but I do not know everything.
The part, which I would call a 80's fashion is the folded cascode only.

@Adam:
Yup, inverse shaping of the triangle is also sounding like a simple and nice idea
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Last edited by ChocoHolic; 16th October 2012 at 07:41 PM. Reason: ...thinking twice....
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Old 16th October 2012, 08:38 PM   #17
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Quote:
Originally Posted by Tekko View Post
Would you say clocked class d should be given more attention than it gets ?
It depends on the final application.
As long as you are not going to run two self oscillating amps in bridged configuration, I have no concerns against selfoscillating.
If you bridge them all non linearities behind the amp (connectors, x-over, speaker) will act like as AM demodulator for the beating frequency of both amps.
It will depend very much on the specific amps and speaker if this will turn audible or not. Have a look to my thread 'to clock or not to clock'.

Watching size, complexity and possible loop gain the selfoscillating approaches are hard to beat. My PCB also contains an option to go for a selfoscillating System D MD.
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Old 18th October 2012, 08:04 PM   #18
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... piece by piece...
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Old 19th October 2012, 03:50 AM   #19
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I like the way you have soldered SMD caps over drivers
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Old 19th October 2012, 06:52 AM   #20
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3d board population
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