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Old 28th August 2006, 01:14 PM   #1
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Default all digital pwm Open Loop or withfeedback

When designing an all digital system (i.e. PCM in to PWM) how can feedback be deployed? There's been a lot of discussion within this forum regarding feedback - take it prior to, or post LC filter. But how do you do it for a PCM design? I imagine using some DSP, perhaps. To take into account the post LC filter requires an A/D. Adjusting the PWM is going to be cycle by cycle, I suppose. This would take considerable thinking on my part, to come up with a circuit for this. But, the commercial world has attempted to help out.

Looking at the commercial offerings, here's what I've found:

1. TriPath TCD6000 - Claims to have pretty low THD, and SNR. Uses digital feedback from the MOSFET output. It is supposed to compensate for MOSFET differences, and power supply affects. Tripath scares me though - it looks they are not doing very well as a company, stock price is in the toilet, I can't get a reply via email or voice mail, it's difficult to get a live person on the phone, and they've had some turn over of key personnel.

2. TI TAS5518 - nice feature set (besides the pwm), claims to low THD. No feedback. Digikey shows the part in-stock. I haven't tried tech support yet, but it's TI and they are a stable company.

3. Freescale Symphony FSA95601 - This is a new part on the scene, it's just sampling now and literature is scant at the moment. It does use feedback, presumably from the MOSFET output. Not much else to say.

4. D2Audio - I don't know much about these parts, anybody want to chime in?

5. Cirrus Logic CS44800 - somewhat interesting part, but I haven't looked all that closely. It uses some proprietary PSRR logic, which requires an aditional A/D for the power rails. Other than that, no feedback. Dynamic range is down a tad from other vendors offerings, and THD spec is a tad worse 0.03% vs 0.01 or better from others. Cirrus logic is also stable company.

6. Wolfson - another company that I haven't scrutinized. No feedback on these parts.

BTW, what is so bad about open loop on class-D? Is it the power supply problem? What if it's regulated really well?
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Old 31st August 2006, 02:29 PM   #2
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Hi,

It's party the power supply problem. Regulating the psu could work but not without great cost, complication, efficiency, reliability.. and possibly performance.

Another factor is that you can't really correct for output stage errors strictly in the digital domaine... because really, it's analog, so at most you'd be taking delay into account and nothing else if you tried digital feedback only. That forces the use of an ADC as you've seen... is it still true digital?

I think to get away with open loop you'd need to switch hella fast... one amp pulled it off thus far, said to sound inferior to a much simpler, more robust, fully analog, self oscillating design
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Old 9th September 2006, 12:31 AM   #3
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Originally posted by classd4sure
. one amp pulled it off thus far, said to sound inferior to a much simpler, more robust, fully analog, self oscillating design

Is TACT the only company that has pulled off the no feedback digital amp?


What would be the best design to take a digital signal input with digital Xover and room compensation, and drive a 0.5 ohm resistive ribbon with good fidelity?? Is the TI chipset the best path? What would be better? Does TACT use TI chips?


" TacT True Digital Amplificiation: The Sound of the Future

Tact True Digital Amplification, the sound of the future. The Tact Millennium is recognized as the first true digital audio amplifier. Other so- called Digital or Class D amplifiers have been various combinations of Analog and Digital technology, in most cases using analog feedback. TacT's True Digital Amplification is conceptually different from any other amplification in almost every area of design. Once you understand the fundamental differences you will realize that Tact's complete line of True Digital Amplifiers offer the potential of much higher fidelity than any other digital or analog design.
Having said all of this it is worth pointing out, that the Tact's True Digital Amplifiers are not amplifiers at all. They are D/A converters that just happen to put out enough current and voltage to drive speakers directly, without the need of any amplification. This way it will perform the functions of: D/A converter, Preamplifier and Power Amplifier, without having any analog circuitry after D/A conversion other than one coil and one capacitor performing a 60 kHz, 2nd order low-pass filter. "
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Old 9th September 2006, 01:32 PM   #4
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Ah I could have been in error saying only one has done it, tact maybe true digital and it's possible they do use TI chipsets, it's been discussed here before by people who've taken a closer look at it than myself. However the mere fact it has "room compensation" I think goes to show its' not without feedback.

The one I had in mind is an amp by Bruno Putzey, can't recall the name of it, but it's a DSD multi phase amp. Each phase handles a portion of the DSD signal so they can switch at a slower rate than the DSD frequency. The outputs are combined and there is no feedback at all.

Tact's method would have to be somewhat complex, and it may not sound bad at all, but is it worth it? Competing non "true digital" methods are so much simpler and more robust.
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Old 9th September 2006, 02:02 PM   #5
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Originally posted by LineSource



Is TACT the only company that has pulled off the no feedback digital amp?

No! The panasonic digital amplifier SA-XR10 and his successors (50/70 ) all use the some equibit system , with no feedback .

See the thread :

http://www.diyaudio.com/forums/showt...080#post114080
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Old 9th September 2006, 02:47 PM   #6
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Right,

So, these are the no feedback amps with 0 PSRR..

Do you really think it's worth it?

The one I mentioned by Bruno is possibly the only other "True digital" that doesn't use Equibit.
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Old 9th September 2006, 02:56 PM   #7
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Originally posted by classd4sure
Right,

So, these are the no feedback amps with 0 PSRR..

Do you really think it's worth it?
I can only talk about my experience with my Panasonic XR10.

The power supply is variable and with a tight regulation.
As it have been designed for five channels and in my Panasonic is only feeding two , with a 8 Ohms load and the amp playing very loud, I can't see any ripple or other artifacts in the rail , with my scop...
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Old 9th September 2006, 03:28 PM   #8
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Sounds like some worthy mods to it anyway, I like your idea of ridding yourself of the unused channels and freeing up power.

How do you like the effect of such a tightly regulated supply? Is it fast, responsive, dynamic??

What sort of regulation do they use? Must be fairly complex ?

Really my only beef with Equibit is you need to be an exclusive member, how hard is it to get samples I wonder.

Cheers
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Old 9th September 2006, 03:59 PM   #9
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Originally posted by classd4sure


How do you like the effect of such a tightly regulated supply? Is it fast, responsive, dynamic??

What sort of regulation do they use? Must be fairly complex ?

Yes, the sound is very fast and specially the bass is very "clean" and have lots of punch and speed.

They use a IC feeding a Half Bridge Fet drive , that feed a pair of Mosfets.Is similar to the audio channels output stages , but this time feeding the electrolytic power supply capacitors...
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Old 9th September 2006, 04:00 PM   #10
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Quote:
What would be the best design to take a digital signal input with digital Xover and room compensation, and drive a 0.5 ohm resistive ribbon with good fidelity?? Is the TI chipset the best path? What would be better?
I dont know, but if you use TI chips, then I recommend you to couple via transformator.

ON:

Effect of varying supply can be cancelled out by dividing with its voltage, and this needs only relatively slow A/D. It's not feedback, I'd rather say feedforward. The other problem is crossover distortion caused by dead time. To decrease it one can measure delay between gate driver input and mosfet output cycle by cycle, and differences can be compensated in next cycle. This needs a fast comparator and a very very fast counter (some dosen gigahertz), or an analog time-multiplier (time-to-charge-charge-to-time conversion). This is not a real feedback either.
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