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Discussion Starter #1
I posted this in another thread but thought it might get lost a bit there. So I'm copying it to it's own thread!:


Hey Guys,


I just got off the phone with a Pioneer Tech. I was asking if he knew what kind DACs and OpAmps were in the new 74TXVi. The bad news is, he said he wouldn't know until the service manual came out (about a month or two from now).


BUT, the good news is, I had foresight to ask him about the amp section and the power supply. After reading a post from "Sounds Simple" (Bob) that the 74TXVi has "apparent improvements in the audio section of the amp" and reading it was much lighter than the 56TXi, I thought it was worth it to ask if the Tech. knew anything about it.

Guess what they do NOT use MOSFET amps in the 74TXVi. I asked him directly if they were digital amps, he said he did not think so. But they are a totally different design. He said he just had the oppurtunity to look inside one and verified they are definatley not the MOSFET design (I should have asked him to go look at the DACs and OpAmps too). From what he has been told, these new amps are much more effiecient in regards to heat. He explained briefly why this was so, but I can't remember the details. He said they are much smaller, and put off much less heat, and they do not require as big of a heatsink as a result. He mentioned something about thier design with regards to current and voltage that also meant that the amp requires less capacitance in the circuit, yet still can supply as much power (Maybe one of the reasons the Caps are smaller than the ones in the 56TXi). He said in his opinion they sounded just as good as the MOSFET design, and maybe a little better. I also asked him about the power supply, he said "oh yes, that is different too!" He said mostly in part to the different amp design.


Interesting stuff for sure! Hopefully in a good way! Maybe it is some type of hybrid digital amp, and he is just not up to speed on the technology yet! Now wouldn't that be cool! :cool:


One thing is for sure, with so many changes in this Elite model, it does make me even more curious if they also changed things around with the DACs and OpAmps.


uzun (one of the few owners of the 74TXVi), regarding your installer looking at the DACs and OpAmps, would a little begging help? How about ALLOT? I can swallow my pride for awhile! :D


Best Regards,

Patrick
 

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As in the other thread, I'm betting it's not digital (would require different DAC stage/layout) but a class-D type amp.


10% less tranformer size corresponds to the added efficiency of class-D
 

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Discussion Starter #3
Yves,


Yeah, you are probably right about it not being digital (like the Tech. said), but wouldn't the class D amp section be more expensive to implement than the AB (in this case MOFSET) deisgn that the previous models had?


Patrick
 

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Discussion Starter #5
"Class D" can be Digital controlled or Analog controlled. The "analog version" uses MOSFETS and is not digital!


Here's a couple of links:

Truth about Class D

"MOSFET" Class D
 

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Quote:
Originally Posted by avaholic
"Class D" can be Digital controlled or Analog controlled. The "analog version" uses MOSFETS and is not digital!


Here's a couple of links:

Truth about Class D

"MOSFET" Class D
Uhh... they're still both "digital" no matter how they're controlled. When a transistor is operated in the following state, it's digital.
Quote:
A class-D amplifier is one in which the output transistors are operated as switches.
As transistor with only two finite states is the very definition of digital.
 

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I wouldn't put to much faith in what you hear from a "so called" Pioneer tech. From what you told us, It sounds a little wishy washy
 

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Quote:
they're still both "digital" no matter how they're controlled
It's nice to have an opinion on things, but please read the two links that are provided to you. True digital amps are "power DACs" that effectively use the digital source stream to pulse the transistors - Tact millenium for example.


self-oscillating designs in class-D, like the UcD's, are, for all what it's worth, analog devices - analog input and analog output. No digital stage, no buffers, no microprocessors or DSP's.


Please read Bruno Putzeys' story on class-D
 

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Quote:
Originally Posted by Yves Smolders
It's nice to have an opinion on things, but please read the two links that are provided to you. True digital amps are "power DACs" that effectively use the digital source stream to pulse the transistors - Tact millenium for example.


self-oscillating designs in class-D, like the UcD's, are, for all what it's worth, analog devices - analog input and analog output. No digital stage, no buffers, no microprocessors or DSP's.


Please read Bruno Putzeys' story on class-D
I did read the two links.


Perhaps you should aquaint yourself with the meaning of the word digital. "A digital system is one that uses discrete values rather than a continuous spectrum of values:" So, a class D amp which uses a switching transistors that are either on, or off, ie: have only 2 discrete values, are by their very nature digital.

http://www.answers.com/digital%20amplifier
 

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Hmm, ok your opinion is noted. I don't feel like going into a long discussion, but not everything is black and white.


Here's my source on why I believe that class-D is not to be seen as a purely digital amp:


No need to discuss further, our fellow readers will make up their own mind, as have you and I, stereodude.


Quote by Bruno Putzeys, designer of the UcD designs, taken from DIYaudio dot com.

Quote:
After reading some discussions on the forum it became clear to me that the notion of "digital" is not entirely clear to everyone. Time to explain the basics.


Digital means "symbolic". A digital signal symbolises a stream of symbols. A symbol is an element of a limited set. The nice thing about digital is that inaccuracies in the transmission of an analogue representation can be rectified as long as you can still recognise what the symbols are. Only when the inaccuracy is so great that you can no longer tell what the transmitted symbol was, the message gets corrupted.


It's like text. Text is digital. Suppose you photocopy a typescript. OK it's still readable. A 5th generation copy may be still completely readable, a 10th generation might not. But wait, you can preserve the text from a 5th generation copy better if you get out a typewriter and re-type it on a clean sheet. As long as you've been diligent in copying the original to the letter (including the typos). The newly typed sheet is now as good as the original typescript and can be photocopied again a number of times if you wish.


The printed page analogy illustrates the distinction between analogue and digital. As concepts, the letters ("glyphs" in print lingo) are digital. In the english alphabet, there's only one uppercase "A". There isn't another one which is slightly different, like having a "B"-like tinge to it. You can, however, represent it using another font, in different sizes or colours, but it remains what we call "A".

The letters on the page are digital information. The shapes of the letters are analogue. The printed page is analogue. Xeroxing is an analogue process. Copying a page using a typewriter is converting it to digital (back to the letter concepts) and forming a better quality analogue representation.


It's not just paper. You might spell out the text on a cassette tape and send it to a secretary. If you've been precise in reading out (including interpunction, typos etc) and the secretary is precise in typing, you get the same text.


That's what is meant by symbolic. It doesn't matter whether our "A" had been typewritten, hand-written, carved in stone or transmitted by smoke signals. As long as it is clearly distinct from any other letter, you get an "A" at the end of the chain.


Back to the paper. Whether the text you're copying (by typewriter) is an original or a 5th generation photocopy will not have any effect on what your copy will look like.


This is a crucial test in determining whether you have an analogue or a digital signal.


"What? You can't just look at a signal and see if it's digital?"


No. The distinction lies in who receives the signal, not in what the signal itself looks like.


Re-read the above sentence. It's the most important sentence in this whole post. Never confuse form with content. In digital signals, form and content are two entirely different domains. In analogue signals, form and content are inextricably intertwined.


Suppose you are handed a typescript and are asked to pass it on. Instead of passing it on, you intercept it and distort it in some way, but only such that it looks a bit different but the text remains perfectly readable. For example, you crumple the page up and straighten it again. Then you pass on the paper to the recipient.


If the result of the subsequent process is in anyway different from what it would have been had you not crumpled op the paper, the paper was an analogue signal, meaning that the way it looked was important, not just what it said.

Instead, if the outcome of the next process is completely unchanged, it means that only the content mattered, not the form. The paper was a digital signal.


OK. Back to electronics. You have a device with all sorts of signals in it which you're tracing a signal with a scope. How do you tell it's digital? It might look very digital, for example a square wave, but by now you know that form and content are not to be confused.


Well, simple. Add some noise. Amplitude-modulate it. Add a small amount of time-variant delay. And make sure you can vary the amount of error you introduce at will. Then look at the output of the device.


There are two possible outcomes.

1)The first is that the output of the device remains to-tal-ly unchanged for all types of error. The signal is digital. Only when you crank up the noise level, suddenly very dramatic errors occur in the output. That's when the symbols could no longer be read.

2)The second possible outcome is that no matter how small the error you put in, you'll see it in the output to some extent. Some errors may have very little effect (e.g. amplitude modulation) while other errors (e.g. jitter) may have a clearer influence. Of course, if you make the error smaller, its effect on the output is smaller too, but it is never completely gone. The signal is analogue, even if it looks like a square wave or a serial bit pattern.


Let us take a few examples.

1) A class D amp consisting of a triangle wave oscillator, a comparator and a switching power stage.

1.1) is the switching output digital? Add noise or timing jitter. After passing the LC filter, the noise shows up at the loudspeaker terminals. Low-pass filtered but always there. The switching signal is analogue. Furthermore, the power stage makes errors of its own, which show up mostly as distortion.

1.2) is the comparator output digital? Add timing jitter. The jitter is faithfully reproduced by the power stage and is delivered to the load as noise.

1.3) is the comparator input digital? Obviously not.


2) A class D amp consisting of a digital PWM modulator and a power stage.

2.1) is the switching output digital? Add noise or timing jitter. After passing the LC filter, the noise shows up at the loudspeaker terminals. Low-pass filtered but always there. The switching signal is analogue. Again, the power stage already does enough to distort the signal to make that point.

2.2) is the PWM output from the modulator digital? Add jitter. The jitter is faithfully reproduced by the power stage and is delivered to the load as noise. You could, however, add a flip-flop clocked by the master clock to get rid of the effect of the jitter. The signal before the flipflop is now insensitive to jitter (=digital) but unfortunately the signal after the flipflop is still jitter sensitive. We've just moved the problem a station.

2.3) is the input to the PWM modulator digital? Add jitter. Now change to the PWM until the whole thing starts crackling. OK, the input signal to the PWM chip is digital, as are all signals inside that chip.


3) A pulse-edge delay error compensating amp. The idea behind such amplifiers is to measure the difference between the power stage and a "reference pwm node". The reference PWM node is a small switching stage that delivers no current and can be made very precise. When the power stage makes a timing error or an amplitude error (which it does) the error correction circuit will modify the duty cycle to compensate for that, so that the low-frequency content better matches that of the PWM reference node.

3.1) Is the PWM reference node digital? Clearly not. The correction scheme explicitly looks at the low-frequency content at the PWM node so any noise added there will be reproduced at the output with great fidelity. Actually you could replace the PWM input with a stable square wave and superimpose an analogue input signal and you'll get that signal at the output.

The PWM reference node may look like a flip-flop, it functions as a DAC. The power stage works synchronously to the DAC but this is merely confusing matters (apart from removing ripple aliasing, which is outside the scope of this post).

3.2) Is the signal going to the reference flip-flop digital? Yes. (explanation is left as an exercise to the reader).


What this actually means is that since the power stage is an analogue affair, as are all the error mechanisms, the concept of a "digital amplifier" is a mirage. All class D amplifiers are analogue, only some will try to move some signal processing into the digital domain.


This, so far, hasn't worked yet. Directly driving MOSFETs with a digitally generated PWM signal abolishes the analogue control loop, but does not replace it by a digital one. This is why such amplifiers are inherently inferior to designs that do have a control loop.


Adding delay error postcorrection reinstates the analogue loop and turns the digital PWM section into a DAC. Luckily, PWM is a good way of making DACs. This kind of amplifier analogue, but it is a valid way of making good analogue class D amps (the lowest distortion class D amp ever made is constructed along these lines).


I hope that this makes clear that analogue controlled class D amplifiers have nothing to be ashamed of. Those called "digital amplifiers" usually do. Paradigm reversal this is called.
 

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Quote:
Originally Posted by Yves Smolders
Hmm, ok your opinion is noted. I don't feel like going into a long discussion, but not everything is black and white.


Here's my source on why I believe that class-D is not to be seen as a purely digital amp:


No need to discuss further, our fellow readers will make up their own mind, as have you and I, stereodude.


Quote by Bruno Putzeys, designer of the UcD designs, taken from DIYaudio dot com.
For some reason you keep posting comments by someone who has an axe to grind against digital amps. So, as a result he feels the need to dummy up a category of "Class-D, but not digital".


Sorry, but if it has a switching transistor that uses a PWM or PDM at the heart of it's amplification, then it's "digital" regardless of how the control loop is done (or isn't done).
 

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Discussion Starter #12
Quote:
Originally Posted by Tom Grooms
I wouldn't put to much faith in what you hear from a "so called" Pioneer tech. From what you told us, It sounds a little wishy washy
Tom,


I waited over an hour, being routed to several different deptartments, before they finally got me to a real service level Tech. This was not your standard "Customer Service" representitive. In other words he was not fumbling through his computer files looking for answers. He went into a long explaination of how the new amp worked (wished I could have recorded it, to accurately describe), and did not "stutter" at all while he was explaining it. He was very clear though that it was not a MOSFET based design. If anything it's my interpitation of what he said that is "wishy washy". :) It may very well be some type of Class D design that uses MOSFETs, but one thing should be pretty clear, it is not the standard MOSFET "based" AB design that previous Elites used.


In anycase hopefully we'll get a firm answer from one of DOCDVD's Pioneer contacts on what the real scoop is.


Best Regards,

Patrick
 

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Quote:
Originally Posted by Stereodude
For some reason you keep posting comments by someone who has an axe to grind against digital amps. So, as a result he feels the need to dummy up a category of "Class-D, but not digital".


Sorry, but if it has a switching transistor that uses a PWM or PDM at the heart of it's amplification, then it's "digital" regardless of how the control loop is done (or isn't done).
Sorry, that is your opinion. End of discussion for me. Sorry for highjacking the thread ppl.
 

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There are 2 categories of PWM class-D systems. One is working with analog input and analog natural sampling normally. The output pulse has discrete voltage value (+Vr and - Vr or 0). But the edge of PWM pulse can happen any time in analog time domain. So, this is discrete space, continuous time system.

The other is working with digital input and both voltage and edge timing are discrete. They rely on and synchronize the PWM pulse edge to high-speed clock, and use some DSP to do the similar thing the analog modulator above do.

Technical people normally refer the former analog PWM or analog class-D, and the latter digital PWM, digital class-D or just digital amplifier.
 

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True.


The "sampling" of the signal doesn't occur on a set time interval. The UcD class is of the analogue class-D type. A tact Millenium is the digital type (a Power-DAC, if you like)


If you would take this sampling as being "digital", then analogue audio tape is digital too: You're magnetising tape crystals into a certain structure, which you can break down to being 0's and 1's also - but this is not digital. There's no set time resolution.


I don't have an axe to grind either - I'd love to have a Tact Millenium if I could afford one! :D
 

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Quote:
Originally Posted by Bob Lee (QSC)
Class D is not digital.


A class D amp could be digital--if the PWM were controlled by a digital or numeric process--but by far most are analog.
I think you should add "necessarily". Probably the biggest class-D solution so far sold is Tripath, which is analog class-D. But recent development in this space makes the digital class-D the mainstream. Chips from TI, STMicro are put in the low-end class-D such as high-volume DVD receivers. I think you will actually see more digital PWM systems than analog PWM systems, in sheer # of units.
 

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Quote:
Originally Posted by jheoaustin
I think you should add "necessarily". Probably the biggest class-D solution so far sold is Tripath, which is analog class-D. But recent development in this space makes the digital class-D the mainstream. Chips from TI, STMicro are put in the low-end class-D such as high-volume DVD receivers. I think you will actually see more digital PWM systems than analog PWM systems, in sheer # of units.
True, but my point is that despite many peoples' claims that class "D" stands for "digital," it actually doesn't. It just happened to be named after A through C.
 

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Quote:
Originally Posted by Bob Lee (QSC)
True, but my point is that despite many peoples' claims that class "D" stands for "digital," it actually doesn't. It just happened to be named after A through C.
Yep, I agree.
 
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