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Old 15th January 2012, 06:07 AM   #131
regal is offline regal  United States
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Originally Posted by abraxalito View Post
I've done more than look at it, I've measured it (in single-ended output mode). Its of the order of 0.05% at full output, falling to bellow the FFT plot's noise at lower levels. Which pretty well agrees with the datasheet - predominantly 2nd harmonic, 3rd harmonic about 6dB down on 2nd. Now I'm running the part with differential output, I have yet to measure but expect to get 0.03% of relatively pure 3rd harmonic at full output.



I'm not familiar with tubes myself. In any case the THD figure is way less important than the IMD - even for normal audio band duty. Here we have an amp exposed to extreme levels of RF into the 10's of MHz.

I don't know Abraxilito you have posted some great ideas over the years but I don't know about this one You have distortion -65db, is this at 2Vrms output? Also IMD is just about directly proportional the THD unfortunately even when its only 2H & 3H, so you will find high IMD as well
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Old 15th January 2012, 06:16 AM   #132
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Originally Posted by regal View Post
I don't know Abraxilito you have posted some great ideas over the years but I don't know about this one You have distortion -65db, is this at 2Vrms output?
I'm comforted by the motto 'All new truths begin as blasphemies'

No the AD605 has only 5V supply, so it can just about get up to 1VRMS output per side. 2VRMS in balanced, using both halves of the chip.

Quote:
Also IMD is just about directly proportional the THD unfortunately even when its only 2H & 3H, so you will find high IMD as well
Reference for this claim? I'm all ears. Stuff designed for RF duty tends to sound fairly good to my ears, this is no exception. I'm still getting audible improvements from applying better filtering to the input of this chip which tells me intermod with out-of-band signals is a major sound quality issue, rather than in-band THD. Of course THD might be an issue too, but I'm sure my speakers are considerably worse at distortion than this device.
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Old 15th January 2012, 06:23 AM   #133
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Originally Posted by regal View Post
Still the biggest hurddle is finding a means to have the masterclock in the DAC, with the I2S source galvanically isolated.
Isolation - use ISO/ADuM chips available from TI/AD, these are made for it.
Reclock the i2s near the TDA with local clock.
Send the clock thru other ISO/ADuM and some glue logic to the transport, which can take the sync signlal.

That's easey!
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Old 15th January 2012, 06:49 AM   #134
regal is offline regal  United States
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Originally Posted by abraxalito View Post



Reference for this claim? I'm all ears. .
Well its simple maths, I read a great illustration on the web and will try to find it. Now of course higher order harmonic distortion creats proportionally worse IMD, but with THD at .05 the IMD is still going to be high (for a DAC) just from the heavy 2H & 3H.

When I read your blog I thought of the the old TDA1543 as a DAC that most now consider to have too much distortion, Its THD at full scale is -75dB, the benefit this DAC has is it can perform when paralleled to give decent output with a simple I/V resistor (i.e. doesn't add distortion with a fairly high impedance on its output.)

So to an outsider looking (not knowing which DAC chip you are using) at your I/V proposal with -65db THD is going to be similar to the the once popular multiple tda1543's + resistor. I would imagine though that your proposal would have lower noise and better resolution and also the TDA1543 distortion harmonics are different so your proposal is interesting. For me personally I am always looking for low THD prior to my amp, since its obviously going to be multiplied (IMD too.)

I agree on the philosophy that stuff designed for RF can be a good match for DAC's because the high bandwidth of the dac output is handled properly, this is where the discrete vs opamp IV battle started on here almost a decade ago.
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Old 15th January 2012, 07:01 AM   #135
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Hi,

Quote:
Originally Posted by regal View Post
Still the biggest hurddle is finding a means to have the masterclock in the DAC, with the I2S source galvanically isolated.
Let me put it this way. If you use a WM8805 and you implement it well and take a little care at the source you can get well under 200pS Peak-Peak jitter and RMS jitter below what is needed to keep the degradation for 16 Bit digital audio to a minimum. I would not as such argue that further improvements are not audible, but they are nit great.

Equally, Joseph K. measured the M2Tech HiFace AFTER modifying it with clean power supplies (and I think better clocks) at even lower jitter than that, so I refuse to loose sleep about this when using a 16 Bit DAC.

With the Hi-Face no-one stops you placing the clock circuitry directly at the DAC and sending it back to the HiFace. With the WM8805 you need to program it so the high drive clock output outputs MCK and use that to drive your reclocker (needs programming). Sure, it is not going to be the lowest possible jitter, but it's going to be a darn side better than mosbunall SPDIF input out there.

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Originally Posted by regal View Post
Would it be nuts to proposed galvanic continuity all the way from the usb ground thru the DAC and provide galvanic isolation on the analog side with good transformers?
Sure, why not? Cannot do any harm.

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Originally Posted by regal View Post
If we are careful with keeping the current loops of all our decoupling small maybe this is a decent route for the DIYer?
I think so.

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Or likely still no better than using wm8805 spdif for the galvanic isolation from the computer/transport and giving up on a real master DAC?
That is an option that is certainly workable.

Remember, Jitter of the datasheet/eval fixture implemented WM8805 is around 50ps when fed a jitter free signal... So it can provide very low jitter if given good power supplies and a low jitter source to start with.

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Also you haven't talking about the analog section,
I did. I said I'd probably use something like the CEN.

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I posted my thoughts in the big thread. In my head I can't accept that a single common gate mofset is suitable solution for an ultimate DAC.
Why not? As said, personally I'd go CEN, which is basically the same 2-way.

The CEN using 2SK170BL and 2SJ74BL has around 12 Ohm input impedance which keeps within the stated +/-25mV compliance. Incidentally, I would not put to much weight on the +/-25mV spec, you can have a fair bit more before distortion rises measurably.

For anyone with a pathological fear of coupling cap's they may be replaced with current mirrors, which incidentally create then the correct polarity, another pair of 2SK170BL and 2SJ74BL can then be used as followers to create low output impedance (12 Ohm).

My various Tube Stage option are widely documented, so I do not repost them.

Ciao T
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Old 15th January 2012, 07:07 AM   #136
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Quote:
Originally Posted by regal View Post
Well its simple maths, I read a great illustration on the web and will try to find it.
I've been searching for an understandable treatment of this for quite a while. So if its really just 'simple math' its surprising nobody's run with it and produced an idiot's guide.

Quote:
Now of course higher order harmonic distortion creats proportionally worse IMD, but with THD at .05 the IMD is still going to be high (for a DAC) just from the heavy 2H & 3H.
But my measurements show its only high at high level. Just pull up any track on Audacity and see where most of the content is - on the kind of music I listen to, which is classical and acoustic, there's only the occasional peak close to 0dBFS. The real action is almost all happening down below -6dB. So to me its a nonsense to dismiss a part solely on its full level distortion - how that figure changes with level is way more important.

Quote:
When I read your blog I thought of the the old TDA1543 as a DAC that most now consider to have too much distortion, Its THD at full scale is -75dB, the benefit this DAC has is it can perform when paralleled to give decent output with a simple I/V resistor (i.e. doesn't add distortion with a fairly high impedance on its output.)
But if most consider it has too much distortion, why do most also use it in a way which makes that -75dB figure even worse? That is by using passive I/V with large changes in the output compliance (several volts in many cases, running on 8V supply). Using a DAC in this way is well known to generate some significant low-order distortion due to less than infinite output impedance. Again it doesn't sound too bad - probably because for real world signals the distortion most of the time is much lower.

Quote:
So to an outsider looking (not knowing which DAC chip you are using) at your I/V proposal with -65db THD is going to be similar to the the once popular multiple tda1543's + resistor.
Yep - accepted. Except those approaches put the burden on whatever the next stage is - so the results will vary a lot, depending on what its feeding. I'd like my design to be a bit more consistent, hence my approach.

Quote:
I would imagine though that your proposal would have lower noise and better resolution and also the TDA1543 distortion harmonics are different so your proposal is interesting. For me personally I am always looking for low THD prior to my amp, since its obviously going to be multiplied (IMD too.)
I'm a bit different - I'm looking for low enough THD, not low THD. THD is well known not to correlate with sound quality. So with this design I'm paying attention to IMD with OOB signals and working to minimize that as I have found significant correlation with perceived SQ.
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Old 15th January 2012, 07:13 AM   #137
regal is offline regal  United States
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Originally Posted by s3tup View Post
Isolation - use ISO/ADuM chips available from TI/AD, these are made for it.
Reclock the i2s near the TDA with local clock.
Send the clock thru other ISO/ADuM and some glue logic to the transport, which can take the sync signlal.

That's easey!
Thing about thos adum chips, if you read the datasheet I think they are going to do more damage than good. Look at their concern for Rf interference and how much it emits, they run at some 100 mhz? I mean when was the last time you saw warnings about RF emmsion in a datasheet for a compnent you are using in an audio circuit? Plus I'm pretty sure a lot of the 400mhz noise from the computer can jump right thru the adum's. To me using one of them isn't much different than placing a pentium cpu on board next to your tda1541.

Now I have heard your idea proposed to work with optocouplers which I think is probably worth trying. The jitter they add would be eliminated by the alignment at the DAC.

Of course we still have the open issue that with asynch USB, a TDA1541 becomes a 15 bit DAC do to its incompatability with 64bit I2S, so more glue logic on the DAC side.

But I think a project like you propose only using GMR's instead adums and coming up with the 64 to 32 bit conversion would be a great TDA1541 PCB project, possibly leaving the analog stage off for people to work out on their own. Ideally someone with the Xmos or whatever asynch usb chip rights would could offer this as a single PCB.

Last edited by regal; 15th January 2012 at 07:17 AM.
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Old 15th January 2012, 07:31 AM   #138
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Hi,

Quote:
Originally Posted by abraxalito View Post
But if most consider it has too much distortion, why do most also use it in a way which makes that -75dB figure even worse? That is by using passive I/V with large changes in the output compliance (several volts in many cases, running on 8V supply).
Doing this (passive I/V conversion with around 2V RMS out and around 8V supply) with the TDA1543 with the reference resistor trimmed for lowest HD can give well below -80dB 2nd Harmonic with all else lower (some chips are outliers though with much higher HD). You can go lower using multiple chip's in parallel but that somehow does not sound as good as a single.

It does not sound bad at all and trying this by now nearly 14 Years ago convinced me that there was something very wrong with the then (and still) dominant way of doing digital audio...

Ciao T
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Old 15th January 2012, 08:02 AM   #139
regal is offline regal  United States
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Originally Posted by abraxalito View Post
I'm a bit different - I'm looking for low enough THD, not low THD. THD is well known not to correlate with sound quality. So with this design I'm paying attention to IMD with OOB signals and working to minimize that as I have found significant correlation with perceived SQ.
I wished I had bookmarked that IMD explaination, it was from a musician/studio perspective and made a lot of sense. Basically when you have two tones the 2H and the 3H add up to give frequencies that aren't harmonically related to either of the first two tones because you have distortion of the distortion, and when playing music with multiple notes this all adds up. Basically THD can be the direct source of IMD, put another way any non-linearities be them harmonically related or not add to the IMD quotient when dealing with more than a single frequency.

Its easy to see when you have a simple tube stage with low noise and only 2H &3H, run the two tone imd test and you will see the non-harmonics pop up (obviously more when their is 4H and especially 5H.)

I agree that THD doesn't correlate well with sound quality but I don't think IMD does either so my questions about your I/V idea are just discussion and curiosity.
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Old 15th January 2012, 08:34 AM   #140
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Quote:
Originally Posted by ThorstenL View Post
Doing this (passive I/V conversion with around 2V RMS out and around 8V supply) with the TDA1543 with the reference resistor trimmed for lowest HD can give well below -80dB 2nd Harmonic with all else lower
What's the theory behind trimming the reference resistor for lowest HD? Just getting the waveform centred between the two end-stops?

Quote:
(some chips are outliers though with much higher HD). You can go lower using multiple chip's in parallel but that somehow does not sound as good as a single.
To me the advantage of paralleling the chips is to get better low-level distortion. Already I feel -75dB is plenty good enough at full-scale. Haven't yet noticed a single chip sounding better than my paralleled set-ups (which allow more flexibility in choosing the I/V resistor value - the AD605 already contains its own resistor). Perhaps I haven't been listening hard enough....

Quote:
It does not sound bad at all and trying this by now nearly 14 Years ago convinced me that there was something very wrong with the then (and still) dominant way of doing digital audio...
Yeah, just playing catch-up here some 13 years behind you
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