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Old 5th December 2011, 01:51 AM   #1141
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Just to be really clear, the junction temp is the internal temp of the regulator right at the little hunk of silicon that's doing all the heavy lifting. It's the hottest part of the regulator. The "case temp" is the outer heat sink contact area of the regulator just like sofaspud suggests. The regulators in the O2 are rated for a junction temp rise of 40 - 60 degrees C per watt of power they have to dissipate when used without a heatsink.

The "rise" means above whatever the ambient temp is. With the board sitting out in free air, that temp is typically only about 5, or worst case, 10 degrees C above the room temp so 25 or 30C. Even with the board in the case, it doesn't change that much as there's not much else in the case to produce significant heat.

I have a remote temp probe and have measured the actual temps in an out of the case with the amp working far harder that it can into HD650s. The draw of the amp, and hence load on the regulators, is directly proportional to the headphone impedance and output level. The actual measurements, using the WAU16-400 that gets the regulators quite a bit warmer than a 12 VAC transformer, are consistent with what I posted two pages ago.

CSA's claim of 77 C on the outside of the regulators would mean a junction temp well over 80C. Using his own current draw numbers the regulator tab temp should be under 50 C which is a fairly large difference and not at all "inline" with my numbers. And I've explained why.

The fact is, several here and elsewhere have posted using the HD650's or very similar headphones with nothing getting too warm. Instead of accepting something is obviously wrong with his amp, and providing the requested information to find a solution, CSA seems much more interested in continuing to throw rocks as he's done in dozens of posts on another forum and just did again above.
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Old 5th December 2011, 03:13 AM   #1142
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I just noticed that JDS Labs posted a front panel for the O2 for $10 on preorder.
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Old 5th December 2011, 03:40 AM   #1143
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I just read the new diyAudio newsletter and Jan Didden's article on the DSOnano. Luv it! I truly hope this catches on. A diy'er doesn't have to have a $600 Fluke and a $6000 LeCroy. Even some basic modest test equipment will return dividends. Check out Jan's review if you haven't already!
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Old 5th December 2011, 04:14 AM   #1144
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I'm planning to have one of the O2's gain switch positions at 1.6x so I'm thinking that 2.5kΩ (2.49) resisters are in order for R19 and R23. I'm planning to get Vishay RN60D2491FB14 which are rated to 1/4 Watt. I'm also running unity gain in the other position, so no R17 or R21.

Is there anything I need to know that would cause me to change my plan?
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Old 5th December 2011, 04:59 AM   #1145
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Quote:
Originally Posted by RocketScientist View Post
@DubiousMike. You don't need to cancel the entire order but you can cancel just the 2.7M resistor if you want and have a source for any resistor in the 1.2M - 1.8M range--even a 1/4 watt (stand it on end). It's not part of the audio circuit so it's not a critical part. You can also leave R9 at 40K if you want. I explained the 11/30 posts here that same day in this thread. The 12/2 changes were also explained here on 12/2 and involved increasing C6 and C7 to 1.0 uF just as a precautionary measure. I don't have any evidence there's any benefit to that change. You can use the same cap as for C1 in the C6 and C7 locations by re-bending the leads or the part in the 12/2 BOM with the correct lead spacing.
Much appreciated!
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Old 5th December 2011, 06:40 AM   #1146
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Quote:
Originally Posted by RocketScientist View Post
CSA seems much more interested in continuing to throw rocks as he's done in dozens of posts on another forum and just did again above.
Along with personal smear:

CSA"...So I found your response a tad smarmy."

This following on from "Not trying to be an *** ..."

hmmm.
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Old 5th December 2011, 01:30 PM   #1147
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Quote:
Originally Posted by sofaspud View Post
I just read the new diyAudio newsletter and Jan Didden's article on the DSOnano. Luv it! I truly hope this catches on. A diy'er doesn't have to have a $600 Fluke and a $6000 LeCroy. Even some basic modest test equipment will return dividends. Check out Jan's review if you haven't already!
While I love Jan Didden's work, and I applaud affordable test equipment, it's worth pointing out that scopes are only useful for a few measurements of high quality audio gear. They're great for evaluating stability problems, clipping behavior, basic square wave response and severe noise above 96 Khz. But nearly all use 8 bit ADCs and even the "high resolution" versions rarely offer more than 12 bit (ENOB) performance. While the DSOnano uses a 12 bit ADC (built into the MCU) it will be lucky to offer 10 or 11 bit analog performance which is only 60 to 66 dB of dynamic range. CD quality audio has 96 dB of dynamic range and most audio gear aims to at least get close to that if not exceed it. That renders scopes virtually useless for evaluating noise, distortion, crosstalk, etc. which are all typically in the range of -80 dB to -130 dB. The DAC's inherent noise, and the noise in the wideband input amplifier, prevents measuring such low level signals.


Quote:
Originally Posted by ethanolson View Post
I'm planning to have one of the O2's gain switch positions at 1.6x so I'm thinking that 2.5kΩ (2.49) resisters are in order for R19 and R23. I'm planning to get Vishay RN60D2491FB14 which are rated to 1/4 Watt. I'm also running unity gain in the other position, so no R17 or R21.

Is there anything I need to know that would cause me to change my plan?
That seems OK to me assuming you don't need more gain or at least some more "excess gain" for quiet tracks, etc. See my All About Gain article if you haven't already.

Also, FWIW, there's a false myth circulating (from H-F) you shouldn't leave out the gain resistors in the O2 for 1X gain. It's been suggested to use high value resistors instead (i.e. 10K - 100K) to prevent the input from being an "open circuit". This is pure nonsense. The low impedance 1.5K feedback resistor is what the negative input "sees" when you leave out the gain resistors. Besides increasing the gain every so slightly, there's no difference in performance adding a large value to ground. It's no different than a voltage follower buffer (as the 4556 output stage op amps are configured). Those don't have any resistor to ground on the negative input either.
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Old 5th December 2011, 01:57 PM   #1148
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Quote:
Originally Posted by RocketScientist View Post
.. the DSOnano uses a 12 bit ADC (built into the MCU) it will be lucky to offer 10 or 11 bit analog performance which is only 60 to 66 dB of dynamic range.

The 66dB dynamic range figure quoted here is within bandwidth under consideration - half the sample rate of the ADC. However, if the output of the ADC is run into a 4k (say) point FFT there's an effective gain in the dynamic range related to the averaging going on by virtue of acquiring that length of sample. So if you're a DIYer wanting to look for harmonic distortion, the picture is not as bleak as RS suggests because the FFT will allow you to see well below the wideband noise floor - assuming that your MCU has the RAM buffer available to store that length of sample.

As a rough figure 4k points in an FFT gives you around sqrt(2k) gain for each bin in the FFT - in this case more than 30dB. That turns out to be enough to get a reasonable THD measurement, even with only an 11bit ADC. In reality it might be a bit more difficult to 'grab' the spectral components because they're likely to span more than one bin of the FFT output. But the noise 'floor' in each FFT output bin turns out to be below -90dB by virtue of the sharply reduced bandwidth.

I have no idea if this DSOnano has the FFT software installed to do such a thing, or the RAM size to support it. Anyone checked out the detailed specs on the MCU?
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Old 5th December 2011, 02:16 PM   #1149
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Abraxalito brings up a valid point, with a repetitive signal (like a sine wave) you can get significant dynamic expansion through post processing math and/or averaging. But it's only useful for extracting periodic signals from the ~60 dB noise floor. It's useless for extracting random noise and hence many measurements (such as THD+N, noise, dynamic range, etc.). The FFT cannot distinguish between the uncorrelated noise in the device under test, the scope's input amp noise, and the noise in the ADC.

I have half a dozen digital scopes, including a few with high resolution DACs, and none of them do a reasonable job of THD measurement even with their software set up optimally for FFT THD processing. Even a rather lousy $12 USB DAC, like the Syba CM119 I tested, will far outperform all my scopes. And, FWIW, most of my scopes have four and five figure price tags.

So, yeah, if you're trying to measure 0.5% THD from a SET tube amp, it might be possible. But if you're trying to verify a piece of gear is comfortably below the 0.01% (-80 dB) guideline threshold for THD+N, I've yet to see any scope pull that off.
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Last edited by RocketScientist; 5th December 2011 at 02:23 PM.
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Old 5th December 2011, 02:35 PM   #1150
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So your next project is ADC USB scope then?
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