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Old 23rd May 2013, 05:55 PM   #501
qusp is offline qusp  Australia
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Quote:
Originally Posted by jan.didden View Post
Actually the ESS DAC is a bit of the odd man out, sort of schitzophrenic, not able to decide whether it wants to be a voltage out DAC (Zout = 0) or a current out DAC (Zout = infinite).

The cleanest option, if you insist on buffering the output, would be the following.
Use an I/V opamp (discrete or chip as you fancy) like Coris showed. Use a feedback resistor that has the same value as the DAC Zout (around 780 ohms). That will give you current loading of the DAC and an output level the same as the open output level of the unloaded DAC. Also, (if you use a good opamp), no noise and distortion penalty!

It may not conform to current fashion, and you may subjectively prefer an other configuration, but for the cleanest sound there's nothing better. The best way to reap the benefits of Joe's clock and low jitter mods!

jan
which is exactly what I do on one of my ESS, but with 195Ω on a stereo out (781Ω || 781Ω || 781Ω || 781Ω = 195) buffering the output to provide no loading/low impedance does net you another ~10dB THD+N versus an ideal voltage output scenario, which is NOT just connecting a resistor to the output, that will not get near that, DNR is roughly the same though.

speaking of which, that means this 20Ω resistor is another 4x lower given this dac doesnt parallel the outputs.

on my other 2 ESS Irun common gate IV with high GM mosfets and set the gate to AVCC/2 a tiny touch shy (-115dB THD+N) of top flight opamp performance, but a bit more fun

in both cases I LP it, multiloop FB on the opamps, mosfet gate/out cap (parasitic ie. Ciss/Coss) for the common gate

Last edited by qusp; 23rd May 2013 at 06:20 PM.
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Old 23rd May 2013, 07:24 PM   #502
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BTW are the circuit diagrams of the Oppo in this area available in the open?

jan
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Old 23rd May 2013, 07:51 PM   #503
Coris is offline Coris  Norway
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Quote:
Originally Posted by jan.didden View Post
BTW are the circuit diagrams of the Oppo in this area available in the open?

jan
No. Unfortunately not leaked...yet
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Old 23rd May 2013, 07:52 PM   #504
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They will - eventually
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Old 23rd May 2013, 07:56 PM   #505
jvd is offline jvd  New Zealand
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I'm a lurker and a plodder - I'm so pleased to see open (and valued) discussion. It's how these threads move forward I believe.
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Old 23rd May 2013, 08:34 PM   #506
Coris is offline Coris  Norway
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Default Now it come some documentation from me...

Maybe I should precise before this "dispute" began here, that this setup with 20 ohm resistor it was an experimental one. This kind of setup it may not be functional of course, because some facts: necessary too much gain after I/V, which it increase the noise in the system, offset issues due to the same very high gain, and few other negative things.
Even though all those negative sides, I could not notice (perceptual) bead sound (FM radio like...), but very good quality one.
I think to present you here those results and "a vous" to analyze and comment.

There are scope plots for the ("worst case") 20 ohm (feedback) I/V resistor setup with x50 gain on final opamp, on the Stereo stage which were modified like this, and a "witness" one which is Front L/R stage very intact and with similar quality with the original stereo one.
Then the same measurements with an 750 ohm I/V resistor and 1,3 gain on final opamp (the same procedure, comparing with Front L/R stage).

It were used for test 1Khz and 80Khz -3dB wav files (192/24) which were played back as normal on the player. The scope is set mainly to High Resolution Acquiring data, but it is used in some cases Normal too..

I use 108 Mhz clock for the DACs, divided to 27Mhz for the processor. This is an temporary clock setup, which is not very performant (specially the 27Mhz part one). There are first some plots of the clocks outputs, to make you an idea about how my scope it works with those clocks and the quite standard frequencies.
For the same reason I publish here some plots with the noise floor of my scope (shorted probe, 4mV scala (lowest not software amplified level), and so on. Also a picture of the measurements setup.
It will be quite many pictures, so many posts which it belong to this main one.
To see better the pictures, please download and zoom it.
Attached Images
File Type: jpg measSetup.jpg (590.4 KB, 285 views)
File Type: jpg ScopeNoiseFloor.jpg (916.8 KB, 272 views)
File Type: jpg clocks.jpg (762.6 KB, 257 views)

Last edited by Coris; 23rd May 2013 at 08:59 PM.
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Old 23rd May 2013, 08:36 PM   #507
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Would replacing any of the regulators with a super-regulator like Dexa or Burson do any good you think? That, together with bypassing the output caps, was the mod I did to my Cambridge Dacmagic some years ago, with good results.
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Old 23rd May 2013, 08:43 PM   #508
Coris is offline Coris  Norway
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20 ohm I/V resistor and x50 gain on Stereo and Front L/R (High resolution Acquiring). Noise Floor of the outputs and with signals.
Attached Images
File Type: jpg NoiseFloorFl-rStage.jpg (804.0 KB, 249 views)
File Type: jpg signalsFl-r.jpg (870.6 KB, 232 views)
File Type: jpg NoiseFloorStereoStage.jpg (959.7 KB, 17 views)
File Type: jpg signalsSTEREO.jpg (930.0 KB, 14 views)
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Old 23rd May 2013, 08:53 PM   #509
Coris is offline Coris  Norway
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750 ohm I/V resistor and x1,3 gain on Stereo and Front L/R (High Resolution and Normal). Plots for Noise Floor of the outputs and with signals.

PS. My first comment is that are not big differences between an "bad" I/V setup and a "good/normal" one...
Attached Images
File Type: jpg NoiseFloorFl-r.jpg (405.8 KB, 30 views)
File Type: jpg signalsFl-r.jpg (803.6 KB, 25 views)
File Type: jpg noisesStereo.jpg (407.4 KB, 15 views)
File Type: jpg signalsStereo.jpg (823.5 KB, 15 views)
File Type: jpg Fl-rSignalsNORMAL.jpg (624.4 KB, 11 views)
File Type: jpg StereoSignalsNORMAL.jpg (654.7 KB, 14 views)

Last edited by Coris; 23rd May 2013 at 09:06 PM.
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Old 23rd May 2013, 09:16 PM   #510
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Coris and Joe, you have been very informative as to the 'problems' with the OPPO 105.
About 6 mo ago, I was asked by OPPO to look at their audio output, and I have studied the schematics, and did listening tests on both the normal and the headphone outputs.
These designs are MADE TO A PRICE and while they are trying their best, OPPO also needs feedback as to how to make a better product. Your measurements have been very helpful to me to ascertain what is really effecting the sound of the OPPO from ideal. Keep up the good work, and don't worry about criticism. You are doing something useful.
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