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Old 24th September 2010, 04:44 AM   #21
anatech is offline anatech  Canada
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Hi Spencer,
Sounds like a good design in that case. There are about as many ways to go about I/V conversion as there are designers.

You should listen to that DCD-S10. Did you know it also has external digital in and out? It also has a balanced output, so it's very versatile on the bench (where mine started life, it's now on my best system). That should allow you to compare it on a one on one basis. Like your friend, I'm still extremely pleased with it. I've also had other DACs that friends have brought over to compare. So far the Denon has sounded the best - overall consensus from many people. It's a winner.

I hope to hear a better DAC section some day. Something I can use in other places (I miss it on the bench).

-Chris
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Old 24th September 2010, 08:33 AM   #22
spencer is offline spencer  Hong Kong
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I have two pictures to show how I take the measurement of the voltage Vpp at the output pin of the PCM1794A. It is about 60mVpp with Ipp 7.8mA and thus giving an impedance of 7.8 ohm. Minus the 4R99 in series at I/V input and thus the input impedance is about 2.8 ohm at the BJT I/V. Note that the noise at the PCM1794A Iout pin is not that much as the scope bandwidth is set to 400MHz.
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Old 24th September 2010, 10:40 AM   #23
TC Goh is offline TC Goh  Hong Kong
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Quote:
Originally Posted by spencer View Post
I have two pictures to show how I take the measurement of the voltage Vpp at the output pin of the PCM1794A. It is about 60mVpp with Ipp 7.8mA and thus giving an impedance of 7.8 ohm. Minus the 4R99 in series at I/V input and thus the input impedance is about 2.8 ohm at the BJT I/V. Note that the noise at the PCM1794A Iout pin is not that much as the scope bandwidth is set to 400MHz.
Attachment 189134

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Hi Spencer

I am assuming you are asserting that a lower impedance value is better. If that is the case, why not take the series resistor out completely?

Best wishes

TC
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Old 24th September 2010, 11:42 PM   #24
anatech is offline anatech  Canada
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Hi Spencer,
Good idea to allow a resistor to linearize the input resistance of the transistor I/V. Can you please clarify your current measurement for me? Is that a peak to peak value of current change?

Hi TC,
I guess from my post you had some idea why the resistor exists. There may also be some minimum resistance that something wants to see. Not being familiar with these exact parts, I'm not going to hazard any guesses there.

-Chris
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Old 25th September 2010, 12:01 AM   #25
spencer is offline spencer  Hong Kong
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Chris,

I inject 0dBFS 1kHz digital signal to the SPDIF input from my AP and check the waveform at pin 3 of PCM1794A and you will see 60mVp-p signal as shown on scope. If you prob the input of the Jfet I/V (after 4.99ohm resistor), the wave form become about 20mV. So the current across the 4.99 ohm is about 40mA/4.99ohm or 8mA peak to peak.

I also check the waveform at the 472 I/V resistor and it is about 3.8Vp-p. So it confirm the 8mAp-p current again.

Let me know if you have other questions or measurement that you want to know.

Spencer
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Old 25th September 2010, 01:47 AM   #26
TC Goh is offline TC Goh  Hong Kong
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Originally Posted by anatech View Post
Hi Spencer,
Good idea to allow a resistor to linearize the input resistance of the transistor I/V.....

Hi TC,
I guess from my post you had some idea why the resistor exists. There may also be some minimum resistance that something wants to see. Not being familiar with these exact parts, I'm not going to hazard any guesses there.

-Chris
Hi Chris

Thanks for your reply. I think you said in an earlier post there are numerous designs out there. Perhaps each is unique in its own way catering for a particular audio need. I guess that is why hifi is personal in taste.

Anyway, I am getting a little restless and looking at ways to play around with the board. In my mind, reducing the impedance should improve performance but perhaps this may introduce other problems.

Oh, why do NP's designs have 15 ohm impdeneces? By design or due to the components he uses.

Have a good weekend.

Best wishes

TC
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Old 25th September 2010, 07:37 AM   #27
TC Goh is offline TC Goh  Hong Kong
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Default Big Caps!

Hi

Look what the Postman brought today!

I will be replacing the 8 6800pf caps with these 1600V Teflon ones ….whoa!! Yes 1600V – crazy, I know. I will also be trying out the monster K75-10’s too.

I think the Teflons should be very interesting and the K75 is a hybrid PIO - PLIO??

This is going to be an interesting weekend!

Best wishes

TC
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Old 25th September 2010, 07:20 PM   #28
anatech is offline anatech  Canada
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Hi TC,
Don't forget that large capacitors are basically antennas that might either pick up or radiate stuff you don't want to know about. That seems to be one of the huge mistakes that are made around the world. If you look carefully at circuits designed for these capacitor types, they are either not at risk for noise pickup (radiation is another matter completely), or they have designed a space for those parts where they are shielded from other signals, or signals that may be radiated from those caps. I'm talking about metal shields or metal enclosed areas in the circuit.

Component selection can be tricky business. Don't forget about the obvious problems either.

In case you are wondering why large capacitors can be a problem, think about one of the two capacitor connections. It is connected to the outer foil, and so acts in the same way that a sheet of copper would the same outer dimensions. Now, if you would never connect a sheet of copper in that location, why would you do the same thing using something that simply looks different?

-Chris
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Old 26th September 2010, 03:24 AM   #29
qusp is offline qusp  Australia
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indeed, I often wonder about the use of these huge NOS caps in noise and induction sensitive circuits, seems people who know better just seem to forget
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Old 26th September 2010, 05:53 AM   #30
TC Goh is offline TC Goh  Hong Kong
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Default Bigger Not Better?

Hi Chris

I am aware of the interdependencies and how they can affect sound but I thought the main culprit were the TXs. That was why when mounting the two boards, I was concerned that the TXs were too close and whether using two would add more noise (phase, EMF interaction, etc) as I have no equipment to measure it.

As far as my audible experience goes, I don't believe these biggies have had a negative impact. In fact, the experience has been to the contrary. However, that is not to say there isn't or the designs were optimized to begin with. Maybe I am just not hearing them – selective listening ha ha!!! Perhaps on the bench, the measurements may be worse, and I am sure they will be L.

Anyway, I am glad that you raised this. At least I am more aware of the possible downside. However, as I have them already and they are very easy to install, I shall give them a try anyway ….. Perhaps someday, I will have the ability to measure the impact.

Thanks again.

Have a good weekend…

Best wishes

TC

PS. I like your tag line ... how true .. how true. That's why many of my purchases have been vetoed! "No, not another box in the house, dear!!!"
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