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Old 7th August 2012, 02:56 PM   #11
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Quote:
Originally Posted by zinsula View Post
You are using a perfect current source, infinite z out. DAC's in real life are worse.
Use a voltage source instead, and put a 1k resistor between it and your input. This is closer to reality. Use 1Volt per ma the DAC puts out.
Better would be to build and measure though...not much sense to believe these Simulation numbers, you might be disappointed.
I tried your suggestion and got even better performance for my circuit in simulation. I was using a current source with 1Mohm impedance originally.

If I had an Audio Precision analyzer then I could verify the design in real life. The best I have is a computer based real time spectrum analyzer with a higher noise floor than the AP analyzer. Nothing I have has a higher spec than the PCM1794a chip, so there's no way I can verify or optimize what ever I build with it.
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Old 7th August 2012, 03:11 PM   #12
zinsula is offline zinsula  Switzerland
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You may try to convert a sine to a .wav in LT Spice, and then feed the I/V input with the .wav
It's possible in LT Spice, but I never tried this.

Generally, I wouldn't trust sim distortion numbers anyway. It can give you a hint about the direction you can take, but exact numbers?

A common base I/V is a good starting point to build upon anyway.
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Old 7th August 2012, 03:14 PM   #13
zinsula is offline zinsula  Switzerland
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Quote:
Originally Posted by dirkwright View Post
[...] Nothing I have has a higher spec than the PCM1794a chip, so there's no way I can verify or optimize what ever I build with it.
Maybe not with gear, but if your ears are happy, go for it...

Oh, btw, open loop common base stages are not THAT low distortion, you may well be able to measure it.
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Old 7th August 2012, 03:25 PM   #14
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Quote:
Originally Posted by zinsula View Post
Maybe not with gear, but if your ears are happy, go for it...

Oh, btw, open loop common base stages are not THAT low distortion, you may well be able to measure it.
Well, I think that any I/V stage should be measured with the DAC in real life, since they are interrelated (the performance of the DAC is somewhat dependent on the characteristics of the I/V stage it's driving, for example). So, measuring the performance of an I/V stage in a vacuum is not very meaningful in my opinion, but can point in the right direction, as you suggested.

I have read that high quality DAC's are trimmed at the factory while they are driving the opamp I/V stage shown in the datasheet, so that's something to consider when designing a different I/V stage.
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Old 7th August 2012, 03:28 PM   #15
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Quote:
Originally Posted by zinsula View Post
You may try to convert a sine to a .wav in LT Spice, and then feed the I/V input with the .wav
It's possible in LT Spice, but I never tried this.

Generally, I wouldn't trust sim distortion numbers anyway. It can give you a hint about the direction you can take, but exact numbers?

A common base I/V is a good starting point to build upon anyway.
Yeah, there is a facility in the SPICE version I use that allows a custom waveform. I have not explored that yet.
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Old 7th August 2012, 03:34 PM   #16
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Quote:
Originally Posted by dirkwright View Post
I have read that high quality DAC's are trimmed at the factory while they are driving the opamp I/V stage shown in the datasheet, so that's something to consider when designing a different I/V stage.
Where did you read that? It might have been true for the old style multibit DACs - they relied on laser trimming of their resistor networks. That's one reason for why PCM1704 is so much more expensive than say PCM1792. But CMOS DACs such as the PCM179x series are designed not to need trimming - that's one of the big advantages in manufacturing for S-D type DACs.
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Old 7th August 2012, 04:41 PM   #17
smms73 is offline smms73  Portugal
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Quote:
Originally Posted by zinsula View Post
You are using a perfect current source, infinite z out. DAC's in real life are worse.
Use a voltage source instead, and put a 1k resistor between it and your input. This is closer to reality. Use 1Volt per ma the DAC puts out.
Better would be to build and measure though...not much sense to believe these Simulation numbers, you might be disappointed.
Hi Zinsula,
i was told that the output impedance of the pcm1794 is 2M with 22pf, try to find in internet for the real value, but no luck .
this circuit was design with that in mind, with a source of 1k the distortion is a lot worse (0.008%). but i have made some changes in the input and got 0.000064% thd with 1k input impedance see circuit below

how do you come to 1k , i think is very low for a current source, if someone knows the real value please post.
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Old 7th August 2012, 04:49 PM   #18
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I think this topology is also very interesting for a Transimpedance MC phono stage.
The only thing that worries m is noise into low impedance, say an MC cartridge that has 6 Ohm DC impedance. On one hand the ist the R9 4kOhm resistor. On the other hand there is Q2, Q5. The Base-Emitter resistance is in the noise loop even when the bias to that transistors is made very low impedance. What whould happen if R9 will be shunted by a big Electrolytic ?
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Old 7th August 2012, 05:06 PM   #19
smms73 is offline smms73  Portugal
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Quote:
Originally Posted by abraxalito View Post
My doubt is - DAC chips don't put out sinewaves, yet you're simulating with a sinewave source. So I can't but help thinking all those zeroes in your distortion figure are rather meaningless.

A modern CMOS DAC such as the PCM179x puts out some very fast edges with rise times perhaps <1nS. Have you considered using transient simulation ? - you can input a .wav file containing a sinewave which will be somewhat closer to reality because it'll have steps. It will still not really be close enough though as it won't have all the out of band hash that's intrinsic to S-D converters.
sorry to disagree with you. but simulating this circuit with a sine source is enough.
this circuit is very fast and dont have slew problems, if you care to simulate in ltspice, you will see that there is no voltage variations, only in the output so there is no parasitic capacitors to charge and discharge. i will explain better how this topology works.
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Old 7th August 2012, 05:10 PM   #20
xslavic is offline xslavic  Moldova
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Wouldn't this be better
Simple and convenient,if dual mon o then xlr
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