Zen -> Cen -> Sen, evolution of a minimalistic IV Converter

One day on the return journey of a business trip, I had an idea of using a JFET version of Leach’s MC headamp as IV converter for current-out DACs. A month later, various circuits were simulated, bread-boarded and measured, and prototypes with proper PCBs were built and tested. We also did listening tests using our own SD player and AD1865 DAC, and were very pleased with the results.

I took them to Jan Didden one evening for some distortion measurements with his Audio Precision System One. He was so impressed that he wanted to include the article about this development in his Linear Audio Magazine, Volume 2, due out at the beginning of September. In support for Jan’s enthusiasm in running the magazine, I duly obliged.

So you can now read the entire story in detail in Linear Audio. To facilitate experimentation of the circuits, we shall make available an evaluation kit, but only if there is sufficient interest (minimum 60 sets).

Jan has generously agreed to me publishing the schematics, bill of material, PCB layouts, and some measurement results here. So in theory you can build the circuit without reading the article. But I strongly suggest you do, not only to understand how it works, but also how to adapt it in the many possible configurations to your need.



Patrick
 
The Schematics

Here are the schematics as published in Linear Audio, courtesy of Jan.

CEN for complementary, SEN for single ended.


Patrick
 

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FFT of Sen IV using 20k series resistor at input (Audio Precision System 1)
(Blue : Signal Source, Magenta : IV Voltage Output)

You need to refer to the article to see why we use this as the I input, as compared to a low distortion VCCS.
The VCCS turned out to be not low distortion enough at about -100dB.


Patrick

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Note that the performance in post #6 was based on a transimpedance of 2700R, as compared to 500R of the original ZEN IV.

On Spice simulations using identical component models (active and passive), the CEN and SEN has a factor of 4 to 6 less distortion than ZEN, using 500R transimpedance in all 3. The distortion is too low for me and Jan to measure because we simply do not have a current source with low enough distortion (-130dB).


Patrick
 

Tazzz

Member
2007-06-15 8:04 am
Note that the performance in post #6 was based on a transimpedance of 2700R, as compared to 500R of the original ZEN IV.

On Spice simulations using identical component models (active and passive), the CEN and SEN has a factor of 4 to 6 less distortion than ZEN, using 500R transimpedance in all 3. The distortion is too low for me and Jan to measure because we simply do not have a current source with low enough distortion (-130dB).


Patrick

This will be the first article i read in the new issue..!

Perhaps you could use the AN67 oscillator and an R2R like structure to generate a clean current source with some effort it should be possible to select the output impedance and the current swing you want.
 
Jan & I were using the Audio Precision Oscillator which is as good as if not better than AN67, which I also have.
The Audio Precision can output up to 20V, which the AN67 cannot.

I can only ask you to read the article. It is unfair to Jan if I tell more here.

And I do want you to support him. :)


Patrick
 

Tazzz

Member
2007-06-15 8:04 am
Jan & I were using the Audio Precision Oscillator which is as good as if not better than AN67, which I also have.
The Audio Precision can output up to 20V, which the AN67 cannot.

I can only ask you to read the article. It is unfair to Jan if I tell more here.

And I do want you to support him. :)


Patrick

I seem to remember FREX posting measurements on the AN67 displaying distortion of less than -160dB down from full scale.
 
FFT of Sen IV using 20k series resistor at input (Audio Precision System 1)
(Blue : Signal Source, Magenta : IV Voltage Output)

You need to refer to the article to see why we use this as the I input, as compared to a low distortion VCCS.
The VCCS turned out to be not low distortion enough at about -100dB.
Regardless of why you use any particular input, how is it that your output has less noise than your signal source? Is there negative gain?
 
The input was a 20Vrms sine wave from Audio Precision's functions generator, which was converted to 1mA rms input to the IV via a 20k series resistor.
The IV Converter output had a 2.7k resistor, and hence the output was 2.7Vrms.

Because the functions generator voltage was some 17dB higher than the IV Converter output, it was no surprise that the noise levels differed by about the same amount.

These are all explained in the article.


Patrick

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Thank you all for your support and interests in the Evaluation Kit via PM, despite not having read the article.

The article would actually answer many of your questions (such as which DAC type, ..., etc.). But it is not difficult to see that the original CEN or SEN circuit, like Nelson's Zen IV, is designed to work with bi-polar current output DACs with no offset voltage, such as PCM63, PCM1704, AD1865, ..., etc. With simple modifications, it can also work with DACS with either current offset, or voltage offset, or both.

We have tested and listened with AD1865 and are absolutely pleased with the results, especially with LCR polystyrene as C_iv. But of course we are always totally biased, and the truth is only when you get a chance to try for yourselves. There are still plans to try it with PCM1704, though we have some more work on the power supply for that DAC chip.

I think it is too early to start a GB of the Evaluation Kit. The article gives you a lot more info as to how the idea came about, how the circuit works, what the performances, are, what other circuit variations you can choose to suit your specific needs, ...., etc.

And there are many other articles in Volume 2 that it is well worth the very small price being charged.
Please give Jan your support. It is IMHO a noble course.

http://www.linearaudio.net/


Patrick

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