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Old 9th September 2010, 05:51 PM   #1
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Default Zen I/V Converter

The Zen I/V Converter article is posted here at DIYAudio,
and I am establishing this thread for discussion.

At this moment, it seems to have lost the first diagram, that
of a typical inverting op amp with a virtual ground input,
but I'm sure that will be rectified shortly.

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Old 9th September 2010, 06:06 PM   #2
EUVL is offline EUVL  Europe
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Very nice circuit. I like it a lot better than the D1, but .....

1. Highish input resistance (> 10R) and capacitance ? Some DACs like PCM1704s might not like it.

1a. Highish output resistance especially if more gain required, but solvable by adding a JFET follower.

2. A pity you could not get rid of the coupling caps. Maybe a folded cascode ?

3. And how about a balanced version that has some extra distortion cancellation trick ?


Patrick

Last edited by EUVL; 9th September 2010 at 06:21 PM.
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Old 9th September 2010, 06:59 PM   #3
EUVL is offline EUVL  Europe
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I am not home right now, so I cannot post a schematics. Also I have not been thinking about it for weeks, so this may not be ultimo version.

Take a PCM1704 with +/-1mA output as example. To get 2Vrms standard CD output, one would need to use 5.6k drain resistors for the JFETs. Somewhat high for driving cables, so a buffer maybe desirable. One can use a single ended JFET follower, or a complmentary one to go with the rest of the circuit. Input resistance is about 33R // 40R, i.e. around 17R.

But if we accept the complexity of a folded cascode to get rid of the coupling caps, we might even get away with lower rail voltages. Using 2SK2013 / 2SJ313 as cascode devices at say 50mA with the source at say +/-12V (gates at ca. +/-9.5V), we can even use +/- 18V rails. And the cascode would reduce input capacitance (so we can consider paralleling JFETs to reduce input resistance as well as noise) and improve PSRR. The output resistor is now reduced to a single 2.8k for 1 complemetary JFET pair, which can be placed at the output socket, or even at the end of the interconnect cable.

Mirror imaging will get me a balanced version, and if properly match they will cancel even harmonics.

Yes, I have a single ended, balanced version of this in the process of being built and tested, but I have not yet been able to come up with a means of cross coupling the 2 differential halves in any beneficial way.

Your comments ?


Patrick
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Last edited by EUVL; 9th September 2010 at 07:15 PM.
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Old 9th September 2010, 07:51 PM   #4
DougL is offline DougL  United States
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Great article. Thank you.

Now for the dangerous part. I assume (most dangerous word in engineering) that for a DAC chip like the PCM1794 that has differential current outputs, I would simply use 2 circuits per channel. Also, because the DAC chip puts out almost 8 ma of current, the 10 ma jfets would be sufficient.
If I needed single ended operation, I could convert it by the usual tricks.

Here is the data sheet for the PCM1794 focus.ti.com/lit/ds/symlink/pcm1794.pdf

Doug
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Old 9th September 2010, 07:59 PM   #5
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I will read through the article now.
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Old 9th September 2010, 08:22 PM   #6
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What about 4 parralleled 2sj74/2sk170 to reduce input resistance.
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Old 9th September 2010, 08:26 PM   #7
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I have been thinking of using the Buffallo II DAC chip with this, but what are other peoples favourites?
Also what other new chips are worth considering?
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Old 9th September 2010, 09:43 PM   #8
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Neat circuit! When I saw it I thought it looked familiar. I was doing some searching for common-base circuits for I/V about two days ago and this caught my eye. Notice any resemblance?

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Old 9th September 2010, 09:59 PM   #9
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Hi Patrick
Are you planning on using lu1014d in common gate mode?
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Old 9th September 2010, 10:10 PM   #10
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i will definitely build this for my akm4396 :P
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