Please comment on this I/V converter

[andy2]

Hello,

Could you comment on this particular I/V converter? I plan to use this on my future DAC (if I ever got around to building one). Has anyone used this type of I/V converter before and could you comment on the sound?

This is basically a differential cascode sort of modify a bit from Douglas Self book (or semi cascode since it is not quite a cascode). Once nice thing about the differential input is that the input is at 0v reference so it is easy to interface with a current output DAC. I was looking at some other I/V and the inputs on some of them is not at 0V so this could help a little.

I looked at the datasheet of the BurrBrown PM1704 and it appears that the output impedance is 1K and the current is about +/-1.2 ma which gives an output voltage of around +/- 1.2V.

On the schematic the RFB resistor (among other things) can be used to adjust the gain. The V marker is used for the output and can be connected to a lowpass filter. Later on, I will add a level shifter after the diff stage to shift the voltage down to 0V so that a cap is not needed.

Any comments would be appreciated.

[relder]

Originally posted by andy2

Once nice thing about the differential input is that the input is at 0v reference so it is easy to interface with a current output DAC. I was looking at some other I/V and the inputs on some of them is not at 0V so this could help a little.

(Note: I'm learning myself, and don't claim to be 100% correct with what follows )

This circuit appears to want a voltage input not a current input.

The other half of the diff pair is referenced to ground, but what's to keep the other half, DAC side, at 0V? (assuming the input is V3 (a voltage source representing a current out?) and the output impedance of the DAC is represented by Rin, 1K?) The DAC would prefer it's output kept at 0V and I can't see how this circuit does that, if that's a goal. Is all the DAC's current supposed to go into the base of Q1? I don't think you can just cram 2.4ma into the base, no?

Rob

[zinsula]

Rob is right, it is not an I/V converter.

But if you would like to do a discrete one, (which sure is a good thing http://www.diyaudio.com/forums/showt...844#post396844) , take a look at that:
Easy-to-build I/V stage

Or the D1 output stage from Pass Labs:
http://www.passlabs.com/downloads/ol.../d1_servm.pdf,
a thread about this one exists, but it seems it takes time to find it here ....

Promitheus has built one: D1 I/V Stage Finished.


Ciao
Tino

[andy2]

Originally posted by zinsula
Rob is right, it is not an I/V converter.

But if you would like to do a discrete one, (which sure is a good thing http://www.diyaudio.com/forums/showt...844#post396844) , take a look at that:
Easy-to-build I/V stage

Or the D1 output stage from Pass Labs:
http://www.passlabs.com/downloads/ol.../d1_servm.pdf,
a thread about this one exists, but it seems it takes time to find it here ....

Promitheus has built one: D1 I/V Stage Finished.

Ciao
Tino

Tini,

Some of the links do not work ... for example the D1 Pass labs link is not correct.

Anyway, I actually looked at some of the I/V here but I was trying to come up with one that has 0V reference. I haven't seen the D1 version yet, but most of the I/V converter here have some voltage offset at the input. I am trying to come up with one that is easy to use like an opamp but hopefully has better performance.
I am trying to modify my existing circuit to include some types of feedback to reduce the input impedance.

By the way, anyone has the link to the Pass D1 version?

[relder]

Originally posted by andy2

Some of the links do not work ... for example the D1 Pass labs link is not correct.

Anyway, I actually looked at some of the I/V here but I was trying to come up with one that has 0V reference. I haven't seen the D1 version yet, but most of the I/V converter here have some voltage offset at the input. I am trying to come up with one that is easy to use like an opamp but hopefully has better performance.
I am trying to modify my existing circuit to include some types of feedback to reduce the input impedance.

By the way, anyone has the link to the Pass D1 version?

The service manual is here:
http://www.passlabs.com/downloads.htm

The D1 maintains 0V, or whatever voltage your DAC prefers, at the DAC out via tying the gate to a static voltage equal to the Vgs of the FET, adjustable via the pot.

Jocko's (do a search) does much the same thing, except another BJT is used to set the Vbe to the correct point (though no adjustment necessary, temperature compensated.)

I'd look at those two to start off with, they are similar with different transistors.

Rob

[zinsula]

Originally posted by andy2
.... Some of the links do not work ... for example the D1 Pass labs link is not correct.....

Ooops, the link is corrupt. Just delete the comma at the end of the link....
Maybe a mod could do the job?

Originally posted by andy2
Anyway, I actually looked at some of the I/V here but I was trying to come up with one that has 0V reference. I haven't seen the D1 version yet, but most of the I/V converter here have some voltage offset at the input.

You may adjust the voltage as described in the first post of Jocko's easy I/V....

Tino