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Try this one instead ...

Posted by Dave I on March 24, 2002 at 14:29:15

In Reply to: Re: 1K is a bit too high for I/V resistor posted by Spartacus on March 24, 2002 at 09:24:28:

The other post seems to have some kind of problem that won't let it display the image and won't let me delete or respond to it so this is a repost.

I did use a fairly simple discrete stage after the I/V resistor and was actually quite happy with the sound. It was a no feedback type design with a gain of about 30 or so and easily bettered opamp type equivalents. It was only when I couldn't get an answer from BB about voltage compliance that I decided to conduct my own tests and that led to my converting over to the current mirror design which sounded much better.

Above is a simplified schematic of my present output stage. I also use an offset control circuit which acts on the current source to avoid any coupling capacitors but would prefer not to show that part since it required quite a bit of development to implement one that didn't significantly degrade the sound, especially in a no feedback design. I also have left the values off of the filter components since they are somewhat dependent on other circuit values and I don't generally use them on my own DAC. I directly drive the amp via a low resistance precision wirewound pot so it is biased very high, almost like a small power amp.

The transistor pair Q1/Q3 is just a classic current mirror. Since the bases are common, the emitters must stay close to the same potential as well. The emitter of Q3 is connected to ground and hence Q1 must follow which keeps the DAC output at a virtual ground. This is a no feedback design (other than the local resistor degeneration in the current source) and sounds very good. In my tests it sounded better than a resistor I/V and discrete gain stage, but remember with a 10 ohm resistor I/V you would need to follow it with a voltage gain of about 300 so it would actually be more complex than this circuit and would still not be ideal. When testing different values for the I/V resistor, I kept the gain stage constant and adjusted volume with a pot on the amplifier input to match levels. The values in this simplified circuit are for a PCM1704 DAC with a full scale current output of 1.2mA. The current from the DAC output will be reflected through R7 (773 ohm) which you could call the I/V conversion resistor. The voltage across R7 will then be amplified a bit further by Q6. The gain of the cascoded, single-ended output stage is set at about 3 by the source and drain loads. One could always skip the output stage and increase the value of R7 and take the output at the junction of R7 and Q1 through a large value capacitior (and I believe Jan Didden presented a design that did just that many years ago in The Audio Amateur) but I prefer to avoid capacitors whenever possible and I like having a high current stage to drive the volume control, cable and power amp.

One should note that there are many other different current mirror configurations that will work well and many will deliver a lower dynamic impedance because of negative feedback, but this no feedback design was the best sounding of the large number of designs I tried.

Dave

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