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In Reply to: IV conversion for 1704 DAC posted by Martin on August 08, 2001 at 02:16:14:
I'm sure many have done it before and from my experience you need to use the smallest value resistor you can to minimize the voltage on the output of the DAC (normally specified as voltage compliance but not spec'd on the PCM1704 since it is designed to work with the virtual ground provided by a high gain opamp). I originally used 100 ohms and then 50 ohms but later tests showed me that the sound improves down to at least 10 ohms so I finally went with a current mirror utilizing a couple common low noise bipolar transistors that gets down close to 3 ohms dynamic impedance without feedback (you can of course get much lower with feedback). Experiment yourself and come to your own conclusion.Dave
Follow Ups:
Hello Dave,What it be a good idea to use such a low value(10 ohm)
when using an opamp for gain? Currently using a 50R
to ground feeding into a OPA624 to sum and provide gain.
Any advice is much appreciated.Thanks
Kyle Takenaga
I'm not sure what you mean by sum and provide gain. You can configure the opamp to do the I/V conversion as an integrator with an appropriate feedback resistor for the output voltage you want and provide a virtual ground for the DAC. Most people using a resistor follow it with some kind of a discrete gain stage since the main goal (IMO) is to avoid using an opamp. I'm not sure that with the modern and extremely fast current feedback opamps which are readily available that you gain anything using a resistor for I/V conversion in an opamp based system because of the penalty you pay for having voltage on the output of the DAC.But in any case, if you are using the opamp strictly as a voltage gain stage following the I/V conversion resistor, I still believe that the lowest value possible sounds best (and 50 ohms will lead to 60 mV on the output of the DAC at full output - some of the DACs are specified for less than 10mV to maintain linearity spec). It's easy to experiment though and everyone should try it for themselves if interested. Just remember to carefully match levels in a way that doesn't adversely affect the sound when comparing.
Dave
Hi Dave,I am modding a parts connection dac3. The output stage consist
of a OPA627 for I/V conversion followed by another OPA627 which
is fed by positive and negative signal(2 pcm 1704's per channel)
(Is this called a summing opamp??)then into a 634 buffer. So
basically 3 opamps in series. I removed 2 of them along with all
the analog filtering, muting circuit,servo circuitry.
I may try the LC audio circuit to rid the last opamp if it can provide
enough gain. Thanks for the info.Kyle
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