I want to build the DAC with design below. I can see that the design use the dac voltage output instead of using the current output with passive I/V, my question is, will the current output with I/V gives more benefits such as lower THD than voltage out? Also, do I need to add a LPF right after the output?
http://www.pavouk.org/hw/audiosystem20/en_ad1865dac.html
http://www.pavouk.org/hw/audiosystem20/en_ad1865dac.html
Build it, experiment with it, compare it to some cheap ES90xx boards. That is the only way to find out if old-school multibit DACs suit your hearing. Honestly, you can ask questions here until you're old and you'll get answers pro & con. Just build it, then judge by yourself. There are a thousand factors beside the ones you questioned that decide "good" sound quality.
Cheers, Markus
Cheers, Markus
I heard that the problem for current out with I/V will give a weak voltage out, can I simply increase the value of resistor say 330ohm?
As I recall, 330ohms is okay to use with the AD1865. However, use caution if trying to increase the I/V resistor to some higher value. The I/V resistor cannot be arbitrarily increased, because the DAC chip's current outputs have a voltage compliance limit. The exact value of which depends on which DAC chip you are utilizing. However, the THD will increase significantly before the compliance limit is reached.
I have used 500 ohm plus a capacitor for low pass, and the output was high enough for me to run directly to my preamp.
I also tweaked the distortion with the MSB adjustment circuit that is shown on the AD1865 data sheet. That is quite easy to do with FFT distortion software such as Arta, an USB audio interface, and a -60dB 1000Hz digital file. In my case it reduced the THD substantially.
Unfortunately, I no longer have this dac running so I cannot measure its distortion, and I can't remember what I had measured previously.
I also tweaked the distortion with the MSB adjustment circuit that is shown on the AD1865 data sheet. That is quite easy to do with FFT distortion software such as Arta, an USB audio interface, and a -60dB 1000Hz digital file. In my case it reduced the THD substantially.
Unfortunately, I no longer have this dac running so I cannot measure its distortion, and I can't remember what I had measured previously.
That parameter is not specified because it's not supposed to be relevant. The current outputs of most DAC chips are intended by design to be held at virtual ground. The current output pin compliance voltage only becomes an issue with passive resistor I/V. Which inherently develops an AC signal voltage on the current output pins that scales with the resistor value.
The placement of the IV resistor and capacitor is at the Iout of the AD1865 as shown on this dac schematic:
Audio DAC with AD1865
I checked and I had actually used 435 ohms and 4.7 nF. The -3dB cutoff is about 78 kHz. A 2.2 nF cap would give approximately the same cutoff as 200 ohm and 4.7 nF (169 kHz).
Thanks for you information. But the Voltage out is still weak when using 435ohm resistor. Perhaps I need to find a way to solve this issue. Maybe use 4 DAC chips to run in parallel is a solution.
You can try a simple IV stage such as this:
Fet-ishizator
I have not tried it but it is simple to do.
Fet-ishizator
I have not tried it but it is simple to do.
Multiple DACs don't help in this case. The issue is the voltage compliance of the current outputs, which will be the same for multiple DAC chips in parallel as it is for 1 chip. If you have access to an distortion analyzer, you could progressively increase the value of the I/V resistor until it reaches a threshold which you feel is an acceptable limit. If you don't have an analyzer, then you might try using your ears to subjectively perceive when the distortion reaches an acceptable limit, then use a a little bit lower value resistor than that for margin.
At the point of maximum distortion which you judge acceptable, the output signal voltage amplitude will be whatever it is, and you will just have to live with it. I suspect, however, that you will find the resulting signal amplitude is high enough if it subsequently feeds a line level type preamp box with some gain. However, if it's still not high enough, you then will need to amplify the signal voltage via some low-ish signal gain preamp circuit.
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I am thinking if I can parallel multiple DAC chips together, I am able to increase the current output and hence allow me to use resistor with smaller value. Don't know if it works or not.
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