Has anyone using a passive I/V DAC tried using the I/V resistor as the attenuator? In other words, using a switched attenuator or pot configured as a rheostat for the I/V resistor.
Since a current-output DAC outputs a given current for a given signal level, reducing the value of the I/V resistor will reduce the voltage dropped across it and hence the signal level seen by whatever the DAC is driving.
One interesting aspect of this would be that the source impedance of the DAC would behave like a transformer volume control with the source impedance progressively dropping as the signal is attenuated more and more.
Of course unlike a TVC, the DAC would see a lower and lower load impedance as the signal was attenuated but then the output of the DAC is a current source (or sink depending on the DAC chip) which ideally wants to see a short to ground.
se
Since a current-output DAC outputs a given current for a given signal level, reducing the value of the I/V resistor will reduce the voltage dropped across it and hence the signal level seen by whatever the DAC is driving.
One interesting aspect of this would be that the source impedance of the DAC would behave like a transformer volume control with the source impedance progressively dropping as the signal is attenuated more and more.
Of course unlike a TVC, the DAC would see a lower and lower load impedance as the signal was attenuated but then the output of the DAC is a current source (or sink depending on the DAC chip) which ideally wants to see a short to ground.
se
Wouldn't you need incredibly small resistors for the smallest settings?
If we limit the 0db setting to 100 ohms (and some people recommend even smaller values, like 10-20) then wouldn't say a -60db setting need something like... well I forget the math offhand but some tiny fraction of an ohm?
If we limit the 0db setting to 100 ohms (and some people recommend even smaller values, like 10-20) then wouldn't say a -60db setting need something like... well I forget the math offhand but some tiny fraction of an ohm?
I would guess you can get down to the sub-Ohm level by switching in more parallel resistors. Eventually you'd approach the resistance of the switch/relay or the pcb traces.
Anyway the original idea seems weird, since you'd get the maximum output at the highest resistance and hence the furthest from the ideal virtual ground.
Anyway the original idea seems weird, since you'd get the maximum output at the highest resistance and hence the furthest from the ideal virtual ground.
jwb said:
Yeah, but if you're going to use passive I/V conversion at all you've already thrown that "ideal" virtual ground out the window, yes?
At least in this case, everything except 0dB of attenuation will have you closer to that ideal. Otherwise, the I/V resistor remains fixed at all times.
se
My understanding is as follows:
1. The process used for DAC chips is not typically good for analog circuitry such as amps - hence current output amps typically offer the best performance
2. The current output from such a DAC is very sensitive to impedance and needs to be buffered so it sees a "ligh load", typically held at a particular voltage. Hence a resistor as an IV without a buffer in front is not considered optimal in traditional datasheet terms.
Petter
1. The process used for DAC chips is not typically good for analog circuitry such as amps - hence current output amps typically offer the best performance
2. The current output from such a DAC is very sensitive to impedance and needs to be buffered so it sees a "ligh load", typically held at a particular voltage. Hence a resistor as an IV without a buffer in front is not considered optimal in traditional datasheet terms.
Petter
Spartacus said:
True. Though it ain't exactly distortion-free voltage. Even if the resistor were perfectly linear, the current output circuitry of the DAC will fare worse in terms of distortion as you increase the load impedance that it's driving.
Increasing the load impedance seen by the DAC's output is rather like decreasing the load impedance seen by a voltage source.
But there is a certain elegance to replacing an active I/V stage with a simple passive resistor.
se
Petter said:
Well, yeah, typically the I/V stage is an active one with the output of the DAC driving the virtual ground at the inverting input of an opamp. But many prefer the sound of passive I/V conversion using nothing more than a fixed resistor.
I'm just thinking that if one is going to take that approach, instead of using a singular resistor of a fixed value for the I/V conversion, you use a variable resistor or switched resistances? This would seem to optimize the performance of the passive I/V approach.
se
Hi!
If one would change that resistor in the I/V conversion the dac chip would not have the same working point at all levels of the output. So , it won't work the same if it is working loud and if it is working low. Like said, dac's output is very sensitive and in my humble opinion the dac wouldn't like that idea in the first post of this thread..
daniel
If one would change that resistor in the I/V conversion the dac chip would not have the same working point at all levels of the output. So , it won't work the same if it is working loud and if it is working low. Like said, dac's output is very sensitive and in my humble opinion the dac wouldn't like that idea in the first post of this thread..
daniel
Doede Douma (http://www.dddac.de/) exerimented a lot about I/V resistor. He proposed also a project with a I/V resistor as the attenuator (http://www.dddac.de/files/DDDAC31.pdf).
I think that your idea is not so bad but the highest value of R should be lower than 10 ohm (ideally ...4-5 ohm). It has been demonstrated that > 100 ohm produces degradation of the signal (http://www.dddac.de/pcm63/DAC_I-V_Resistor.htm).
Unfortunately, a low R value gives a small signal and, consequently, the signal/noise ratio is reduced.
Probably a low value resistor (4-5 ohm) followed by a step-up transformer could perform better ... (http://hjem.get2net.dk/aaholm_audio/index.htm).
I think that your idea is not so bad but the highest value of R should be lower than 10 ohm (ideally ...4-5 ohm). It has been demonstrated that > 100 ohm produces degradation of the signal (http://www.dddac.de/pcm63/DAC_I-V_Resistor.htm).
Unfortunately, a low R value gives a small signal and, consequently, the signal/noise ratio is reduced.
Probably a low value resistor (4-5 ohm) followed by a step-up transformer could perform better ... (http://hjem.get2net.dk/aaholm_audio/index.htm).
sparkle said:
Well that pretty much holds true for most everything.
Like said, dac's output is very sensitive and in my humble opinion the dac wouldn't like that idea in the first post of this thread..
Hehehe. Well I really don' t care what the DAC likes. I'm only concerned with what I like.
There are already plenty of passive I/V DACs out there and many people like them and even prefer them over more technically "ideal" solutions.
se
Antonio Tucci said:
Thanks, Antonio! That's just the sort of thing I was looking for!
I was pretty sure someone had done it before, it's too much of a no-brainer. Just wasn't aware of anyone who had.
Thanks for the info! Though I'm not looking to win any meter reading contests. I'm looking for something that sounds good to me.
I'll be doing some experimenting with this (thanks again for the 1543, Peter!) and deciding for myself how I like it or not. I started this thread primarily to see if anyone had done it before and if there might be something I was overlooking which would make it a waste of time giving it a try.
Thanks. Though step-up transformers were always going to be involved with what I had in mind.
And thanks again for the link to the DDDAC page and the switched resistor schematic. Much appreciated!
se
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