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In Reply to: RE: Output impedance of a DAC to input impedance of Power amp?????? posted by John Elison on August 10, 2014 at 15:59:22
Thanks John. That is encouraging. I'll post back here if it works out ok.
I love my tubed pre-amp but really want to simplify my system. Having a tubed DAC I don't feel I need any more 'tubeness'. The DAC is very smooth in the treble. It seems with high output sources nowadays, and if you are using one source either a DAC or streamer, the old style pre-amp is going out of fashion? I think the passive pre-amps that don't work out are fitted to high ouput impedance sources (500 ohms+) or using long interconnects.
Live long and listen!
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So true Astrostar, you need to keep in mind the whole "load" comes back the the source to be able to drive the "combined load" of the passive and amp, to be able to keep the full dynamic range the source is giving out, without the load becomming a bottle-neck.
The general accepted rule is a "source output" to "combined load" ratio of 1:10> or higher which it usually is as shown in the next parragraph.
So if your source is eg: 100ohms output impedance (which most good ones are and even less save for a few doggy tube output stages) the the 1:10 ratio for this "source" to "combined load", the load should be 1000ohms (1kohm) or higher.
EG: Say the amp is the industry standard of 47kohm input impedance and your passive pre is also 10kohms then the combined load the source will see is 8.2kohm, this is a ratio (with 100ohm source) of 1:82 which is no problem at all.
All you need then is <1.5mts, of low capacitance <100pf per foot cables as well, so the highs are not rolled off and all is fine to use a 10kohm passive pre.
Cheers George
It's not the output impedance of the source, it's the output impedance of the passive attenuator that causes the problem with long cables.As long as the source is "happy" with the input impedance of the attenuator that will not be the problem but the output impedance of the attenuator vs. the total shunt capacitance that has to be driven is where the problem lies.
That capacitance includes the cable capacitance plus the input capacitance of the power amp.
The output impedance (of the attenuator) vs. the total shunt capacitance creates a low pass filter and if you don't want the volume control to "change" the sound of the system then that low pass filter's -3db point needs to be at or greater than 200kHz otherwise there will be phase shift at or below 20kHz.
P.S. I started by saying that it's not the output impedance of the source but in a way it is.
If your DAC's output impedance is really 10 ohms then there's no reason you can't use a very low impedance attenuator.
10 ohms can drive 1k ohms all day long without causing any problems at all.
A 1k attenuator will have a worse case output impedance of 250 ohms and that would drive long cable just fine.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/12/14
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