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atupi 6th May 2012 08:42 AM

Output impedance of a DAC analog stage
Could someone tell me what is the output impedance of this analog stage and maybe how can be calculated ?

I'm asking this because when i'm testing my B1 buffer with my 100k input impedance amplifier sounds better if i put the buffer before my volume pot even if everyone is recommending to have buffer after the volume pot and i suspect something wrong with output impedance of this analog stage of my DAC.

With the buffer after volume pot the bass and stereo image is ruined.
Instead of LT1028 i'm using LME49720.
My volume pot in 20k log.

wwenze 6th May 2012 09:11 AM

The above stage's output impedance is dominated by the value of R7, so it's just a bit higher than 100ohm.

martin clark 6th May 2012 09:51 PM

+1 to that; and by 'just a bit' as wwenze refers you should understand that as 'only a few tens of ohms, plus R7, in the audio band'. The excess mostly depends on the exact opamp chosen for U3, but even a poor choice will be well within 100R.

So this analogue /filter stage will be perfectly happy driving your 20K pot on its own and. I think you may need to review what is going wrong in your buffer stage if putting after the pot spoils things.

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