Hi Cable designer,
A dac with a digital domain volume control is the best way to go, but some need to be used in the top 25% of their full output otherwise "bit stripping" may occur.
Failing that a passive pre is the next best.
A couple of parameters need to be observed for good impedance matching, most systems comply to this.
1: The source should have an low’ish output impedance of 1kohm (1000ohms) or less, which most are.
2: The passive pre should be around a 10kohms to 20kohm potentiometer (pot).
3: The input impedance of the poweramp/s should be 33kohms or higher which most are.
4: The interconnects from the passive pre to the amp/s should be low capacitance (<100pf per foot) which most good one are and kept to 1.5mt or less.
A Quote from the master Nelson Pass on passive preamps regarding gain.
Nelson Pass,
"We’ve got lots of gain in our electronics. More gain than some of us need or want. At least 10 db more.
Think of it this way: If you are running your volume control down around 9 o’clock, you are actually throwing away signal level so that a subsequent gain stage can make it back up.
Routinely DIYers opt to make themselves a “passive preamp” - just an input selector and a volume control.
What could be better? Hardly any noise or distortion added by these simple passive parts. No feedback, no worrying about what type of capacitors – just musical perfection.
And yet there are guys out there who don’t care for the result. “It sucks the life out of the music”, is a commonly heard refrain (really - I’m being serious here!). Maybe they are reacting psychologically to the need to turn the volume control up compared to an active preamp."
Cheers George
