attenuation on dac output with U-pad


2010-10-11 7:11 pm

I have a U-Pad between my dac and amp and I hope to get some confirmation on my setup:

active system with source -> minisdsp -> dacs -> amps.

Because of the digital volume control, the CS4398 was a bit noisy, because of the high gain of the Hypex amp. So I put an U-pad in between the dac and hypex ucd180 (balanced). The levels of both amps are now equal and I don't have to apply much digital attenuation.

The numbers:
I used RSeries=1000ohm
Attenuation is around -12db.

Output impedance CS4398 dac: I think 50ohms
Output impedance Hypex UCD180: ? (probably higher then 50k, they specified a maximum of 7k)
Cable capacitance is no issue (less then 10inch between dac and amp)

What I understand from U-Pads and please correct me if I'm wrong:
The dac sees 2 parallel impedances: the U-Pad Shunt resistor and the Amps input impedance. So the impedance it sees is lower then the shunt resistance. In this case max 680Ohm. Plus the series resistance of the Rseries resistors.

So my dac has a load of around 2000 + 680 Ohm (?)
so thats 53x the output impedance of the DAC.
And the Amp input impedance is 37x times the source impedance.

is this correct?