Owner's Manual
3
Product information
05. 2017
Update: FG Datum: 23.03.2017
PRO-JECT AUDIO SYSTEMS
a division of AUDIO TUNING GmbH
1050 Wien, Margaretenstraße 98,
Double filtering for a noise free USB input
One of the major problems for USB DACs is the transmission of unwanted noise coming through the USB output. A lot
of research of Pre Box S2 Digital went into how to make this noisy connection more quiet. After a lot of headscratching
trial and error we found the solution to lie in active and passive filtering of the output.
We can see that the noise floor at 1kHz is about –75dB and at
10kHz it is about –110dB.
The results can be seen here:
After applying passive RF interference filtering the results are
already remarkable. Noise on 1kHz drops to –125dB, at 10kHz
Noise comes even close to –160dB. But they improve even further
when applying an active filter after the passive filtering.
This is the USB output noise after double filtering:
The results are limited by the capabilities of the measuring
equipment and noise floor lays at –160dB or even lower. There are
visible peaks at 50Hz and 100Hz which are caused by limits of the
measuring instruments. However, they are at a level of –130dB and
–140dB.
Typical USB output from personal computers:
USB output aer passive filtering:
USB output aer acve filtering: