Operating Manual

Fredenstein HD Reference Preamplifier
Unfortunately, standard audio test equipment is unable to cover the complete range of the HD
Reference. The frequency response of the HD: 10Hz to 50kHz within 0.1 dB at ALL gain settings,
-3dB at 2Hz and 900 kHz. At most gain settings, it becomes even flatter.
Figure 1: Frequency response. At first glance, it may look like the HD Reference is down 13-16 dB
at 2 Hz, which might not be so bad because the graphs are still dead flat at 10 Hz, but there are 3
lines virtually on top of each other and one is the graph of the Audio Precision APx515 measuring
itself. Only the +60 dB gain setting (red line) shows any visual departure from the others (-0.5 dB at
5 Hz, -2.5 dB at 2Hz). With 40 dB gain, you can see the HD Reference measured as flat as the AP
within 0.1 dB from 2 Hz to 80 kHz, which were the frequency limits of the AP. Actually it stays
very flat to about 10 times that. Even 10 Hz and 100 kHz square waves still look respectably square.
Figure 1
Figure 2: Noise floor at 64 dB gain. The most remarkable quality in this graph is the flatness. All
amplifiers are guilty of 1/F noise which shows up as increased noise at low frequencies. The 1/F
noise corner of the HD Reference is typically 10Hz which ranks among the best. Very flat HF too.
Fredenstein HD Reference Manual V1.0 Feb 5, 2016 Page 5