Operating Manual

Fredenstein HD Reference Preamplifier
Figure 2
Figure 3: Total Harmonic Distortion. This graph shows the 1 kHz THD as the input signal is
stepped over a 86 dB range. Two gain settings are shown, 0 dB and +40 dB. Even with 40 dB of
gain, distortion is typically near -100 dB which translates to 0.001%. You can see a smooth gradual
climb after the output passes +5 dBu on the +40 dB gain setting.
Figure 3
Figure 4: THD&N Ratio.
Changing the measurement to “percentage” and allowing for “noise”. With the noise floor, the
actual THD&N becomes 0.003% near +10 dBu for the 40 dB gain setting, and 0.001% near +15
dBu output level for the 0 dB setting. The rising slope towards the left simply shows that when the
input signal is very low, that the noise floor represents a relatively a larger percentage of the output.
We can point out, that if you compare other graphs of this type, most show a lowest point between
+20 dBu and +25 dBu, then rise suddenly and sharply as the device nears hard clipping. Notice the
gentle rise in the HD Reference. Tube circuits also often tend to have this kind of curve.
Fredenstein HD Reference Manual V1.0 Feb 5, 2016 Page 6