Fredenstein HD Reference Preamplifier
(Will not apply to next revision with a single PCB) There is another LED on the Gain Display between the 2 digits that is used to indicate if a pre-
amplifier board is set to an invalid address. This is an internal setting that should be properly set by the factory. Channel 1 needs to be set with both
dip switches set to “ON”. Channel 2 needs to be set with 1 to “OFF” and 2 set to “ON”. “ON” is towards the back of the PCB and “OFF” towards
the front edge.
Technical Data:
Frequency Response :
2 Hz – 700,000 Hz, - 1 dB
1 Hz – 920,000 Hz, - 3 dB
Distortion
:
< 0.0082 at +4 dBu Output Level (31 dB Gain)
Input Noise
(EIN) :
< -128.5 dB (150 Ohms Source Impedance 64db Gain)
<-130 dB A Weighted
Input Impedance
:
> 200 kOhms or 1000 or 300 Ohms selectable
CMRR
:
> 60 dB at 54 dB gain
DI Impedance
:
> 2 MOhms
Max. Input Level
:
+ 20 dBu,
Output Impedance
:
600 Ohms
Max. Output Level
:
+ 30dBu
Max Input Signal
:
+20 dBu
(DI or Microphone Input)
+ 0dBu
Note: It bears pointing out that the HD Reference may be unique in the actual combination of the
ultra-wide bandwidth, along with the extremely low noise floor, impressive distortion specifications
plus the spectacular impulse response. Some designs sacrifice bandwidth or distortion to keep the
noise low, others have low distortion or flat high frequency response only for the low gain settings.
And sometimes the published test numbers and listening test results just do not seem to agree.
Beyond the one dimensional numbers, there are graphs and FFT plots that are even more impressive
to those familiar with testing high-gain audio amplifiers. For example, the distortion remains low
and flat well beyond 20 Khz, unlike most pre-amplifiers. Similarly the noise floor is flat and doesn't
appreciably increase in the lowest octaves (1/f noise). Also rare is that both the specifications and
the audible performance stay remarkably constant across the full 64 dB gain range. Pre-amplifiers
that depend on negative feedback to achieve good specifications (and sound) at low gains, reduce
that corrective feedback to get high gains. The HD Reference does not need or use negative
feedback.
While these specifications are impressive, this mic pre-amplifier's purpose is to facilitate recording
music and, in the final analysis, a recording engineer generally judges the level of electronic art “by
ear” and not with a test bench. The topology and feature set of the HD Reference were chosen for
reasons related to recording inspired performances and capturing music as perfectly as possible and
the nice numbers are just a byproduct.
What is an unfortunate byproduct is the heat. With positive and negative 24 volt rails, we have 48
volts and 4 true Class A amplifiers capable of driving 600 ohms. Class A amplifiers dissipate the
same heat whether there is no signal or the signal is fully driven. In the case of the HD Reference,
this means about 16 watts (8 watts per channel) of heat, so there are heat-sinks inside, ventilation
slots in the top and bottom panels and a serious recommendation in the beginning of this operating
Fredenstein HD Reference Manual V1.0 Feb 5, 2016 Page 10
Содержание HD Reference
Страница 1: ...HD REFERENCE Operating Manual...
