Fredenstein HD Mic Pre

The Fredenstein HD microphone preamplifier utilizes a completely new approach to microphone
level amplification. Everybody in the recording industry is now using 96 kHz or 192 kHz sampling
rates while tracking, but still using equipment, which cannot provide the frequency response or
bandwidth provided by modern A/D converters. The HD Pre overcomes these limitations and yields
a new sound quality. Especially for wider bandwidth sources, like acoustic instruments, voices, and
drums, the HD pre plays in its own league. It is the preferred choice for classical recordings as
well.
Besides incorporating a fully discrete signal path and a Class-A balanced output stage, the HD Pre
uses a revolutionary current amplification circuit, which neutralizes limiting internal transistor
capacitors. As a result, the HD Pre features an unprecedented bandwidth of over 900 kHz. The
screen shot below shows the -3dB point at 917 kHz.

The new current amplification circuit does not need any negative feedback which improves the
impulse response dramatically. The next screen shot shows the square wave response at 106 kHz.
The slew rate of the output stage is less than 400ns for a 12 Vpp output signal, which equals to 30 Volts per microsecond.

The input impedance of the HD Pre is standard 200 kOhms and can be reduced to 1 KOhm by
activating the LOW-Z function. This high input impedance avoids loading the microphone output
transformers and yields in most cases sonically superior result. In any case, the engineer can switch
between the two settings to find the desired result without any fear of damaging the microphone.
But the HD also performs excellent in more traditional amplifier tests:
Unfortunately, standard audio test equipment is unable to cover the complete range of the HD Pre.
Above is the frequency response of the HD: 20Hz to 20kHz within 0.1 dB, -3dB at 7Hz and 900
kHz.

The output reference level is + 4dBu, the standard American studio reference level.
So THD plus Noise is at -71 dB at 64 dB gain which equals to an input noise of -135 dB at 50
Ohms generator output impedance. The total distortion and noise ratio is excellent at 0.0082 %.