MODEL 408A – OPERATION
The model 408A is designed for moving magnet (MM) cartridges only.
To operate the preamp connect your turntable output cables to the RCA input connectors on the
preamp’s rear panel. Connect the preamp’s output RCA connectors or the stereo phone jack to your
amplifier, recorder or sound card line-input. (Red connectors are the “right” channel and black or
white connectors are the “left” channel.) Plug the 24 VDC wall power supply into an outlet on the
same circuit and as physically close as possible to the outlet supplying power to your turntable
because this will (probably) help minimize hum pickup. The power-on LED on the 408A will light.
To make the preamp as quiet (low output noise) as possible there is no on-off switch because
operation from a switched power strip is assumed.
There is a blue 5-way binding post near the input connectors that connects to only the cast aluminum
enclosure. You may get a lower amount of hum at the preamp’s output if the binding post is
connected to the turntable’s frame with a separate wire. Also, the turntable’s power plug polarity
can make a difference in hum pickup so try reversing the plug in the outlet and keep the position
with the lower hum. It is helpful to look at the preamp’s output with a computer sound card (use the
line-input) and a spectrum analyzer program.* This lets you see and minimize the output hum and
noise. Don’t forget to turn on the turntable motor before making you hum measurements.
Adjust the Volume control (gain) as needed. The maximum gain is 52, 56 or 60 dB as set by four
easy-to-change jumpers on the preamp’s circuit board. There is a diagram showing how to set the
jumpers on the under-side of the top cover plate and also in this User Guide. Don’t set the max gain
higher than necessary because this can cause output signal distortion. 52 dB (the factory setting)
should be sufficient for a pickup cartridge with a 5 mV output (see your cartridge spec sheet). Use
a higher max gain if your cartridge has a lower output or if your sound card’s line-input has a low
sensitivity.
Turn on the 20 Hz Rumble Filter by flipping its toggle switch up. The filter is bypassed when the
switch position is down.
The original model 408 was designed at the time when CD’s were the main restoration media so
lowpass filters were included in a preamp’s output to limit the frequency response to 22 kHz. This
corresponds to the Red Book CD standard which uses a 44.1 kHz sampling rate. The 408A filters
can be bypassed with the front-panel toggle switch for wider-range listening and recording higher
resolution digital files. The filters are bypassed when the switch is in the down position.
* A good choice is TrueRTA (TM), Real Time Audio Spectrum Analyzer. It can be downloaded
from www.trueaudio.com.
(26 Seiten)
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