Preamplifier For Soundcard

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This circuit can be used
for inductive pick-up elements and dynamic microphones Most soundcards
have a ‘line’ input and one for an electret (condenser) microphone. To
be able to connect an inductive tape-recorder head or a dynamic
microphone, an add-on preamplifier is needed. Even in this day and age
of integrated microelectronics, a transistorised circuit built from
discrete part has a right of existence. The preamplifier described in
this short article goes to show that it will be some time before
discrete transistors are part of the silicon heritage. The preamplifier
is suitable for use with a soundcard or the microphone input of a modem.
As you will probably know, most sound-cards have input sockets for
signals at line level (stereo), as well as one for a (mono) electret
microphone.

For the applications we have in mind, connecting-up an inductive
pick-up element or a dynamic microphone, both inputs are in principle
suitable, provided the source signal is amplified as required. The
author eventually chose the microphone input on the soundcard. Firstly,
because the line inputs are usually occupied, and secondly, because the
bias voltage supplied by the micro-phone input eliminates a separate
power supply for the preamplifier. The microphone input of a soundcard
will typically consist of a 3.5-mm jack socket in stereo version,
although only one channel is available. The free contact is used by the
soundcard to supply a bias voltage to the mono electret microphone. This
voltage is accepted with thanks by the present preamplifier, and
conveniently obviates an external (mains adaptor) power supply.

Circuit diagram:

Preamplifier For Soundcard Circuit

Preamplifier For Soundcard Circuit Diagram

A classic design:

In true transistor-design fashion, the preamplifier consists of
three stages. Capacitor C1 decouples the signal received from the
microphone or pick-up element, and feeds it to the input of the first
stage, a transistor in emitter configuration, biased to provide a
current amplification of about 300 times. Together with the source
impedance of the microphone or pick-up element, capacitors C2 and C3
form a low-pass filter which lightly reduces the bandwidth. In addition,
the output low-pass, R2-C3, reduces the dynamic collector resistance at
higher frequencies. In this way, the filter reduces the gain in the
higher part of the frequency spectrum and so helps to eliminate any
oscillation tendencies.

The first, high-gain, stage is terminated by T2. Unlike T1, this
transistor does not add to the overall gain, because the output signal
is taken from the emitter (common-collector circuit). T2 thus acts as an
impedance converter, with C4 reducing any tendency to oscillation. The
output stage around T3 is a common-emitter circuit again. In it, preset
P1 determines the voltage amplification. T3 is biased by means of a
direct-current feedback circuit based on components R7 and C5. To this
is added an ‘overruling’ dc feedback path back to the input transistor,
via R6. This measure guarantees good dc stability in the preamplifier.
The circuit is small enough to be built on a piece of veroboard or
stripboard, and yet remain reasonably compact.

To prevent interference from external sources, the completed board
should be mounted in a properly screened (metal) enclosure, with the
connections to the input source and the sound card made in screened
cable. The preamplifier provides a frequency-linear response. In case
the source signal is marked by frequency correction (e.g., RIAA), then a matching linearization circuit should be used if the relevant signals are used by the computer.