Audio Engineering

Preamplifi ers and Input Signals 183

with the gain of the second stage amplifi er (a TL071 IC op-amp) switchable to suit the
type of input signal level available.

7.7 Input Connections

For all low-level input signals, care must be taken to ensure that the connections are
of low contact resistance. This is obviously an important matter in the case of low-
impedance circuits such as those associated with MC PU inputs, but is also important in
higher impedance circuitry, as the resistance characteristics of poor contacts are likely to
be nonlinear, and to introduce both noise and distortion.

In the better class modern units, the input connectors will invariably be of the “ phono ”
type, and both the plugs and the connecting sockets will be gold plated to reduce the
problem of poor connections as a consequence of contamination or tarnishing of the
metallic contacts.

The use of separate connectors for L and R channels also lessens the problem of
interchannel breakthrough due to capacitative coupling or leakage across the socket
surface, a problem that can arise in the fi ve- and seven-pin DIN connectors if they are
fi tted carelessly, particularly when both inputs and outputs are taken to that same DIN
connector.

0V

470 K 2 μ (^2) Out
6n8
10 K
150 R
1 μ 5
47 K
47 K
PU
4K7
33 n
100 K
9 K47
180 p 100 R
ve
ve


0–1 K4
A 1
Q 2
Q 1
3K0
1K1
0 μ 1
2 μ 2
1 μ 5
Figure 7.12 : The “ Quad ” ultralow noise input circuit layout.