Audio Engineering

Preamplifi ers and Input Signals 173

cartridge), and this sets an input “ resistor noise ” threshold, which is higher than desirable,
as well as requiting inconveniently high values for R 1 and R 2.

For these reasons, the circuit arrangements shown in Figures 7.4(e) and 7.4(f) are found
much more commonly in commercial audio circuitry. In these layouts, the frequency
response shaping components are contained within a “ series ” type feedback network
(i.e., one in which the negative feedback signal is connected to the amplifi er in series
with the input signal), which means that the input circuit impedance seen by the amplifi er
is essentially that of the PU coil alone and allows a lower midrange “ thermal noise ”
background level.

The snag, in this case, is that at very high frequencies, where the impedance of the
frequency-shaping feedback network is small in relation to RFB , the circuit gain

Out

(h)

R 4

R 3

R 2

R 1

RFB

CFB

Rin

C 1

A 1

A 2

C 2

0V









332 K8

NE5534AN

NE5534AN

100 R

2K7

470 R

47 K 47 R

180 R

200 p

(i)

PU

4 μ 7

3K3

3no 8h8

26 K7 Out

A 2

A 1

0V



 

Figure 7.4 : (Continued)