Audio Engineering

878 Chapter 30

isolate the oscillator circuit from the load and to increase the output voltage level to, say,
10 V rms. An output attenuator of the kind shown in Figure 30.5 would then be added to
allow a choice of maximum output voltage over the range 1 mV to 10 V rms, at a 600- Ω
output impedance.

A somewhat improved performance in respect of THD is given by the “ parallel T ”
oscillator arrangement shown in Figure 30.6 , and a widely used and well-respected low-
distortion oscillator was based on this type of frequency-determining arrangement. This
differs in its method of operation from the typical Wien bridge system in that the network
gives zero transmission from input to output at a frequency determined by the values of
the resistorsRx , Ry , and Rz and the capacitors Cx , Cy , and Cz. If Cx  Cy  Cz /2  C and

 





C 2

C 1

C 4

R 1

R 3

R 5

R 9

Q 1

JFET N

A 1

Op. amp.

R 2

R 4

R 8

A 2

Op. amp.

D 1

C (^3) D 2
C 5
X
Y
R 10
(R 7 )
R 7
R 6
Sinewave output
0V
Figure 30.4 : Amplitude control by FET.