A useful test box to have handy while designing and building phono preamps
is one which will yield the opposite of the playback characteristic, i.e.,
an inverse RIAA (or record) characteristic. The circuit (Fig. 1) is achieved
by adding a passive filter to the output of an LM387, used as a flat response
adjustable gain block. Gain is adjustable over a range of 24 dB to 60 dB
and is set in accordance with the 0 dB reference gain (1 kHz) of the phono
preamp under test.
For example, assume the preamp being tested has +34 dB gain at 1 kHz. Connect
a 1 kHz generator to the input of Fig.
1. The passive filter has a loss of 40 dB at 1 kHz, which is corrected
by the LM387 gain, so if a 1 kHz test output level of 1 volt is desired from
a generator input level of 10 mV, then the gain of the LM387 is set at +46
dB (+46 dB 40 dB + 34 dB = 40 dB = X100; 10mV X 100 = 1 volt). Break frequencies
of the filter are determined by equations (1)(3)
f_{1} = 50 Hz = 1/ 2 π R_{9}C_{4} (1)
f_{2}=500Hz= 1 / 2 π R_{10}C_{4} (2)
f_{3} = 2120 Hz = 2 π R_{10}C_{5} (3)
The R7C3 network is necessary to reduce the amount of feedback for audio
signals and is effective for all frequencies beyond 20 Hz. With the values
shown, the inverse RIAA curve falls within 0.75 dB of ideal.
Fig. 1 Inverse RIAA response generator.
Fig. 2 Square wave oscillator.
The remaining half of the LM387 can be used to build the 1 kHz square wave
oscillator, and the required configuration is shown in Fig. 2. Briefly, operation
is as follows.
Resistor string R2 and R4 sets up an approximate reference point of 1.8
volts with the output low. When the output switches to high, a hysteresis
effect is achieved via resistor R5, when an additional amount of current
is fed into R4, thus raising its potential to about 2.0 volts. The 0.47 /IF
capacitor therefore alternately charges through R1 up to 2.0, volts, and
discharges through R2 down to 1.8 volts. This portion of the circuit is thus
a freerunning, regenerative comparator with hysteresis. The resistor network
comprised of R6 through R8 constitutes a 60 dB pad with equal port impedances
of 10 kilohms. This reduces the 30volt, peakto peak output level down to
about 30 mV, so it can drive the inverse RIAA generator half of the LM387
without overloading it. While this is not the world's very best square wave
generator nor most accurate inverseRIAA generator, it is economical, and
the design equations are included for those who wish to alter the oscillation
frequency of the square wave generator.
(Source: Audio magazine, Feb. 1977; by
Dennis Bohn  Written while at National Semiconductor, Santa Clara, Calif.;
now with Phase Linear, Lynnwood, Wash. 98036, USA.)
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