Allen Lein has created another handy device that can be built in an afternoon with a minimum number of readily available components. His audio generator is an essential tool for measuring distortion in audio amplifiers.
Al’s design is based on the classic Wien Bridge circuit that was the foundation of Hewlett-Packard’s 200 series of audio generators.
Ironically, it was H.H. Scott, of Dynaural Noise Suppressor fame, who came up with and patented the basic idea while working for the General Radio Company. G.R. licensed the idea to a couple of college kids from California, William Hewlett and David Packard in 1939.
“Hewlett and Packard had been casting about for products that might be manufactured by their newly formed firm.
Hewlett was working on his master’s degree with a small group of students studying various uses for feedback in amplifiers. Hewlett’s contribution was a feedback oscillator stabilized by a nonlinear resistance (a tungsten-filament lamp) in one arm of the Wien bridge. Tungsten lamps had been used a year before in a crystal oscillator by a Bell Labs researcher, and of course the Wien Bridge was not new. But Hewlett was the first to bring all the elements together in a practical RC circuit.” The Model 200 was a featured product for the company for nearly a half-century. “By the time the 200CD had disappeared from the H.P. catalogue in 1986, perhaps 500,000 of that model alone had been produced.” (from an Alan Douglas article in the Antique Wireles Association's Old Timer's Bulletin, May 1996, pgs. 50-51)
Here's a stripped-down version of the oscillator section of an H.P. 200 A/B. Tubes V1 and V2 comprise a two-stage resistance-coupled amplifier which is caused to oscillate by the positive feedback network (R1, C1, R2, C2). This is a classic Wien Bridge circuit. By using variable capacitors it is possible to tune the oscillator over a wide 10:1 range, and by using a switching arrangement to select different values of resistance for the network, several ranges are given to the oscillator.
Negative feedback is used in the oscillator section in order to minimize distortion and to obtain a very high order of stability. The amount of negative feedback is determined by a resistance network (R12, R11, R7), one element of which is nonlinear (R7, the 3-watt lamp in the cathode of V1). The lamp controls the amount of feedback in accordance with the amplitude of oscillation and consequently maintains the amplitude of oscillation substantially constant over a wide frequency range. The negative feedback also keeps the operation of the system on the linear portion of the tube characteristic. It is notable that the lamp has sufficient thermal inertia so that it operates well even at low frequencies.
Here's a Wien Bridge circuit defined:
The Wien Bridge Oscillator uses a feedback circuit consisting of a series RC circuit connected with a parallel RC of the same component values producing a phase delay or phase advance circuit depending upon the frequency. At the resonant frequency ƒr the phase shift is 0. Consider the circuit below: The above RC network consists of a series RC circuit connected to a parallel RC forming basically a High Pass Filter connected to a Low Pass Filter producing a very selective second-order frequency dependant Band Pass Filter with a high Q factor at the selected frequency, ƒr.
If we redraw the above RC network as shown, we can clearly see that it consists of two RC circuits connected together with the output taken from their junction. Resistor R1 and capacitor C1 form the top series network, while resistor R2 and capacitor C2 form the bottom parallel network. This is starting to look a lot like the H.P. circuit.
Lein's oscillator uses an operational amplifier circuit that starts something like this:
The output of the opamp is fed back to both inputs of the amplifier. One part of the feedback signal is connected to the inverting input terminal (negative feedback) via the resistor divider network of R1 and R2 which allows the amplifier's voltage gain to be adjusted within narrow limits. The other part is fed back to the non-inverting input terminal (positive feedback) via the RC Wien Bridge network.
The RC network is connected in the positive feedback path of the amplifier and has zero phase shift at just one frequency. Then at the selected resonant frequency, (ƒr) the voltages applied to the inverting and non-inverting inputs will be equal and “in-phase” so the positive feedback will cancel out the negative feedback signal, thus causing the circuit to oscillate.
Here's Lein's oscillator in the 400Hz position. Point A is a virtual ground, established by two 10K resistors (R4 & R5) across the 9-volt supply. The bottom line of the diagram is negative in respect to point A.
Compare this to figure 5 and the Wien Bridge components are arranged the same, with the high-pass filter from the output of OA1b (pin 7) to the non-inverting input (pin 5), and the low-pass filter from pin 5 to ground (point A).
The voltage divider (R1 & R2) in figure 5 establishes the gain of the opamp. In Lein's design, R1 is 10K but R2 is replaced by a 2N3819 JFET in series with a 4.7K resistor. The JFET acts like a variable resistance controlled by the second opamp, OA1a.
An LM385, precision "voltage shunt-regulator" IC, is used like a zener diode in this circuit, presenting negative 2.5 vdc at the bottom of the 220k resistor. The output from pin 7 of the first opamp (OA1b) is rectified by the diode (D1) and filtered by the .1 uf capacitor (C10). When the voltage from 100k resistor (R10) pulls pin 2 up to zero volts, both the inputs of the op amp are equal. The voltage from the 330k connected to output pin 1 working against the parallel combination of the 100k and 220k determines the gain of OA1a. The .1 uf cap across the 330k is to slow the response of OA1a.
Here's the complete schematic of Allen Lein's 400/1000 Hz generator.