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1960’s Style Speech Filter Circuit

Updated on February 26, 2013

1960’s Style Speech Filter Circuit

Speech filters are useful for communications equipment. High pitch heterodyne tones are a common problem with both super-heterodyne and regenerative receivers. For small transmitter circuits a speech filter will help restrict the bandwidth of the transmitted signal. These days audio filter circuits are usually constructed using op-amps. However discrete transistors are lower cost and have fewer leads to solder.

Figure 1

An easy to make speech filter circuit.
An easy to make speech filter circuit.

The Circuit

Figure 1 shows a simple speech filter of the sort that was commonly used in the 1960’s.

Q1 is arranged as an emitter follower circuit. R7, C3 and C4 create a high pass filter. Q2 together with R2, R3, C2 and C5 form a low pass filter.

For an audio source impedance of 10k the frequency response curve of the circuit is shown in figure 2.

If the audio source impedance is lower than 10k there will be some additional peaking of the response around 300Hz. This filter circuit has some peaking of the response around 3KHz with can be helpful with regenerative receivers.

Figure 2

The frequency response curve of the 2 transistor circuit.
The frequency response curve of the 2 transistor circuit.

A one transistor circuit

Figure 3 shows a one transistor circuit with gain. This circuit is ideal to use with a regenerative radio receiver as the sharp rise in gain approaching 3KHz compensates for the inherent low pass filtering action of the regenerative action.

Figure 3

A simple 1 transistor speech filter with gain.
A simple 1 transistor speech filter with gain.

Figure 4

The frequency response of the 1 transistor circuit.
The frequency response of the 1 transistor circuit.

Designing similar circuits

These days it is much easier to design similar filter circuits than it was in the 1960’s. Using tools such as the circuit simulator LTSpice and EvoSpice 4.1 (a numerical optimizer for LTSpice) you decide a suitable circuit arrangement and the target objective (frequency response curve). Then you run EvoSpice and let it find the optimal component values. For the circuits above it took less than 5 minutes per run. However I did have to change the target objectives a few time to get exactly the frequency response curve I wanted. Overall though, the design process was very simple and enjoyable.


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