Following Dr. Simon Bateson's line level matching circuit, (H&SR Dec 85), here is a 1 kHz test tone oscillator module to further increase the flexibility and usefulness of the H&SR patchbay.

The test tone oscillator is an invaluable tool to line up and calibrate recording equipment.

The circuit described here has a single fixed output 1 kHz at 0dBm (0.775mV RMS), but this can be modified to meet your exact requirements. Although the PCB was designed to fit the H&SR patchbay, it could also be mounted in a separate box with a switch to allow a range of frequencies to be covered. The circuit board also has provision for a battery holder and a battery low indicator, but these can be omitted if an external power supply is available. To eliminate the need for a power switch, stereo jack sockets were used and power is only applied when a mono jack plug is inserted.

The Circuit

The oscillator is based upon a Wien network built around an op-amp. R6, R7 and C1, C2 (see Figure 1) form the Wien network and this applies positive feedback at the frequency selected by the component values. If R6 = R7 = R and C1 = C2 = C then the frequency of oscillation is equal to: ¹/_2πRC.

1 kHz was used as it lies in the middle of the audio range, but references are often made at the top (10kHz) and bottom (100Hz) end of the audio range. The capacitor values C1 and C2 were chosen to allow the audio range to be covered easily by adjusting the resistor values (R6 and R7) only, (see Table 1).

To ensure stability and purity of tone, negative feedback is applied via a thermistor. If the output level increases, the thermistor's resistance decreases, which in turn decreases the output and so produces a constant output level. The output level is set by R5 at 0.775mV RMS (0dBm) into a 600Ω load.

The LED of the battery indicator will switch off when the supply falls below about 7.5v. This is because the base of the transistor, Q1, is held at 6.8V by the zener diode, D1, and the transistor will only conduct if the voltage between the base and the emitter exceeds about 0.6V. A low power LED was used in the prototype to conserve battery life. If you are unable to obtain a low power LED then a standard LED will do but the resistor R2 must be changed to 390Ω.

If an external power supply is to be used you could omit R1, D1 and Q1, and short the collector and emitter of Q1 with a wire link. The LED will then act as a Power On indicator.

Construction

Construction is very straightforward, but care must be taken to ensure that the IC, transistor, diodes and capacitors are inserted in the correct way. Care should also be taken with the thermistor as it's an evacuated glass type which are very fragile. It is best to wire through the holes provided. On the prototype the thermistor was also covered with a piece of rubber sleeving, but this is not essential. If used, the battery holder should be bolted to the board before soldering. Connections to the LED should also be insulated wire, the bare ends of the LED should also be insulated, and make a careful note of the polarity. A 6.3mm (0.25in) hole is required to mount the LED on the patchbay panel.

The module is best located at one side of the patchbay panel, where the LED can easily be fitted. For the more ambitious, a switch could also be mounted to the patchbay panel to switch frequencies.

In Use

Before connecting up check the circuit through once more. If you are satisfied then away you go!

As previously mentioned the thermistor stabilisers the output, but this takes a short time to warm up, so leave the oscillator switched on for three to four secs before taking any readings. The PCB has two output jack sockets to allow mounting in the H&SR patchbay. These outputs are connected in parallel, but please note, if they are used simultaneously then the output will be greatly reduced due to the different loading.

To calibrate your gear, first patch the oscillator's output to your mixer and note the meter reading, then connect the mixer output to the tape input and adjust the record level so that both the mixer and tape meters read the same level. You have now calibrated your mixer and recorder and, if the record levels are left untouched, no further reference need be made to the recorder's meters. You can now record a short burst of the test tone on tape and check the playback level is the same.

When you have finished remember to unplug the jack lead from the oscillator to conserve battery life. With less than 10mA consumption, (when using a low power LED), an alkaline battery should last for about 50 hours.

PCBs are available at a cost of £2.50 (including P+P) from Clive Keeler, (Contact Details).