Hello,
I'd like to share a simple, passive circuit that turns a Pultec-type equalizer into oscillator. The purpose of it is to measure the peak frequency of the high frequency boost.
When we connect the output with the input (with proper phase) through simple attenuator, the positive feedback occurs resulting in oscillation at the peak frequency which can be measured with a frequency counter.
The 2200 resistor isolates the input of the frequency counter from the output of the equalizer. Diodes (silicon) limit the input voltage, so that the equalizer works in its optimum range.
If the circuit doesn't excite itself check the phase or adjust the value of the resistor that is paralleled with diodes.
The output wave is almost perfect sine wave at several volts.
To excite oscillations the knobs have to be set adequately. All pots should be set fully counter-clockwise, except the peak boost knob, which has to be set at full. The bandwidth of course should be set at the sharpest setting (high Q).
This is the perfect tool for pre-aging inductors and the make-up amplifier. Changes of the resonant frequencies can be observed at the beggining, but after a few days they settle down.
I'd like to share a simple, passive circuit that turns a Pultec-type equalizer into oscillator. The purpose of it is to measure the peak frequency of the high frequency boost.
When we connect the output with the input (with proper phase) through simple attenuator, the positive feedback occurs resulting in oscillation at the peak frequency which can be measured with a frequency counter.
The 2200 resistor isolates the input of the frequency counter from the output of the equalizer. Diodes (silicon) limit the input voltage, so that the equalizer works in its optimum range.
If the circuit doesn't excite itself check the phase or adjust the value of the resistor that is paralleled with diodes.
The output wave is almost perfect sine wave at several volts.
To excite oscillations the knobs have to be set adequately. All pots should be set fully counter-clockwise, except the peak boost knob, which has to be set at full. The bandwidth of course should be set at the sharpest setting (high Q).
This is the perfect tool for pre-aging inductors and the make-up amplifier. Changes of the resonant frequencies can be observed at the beggining, but after a few days they settle down.