by Victor Kampf
Despite the simplicity of its circuitry, the Speaker Emulator has the characteristics of a much more complex circuits. The frequency characteristics of the emulator are shown in the figure below.
The emulation is based on the specifications of well-known Celestion V30 speaker.
The Device Circuit Diagram
All the stages are distinguished by utmost simplicity while fulfilling all the necessary functions. When using these transistors the direct current modes are set automatically within 3 … 6V.
The first stage of T1 is a repeater with self-bias operating at current equal to the transistor initial current sink. High input impedance (about 1MOm) allows you to connect the simulator directly to high-impedance signal source.
Low-frequency response is formed by means of two filters: passive high-pass of the 1st order formed by C6 and the input impedance of the stage at T3 provide LF downturn within 6db/oct; FHP 0f the 2nd order on T2 creates a “resonant hump” within 130-140Hz, characteristic, for example, for guitar-intended Celestions.
Frequencies above 5kHz are attenuated by LF passive filter of the 1st order R6, C5. Chain C7, R12 emulates a slight increase in the speaker signal, caused by the growth of induced resistance of the speaker at frequencies above 500Hz, which occurs during connection to a tube amp.
At T3, switched by the circuit with a common source, low-pass filter is implemented with weakly pronounced “pseudoresonance” within 4kHz and the decline above this frequency -14dB/oct. T3 source is connected directly to the ground, so the stage works with substantial non-linearity, partially offset by a negative feedback. On the one hand this switching enriches the low-frequency signals with “lower” harmonics, since the compensation at low frequencies is attenuated, on the other – softens “resonance” properties of the stage in comparison with the filters on OC, resulting in more natural sound. The cutoff frequency of the filter is adjusted by resistor R14.
Notch filter based on T4 is connected directly to the previous stage and is designed to create additional “notch” frequencies within 6-8kHz, which in turn “softens” the residual high-frequency components of the filtered signal. The frequency of rejection is changed by resistor R18′.
Any low-power n-channel FET with IDSS =0.4 … 0.6 mA (zero-gate voltage current) of can be used as T1-T4.
© AMT Electronics
© Victor Kampf