B² Spice

Case Study: Modeling Headphone Amplifiers in B² Spice 

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The DC voltage is set to 0V and the peak amplitude of the AC signal is set to 1V. We can change the frequency to 100Hz or 120Hz to mimic an actual power supply’s noise frequency, but as the Fourier analysis is already set to 1kHz, we will leave that value in place. Now we run a “Transient and Fourier” test on our circuit (with a 300-ohm load).

To convert the Fourier graph to a PSSR graph, we need to define a new plot line: db(mag_v11). Looking at this new plot reveals that the PSRR figure for this amplifier is –20dB (if the AC voltage source’s voltage doesn’t equal 1 volt, the results will be wrong, as 1V is the reference voltage). Now, while –20dB is not that not great, isn’t that bad either, but it does imply the need for a well-filtered power supply or even a regulated power supply. As for the frequency response, it is plenty wide (within 1 dB from 10Hz to over 100kHz), as can be seen below.

In fact, capacitor C3’s value is needlessly too high for a 300-ohm load; 47µF would be a better value. However, if you plan on playing a variety of headphones on this amplifier, then keep the original value, as it is needed for the 32-ohm headphones. Really, the moral of this story is that driving a 32-ohm load requires using multiple output triodes in parallel, say three 6922s, to increase the gain and lower the distortion and output impedance. But with 300-ohm loads, this tube headphone amplifier will please many picky listeners.