B˛ Spice
Case Study: Modeling Headphone Amplifiers in B˛ Spice
Since 1990
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Windows:
B2 Spice
A/D v4 Pro
B2 Spice A/D v4 Std
B2 Spice A/D v4 Lite
Digital Logic
Macintosh:
B2 Spice
2.1
B2 Logic 3.1
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Beige
Bag Software, Inc.
phone 734.332.0487
fax 734.332.0392
info@beigebag.com
Because the amplifier uses a global feedback loop, it is sensitive to internal time constants. If the amplifier shifts the phase of the output too far of the mark, the amplifier becomes an oscillator, but by carefully choosing internal time constants, we can “tune” the low frequency response in much the same way as a low-pass filter’s peaking is tuned. For example, decreasing the output capacitor’s value by tenfold (47µF) causes a +4dB peak at 30Hz, as shown below.
What is the advantage to creating a peaking low-pass filter out of our headphone amplifier? Two advantage result: the first is that the output coupling capacitor value and, hence, its size (and cost) can be reduced; second, a bass boost is often needed to compensate for the thin sound most headphone produce. The remaining questions are how much peaking and at what frequency. Because the ear is less sensitive to low frequencies, it takes a few dBs of boost for us to hear the effect, say +3 to +9dB; and because little recorded music (excepting Classical and Rap) contains much information below 50Hz, 50Hz to 70Hz would be a good target frequency. B2 A/D Spice’s “Parameterized AC Sweep” test comes in handy here, as we can specify a beginning and ending value for capacitor C2 and have B2 A/D Spice run a series of simulation with C2 incremented after each run.
It looks like the second plot reveals the right value (10µF), as its displays a peaking of +6.7dB at 58Hz. By the way, in order for this technique to work, the load impedance must remain fixed. In other words, if you use 300-ohm headphones, the peaking will be the same, but it will be at 5.8Hz, not 58Hz. Can we adjust the amount of peaking, while not altering the frequency at which the peaking occurs? No, as altering capacitor C1’s value also shifts the frequency. Still, it is much easier to tweak values in B2 A/D Spice than it is to actually de-solder and re-solder a circuit 20 times to find the right values.
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