Lighthouse Electric Tech Support
STA1 - Reducing Bass Boost
The STA-1 pre-amp has a bass boost of 25dB, which may be to much for someones taste. Changing just one component will lower the boost value to 16dB or 13dB.
Fig.1 shows the original STA1 bass circuit, with the bass-control potentiometer set fully clock-wise = max. bass boost.
The bass boosting occurs as the reactance of C5 increases with lower frequencies.
For clarity, the modified circuit in Fig.2 shows the essential components of the low-pass filter.
The formula for this LP-filter is:
fc = 1 / 2Pi * R * C
By increasing the value of C5, the reactance Xc will decrease, thus reducing the boost at low frequencies.
Using SPICE simulation, it was determined, that increasing C5 to 0.027uF will decrease bass boost to about 16dB @ 25Hz
With 0.033uF bass boost will decrease to 13dB @25Hz.
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