Switched-Capacitor Filter (SCF) based around two Maxim MAX295 8th-order Butterworth filter ICs. These filter ICs have a 48 dB/octave dropoff at audio frequencies (300 - 3000 Hz), or a maximum out-of-band attenuation of around 110 dB. This is perfect for removing 60/50 Hz hum and high frequency hiss.
Controlled with a simple variable clock generator based around the standard 555-timer IC, the 3 dB low-pass Frequency Cutoff (Fc) of the MAX295 filters is varied between approximately 300 Hz and 4700 Hz. This is perfect for most speech-frequency specific applications. The MAX295's Fc is equal to the input clock rate, divided by 50. Example: if the 555-timer is set to 165,000 Hz (165 kHz) then the MAX295's cutoff frequency will be approximately 3300 Hz.
The TL071 op-amps are for additional low-pass filtering of the MAX295's output, which will have some "digital noise" on it. They are not needed but highly recommended. Also, the MAX295s have a fairly high negative voltage current draw, so a Maxim MAX660 negative voltage converter is needed. Standard 7660-types will not work.
Schematics
- Continuously Variable Bandwidth Audio Filter (167k PNG)
- Continuously Variable Bandwidth Audio Filter (30k PostScript)
Pictures
- Picture 1 Outside case overview. Power switch and LED on the left, Fc tuning in the middle, audio input/output via the 1/4" jacks on the right.
- Picture 2 Internal overview. Front panel is on the bottom, the 4 "AA" battery pack is on the left. The +5 VDC voltage regulator and the -5 VDC voltage converter are on a separate PC board. This is done to maintain single point grounding and powering, which avoids ground loops. The PC boards are mounted on double-sided sticky tape. If ground loops persist (hum on the signal), you'll need to use 1:1 audio isolation transformers on the input/output.
- Picture 3 Closeup of the main PC board. The two MAX295s are on the upper left, with the 555-timer below them. The two TL071s are on the right.
Note / Datasheets
