Low distortion analog filters

The D68 series of analog filters from Frequency Devices have Total Harmonic Distortion levels as low as -100 dB with near theoretical frequency responses and low noise.  The D68 filters are available in low pass and high pass versions with Butterworth, Bessel, Elliptic, and Constant Delay transfer functions.  Filters with 4, 6, and, 8 poles with corner frequencies from 1 Hz to 100 kHz can be ordered.  The filters are very small at 1.8” by 0.8” by 0.3” tall.