Changing Hi-Blend to Flat Blend

Some FM tuners provide stereo noise reduction with a switch labeled hi-blend. This function blends the left and right audio channels at high frequencies to reduce background noise on weak signals. Since stereo noise originates mostly in the L-R subchannel, reducing interchannel differences lowers it. Separation at lower frequencies remains mostly intact. The main drawback of hi-blend is an unnatural smearing of acoustic images as instrument harmonics collapse to the center while their fundamentals stay put. In addition, hi-blend does not reduce low-frequency noise, which predominates in stereo. Finally, it rolls off the overall spectrum.

This is a typical hi-blend circuit. When blend is invoked, FET Q202 switches capacitor C215 across the left and right signal paths. In conjunction with the differential source resistance, the capacitor forms a single-pole lowpass filter that reduces the differences between the channels at higher frequencies.

Replacing the blend capacitor with a resistor converts a hi-blend circuit to a flat blend that combines the channels uniformly with frequency. Flat blend narrows the width of the acoustic soundstage, positioning the instruments closer to center, but their spatial images are not unnaturally smeared, the spectrum is not rolled off, and low-frequency noise is reduced. The main drawback of flat blend is that it may provide less noise reduction for a given soundstage width, which is determined primarily by instrument fundamentals. If you do little listening midway between the speakers where the stereo image coalesces, you may be better off with hi-blend.

I usually install a 10kΩ trimpot as the blend resistor and adjust it for 9.4 dB of separation. This small figure provides a surprisingly good stereo image with 6 dB of noise reduction, a noticeable improvement. The following table gives other trade-offs between noise reduction and separation. The soundstage width figures are for the speakers subtending a 60° angle and are based on Fig. 16 here. 

     Noise      Stereo    Soundstage

Reduction  Separation    Width

   3 dB      15.2 dB      74%

  4         12.8         67

  5         10.9         60

  6          9.4         53

  7          8.2         48

  8          7.1         42

  9          6.2         37

 10          5.4         33

 11          4.7         29

 12          4.1         26

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Updated May 11, 2008