| br>This applies to both editions of the board.
U9 should be a NE5532 or OPA2134 instead of a TL072.
U9 is the anti-aliasing filter for the input of all four BBDs. Thus one common output feeds four BBD inputs. Charge injection from each BBD add together on this common line. The clock frequencies are well above the audio range but mixing together of several ultrasonic audio signals results in beat frequencies that are within the audio range. Although, these unwanted charge injections should be nulled by the low impedance output of the anti-aliasing filter, the use of a TL072 means that the output impedance is not low enough at high frequencies. Using a 5532 or 2134 with their higher gain bandwidth allows the effect of the charge injection to be minimised.
This has been corrected in Builder's Guide versions 1.0.10 and 2.0.2.
Tony br> br>