DNL unit developed by Philips back in the old days, found this one in an old cassette deck and decided to trace it. What it does is that it cancels out signals in a certain band (noise, in this case at about 6kHz) that is below a certain treshold (starts kicking in at about 10mV). If the input signal is too weak, a signal of the opposite phase is developed and mixed with the original signal, effectively canceling it out. The DNL function is deactivated by closing the switch.
