Dynamic Bass Boost

Posted on Feb 5, 2014

A while ago I was doing some fairly trivial repairs on a Philips portable radio/CD/cassette player. It had a sticker on it claiming that this particular product features something called Dynamic Bass Boost. From the outside this means an extra switch, that when turned on, causes the music to sound better (well, different). It appears that this is

Dynamic Bass Boost
Click here to download the full size of the above Circuit.

just another marketing name for a filter that tries to compensate for the frequency response of the human ear (sometimes it`s called Loudness, my Sony calls it Dynamic Sound Generator). The idea is that the ear is less sensitive to low and high end of the spectrum and that the sensitivity varies with the volume. So to compensate you insert a filter into the audio amplifier with a transfer function that matches the inverted ear sensitivity function. I was curious how exactly that filter is implemented, so I disassembled the radio a bit further than it was strictly necessary for repairs and had a look at the circuits. Judging by the patent application I was expecting something complicated, with active filters and such. This little passive filter circuit is all there is behind that button. It`s implemented on a small circuit board that`s attached directly to the rotary logarithmic potentiometer for volume adjustment (R1 and R2 on the picture) and is duplicated for each channel. Input is marked Ui and output is Uo. The button merely shorts C2 to the ground. Unfortunately, the capacitors and resistors didn`t have any recognizable markings on them, so I wasn`t able to simply read their values. Instead of measuring each component separately, I measured the circuit`s frequency response using a signal generator with frequency sweep and a digital oscilloscope with a Fourier transform: The potentiometer has a tap at around on third of...

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