Small Signal Audio Design by Douglas Self

Author:Douglas Self [Self, Douglas]
Language: eng
Format: mobi, epub, pdf
Publisher: Taylor and Francis
Published: 2014-08-06T16:00:00+00:00

Figure 13.33: Frequency response of the Denon loudness control in Figure 13.32a. Complete pot rotation is from Marks 0–10, curve 0 not shown

The simulated frequency response for the Denon circuit is shown in Figure 13.33. There are three notable features: first, there is a lot of boost applied at HF, despite the fact that, as we saw above, no HF correction is actually wanted. Second, the correction curves only change in shape over the top half of the control rotation, the response being fixed below that. Third, there is a rather dramatic change in mid-band level between Marks 5 and 6, as the pot wiper approaches the centre-tap.

The almost universal inclusion of HF boost in these loudness circuits is a bit of a mystery. I can only assume that people took a quick look at the Fletcher and Munson curves, and saw increases at the HF end, but did not notice that the curves are essentially equally spaced, not varying in shape with level. I wonder if this unwanted HF boost was one of the reasons why loudness controls were so poorly regarded; in their heyday, the primary source was vinyl, with bad distortion characteristics at HF. It seems likely that the HF boost would have accentuated this.

A few other loudness schemes were used. The Sansui A60 and A80 amplifiers had separate loudness and volume pots. Newcomb and Young [9] put forward a very ingenious active control that compensated at LF only.

These loudness controls must represent the widest ever use of centre-tapped pots, which are otherwise rarely encountered. Pioneer used a centre-tapped pot with a simple loading resistor to ground connected to the tap, to emulate a logarithmic law; there was no loudness facility (see the Pioneer A-88X and the A-77X (1985)).

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