What happened to the loudness control?

Some of my preamps (I have many, mostly vintage) have a loudness button. I lean a little to the purist side and I do not use it because I can listen at higher levels. At low levels a well-designed loudness function does improve the frequency balance, imo. Some are better designed than others. For instance H-K designed a "Phase-Correct Loudness" (as they call it) feature for their Citation 21 preamp. It boosts only the very low end and can be used to extend the low frequency response of smaller speakers, as they say in the manual. They go on to say that high end boost is not necessary, from their listening tests. A look at the schematic reveals much more than just an RC network. It is an active circuit with four transistors per channel in addition. To my ears it is not unnatural sounding.

IIRC the old Fletcher-Munson curves were superceded decades ago by a study by Robinson and Dadson (spelling may be wrong) which provides more accurate correction for changes in hearing sensitivity at lower sound levels. Of course, these are data averaged over lots of listeners, and your level-specific sensitivity is likely to be a bit different. You could have a simple audiogram to get an idea of your sensitivity across the spectrum, but that wouldn't tell you about your relative growth of loudness across the spectrum at various sound levels.

An equalizer set the way you like it seems more appropriate. I fear the god of audio purity may strike me for writing that.

db

That is what a loudness compensation control is, an equalizer, but variable with the volume. One hopes that it complementary to the frequency response variability of the ear according to volume.
I use the variable loudness compensation control on my Nak 630 pre-amp by setting the compensation at 0 and the main volume control for a natural sounding volume of about 80db or flat frequency response. When I want to reduce the volume to more background levels, I use the loudness compensation control. The main volume setting remains the same.
Works great! I think that the loudness controls on the old Macs worked in the same fashion.
One button loudness compensation is next to useless because it is fixed.
Bob P.

The reason the loudness control was dumped is simply that no one has ever come up with a circuit that worked 100% of the time at 100% of the volume settings. Consumers eventually figured this out and preferred not to own it.

Spatialking...No one uses 100% of the volume control range! The twin control scheme described by inpepinnovations@aol.com works very well.