September 2012
When I was in high school, there was a Newcomb institutional phonograph — 12-inch speaker and vacuum tube circuitry. My friend pointed out that when the volume control was not rotated clockwise far enough, the music was "thin." There came a point, however, that with further rotation of the volume pot the sound became full and lush. I would love to know what was going on in the circuit to cause that behavior.
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I suspect that the speaker cone is warped and sticking. It doesn't produce much sound until the power is enough to break it loose, then it sounds OK.
The circuit probably employed a tapped potentiometer (volume control). After the wiper passed a certain point on the control -- usually the center point on the range -- circuitry connected to the tap became the dominant factor in determining frequency response.
Older amplifiers had a "Loudness" control for the volume control. You may see a "Loudness" switch on some receivers or amps. The loudness control is a pot with a tap to which a resistor/capacitor combination was attached to boost bass at low volume levels to compensate for the human ear's lack of sensitivity.
It would appear that this phono did not have a loudness control, rather a simple three terminal pot.
The "thin" sound being described is not due to a circuit, but due to the fact that our ears hear lower and higher frequency sounds less well at lower volumes. A search of the net on "Fletcher Munson" will show the typical contours.
That said, the question becomes: How do we compensate for it? In audiophile equipment a tapped volume control and a loudness compensation circuit (or digital equivalent) are the preferred method. However, it is easy to retrofit existing equipment to provide very good compensation.
The image attached is a model of a volume control circuit with loudness compensation. Consider the volume control to be an attenuator as a signal source to the next stage, and the low pass filter to be a second signal source.
In practical application, a 100K volume control is typical. At higher frequencies the Cfilter is essentially a short circuit so we need to be mindful of the lower impedance load on the previous stage. In practicality, I find that an Rfilter of 12 to 18K, and Cfilter of .056µF to .082µF work well. Rsum may be from 39K to 120K.
In my 70's Technics reciever, I use values of Rfilter = 12K, Cfilter = .056µf and Rsum = 120K. This results in excellent tonal balance at virtually all volume levels. If I want a warmer tone reminiscent of vintage Grundig tube radios, I either reduce the Rsum or pick a corner frequency that provides more lower midrange boost at lower volumes.
Scale values appropriately based on the volume control resistance value. Starting off with trimpots for the Rsum and Rfilter is recommended to get to the tone effect (or percieved flatness across volume levels) to that desired.
If I understand you, this may be because of the hearing response to various frequencies at different loudnesses. See Fletcher Munson curves on Wikipedia. Essentially the ear hears the middle frequencies on the audible range better than the highs and lows at low volumes. At higher volumes hearing response gets flatter and flatter.
Older sound systems did not have a way of compensating for this except manually with base and treble controls. Some modern ones may, but I don't see how because the sound pressure level is so dependent on loudspeaker efficiencies which now can range from a fraction of 1% for compact speakers to 20% or so for a full-sized corner Klipsh.
Thus, systems in which the amp is sold separately from the speaker cannot know where the sound power level will be. The old system you speak of had a specific built-in speaker and thus would not have suffered that problem. Therefore the thinness of the sound you heard at low volumes could well have been pure Fletcher Munson effect.
It should be noted, as it is on Wikipedia, that those curves have been refined some in later years but the phenomenon still exists pretty much as described by Fletcher and Munson. I do not see why a solid-state amp would not exhibit the same phenomenon.
