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Older amplifiers often employed a volume pot that had a 4th "loudness" pin (which I take it varied with volume up until 40% give or take after which it was fixed).

Unfortunately, newer volume pots do not include this 4th pin, so I want to find a solution through which I can get a fixed loudness control (ie: on/off, without it varying with volume).

Problem is I don't really understand how this works. Does it need a fixed resistor to ground, or a fixed resistor from the input to the loudness circuit?

A piece of the schematic showing such a circuit. I've circled the volume pot and the loudness circuit. Having had another look it it now, it seems like the loudness part is a feedback circuit that takes some signal from the 4th pin and applies it back on the input signal (when switched on).

enter image description here

CatalinM
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  • Just one extra pin? How was it wired up? To a button? – DKNguyen Dec 29 '21 at 14:49
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    @DKNguyen 1 pin per channel, to be specific. It's usually connected to a "loudness" EQ circuit, through a switch. I'll try to add a schematic to the question to show this. – CatalinM Dec 29 '21 at 14:57
  • Please do. You could also use a SPDT button or toggle to swap the pot for a resistor if all you want is a fixed volume button. – DKNguyen Dec 29 '21 at 14:58
  • @DKNguyen I still need a working volume pot. I just need the loudness part to be fixed, but I'm not sure where to add the extra resistor (pot input or pot gnd) and what value should it have? – CatalinM Dec 29 '21 at 15:32
  • I said *swap* so make the switch electrically replace the pot with the resistor. That's why I keep saying SPDT switch (or DPDT since you have two channels I guess). You'll need to set the volume you want with the pot and measure that for the resistor. – DKNguyen Dec 29 '21 at 15:33
  • I can measure the pot, sure, but which part is actually relevant? Loudness tap to input or loudness tap to GND? – CatalinM Dec 29 '21 at 15:38
  • Oh, if the pot is wired as a divider and not a resistor, then swap the pot out for a two resistor divider. So both. Whatever the pot is, replace the entire thing with something else. – DKNguyen Dec 29 '21 at 15:46
  • How will a resistor boost low frequencies at low volume? It seems to me that the single resistor bypass will just increase gain for all frequencies and risk clipping at high volumes. – GT Electronics Dec 29 '21 at 18:43
  • @GTElectronics I don't think there _any_ way a resistor can boost a frequency... we're talking about a passive circuit here - have a look at the attached schematic. From what I can understand, the circuit used to attenuate mids more when it was engaged, with a varying degree based on the loudness tap. Lacking this, I want to add a resistor and have it attenuate the mids with a fixed degree - just can't figure out the value and how to attach it. – CatalinM Dec 30 '21 at 10:49

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A "loudness" control was intended to be "bass boost" circuit - increasing the bass component of the input audio at low volume levels. Typically it was just a switch.

Treble and bass were intended to be set for mid-volume (normal) listening levels and loudness set off. Then loudness turned on if listening at low volume levels.

The more descriptive, Bass Boost (or some permutation) was eventually used as the name of this button in some brands.

Back in the 1970s, amplifier manufacturers and the engineers who designed them and reviewed them were focused on exactly amplifying the input signal (same gain across the input frequency spectrum). Humans like a bit more gain at low frequencies when the volume is low so they need to meet the specs (without loudness button) AND make the bass feel "normal" at low volume. Hence the loudness button concept was created.

Now, many TVs and audio amplifiers have various modes to do this with software. I don't think a button is needed. Just select the right mode on the device.

GT Electronics
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  • Thanks. I am aware of the why's of it (including the Fletcher-Munson curve)... it's the how that kind of escapes me. Looking at that schematic I just added, it feels like I could add a fixed resistor to the input pin of the volume pot and derive the signal from there. – CatalinM Dec 29 '21 at 15:19
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A loudness control is usually just a volume control with a tap, usually around the middle of the control's range. The tap is connected to a tone compensation network of resistors and capacitors so that at low volumes the treble and bass are boosted, and as the volume is raised they are boosted less and less.

GodJihyo
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  • Yes... that I know. But how can I "fake" that missing tap from a newer volume pot? – CatalinM Dec 29 '21 at 15:19
  • @CatalinM You can't really. So swap out the entire pot for a resistor with a SPDT button. – DKNguyen Dec 29 '21 at 15:25
  • You said in your original post that you want a fixed one, you could just make a bass/treble boost circuit and switch it in or out. Pots with a loudness tap are available, so that would be the best way to do it. The whole point of a loudness control is that it compensates for different amounts of boost at different volumes as you're adjusting the volume. – GodJihyo Dec 29 '21 at 15:28
  • @GodJihyo fixed in the sense that it will not vary with volume as the original did. I haven't found a good quality pot with a loudness tap unfortunately. ALPS seems to make one, but no one is stocking it. Therefore, I'm using a normal pot and I just need to understand how to replace this loudness tap with a simple resistor. – CatalinM Dec 29 '21 at 15:31
  • There is the Pye Black Box circuit, but it requires a dual pot. It's basically two ganged pots, the first one wired as a normal volume control, with the wiper going to the top of the second pot and a resistor and capacitor between the bottom of the second pot and ground. – GodJihyo Dec 29 '21 at 16:08
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It's going to be hard to fake.

Maybe you could use 2 potentiometers with some sort of mechanical linkage,

Or disassemble and modify a regular potentiometer using conductive ink, or Lichtenberg effect, to add a loudness tap.

Or use a microcontroller and digital potentiometers

perhaps contact a potentiometer maker that does tapped potentiometers and try to get some engineering samples.

Jasen Слава Україні
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  • It's not hard at all, as long as I accept the fact that it won't turn out the same. In the end I just want to replace the variable resistance that loudness tap from the volume had with a fixed one. – CatalinM Dec 30 '21 at 10:46