First post, by clb
recently I learned something new about potentiometers that I did not know before, so thought to write a story about my experiences, and where my learning ended up with.
On a 8088 Turbo XT clone that I am restoring, there was an audio volume knob set up at the ISA slots on the back. Some 30 years out in cold storage had broken the potentiometer and it would no longer work properly. See the first picture "volume_knob_and_speaker.jpg".
The PC speaker looks like an 8Ohm impedance 2.25" speaker, has the writing "8Ohm 0.25W" on it. In particular, not the piezo buzzer. The speaker connects to pins 1 and 2 of the four-pin connector on the motherboard, and sounds great at max volume when there is no potentiometer connected in between.
Having a volume knob on the PC speaker seems like a great idea, and I thought that I'll do a great improvement and switch the volume knob to the front, so that adjusting audio volume will be really easy to do then. "I'll just go and order a suitable pot and all job done", I thought.
Ouch, turns out that sourcing a suitable pot seems like an impossible task.
First, measuring the old pot gave a resistance of ~10kOhms max, and given that this is an audio application, it seemed obvious that I'll order a 10kOhm logarithmic pot and that'll fit nicely.
When my order arrived and I tried it out, I would get good max volume at 100% of angle, and practically silent at 0% angle (though still tiny bit audible if one sits next to the machine and listens carefully, and there's no background noise), but surprisingly a sharp cutdown of volume when turning ten degrees or so, and already at 90% max setting, audio would be really quiet.
I was not really able to understand this, I thought that logarithmic pots (or "audio pots" as they are sometimes called) are exactly for this kind of volume setting purpose, to avoid the sharp audio volume cutoff that linear pot would have.
At first I thought that maybe I got the wrong pot and it was not a logarithmic pot after all. So I measured its resistance response curve between 0% and 100%, and got the leftmost graph in "pot_response_curves.png".
Shrugging it off, thinking that maybe I got confused somehow on the theory - maybe something is different since this is a PC speaker we are talking about and not "real" audio (whatever that distinction would mean...), I ordered a linear 10kOhm pot, thinking that that's what I'll need then.
Well... that potentiometer arrived and tested it, quickly realizing that it is no better - in fact - it was even worse than the logarithmic potentiometer: audio volume would fall off really sharply in a few degrees. Measured the linear pot's response curve, which naturally was linear. See the middle graph.
What on earth's going on?! Only here, (after also testing 5kOhm linear and log potentiometers, with only marginally better success) I actually paused to think what kind of response curve I'd really want to have to get a linear sounding audible response.What I would have liked was a pot that at the loud volume end, the resistance should change modestly, and at the quiet volume end, the resistance should change sharply.
Referencing that to the response from the logarithmic pot, I realized that if I wired the pot from the opposite side from the three legs, that would give me perfectly linear audible response, but with the adverse effect that the handedness would now be wrong: max volume would occur at the far left end, and minimum volume would be at the far right end, opposite to how volume knobs are generally expected to work. The response curve from this wiring is shown on the far right in the graph.
What I'd want is that same response curve, but mirrored to opposite handedness. Googling around, I learned that this kind of opposite handedness potentiometer is called "reverse logarithmic" or "reverse audio" or "anti-log" potentiometer - so that's what I'd need.
Got a 10kOhm reverse audio potentiometer, wired it in, and success! Well, 90% there. Handedness was now right, but I was a little bit underwhelmed about the volume range.. the PC speaker becomes quiet at maybe around 7kOhm or so. So the 10kOhm pot turned out to be a little bit too much, so that maybe 25-35% of the range at the end wasn't that useful. It works great, but since retro restoration projects like this end up seeking for perfection, I did not stop there.
That converged me to use a 5kOhm reverse audio potentiometer, which is just perfect! Well, 95% I'd say. The volume range is really nice, no sharp cutoff, perceptually linear, and the whole range gives a meaningful control. However at 0% volume, the 5kOhm resistance is not quite enough to completely mute the speaker. Possibly when the speaker is enclosed inside the chassis, it might sound practically mute (haven't tested closed chassis yet).
While I had been searching for different potentiometers, I learned about some pots that have on-off switches in them, such as this Taiwan Alpha 5kOhm Audio potentiometer seen in image "5k_switch_pot.jpg". The on-off switch resides at the left end, so that one can wire it that turning the whole pot off cuts off the circuit.
This kind of potentiometer with an on-off switch would be perfect! At 5kOhm the volume is really quiet, but still slightly audible, so switching the circuit off completely would give a nice way for complete audio control.
However, despite a lot of searching, I have not been able to find such a "5kOhm Reverse Audio Potentiometer With an On-Off Switch at the End" being manufactured anywhere 🙁 It even reads as theoretical like the elusive Rubber Chicken with a Pulley in the Middle. God damn.
So for now, I have had to settle with a 5kOhm Reverse Audio Potentiometer Without an On-Off Switch at the End. You can see the final image at "final_result.jpg". Drilled the pot into the CF<->IDE adapter chassis since I did not dare to drill into the original 5.25"->3.5" enclosure chassis. I'll add a "VOL" sticker below the knob, and it oughta stand out fine from the CF card.
I won't be able to completely mute the speaker on this build then, but why would I want to do such a blasphemy anyway 😜
I am still not 100% sure why this application needs a reverse logarithmic potentiometer rather than a general logarithmic potentiometer, but it works, so I'm happy.
Have you had a pot adventure similar to this? (phrasing..) Which item did you end up with?