I’m working on a project to adjust the volume of an old analog stereo. The volume knob works, I want to adjust the volume on top of that. From some research I found a potentiometer controls the volume. In the case of this stereo a dual gang potentiometer. I expected to be able to measure the voltage across the potentiometer input and output, but there is no measurable voltage, only the resistance is measurable.
My plan was to read the voltage from the potentiometer, make adjustments, then output the adjusted voltage, but that won’t work if I can’t measure the voltage!
Am I doing something wrong? Should the voltage be measurable? And is my approach to adjusting the volume by reading voltage then outputting adjusted voltage reasonable?
While I don't know about this specific stereo I will say that 99% of amplifiers with pots control the actual AC music signal. In addition they are logarithmic tapers which means the resistance is very non linear.
There is a number on the pots, see what you get when you google it.
No. Not using your approach. The POTs are adjusting the sound signal which is likely AC. What you must use is an oscilloscope to see the signal and using that, measure the voltage. No sound, no voltage. Loud sound, higher voltage.
What are you wanting to do? And how does an Arduino fit?
Paul
The stereo doesn't have a remote for volume so I want to try making one. Arduino would receive the remote signal and adjust the voltage.
If the actual sound signal is coming through the pot I don't want to mess with it there... I guess I need to rethink this. Thanks for the tips.
May sound silly, but this is done quite a lot in HiFi circles where they appreciate a GOOD quality pot.
Put a little box in front of your stereo, with an audio input and an audio output. Connect the input to whatever source you're playing from. The output goes to the amplifier. Between the input and output you place a digital volume control IC; there are several kinds. Also include an Arduino of your choice and an IR receiver. Bob's your uncle.
Don't want a separate box? Place the circuitry described above in the audio path just before the existing pot. Simply disconnect the wires/tracks that go to the pot and redirect them through your circuit.
Don't forget the decoupling capacitors to block any DC offset from going into and out of your circuit.
It's not very complicated and actually a nice little project, go ahead!
Yeah! the volume of SONY audio I have is analog, so when I press the button that raises or lowers the volume on the remote control, the motor inside actually rotates the volume dial automatically.
That's it; it was done a lot in commercial equipment in the late 1980s~1990s. In DIY circles it's still done sometimes so people can use a decent pot and still have remote volume.
Regular audio signals are difficult to measure because they are constantly changing and the average voltage is low, and the meter won't respond to the peaks, but you you should be able to measure something and you should measure around 1VAC with a constant test-tone. Usually the controls are running at Line Level.
Don't run full-power test-tones into your speakers!!!!
There are digital pots but they can be tricky with audio. Some may not work with AC (unless they are biased) and you can get clicks, pops, or other noises, especially when you change volume. And you have to make sure it doesn't power-on at full-volume. You might be able to find "special" digital pots designed for audio, or there are VCA (voltage controlled amplifier) and DCA (digitally controlled amplifier) chips.
There was a chip called the LM1035/LM1036 which is a DC-voltage controlled volume & tone control. I don't think it's made anymore but it still seems to be available.
As you probably know, the regular Arduino doesn't have a DAC so it can't put-out a varying DC voltage but the analogWrite() PWM can be filtered/smoothed for an application like this.
Several alternatives (at least for volume control) are still available and being manufactured. I'd suggest sticking with parts that are stil current, especially since there's no need to revert to obsolete parts in this case.
Heck, I must have a handful of digital volume control IC's lying around here somewhere...
Most amplifiers have a pre out and main in jumper on the back. You could use an electronic volume or get another dual pot and control it with a motor. But at least it will be hidden.
You could make a LED/LDR combo, using an esp32 etc on bluetooth or wifi to control a pwm LED, which is shrink tubed facing an LDR that then connects to your stereo replacing or possibly in parallel to your pots.
If you do that be sure to use ultrasonic frequency for the PWM (so it doesn't breakthrough to audio) and don't expect great linearity from LDRs, they typically introduce audio disotortion higher than most good amps.
