Hi,

I'm designing a tube preamplifier & I want to add a volume counter.
I'm a complete novice so I want someone to help me with it.
The volume pot is a 250K which has 40 small divisions.
As I cant use any signal from the amp to drive the LED counter I thought of a couple of ideas: To make a kind of ohm-meter circuit connected to either the volume pot or to another VR mechanically connected to the volume pot to avoid signal interference.
The second idea is to fit a shaft extention to the the volume pot which magnitically activates a (solenoid???) switch.
I also need to know how to calibrate the counter for the required reading ie. from 0 to 40.

Please help, thanx

 

If it's going to be a tube preamp, perhaps you might think of Nixie tube readouts.

The part about the 250K pot with 40 divisions does not compute. Please expand on the 40 divisions.

At any case, trying to get the resistance of the pot while in use is going to have noticeable and bad effects on the audio signal. The pot should have an audio (logarithmic) taper, which would vastly complicate the resistance-to-rotation problem.

You might find a shaft encoder which will give some output close to what you wish. 64 positions might be close, as the pot probably has 300 degrees of rotation - close is close.

Most of us are happy with a pointer knob on the volume control and index marks at the min and max points. One judges volume by ear. Some us us likemore bells and whistles. De gustibus non est disputantum.

 

Hi...Yes Beenthere is rite !A couple of small nixie tubes would look great.... Just one thing, I don't look at the volume control when I turn my Hi-Fi up or down, my ears tell me how loud it is !!!! (just a joke)......I did a similar thing, A disk etched as a PCB, attachit to the shaft just behind the front panel, a couple of minature reflective opto switches (one out of phase to the other), a 4027 (for direction), a couple of 4029,s and the display drivers......... That is about it......Daniel.

 

If it is a mono amplifier you can use a double gang potentiometer. One potentiometer will be for the volume itself, and the other, isolated from the first (but sharing the same shaft) and with the extremes connected to the ground and Vcc, could be connected to four cascaded LM3914 (to serve as the input stage of an analog-digital converter). But this requires additional logic to make a proper digital ADC. I think there are also one chip voltmeters, but with these chips (LM3914) you can use custom logic to use with nixie tubes (in which there is one pin for each number, and one common). If you use the voltmeter chip instead, you can use a 7-segment nixie or LED displays.

P.S.: The only problem I see is that the LM3914 is a linear converter, and the dB function is logaritmic. But it is better to use a linear converter when cascading.

 

The LM3915 (or is it the 3916) is logaritmic (well of them......).....You could dismantle the pot and replace one of the tracks with a linear one of the same manufacturer....Daniel..