Tomo
DIY tube amps can be SHOCKING
- Joined
- Jun 22, 2001
- Posts
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Hey,
I found a large quantity of 5kohm RN55 resistors. About 50 of them. And first thing that came to mind was ... Stepped Attenuator. I have 24-step rotating switches too.
And I remembered Rick's schematics he posted few days ago. He paralleled a linear potentiometer with a resistor to get a logarithmic pot.
I decided to give it a try. Here are calculations:
Vo/Vi = ((1/x+1/r)^-1) / ((1/x+1/r)^-1 + (Rpot-x))
Where r is the bypassing resistor value and Rpot is the maximum resistance of the potentiometer.
With my available parts, Rpot is 23 X 5 = 115K ohms. It happens that r=10K. Which gives a decent pesudo-logarithmic scaling with ~1dB per step. dB per step increase near the lowest volumes. Maximum attenuation is -50dB. Between -50~-40dB, the steps are near 2~3dB per step. -40~0dB, the steps are about 1dB per step.
It is not too bad. (I think steps are little too small though.)
Tomo
I found a large quantity of 5kohm RN55 resistors. About 50 of them. And first thing that came to mind was ... Stepped Attenuator. I have 24-step rotating switches too.
And I remembered Rick's schematics he posted few days ago. He paralleled a linear potentiometer with a resistor to get a logarithmic pot.
I decided to give it a try. Here are calculations:
Vo/Vi = ((1/x+1/r)^-1) / ((1/x+1/r)^-1 + (Rpot-x))
Where r is the bypassing resistor value and Rpot is the maximum resistance of the potentiometer.
With my available parts, Rpot is 23 X 5 = 115K ohms. It happens that r=10K. Which gives a decent pesudo-logarithmic scaling with ~1dB per step. dB per step increase near the lowest volumes. Maximum attenuation is -50dB. Between -50~-40dB, the steps are near 2~3dB per step. -40~0dB, the steps are about 1dB per step.
It is not too bad. (I think steps are little too small though.)
Tomo
