One of the enchantments for my 50's turntable is an Arduino Light Organ.
The organ uses FFT to calculate the amount of bass, middle and treble in the turntable signal.
I'm building a simple signal adapter to boost the tiny pickup coil signal.
One Transistor Amplifier
There are three reasons why I can't feed the pickup coil signal directly into the Arduino.
The amplitude is small, it can't drive the impedance of an analog input pin (it has to drive an audio pre-amp too).
And it's a negative signal half of the time.
The quality of the amplifier isn't that important - as long as it doesn't create too much additional harmonics and doesn't feed anything back into the source signal.
The most important thing for this module is that it adds a positive bias to the signal, so that the full signal is within range for the ATMega ADC.
This 5-component amplifier does the job. It has a so-so frequency response due to my choice of capacitor, and it is nowhere near linear.
If necessary, I will make firmware changes to correct for that.
I've measured bandwidth and captured the scope output on 4 spots in the audio range. This is measured without the input resistor of 100 K. Input signal is 350 Vrms.
I don't think I have to expect too much on the range of this turntable (Maybe I can test that later. I have a test record).
But you see that it's a poor performer.
| 20 Hz | 200 Hz | 2 kHz | 20 kHz |
|---|---|---|---|
 |  |  |  |
It may be bad on the pictures above, but when I use it for what it is intended, it is quite ok.
The signal has its necessary positive dc bias. And although I haven't tried it in practice yet, I bet it'll do the job just fine.
I first breadboarded the circuit to do the measurements, but this is a fine candidate for the proto-shield that came with the Enchanted Objects package.
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