Thursday, 9 November 2017

FFT Tests on the tone control

With the tone control board and power supply built, but the case still in the machine shop, it seemed a good opportunity to run some performance tests. The site I got the circuit from only had Spice simulations rather than measured results. So I don't know if this is the first time actual test results from this circuit are available.

Anyway, my testing method was to run a sine sweep into the input from 50Hz to 40kHz, then connect input and output to the oscilloscope, running the output into a 100K dummy load (to simulate the volume control of the amplifier it would be running into)

I ran FFT transforms at tone flat, full treble lift, full treble cut, full bass lift and full bass cut. The results are below. 

Please ignore anything below around 70Hz; my FFT process loses all resolution at that frequency.

There are two traces on each plot; the yellow trace is the input signal, for reference. The blue trace is the output from the tone control.

First up - the testing setup


Ain't it beautiful? Careful where you put your fingers!
The 9V is for the signal relay; if 9V is present the contacts close and the circuit is engaged. If no 9V, then the relay bypasses the circuit and bridges the input and output. When the case is made I'll wire up the 9V supply properly but for now it's Energizer-power


Tone controls flat. As mentioned in the text, ignore the region below 70Hz; my FFT process doesn't have resolution there


Bass full cut


Bass full lift


Treble full cut


Treble full lift


In each graph, scale is dB on the millivolt, 5dB per vertical division, yellow is input trace (for reference) and blue is output. (My FFT math capability to reference one signal off the other doesn't work properly, hence the presentation of both signals).

For completeness, I ran the same tests on the other channel. Results were identical.

Conclusion

This tone control is working exactly as anticipated, and is not far from the predictions on the source site.

Something seems to have gone right.

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