ammoon Guitar Radio Link Filter

 In a previous post, I reviewed the Ammoon Guitar Radio Link.

Although it works well for connecting a guitar directly to an amplifier, I ran into a problem when I plugged into my Zoom Guitar Effects Pedal G1XN. 

The output becomes very noisy. It took a while to work out what was going on, because if the radio link is placed between the Zoom G1XN and the amp no such noise is produced. Both the Ammoon and the Zoom are digital devices which sample the incoming signal at a frequency higher than audio frequency. I have already noted that the output of the Ammoon is poorly filtered and contains high frequency noise - the fuzzy trace on the oscilloscope revealed this. When mixed with the sampling frequency of the Zoom G1XN the ultrasonic noise of the Ammoon is folded back down in to the audio spectrum - it is working rather like a superhet radio receiver which mixes radio frequency signals down to a lower frequency by mixing them with the output of an oscillator. 

To try and fix the problem I designed and built a five pole passive filter with inductors and capacitors. This proved the point, by removing the noise, but it was less than ideal. The inductors picked up mains hum from the mains transformer in the amp, and the filter response was far from flat producing strong peaks in the audio of the guitar. The peaks seemed to be related to the output impedance of the Ammoon, and since I had no data for this I decided an active filter was a better idea, with a unity-gain buffer to isolate the filter section from the output of the radio link.

I simulated the circuit above in LTSpice. The resulting frequency response is shown below.

I built the circuit on veroboard and measured the output with signal generator and oscilloscope. I used Excel to plot the actual response - shown below. It is very similar.

The active filter is powered by a 9V battery. I used a CMOS op-amp a TLC2262, it is low noise, and it has very low supply current. It is not a high speed op-amp, the Gain-bandwidth product is less than 1MHz, however this circuit has very low gain, and we are intending to filter out the high frequencies anyway. R1 and R2 hold the d.c. operating point at mid-rail. The capacitor and resistor values were chosen by experimenting with different values in LTSpice. 

The cut off frequency is, perhaps a little lower than it needs to be, I may raise the cut off frequency a little:

C3=1nF C2=220pF gives -3dB at 8kHz, and -32.9dB at 48kHz.

C3=470pF C2=100pF gives -3dB at 17kHz and -19.5dB at 48kHz

C3=680pf C2=150pF gives -3dB at 11.9kHz and -26dB at 48kHz.

Of course the important thing is ho it sounds, and so far it seems very satisfactory.