Here is how a simple treble roll-off could work. basically like a guitar tone control, with an extra resistor.
Its placed between the mix and volume modules:
The maximum roll off is determined by the 33k resistor R and the 0.1uF capacitor C, and as shown, it will roll off above about 1800hz under the control of the pot. The pot can be a 250k log pot, and you'll probably find most of the action is down the low end. Or try a 100k log, and maybe cut the track at 10 if its still cutting a bit of treble at maximum.
If you want the roll-off to start at a lower frequency, increase the cap, say to 0.18uF to roll off above 1000 hz.
Hi John, thanks a lot. I will add that to my crude lab setup.
On your question about balance: I was aware of that a piezo normally needs a preamp, but, having none at hand, I tried without.
To my surprise and satisfaction the balance is fine.
I think it is due to that there is a "mechanical" amp in my system. I tried different places for the piezos, and the best result was to have both of then glued to the banjo skin close to the bridge. There is a lot of movement in the banjo skin (at least compared to a guitar) so my theory is that the skin gives a much stronger signal in the piezos than in a guitar.
If you are interested I have a few pictures of my project on:
There is a little hum, but I think that is due to the crude lab setup, and will disappear when I do the final assembly inside the banjo, well shielded.
I had some trouble to get it right, but when I made an systematic approach, using a 200k log pot (turned so I got a slow slope in the beginning) and a 100nF capacitor it worked fine.
Should anyone use your sketch, they should be aware that there is a small ambiguity regarding the R4 that can be omitted. In the sketch you say the in the mix unit; in the text you say in the volume unit, the latter is correct (at least it works for me).
(On Tilley hats: they are rare in Europe as well, I ordered mine on the web from Canada)
So now I can go on and finish the woodworking. Should anyone be interested in my project, I will update my site from time to time, and write in English.
I did intend it to be the mix module where R4 can be omitted, but it will work with or without it in either module. Its small function is to control a small pop that you might get when you first plug it, by keeping the output side of the ouput capacitor at ground. i adjusted the text above.
Put it all together today, plugged it in to the amp - not a sound - not even hums or cracks. Back to the lab, multimeter and soldering iron. Guess shortcut somewhere
It turned out to be the signal and ground in the output jack that had been bent together. So I finalized the inside, and could test my banjolintar to my great satisfaction. It sounds really good, with very different sounds from the mag and the piezo. The blender works fine, as well as the volume control and the tone control. There is no humming or hissing or any other unwanted sounds. So now I can stow away my electric tools.....