Help Please

[kenmeisteruk]

Hi All.. Hoping you can help.  I want to build a wooden stomp box for bass drum.  I am torn between using a piezo or a 4 or 8ohm 4" speaker.  Probably going to go the piezo route.

Now.. my question.. I'd like to add the following to the circuitry..

A:  A volume control
B:  Capacitor/s to cut all frequencies above 120hz out, essentially I only want the Low Frequencies.
C:  Possibly a tone control that would allow tone control between 40 and 120hz ...(does this even exist?)
D:  I'd like to also have an XLR and Jack Output

Would appreciate some GURU advice... THANKS In ADVANCE!

[stallik]

I've tried both piezo and speaker types. The deepest bass I achieved with the piezo was by mounting it by the very edge and leaving the remainder to float in mid air. With or without a volume control, I found difficulty achieving a good volume on a normal stomp without overloading the front of the amp when stomped too hard.
By far more successful is with a speaker. After much experimentation, I decided to just buy the guts of a puck 'n' stompa from Australia. This uses a tiny speaker (1 1/2" or so) which is damped with a piece of plastic glued over the cone. A little impedance matching transformer and an output jack are the only other components and the bass is room shaking.
I've been so pleased with it that, if I ever needed another, I'd be tempted to just buy the whole thing which is pre mounted in a hockey puck

Oh, and welcome to the forum

[FUZZZZzzzz]

Does it have to be 'mechanical'?

You could also try something like this with a piezo to trigger it.

[mimmotronics]

B:  Capacitor/s to cut all frequencies above 120hz out, essentially I only want the Low Frequencies.

You can accomplish this many ways, essentially it boils down to a simple Low-Pass Filter with a cutoff frequency set at 120Hz. The order of the filter (steepness of the frequency attenuation roll-off) will determine how you build it. You could take the passive RC approach or you can design an active low-pass filter with an active device (op amp or transistor).

C:  Possibly a tone control that would allow tone control between 40 and 120hz ...(does this even exist?)

Yes, what you've describe is an adjustable low-pass filter that can vary between ~40 and 120Hz. This can also be done passively or actively.

As for the intricacies of filtering low frequencies, I'm not well-versed enough to comment on (I'm certainly not a "guru") but my first approach would be to set up an oscillating sine wave test signal where you can vary the frequency (e.g. from a function generator). Start with the simple passive RC filter, monitor the result by recording the amplitude of the output waveform at different frequencies around the cut-off. If the roll-off is too gradual and if the frequencies you don't want are too noticeable, I'd probably move on to an active low-pass (Sallen-Key) approach.

Once that design is made up I'd move on to the variable cut-off design. I'd consider a passive RC circuit where the resistive portion is composed of a pot and a fixed resistor value, so as to set the minimum and maximum cutoff frequencies, then I'd buffer the output in case of loading effects.

[kenmeisteruk]

Hi Guys,

thanks for the replies and advice!  Stallik, I actually started looking at the Stompa kit from Australia, and I'm leaning more toward this route now..  Can you advise as to what the impedance matching transformer is?  I'd like to source a small 1.5 inch speaker and the transformer myself, rather than buying the kit from Australia.

Thanks everyone for your help!  Really appreciate it!

[stallik]

The transformer is described as a transducer by the supplier. The speaker impedance is either 4 or 8 ohms. I've not measured the transformer as it's covered with goop but I presume it raises the impedance of the output to better match the input of the amp. The whole thing is really simple but I reasoned that by the time I'd tried a bunch of speakers, various ways of damping the cone and different transformers, I'd probably have spent as much as buying the thing ready done.
The seller is a good man to deal with as well

[PRR]

Small speakers do not respond well below 120Hz.

A 120Hz low-pass (high-cut) is a tough design because your ear hears any over-120hz leakage so much more than under-120Hz thud; and because small transducers work so much better over 120Hz.

If there is a ready-made unit, and I was not in for a long-long project, I sure would consider buying.