A Custom Tone Control Circuit for the Busking Amp
I wanted a tone control to fit in the busking amp. This is not to provide EQ for enhancements, but more to be able to increase gain before feedback. In other words to be able to play louder, with the amp howling. The design is based on Rod Elliots circuit fom his High Quality Audio Mixer
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| Rod Elliot's design (Click for larger image) |
I went for this circuit primarily for the variable frequency of the mid tone control. This should help us to control feedback better. It is surprising that in normal (for us) venues and with only a 7.5W output, that we usually cannot turn the gain up full, before an acoustic howl starts.
I decided to use the Roadrunner technique for the layout, as it provides a tighter design than veroboard. Using 2 dual op amps instead of 1 quad is no advantage with Roadrunner so I used a TL074. It feeds into the PAM8620 class D amp which has a 12.5K input impedance.
This meant I had one op amp left over. I decided to use it as an input mixer using fixed values as in the external mixer/preamp. Electronically, it sits between the inputs and the power amp and is powered by the 12V battery. It has inputs for:-
- a mic, which comes from the XLR input. The gain is set to 10, and the maximum gain of the power amp is 40. So it needs a 55mV signal from the mic for full power output. Which really means close miking only.
- a guitar, specifically the Freshman and the K&K pickup
- and an external input which, in practice, is our mixer/preamp.
1. and 2. will not be used much. We really need two mics, a guitar and mandolin, and this is what the mixer/preamp was designed for.
I out the circuit into TinyCad and the final version is here:-
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Tone control with custom fixed ratio mixer. (Click for larger image) |
I found a board on eBay that looked like it would do the job.
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| A suitable board found on eBay (Click for larger image) |
I did not have room for the full board, but there is a convenient place to cut it in half
I used a spreadsheet (LibreOffice) to make a model of the board and to place the components in positions suitable for Roadrunner.
The layout was very tight