Tech Forum

An audio amplifier with a graphic equalizer doesn't require a DSP or FPGA. Quickfilter Technologies (www.quickfiltertech.com) sells a variety of chips aimed specifically at audio applications. The company also has development kits and a neat GUI. No programming needed. A USB cable provides a link between a development board and a PC used to set parameters.

The Quickfilter QF3DFX chip includes a 10-band parametric equalizer and an eight-band graphic equalizer. Cypress Semiconductor's Programmable System on a Chip (PSoC) families (www.cypress.com) include filter blocks, and a graphical drag-and-drop GUI makes it easy to configure a system. These chips include ADCs and DACs, so I bet Monito could create what he wants with only a PSoC chip and a couple of audio amplifier ICs. Find a useful app note at www.cypress.com/?docID=21352, and info about a 10-band graphic equalizer at www.cypress.com/?rID=43672.

If you're generally new to audio processing, I would stick with analog for a while. It's not as cool or widely configurable as a DSP, but it usually does a pretty good job when done right. For your project, you'll probably want:

1. Input gain
2. Three-band or more *parametric* EQ
3. Output gain
4. Power amp

I included two gain stages so that the EQ's working level can be adjusted independently of both the input and output levels. This allows you to overpower the analog noise in that stage without fear of clipping and still get the output level that you want. (Professional analog mixing consoles are basically that on steroids.) The gain stages are the easiest parts of the circuit: Use a dual-gang linear potentiometer (for stereo, single-gang for mono) with an extra resistor between the center-tap and ground. This loading resistor should be roughly 10% to 15% of the pot's resistance, and makes a better audio taper than most audio pots.

The power amp can be a single IC for small speakers or headphones. They're sometimes called power op-amps. The EQ is a little different than I think you had in mind. I think you're imagining a many-band *graphic* EQ, which is easier to understand at first glance, but I think a more useful tool for you would be a *parametric* EQ. Parametric means that you can adjust the frequency of each band, as well as the gain. For example, if you have a problem at, say, 3 kHz, and you have a graphic EQ with adjacent bands at 2.5 kHz and 3.5 kHz, it's going to be hard to adjust that satisfactorily. (This is why professional graphic EQ's have up to 31 bands. Even so, they still don't work well for everything.) With a parametric EQ, you could adjust a frequency knob to put one band exactly where you need it for each problem area. A semi-parametric EQ is easier to build than a full-parametric, and just has the two knobs per band — frequency and gain. A full-parametric EQ has a third knob that adjusts the width of each band. Despite missing a knob though, the semi-parametric version is usually sufficient because it automatically adjusts the bandwidth as a function of gain. Large gain adjustments typically make a narrow spike or notch, while smaller adjustments make a wide hill or valley.

This website (www.sound.au.com) is a great resource for analog audio projects. It has circuits for almost anything you might want to do to your sound, with schematics, detailed descriptions, and usually a PCB for purchase. One minor sticking point for you might be that these circuits are designed for ±15V supplies (30V total, with signals referenced to a center-tap). If all you have is 5V, you might be able to make a switching power supply that provides that, or you might be able to adapt the circuits to run on 5V with a higher noise floor. (Actually, the noise level stays the same, but you're using a smaller signal.)