idea: Petre Petrov (Bulgaria)

Three-way Audio Amplifier

The design presented here may very well belong in the ‘low-fi’ category, but nevertheless has a few interesting features, as is demonstrated by the schematic below.

It is the intention to power this amplifier using the external power supply for a laptop/notebook. These can be obtained from many places for little money and generally supply an output voltage of 19 V_DC. A disadvantage of such power supplies is that they often have too much ‘rubbish’ on their output voltage. That is why an LC filter is used to keep the noise down as much as possible.

For the actual amplifier itself, the well-known LM380 (or LM384) is used; in fact, no fewer than three of them. To build a three-way amplifier, there are therefore separate signal paths for low, mid and high frequencies. The filters that are necessary for this can be bypassed using switches (S3 through S8).

The amplifier also has separate inputs for low, mid and high, but these can be connected together using switches S1 and S2.

The circuit itself is not particularly critical can be built relatively easily on prototyping board.

A few comments: the amplifier ICs can become quite warm; an appropriate heatsink is absolutely essential. Depending on the impedance of the loudspeakers used, the value of the output capacitor may require changing as indicated in the schematic.

Both the LM380 and LM384 have a fixed gain of 50 with an input impedance of 150 kΩ. At a supply voltage of 19 V, the maximum output power (at a THD of 10%) amounts to about 3 watts.

The schematic may be too small to be read easily hence a larger version is available as a free download.