The difference between a servo-amplifier and a regular audio amplifier is primarily that audio amplifiers "roll off" or block very low frequencies that you can't hear, often to keep the amplifier stable. In servo-amplifiers, power must be delivered even at zero frequency (for example, to provide a steady state voltage holding a motor against a torque).
The LM1875 audio power amplifier from National Semiconductor is available in a number of kits for audio amplifier applications and uses an integrated circuit that is internally compensated to be stable for gains of 10 or greater down to 0 Hz. Also, it has the nice feature of being short circuit and thermally protected.
Below is the necessary modification to the DIY Kit 50 25 W Hi-Fi Audio Amplifier Kit available from Q-kits to turn an audio amplifier into a servo amplifier.
In the present design, this amplifier is run at plus and minus 12 volts, derated from its suggested plus and minus 25 V power supply. There is a thermal shutdown on the chip and so you can experiment with how much power you can produce based on your particular heatsinks. Originally, I used CPU heatsinks with fans, but had to remove the fans because the low frequency magnetic waves from the brushless motors were being captured by the guitar-pickup. Using a 12 V power supply cut the response time of the motors but because I was looking for a “slide” sound, I accepted this in the original and so far only design. The crosstalk between the cooling fans and the guitar pickup can probably be avoided by moving the amplifiers away from the guitar pickup or by conducting air to the heatsinks through a tube from a remotely located fan.