The Low Parts Count Audiophile Power Amplifier

Update Aug 2015: there were a few more typos in the parts designation for the output power FETs. You would think that Exicon would prefix their parts with EXC, but alas - it's ECX... Here are the revised figure 3 and figure 9 for the article.

We’re sorry that an error crept into our final amplifier circuit, Figure 9, page 65.  Here is the new, corrected version. The only problem was that C4, 10uF, was improperly placed. It should have been in series with R5, to match C3 in series with R4.  Remember that the hum cancellation of the differential input is effective only if the output impedance of the driving preamplifier adds together with R4 to equal 1K.  As drawn, it is assumed that this output impedance is zero.  If both C3 and C4 are shorted out, the amplifier will be DC coupled but still has full differential character.

The authors also offer an LTspice simulation package, containing two LTspice simulation files, for you to check out the amplifier. The package also includes the spice models for the MOSFETs. One simulation file is the full amplifier with all components, while the second is a simplified version which has the power transformer secondary windings in the model. This allows checking power supply loading, turn-on behaviour, and hum characteristics. In each case the bias resistors will have to be changed to alter the MOSFET bias current.

Although the output stage is inverting, the overall amplifier is noninverting, as are most power amplifiers.  If you know the polarity of the rest of your audio system, then by wiring up your loudspeakers with the appropriate phase, the polarity of your whole system will be correct.