Restoring Vintage Audio Equipment

Phoenix HiFi

Subjectively, I like the sound improvement.  To my ears I hear a more relaxed presentation.  I don't really hear that audible hum and noise are reduced, just that the soundstage opens up a bit more, and there is subjectively less stridency on certain recordings.

These MacKits actually belong to an associate.  He is allowing the new caps to burn in before doing a formal evaluation, but said that after the cap upgrade, the bias reset and the choke mod, even at first turn-on they sounded much better than before.  I'm anxious to get his evaluation after sufficient break in.

My MC30's do have the choke mod so I listen to them almost every day.  My friend's MacKits did get new metal film caps for the plate resistors, while my MC30s retained the original carbon composition types.  I think I like the detail of the metal films better so my Macs will return to the bench.

If you have any questions about this mod, feel free to email me at

This plot shows the results when I installed the choke. The THD number remained the same, but the sidebands are reduced quite a bit.  The table below shows the improvement to the first few sidebands by the desired tone.

The first step was to adjust the bias to compensate for the silicon diode that replaced the old selenium type.This did reduce the harmonic distortion by over half, but the power supply modulation sidebands are still present, and even higher in level than the second harmonic.

Above is an Arta plot of the output of a MacKit 30W amp, essentially the kit version of the MC30.  This is a 1kHz tone at 1Watt output.  We see the second and third harmonics at 2 & 3 kHz as expected and the 0.031%THD, but we also see some sidebands flanking the desired tone at 1 kHz +/- 60Hz, +/- 120Hz, +/- 180Hz and so on at multiples of 60 and 120 Hz.  These sidebands indicate that the residual noise on the power supply actually modulates the 1kHz tone.  Notice that the harmonics also get modulated.