In my November 26th blog post, I described a slow vacuum leak that I thought was due either to reusing the Viton seals (black ring in photo below is rear seal) or a leak in my vacuum testing apparatus. It turned out to be something altogether. After testing all the components individually, I convinced myself the leak is in the cable feed through.

Near the centre of the picture below, there is a flex cable passing through an opening in the aluminum plate. The opening is sealed with epoxy. I speculate that flex cable insulator is just porous enough to allow a small amount of air to enter the vacuum chamber.

A commercial vacuum chamber cable feed through is very expensive but one feature they seem to have in common is metal pins encased in epoxy. The pins don’t have any electrical insulation. I think the electrical insulation on the flex cable is the weak link. My next step is to redesign the cable feed through to use uninsulated metal pins.

As a side note, I use the same cable feed through on the acrylic version of the enclosure so after I redesign the cable feed through, I will test it on my acrylic enclosure design - there may be life in the acrylic version yet.

Rear aluminum plate of vacuum chamber showing flex cable surrounded by epoxy.

My other progress is with the frequency response testing of DÆ Phono Preamp v2.0. In my November 30th blog post, I described the fine tuning of the RIAA filter capacitors using a B&K 880 Precision LCR meter. Following fine tuning the filter capacitors, the filter matches, the ideal RIAA response to within 0.11 dB from 20 Hz to 20 kHz and within 0.03 dB from 50 Hz to 20 kHz. The low frequency roll-off is caused by the DC blocking capacitor and could be further improved with a larger capacitor. The measurement is 0.03 dB may be a limitation of my measurement process. Next, I will look to improve my measurement process.