Bypass caps

48 years ago, in 1974, Stan Warren and I got an insider tip we considered at the time to be solid gold. Bypass caps. This was back in the days when Bob Carver's Phase Linear 400 amplifier was what anyone "in the know" was using to power their speakers. That Phase Linear amp was sooooo much better sounding than any other solid-state amp of that day (SAE, Crown, and a myriad of Japanese brands) that if you could get your hands on one that's what you did (unless you could afford Audio Research which none of us could). No sooner had we acquired a Phase Linear to replace Stan's SAE 31B than a friend came over with his own PL 400 for us to compare. Holy Moley. The friend's identical model amplifier was stunningly more open, airy and musical. And not just by a little bit. Everything was better about his Phase Linear 400 than ours. He popped open the lid and pointed to two small blue capacitors tied across each of the screw terminals of the amp's power supply capacitors. "That's the difference?" we asked. It was and to prove his point he unscrewed one leg of the small blue caps and fired back up the amp. Darkness was again upon us. The airy openness and improved transients we had heard before were muddled and lost. That small 0.1mF film capacitor strapped across the amp's sluggish power supply electrolytics made a huge improvement. They work because electrolytics aren't great at higher frequencies. By paralleling a small film capacitor to fill in where the bigger caps aren't that good makes an amazing improvement. To this day, every electrolytic capacitor in any PS Audio product has been lovingly bypassed with a smaller hand-picked film capacitor. Of course, this drives the measurement crowd crazy since it's not in the signal path and (horrors) only in the power supply. This measurementist's view ignores the notion that an amplifier is a power supply with an output valve that modulates that power supply. But then, you'd know that simply by listening. Because it matters.