Saturday, May 25, 2013

Selenium Rectifiers: The Most Musical Power Supply Rectifiers?

Originally used on Avery Fisher’s Fisher 500-C Receiver, is the selenium rectifier the most musical power supply rectifier ever used for hi-fi applications?

By: Ringo Bones

Sadly, as we headlong into the second decade of the 21st Century, the “failure-prone” selenium rectifier originally used in the Fisher 500-C receiver had now been relegated to the dustbin of history and are the usually the first ones to be replaced with more modern silicon bridge rectifiers whenever a Fisher that dates back to 1964 needs to be brought to present day operating condition. But should everyone wanting their old Fisher 500-C receiver to be brought to present day working condition need to replace the original stock selenium rectifier that still works despite of its reputation of being failure-prone? After all, selenium rectifiers tend to be more musical than their more modern silicon-based counterparts, right?

Given that the thermionic vacuum tube / valve manufacturer Mullard managed to manufacture the most reliable – and most musical sounding – version of the AD 149 PNP germanium output transistor during the 1960s – Mullard doesn’t seem to be able to manufacture their own more reliable, and hopefully a more musical version, of the failure-prone stock selenium rectifiers on the original Fisher 500-C receiver. And if they did, they might be as rare as hen’s teeth this day and age. But given the inherent unreliability of selenium rectifiers, why did Avery Fisher used it as the main rectifier system when he first designed his iconic Fisher 500-C receiver? Is it really more musical sounding in comparison to its more modern silicon counterparts? But first, here’s a primer on what this selenium rectifier business is all about.

Selenium rectifiers belong to the family of metallic or dry-disk rectifiers. A typical selenium rectifier consists of a layer of selenium, a semiconductor, deposited on an iron plate. The selenium acts as a P-region while the iron plate acts as the N-region. And when a voltage is applied in the forward bias direction (positive to the selenium and negative to the iron), current flows readily through the rectifier. When connected in the reverse bias direction, the resistance of the rectifier becomes very high and very little current may flow through it. Several such diodes may be connected in series to increase its voltage-handling ability. This is primarily how the power supply of the original 1964 era Fisher 500-C receiver converts the 110-volt 60-Hz AC of American wall sockets into the various DC voltages required to power it up and play tunes, or of news broadcasts, etc.

Given you are one of the fortunate few to be able to use one as part of the power supply of your experimental DIY hi-fi power amp (tube or solid-state), selenium rectifiers tend to have a “musicality” not normally found in typical run-of-the-mill modern silicon rectifiers that are now de rigueur in mass market audio equipment. Its closest modern equivalent sound-wise and musicality-wise is a high-speed Schottky diode rectifier and a Rubycon Black Gate capacitor equipped power supply. Sadly, high-speed, high current Schottky rectifiers are now a rarity after Mainland China invented a RADAR system heavily dependent on high-speed Schottky rectifiers in its operation that can “see” stealth aircraft.  

The “unreliability” and the “failure-prone” nature of the selenium rectifier may have been due to the still primitive – compared to the standards already prevalent during the 1980s to this day – state of solid-state device mass production of the late 1950s and for much of the1960s. Also, I can only assume that a typical early 1960s selenium rectifier could also be very prone to galvanic corrosion if the selenium and iron plate interface gets exposed to atmospheric moisture – especially in hot, tropical climes. These glaring caveats aside – a selenium rectifier mass produced with high-reliability in mind has a musicality not normally found in today’s mass-market generic silicon-based power supply rectification systems, unless of course you are sold to the still much more musical thermionic vacuum tube rectifier system based on the original Mullard GZ34 or 5AR4.


April Rain said...

I think Mullard never got into the selenium rectifier manufacturing business because even if left unused and in safe storage, selenium rectifiers tend to deteriorate over time.

Ringo said...

You are absolutely right about the "perishable" nature of selenium rectifiers, April Rain - and when they do "give up the ghost" so to speak, they emit a really awful smell (as in selenium mercaptants?).