Friday, November 27, 2009

Boosting the AN214 – How to do it Right

The heyday of the boosted AN214 DIY amp may be over, but has this iconic hobbyist audio engineering design ever been properly implemented for ultimate sound quality?

By: Vanessa Uy

During the past few weeks, I’ve received some disused but still functional “homemade” AN214 IC amplifier paired with its MJ2955 transistor-based transformer-coupled booster amplifier soulmate for possible tweaking to improve their sound quality. Sadly, I’ve found that an overwhelming number of them – though can function as intended – have been insufficiently designed from an audiophile perspective. Either on the basis of saving money as most electronic DIY-ers are not particularly rich anyway, or following the sway of fashion based on audio electronic myths centered on the AN214. Especially concepts that had been perpetrated by very influential – if rather misinformed – self-styled electronic engineering gurus who are to afraid to be adventurous when it comes to spending a bit more to create a more realistic and natural sounding audio amplifier through over-engineering. Despite of money-saving necessity to skimp on better parts and layout hanging like the Sword of Damocles on the prospective hobbyist, is there a better way to design a boosted AN214 amp without breaking the bank?

I’ve mused before on the virtues of using oversized input and output transformers – to the dismay of electronic and electrical engineering degree holders – for the MJ2955 transistor-based transformer-coupled booster amp design to improve its bass output and quality. But the units I received for tweaking not only have standard sized input and output transformers hindering them from achieving their sound quality potential. Some not only have even smaller than standard transformers in order to save money, but their cost-saving measures can only prove to be expensive in the long run due to the potential damage they might inflict to the loudspeakers connected to them. Fortunately, I have two ways of tweaking the boosted AN214 design not only for better sound quality but also to maintain loudspeaker longevity as well. By avoiding – make that not doing the following if funds allow – when constructing your very own boosted AN214 amp design.

The first is you should not ground the primary / input side of the input transformer of the MJ2955 transistor-based transformer-coupled booster amp. Just because in an overwhelming number of applications of the AN214 IC audio amplifier is in its single-ended output configuration, where the minus side of the speaker output terminal is connected to the ground of its power supply. Does not mean one should ground the input side of the input transformer of the booster amp. Some DIY-ers usually ground the input transformer either to minimize – with the emphasis on minimize – the hum of the AN214 stage. Which is probably either caused by insufficient size of filter capacitors in the AC / DC power supply or just a bad circuit layout. Floating the input side of the booster amp not only makes it sound more dynamic, but also allow you to experiment other IC amplifiers other than the AN214 – especially those in bridge configuration where grounding the negative terminal could destroy the IC amp and the booster amp.

The second is please use separate power supplies for the AN214 section and the MJ2955 transformer-coupled booster amp section. Just because these two sections share the same 12-volt battery in automotive applications doesn’t mean you should use a common AC / DC power supply for both. Some farsighted DIY electronic enthusiasts who care about speaker longevity have even configured a power on delay for the boosted AN214 design by allowing the booster amp to turn on several seconds after the AN214 section turns on in automotive applications to prolong loudspeaker life. Those turn-on thumps heard on the partnering loudspeaker produced by simultaneously turning on both the AN214 and the booster amp section does not exactly bode well to the longevity of the connected loudspeaker.

A significant number of boosted AN214 designs have been paired with the venerable budget wonder of the 1990s, the Mission 731 LE speakers. These types of loudspeakers and their ilk are not exactly as indestructible as their raunchy sounding concert public address speaker system brethren. And those DIY electronic enthusiasts fortunate enough not married to bitch wives from hell who have the ideal set up of a domestic listening room for their hi-fi, a separate power supply for the AN214 section and the MJ2955 transformer-coupled booster amp is the perfect way to go. Favorably if equipped with high-speed Schottky Diode / Schottky Barrier Diode rectifiers and Rubycon Black Gate capacitors for the best possible – and tube amp-like dynamics – sound quality.