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.
Showing posts with label MJ2955 PNP Transistors. Show all posts
Showing posts with label MJ2955 PNP Transistors. Show all posts
Friday, November 27, 2009
Saturday, June 20, 2009
A Transformer-Coupled Booster Amplifier for the AN214
Due to it’s inherently low power output design, does addition of a transformer-coupled booster amplifier to an AN214-based amplifier aid or hinder it’s sound quality?
By: Vanessa Uy
Given it’s inherently low power output – between 4 to 5 watts into an 8-ohm load. It was therefore inevitable to find a way of increasing the “meager” output of the AN214 IC-based audio amplifier without sacrificing it’s inherently good sound quality (on the cheap?). I wonder how many ways were “engineered” during the past 30 + years or so history of this device. Surprisingly, a consensus was reached of choosing a “primitive” transformer-coupled PNP transistor-based design that probably dates from the 1950s.
The design configuration of the transformer-coupled booster amplifier used to increase the 5-watt power output of the AN214 IC-based power amplifier for all intents and purposes resembles that of a transformer-coupled 300B-based push-pull tube power amplifier circuit. Except that the 300B tubes (or valves as they say in Merry Old England) were replaced by MJ2955 PNP power transistors in a TO-3 package. From my point of view, this transistor-based transformer-coupled booster amplifier circuit was probably based on the first transistor-based audio power amplifier designs first published in the US-based audio electronics magazine called AUDIO. Probably those issues dated between January 1960 to December 1961 - i.e. the Golden Age of Stereo.
During my experimentation of the MJ2955-based transformer-coupled booster amplifier – which to all intents and purposes is a continuation of the experiment done by my older audio-buddies during the last 25 years or so – I did optimize the input transformer winding design. By eliminating stray capacitance of the input transformer / driver transformer winding and optimizing it for a near-perfect square wave performance and transfer function characteristics. I did manage to reduce the booster amplifier’s recommended negative feedback level, which only improved the sound quality to no end.
Compared to its 300B tube-based sibling sound quality wise, it was a close match. Although the 300B-based push-pull amplifier did manage to display irreproachable beauty where it excels. Like in the Middle-C region of the musical performance – i.e. upper bass and lower midrange region of the audio spectrum. And even though it was criticized during the 1980s that transistor-based transformer-coupled booster amplifiers are notorious for having high total harmonic distortion (THD) in comparison to a full complementary direct-coupled solid-state power amplifier. But if you wind the transformers properly by eliminating stray capacitance – especially the input / driver transformers, the inherently high THD (total harmonic distortion) of transformer-coupled solid-sate booster amplifiers can be dramatically reduced. Although high-quality audio frequency transformers – driver transformers and output transformers - that are wound at artisan levels to optimize their square-wave performance and optimize their transfer-function characteristics across the audio spectrum are magnitudes more expensive in comparison to matched pairs of high-power output transistors.
The bad news is – from a perfectionist audiophile’s perspective – a lot can go wrong, sound quality wise, in the actual construction of full complementary direct-coupled solid-state power amplifiers. Especially if the “engineer” doesn’t trust their own ears and chose to trust on measuring instruments instead could result in a sluggish sounding power amplifier with unlimited reserves of power - but one that is sorely lacking in the pace, rhythm, and timing department. That's why I really hate musically illiterate audio engineers. Looks like the old-school wisdom gained by audio engineers during the Golden Age of Stereo still applies well into the 21st Century. And it also provides a low-cost route to audio nirvana via the AN214 IC-based audio power amplifier route. Ain’t it fun to be an audio cheapskate?
By: Vanessa Uy
Given it’s inherently low power output – between 4 to 5 watts into an 8-ohm load. It was therefore inevitable to find a way of increasing the “meager” output of the AN214 IC-based audio amplifier without sacrificing it’s inherently good sound quality (on the cheap?). I wonder how many ways were “engineered” during the past 30 + years or so history of this device. Surprisingly, a consensus was reached of choosing a “primitive” transformer-coupled PNP transistor-based design that probably dates from the 1950s.
The design configuration of the transformer-coupled booster amplifier used to increase the 5-watt power output of the AN214 IC-based power amplifier for all intents and purposes resembles that of a transformer-coupled 300B-based push-pull tube power amplifier circuit. Except that the 300B tubes (or valves as they say in Merry Old England) were replaced by MJ2955 PNP power transistors in a TO-3 package. From my point of view, this transistor-based transformer-coupled booster amplifier circuit was probably based on the first transistor-based audio power amplifier designs first published in the US-based audio electronics magazine called AUDIO. Probably those issues dated between January 1960 to December 1961 - i.e. the Golden Age of Stereo.
During my experimentation of the MJ2955-based transformer-coupled booster amplifier – which to all intents and purposes is a continuation of the experiment done by my older audio-buddies during the last 25 years or so – I did optimize the input transformer winding design. By eliminating stray capacitance of the input transformer / driver transformer winding and optimizing it for a near-perfect square wave performance and transfer function characteristics. I did manage to reduce the booster amplifier’s recommended negative feedback level, which only improved the sound quality to no end.
Compared to its 300B tube-based sibling sound quality wise, it was a close match. Although the 300B-based push-pull amplifier did manage to display irreproachable beauty where it excels. Like in the Middle-C region of the musical performance – i.e. upper bass and lower midrange region of the audio spectrum. And even though it was criticized during the 1980s that transistor-based transformer-coupled booster amplifiers are notorious for having high total harmonic distortion (THD) in comparison to a full complementary direct-coupled solid-state power amplifier. But if you wind the transformers properly by eliminating stray capacitance – especially the input / driver transformers, the inherently high THD (total harmonic distortion) of transformer-coupled solid-sate booster amplifiers can be dramatically reduced. Although high-quality audio frequency transformers – driver transformers and output transformers - that are wound at artisan levels to optimize their square-wave performance and optimize their transfer-function characteristics across the audio spectrum are magnitudes more expensive in comparison to matched pairs of high-power output transistors.
The bad news is – from a perfectionist audiophile’s perspective – a lot can go wrong, sound quality wise, in the actual construction of full complementary direct-coupled solid-state power amplifiers. Especially if the “engineer” doesn’t trust their own ears and chose to trust on measuring instruments instead could result in a sluggish sounding power amplifier with unlimited reserves of power - but one that is sorely lacking in the pace, rhythm, and timing department. That's why I really hate musically illiterate audio engineers. Looks like the old-school wisdom gained by audio engineers during the Golden Age of Stereo still applies well into the 21st Century. And it also provides a low-cost route to audio nirvana via the AN214 IC-based audio power amplifier route. Ain’t it fun to be an audio cheapskate?
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