Used to be one of the most closely guarded “proprietary trade secrets electronic tweaks” of the AN214 IC amplifier, does the blue input captain micro-transformer tweak still relevant in the 21st Century?
It is probably one of the most coveted and one of the most closely guarded proprietary trade secret when it comes to the electronic tweaking of the AN214 IC amplifier during the Great Car Stereo Wars of Cebu during the 1980s. The results – more often than not – tend to provoke commentaries and in a nutshell it is either you’ll love it or you don’t and for almost a quarter of a century, no particular individual has yet claimed for inventing the famed tweak and announced it, with adequate proof I hope, over the internet. But does the blue input captain micro-transformer tweak of the AN214 IC amplifier still relevant in the 21st Century hi-fi scene?
First of all the “blue input captain micro-transformer” referred to in this procedure looks to me like a balun transformer – i.e. a balanced-unbalanced impedance matching transformer - whose one side is a “balanced” center tapped winding and the other side (primary or secondary winding?) is a two-terminal winding. The blue color denotes the impedance or DC winding resistance of the transformer. In the past, such transformers are used in ultra low noise microphone preamplification systems that allow a balanced three-terminal microphone to be used in an unbalanced two-terminal preamplification system or an unbalanced two-terminal microphone to be used in a balanced three-terminal preamplification system without introducing audible 60-cycle hum. “Blue input captain micro-transformers” are also used as a galvanic isolation transformer of line-level signals in spring reverb systems. I’ve also heard that during the late 1980s – from my older working musician buddies – that blue input captain micro-transformers can be used as an excellent electric bass guitar pick-up, although orientation of the transformer is critical akin to that of the P-90 type electric guitar pick-ups.
I suspect that whoever first formulated the blue input captain micro-transformer tweak of the AN214 IC amplifier – who (Edwin?) probably was a graduate of Cebu City’s Ivy League engineering colleges around the late 1970s to the early 1980s – was an electronics communications engineering student who got the idea of trying to use the working principles behind intermediate-frequency transformers or IF transformers used in AM / FM tuners as a novel tone control system. Given the Baxandall type bass and treble tone control system can only accentuate the low and high frequencies of a line level audio signal by much, a parallel-resonant high-Q band reject filter formed by the blue input captain micro-transformer and whatever value capacitor is connected across it operates the same as an IF transformer found in radio tuners albeit one that is capable of handling line-level audio signals that ranges in amplitude from 100-millivolts to 1-volt peak-to-peak.
The blue input captain micro-transformer tweak of the AN214 IC amplifier is usually placed in the negative feedback section of the Baxandall type bass and treble tone control preamplifier circuit often paired with the AN214 IC amplifier system. As it is in parallel of the bass and treble arm of the negative feedback of an actively amplified Baxandall tone control circuit, the high-Q band reject properties of the blue input captain micro-transformer tweak with a paralleled capacitor that ranges in value from 100-picofarads to 2.2-microfarads are a bit exaggerated due to the active amplification but the inherent 24-decibels worth of signal attenuation is cancelled out due to the active amplification of the said circuit. Resulting in a rubbery Bill Laswell type bass and shimmering treble reminiscent of low powered zero negative feedback single-ended triode amplifiers using 2A3 power triodes.
From my ear’s own perspective, I bet that an overwhelming majority of “affordable” hi-fi audio gear manufactured during the late 1970s and the early 1980s were designed for inherently “colored” hi-fi loudspeakers. Most of them are certainly not equal to the task of driving a genuinely low coloration hi-fi loudspeaker available at the time – i.e. the Quad ESL-63 – although at that time, only a very few hi-fi enthusiasts can afford the famed Quad electrostatics. Basing on most “affordable” still perfectly functioning early 1980s era hi-fi loudspeakers populating the garage sales of my neighborhood, they are quite very colored in comparison to a mid-1990s era Sonus Faber Concerto hi-fi loudspeakers. Thus the blue input captain micro-transformer tweak of the AN214 IC amplifier was primarily designed to “filter out” the most colored band of most affordable audio gear of the early 1980s – i.e. the midrange or the mid-band part of the audio spectrum. Nonetheless, the blue input captain micro-transformer tweak of the AN214 IC amplifier give the masses a taste of that “illuminated from within” musicality of 2A3 power triode equipped zero negative feedback single-ended triode audio amplifiers.