BA512 is a discrete operational amplifier introduced by rupert neve & company ltd. in 1976, as a drop - in r e placemen t for its ba440 serial feedback amplifier. we have re - engineered the BA512 for use in a variety of third - party applications, conforming to a more popular form factor and pinout. specifications are as follows. * serial feedback amplifiers, also known as qu asi - operational amplifiers, lack a true differential input, i n stead utilizing the emitter of a lone input device as an inverting input. BA512 electrical characteristics at vcc = +/ - 15v, tamb = +20 ? c symbol parameter typical unit v io input offset voltag e, rs = 600 3 mv v oo output offset voltage 30 mv i ib input bias current 3 ua e n equivalent input noise, f = 1khz, rs = 25 - 147 dbu cmrr common mode rejection ratio, rs = 600 70 db i cc supply current, no load +/ - 16 ma i cc supply current, 150 load +/ - 36 ma v opp output voltage swing, rl = 150 +/ - 14 v z i input impedance 460 k r o output resistance 3 sr slew rate vi = +/ - 1v, rl = 600 , cl = 100pf, unity gain 10 v/us a ol open - loop voltage gain vi = 100mv, rl = 150 , f = 10khz 72 db gbp g ain bandwidth product vi = +/ - 10mv, rl = 150 , cl = 100pf, f = 40khz 2 mhz all content copyright ?2013 eisen audio, llc and/or respective owners. all rights reserved. terms and spe c ific a tions are subject to change without notice. neve is a t rademark of ams neve limited corp o r a tion united kingdom. eisen audio is not affiliated with this party. www.eisenaudio.com BA512 discrete operational amplifier eisen audio version 1.2 june 9th, 2014
architecture the BA512 discrete operational amplifier is composed of the following three stages: 1. a dif ferential long - tail pair input stage, with constant current source biased for ~2.2ma and emitter degeneration resistors, feeds [2.] from the loaded collector of its non - inverting input device. 2. a voltage amplification stage, with its own constant current so urce biased for ~4.3ma, provides le v- el - shifting and feeds [3.] as follows. single pole miller co m pensation is employed. 3. a voltage source of two times vbe comprises the vas collector load, passing the same constant ~4.3ma branch current. from either side of this "base spreader" a complementary pair of medium power output transistors is connected, yielding class a/b push - pull operation. power supply the BA512 is optimized for bi - polar dc power supplies of +/ - 12 to 20v, with the typical offering of +/ - 16vd c being ideal. it will draw approximately +/ - 16ma of dc current u n der idle conditions. [ note : the BA512 can eas i- ly drive ac coupled output loads presen t ing a nominal impedance of 140 or greater, but will draw more dc supply current, a c cordingly.] when tes ted with +/ - 24v power supplies, the BA512 shows an increase in idle current to +/ - 22ma, and output offset voltage increases to 100mv. with the exception of a greater vop, a u dio performance shows no significant changes. on - board 0.01uf bypass capacitors ma ke local power supply decoupling unnecessary. however, we reco m- mend best practice for decoupling a discrete op amp, e.g. local re s ervoir capacitors of 22uf or greater, in para l- lel with 0.1uf film bypass capacitors. feedback and stabilization typical freq uency response for non - inverting applications is flat from 10hz to 100khz. low value capacitors (e.g. 100pf) at any junction (i.e. power bypass, output load, series feedback, input shunt) will not significantly alter this high - frequency response, and are n ot necessary for stability, so long as the BA512 is operated within application guid e lines from 0 to 50db of voltage gain. application hints the BA512 maintains a dc offset of ~3mv at its non - inverting input, depending on source resistance. while this am ount is negligible in most circumstances, the following configuration considerations must be made: 1. input transformers having a secondary winding dcr of less than 200 , and/or unusually thin and se n- sitive coil wire, should either not be dc coupled to the ba 512, or have a resistor of ~1k inserted in series with the BA512 input. 2. when configured for variable gain, or any fixed gain greater than 1, an adequat e ly sized (i.e. for 1/(2pi r c) = better than - 3db @ 10hz) electrolytic blocking c a pacitor should connec t the shunt fee d- back resistor to ground. BA512 voltage gain thus occurs only in the ac domain, avoiding an increase in dc offset. this also protects a sensitive gain adjust potentiometer from dc damage. all content copyright ?2013 eisen audio, llc and/ or respective owners. all rights reserved. terms and specifications are subject to change without notice. neve is a trademark of ams neve limited corp o- r a tion united kingdom. eisen audio is not affiliated with this party. www.eisenaudio.com BA512 eisen audio page 2
assuming an ac coupled source, the BA512 r e quires a input bias shunt resistor (ideally 47k or less, when gain greater than 40db) connected directly from its non - inverting input to ground. depending on the value, this resistor may provid e an excess of se c ondary termination for input transformer ratios greater than 1:8. although gains of 54db or more are possible, the extremes of high and low frequency response will suffer from decreased bandwidth and increased distortion. recommended gain s should be limited to ~50db, which is the case when using the preferred negative feedback resistor values of 15k series and 43 shunt. for inverting configurations (e.g. active combining amplifier), tie the BA512 non - inverting input to ground, source the inverting input from a resistance of 47k or less, and adjust the feedback resistor for a gain of 1 or greater. there will be dc offset of a p proximately - 30mv at the inverting input, so you may wish to place a blocking capacitor in this node. complete cir cuitry for popular microphone preamplifier configurations is demonstrated in the figures 1 and 2 below. all content copyright ?2013 eisen audio, llc and/or respective owners. all rights reserved. terms and specifica tions are subject to change without notice. neve is a trademark of ams neve limited corp o- r a tion united kingdom. eisen audio is not affiliated with this party. www.eisenaudio.com BA512 ei sen audio page 3
all content copyright ?2013 eisen audio, llc and/or respective owners. all rights reserved. terms and spe c ifications are subject to change without notice. neve is a trademark of ams neve limited corp o r a tion united kingdom. eisen audio is not affili ated with this party. www.eisenaudio.com BA512 eisen audio page 4
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