With apologies to John Hall's memory, some facts gleaned from (recent) personal experience and Altec and Langevin manuals and catalogs in my possession:
John Hall claims to have designed both the Langevin and the Altec preamplifiers in question.
I've dealt with over 40 of these modules in the last few years with only one transistor and several of the power supply electrolytics ever being bad. The current draw is almost always spot-on with the manual specifications, which should refute any notions of poorly matched or selected parts. I'm not sure how one could call this clumsy design, being well-established methodology in tube design and perfectly applicable to SS.
The 470A / 475A / 9470A / 9475A are all the same amp with only the slightest circuit variations, those being a change in some shield connections, an overall polarity inversion, and a switch from matched pair single case 2N2716 transistor pairs to individual 2N3900A's.
NOTE THERE IS A LATER SQUARE 470A THAT IS NOT THE SAME THING! APPARENTLY ALTEC STARTED USING THAT DESIGNATION FOR A TELEPHONE MODULE AFTER THE ORIGINAL 470A WAS CHANGED TO 9470A. LOOK UP PICTURES IF IN DOUBT! A 470A SHOULD LOOK EXACTLY LIKE A 9470A IF IT'S REALLY THE SAME THING!
The 470A / 9470A are meant for rackmounting, the 475A / 9475A were meant for retrofitting 250SU consoles when you were ready to throw out those horrifyingly bad
458A and 459A tube modules. They were also stock in the SS 250T3 consoles. There are less external connections on the 9475A's, but everything is there on the circuit board.
Judging by the catalogs and schematic notations, the Langevin came on the market in 63, and the Altec drawings are dated 9/64. Both types appear to have been marketed continuously until at least 1972.
The Langevin AM-16 is the only other American made preamp I know of using a push-pull class A discrete transistor circuit. John Hall recently stated
"It (Altec 9475A) is an updated Langevin AM-16. Has wider freq. resp and less distortion. The input xfmr was much better than the AM-16." They are the only two stock manufactured circuits with any similarity. Every other manufacturer went with single-ended designs. The Altecs and Langevin both have nearly identical power requirements, harmonic distortion ratings and output power ratings (+27 dbm). Both have -127 dbm noise ratings. -22 dbm is the input overload level for the Altec, which matches pretty much everything else manufactured at the time. The AM-16 has about 7db less gain than the 9475A, as I'll mention again below. This gain difference relates to a measured lower input headroom for the AM-16. Quoted gains are 45 db (input terminated) for the Altec and 45 db (termination unspecified) for the Langevin. My measurements show 51 db for the Altec when wired 150 in and 600 out with no input termination. The AM-16 must have a higher amount of negative feedback and/or lower stage gain due to the use of 8 transistors rather than 6 as found in the Altecs. Gain specs and basic theory are pretty much the same for the input iron (2db more gain in the Langevin input at 19 db), but the outputs are totally different in theory with the Altec being a straight 1:1 repeat coil and the Langevin being 1:1:2 with McIntosh style output primary coupling which makes for very different feedback paths. The audio path crossover in the middle of the AM-16 circuit should contribute to a different push-pull / feedback harmonic distortion cancellation profile, and the additional transistor stage should make a pretty large difference to the AM-16 sound also, but not as much as the iron. The Langevin circuit seems more overkill to me, though in elegant ways. It adds a pot for adjusting harmonic distortion; this is good in theory, but what if it's out of adjustment? The freq. (and implied phase) charts as published by the companies are certainly night and day differences. The AM-16 claims to be down 6db at 40kHz, where the Altec's -3db points are listed as 2.5 Hz and 100kHz.
The Altec circuit would be very easy to build as there is nothing unusual about the transformers. The Langevin input is not unique, but it's output is specialized. The Altec transformers do indeed have Peerless stamps and part #s, and match the profiles of other Peerless products to a very high degree. I see no reason for Peerless (one of the best in the business) to farm that winding work out as John Hall has stated. The Altec input in particular is practically identical to the 458A / 459A tube input and the previous generation 4629 input, at least to the point that one could replace either earlier type with the 9475A type.
As to sound, both are great preamps, but to my ears the 9475A is the more hi-fi unit with an overall faster sound on transients, tighter bass and better top and bottom octave extension. The AM-16 in comparison has a slightly slower, milky/gauzy sound with a slightly more rounded tone; more in the direction of the tube gear that both replaced. The AM-16 has a slight upper mid/treble bump while the 9475A is more purposefully flat. These comparisons may or may not seem hair-splitting to the individual end user. Many people really love the slightly softer more mid-range focused sound of the AM-16. The AM-16 seems to win the midrange competition over the 9475A. The AM-16 had about 7 db less total gain in a direct comparison under identical and matched standards, including same power supply at same time.
The only remaining piece of the puzzle would be the question of how the AM-16 came to be so well known, and the Altec did not. I suspect it relates to fate regarding placement of AM-16's in certain hit producing studios where word would have spread. I would think more Altecs ended up in radio, TV, and film studios due to marketing approach differences between the companies. I would suspect the # of Altecs to be higher than Langevins.
I think I'm working on a book, so compiled contents copywrite 2005 Doug Williams