As far as I can remember (I have the 100A Service Manual somewhere) the Advent 100 and 100A were both discrete transistors and FET's, except that one IC used as a microphone preamp in the earlier Model 100. The Advent Model 100A's had Line 1 and Line 2 inputs (the leftmost pots) instead of a Microphone input and a Line input, and so I believe it lacked the UA-739 IC. If not recording, all signals passed through discrete circuits only.
I also had a Teac AN-60 and AN-180. Not quite sure about what the AN-60 had inside, but I did not like its sound, it soured me on Teac RTR's, which I also tended to think sounded shrill and "transistory". I later found out the AN-180 was stuffed with
nasty sounding UA-709 IC's in the signal path, which makes me glad I never used it, tho it did look nice~!
Anything as old as 1960's-1975 RTR's or Dolby units will be much less than optimum some 30, 35, or 40 years later. Most RTR units I see these days, are not even coming close to meeting original factory specifications for frequency response, distortion, wow and flutter, and signal-to-noise ratio, without significant rebuilds, upgrades, or repairs.
I have some Dolby A-361's, and find it quite interesting that you say the A-301 was much better sounding. Having looked inside, and based upon the massive use of cheapie tantalum caps inside the A-361's, I'm not surprised at all. IC's and Tant caps, there are so many, some have to be in the signal path. I'd love to go in and replace them all with lytics, or polypropylenes (as if there would be enough room...) and then do a comparison, but I don't have any extra A-361 units, or any A-301's to run such a comparison.
Steven L. Bender, Designer of Vintage Audio Equipment
I have used in the past the following: Advent 100 and 100a, Dolby Labs 334, Teac an-180 and Nakamichi NR-200B-c as well as Charlie's Dolby Labs A-301 beast. I really liked the A-301,as it was amazing for Dolby A tapes and pretty damn fine for the B tapes as well.The Dolby Labs A-360/361 pro units did not come close to the sound of the old A-301, which was the original Dolby Labs processor from 1966. Decca started using it in 1966 or 1967. I now have a stock Advent 100, which is all discrete. I think the 100A went to ICs. I find the stock 100 to be on the warm side and the Teacs to be a bit brighter sounding.
It would be interesting to hear one of the Advents in stock versus modified forms.
Rich Brown
Acoustic Arts
Portland, Oregon