Well, well, well….first I must say, thanks Tony for giving me a very entertaining afternoon. I often wondered how long it would be until a thread like this (featuring you) came to be. Now I know!
Anyhow, since there are no “secrets” in what I do, let me enlighten you and anyone who cares to read this. Most all of the info I am about to type, has been on my website for as long as it has been active. So, this is really all old news but I will get you up to speed. The VP26 concept.
After seeing the 100-150 312 style mic preamps (to quote you Tony), that are on the market, I decided to do something a little different. The circuit I use is exactly the same as the preamp circuit that the original API used in their 1604, 2488 and 3232 recording consoles. From what I know, this was from around 72 to the late 70’s, a little after Datatronix took over. What I have I changed?
Besides using modern components…only two things. The coupling cap (ooh nasty!) between the opamp and output transformer was changed from 250uF/4V (bastard value in today’s world) to 470uF/16V. That will slightly lower the knee of the HPF it’s creating with the 20k resistor that is parallel with the primary of the output transformer. Minimal and almost not worth the discussion. The other change was the 250uF/4V (still a bastard value) cap that is series with the gain pot was changed to 330uF/25V. Again, minimal impact on the circuit. Not worth discussing.
Oops, I guess I did add the t-pad after the output transformer. My guess is not many people would want to give that up.
Now, that it’s for the changes. So, I guess what you are really saying is that Saul Walker and the early API team were poor designers and created a crappy preamp circuit and recording console?
Maybe you didn’t like the 33uF coupling cap that is before the opamp? Well Tony, you are right. It is not really necessary. Why is it there? It’s there because it was necessary when the console’s input channel was switched to Line Input. For whatever reason, Saul and company could have maybe switched it out when selecting Mic Input…but they didn’t. That’s why it’s there on my preamp. I mean really, who am I to second guess what they had specifically made the decision to do?
This same thing could be said for a handful of other components that are in the signal path. Like the 1k series resistor or the 150k load resistor or the 27pF shunt cap. Those are before the opamp. You could also loose the 20k R and 100pF shunt cap that are after the opamp. Again, they were used in the console circuit and my goal was to recreate that exact vibe so they are in use on the VP26 card. Transformers.
One of your manufacturers told me a little story about his conversation with the great Saul Walker and the 2622 input transformer. This was a few years ago from an AES show IIRC. What I do recall is what Saul said about the 2622. He said it wasn’t a great transformer (some 40+ years before your findings!). It was just some stock, off the shelf transformer from a now unknown manufacturer. It had issues. He talked about how he added the 1000pF caps on the primaries to tame some nasty HF ringing. That at least made it usable from his viewpoint. The 2622 is a gnarly little bitch of a transformer. It will give you a color like no other. It does not look good on the bench with an AP (is that what you used?). In fact, some might think it is “broken”!! LOL!! I have heard this before. In the late ‘70’s, API switched to Jensen input transformers. Who knows why for sure but cool, good for them. I was however after the sound of my 1976 API console preamp so I wanted a nasty little 2622. Low and behold, I ran across the great Ed Anderson. The transformer (amongst other things!) guru that played a part in the original MC76 as well as designed the Tav and Odd for Purple. Wait, I see you don’t sell Andrew’s stuff so you probably think they are sh!t. Anyways, Ed had already designed his take on the 2622 so he sent me a sample. It was nearly indistinguishable with the original AP2622’s in my console. Exactly what I was after.
Nearly all the same can be said for Ed’s 2623-1 output transformer. Not a great transformer on paper. Some would say it’s only good but, he nailed the old AP2623-1. Again, exactly what I was after. BTW, the AP2623 (some -1's and some -4's) was typically the only
output transformer used anywhere in the old 2488 recording console. Bet you think those suck too?
Now, put all these things together (a complete circuit) and you have a very vintage, 1976 era mic preamp based on my vintage API console. If you would have asked me, “hey Jeff, why did you do this and use that” I would have told you. Any other “design secrets” you think need sharing??
Again, thanks as always for the entertainment!