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Roll your own preamp, anyone?

Discussion in 'Preamps / Channel Strips' started by Bear's Gone Fission, Jun 1, 2001.

  1. Bear's Gone Fission

    Bear's Gone Fission Active Member

    Well, I'm trying to plan my electronics projects for the summer, and I have a nagging impulse to build some high quality mic pre's. I've been using console pre's on my A&H or whatever I can borrow at the time, but I want something good. I'm thinking clear and clean would serve me best, but maybe some flavor too.

    So my thought so far is to build several channels that work with the Jensen 990 / API 2520 style opamp blocks and use different opamps for different flavors. The only 990 based circuit I've spotted at the Jensen site is a simplified twin-servo circuit. So has anyone (besides John Hardy) built the twin-servo? Anyone tried the API 2520 in this application? Are there other circuits I should look for that use this sort of opamp?

    I'm also open to other circuits using different devices, though I'm not hot on figuring out a good cooling scheme for tubes in a home brew.

    Thanks

    Bear
     
  2. Tom Cram

    Tom Cram Active Member

    Bear, my buddy Darrin here at dbx has this to say;


    Hi,
    The Jensen 990 op-amp is a good design and was made to compete with the already existing API2520 op. Speaking as a former member of the API team in Springfield VA., The difference is as follows:
    1) In 1966 the API engineers came up with a class A OP Amp which in the beginning, looked like it had a distortion problem. Rather than throwing it in the trash and starting over, they elected to plug it in and listen. What they found was a pleasant overtone which sounded like a tube amp right as you turn up the volume and that warm sound develops short of sustaining but clear and warm. The best part of this design is it creates that warmth at low input signal as well as when you push it.
    2) Some years later Jensen came out with a design as well. The headroom was about the same ( Within 4db ) but was a more transparent signal path. Still it was a great sounding OP-AMP.

    When Designing a PRE-AMP design you must first start with most companies have forgot. KEEP IT SIMPLE. The API 512 PRE AMP Was designed around the famous 312 preamps used in their vintage consoles. The path is so simple that it runs audio from a 2520 op through either a Iron core Transformer ( Instrument ) or a Jenson Nickel Core ( For Vocals ) and out. If you examine the 512 closely, you will see about 20 support parts and that is all.

    Further, If you design a circuit that does not work the way you want it to, don't add parts to filter the problem. Instead look at the area that works within it, and start from scratch. This is what separates a cheap design with a world class type.

    Most applications within store bought instruments today are not developed for warm sound as they are built for the masses. This is why I would recommend a 2520 over the 990. Tube will produce warmth, but only when you reach a volume level where the tube starts to overdrive. The API250 does that at low levels in and just gets better the more you push it. Also think about making a jumper section so you can switch your unity gain from -10 to + 4 dbu operation. Then you can take your preamp box with you to any location, and it will match up to any recorder you are working with.

    I hope your project is a success

    Darrin Ward
    Harman Music Group
    dward@hmgsl.com
    801-568-7661
     
  3. Bear's Gone Fission

    Bear's Gone Fission Active Member

    Thanks for the info Darrin and Tom. I was thinking a 4 channel unit maybe, 2 warm, 2 clean, so probably two API and two Hardy, though I might try out the MM-99 and Fred Forrsel's take on the 990 for kicks. I'm in analog, so more warmth isn't always necessary, I sometimes want that straight-wire-with-gain thing.

    Is the 312 or 512 topology to be found on the web? (I hear the 212 is a bit thin sounding by comparison.) Or has anyone tried the 2520 in the Jensen Twin-Servo circuit?

    I have the white paper on the Jensen 990 and know I'd do better buying those from John Hardy. Does anyone have the schematic for the 2520? Does API sell them seperately?

    Thanks

    Bear
     
  4. Bear's Gone Fission

    Bear's Gone Fission Active Member

    Found a 312 schematic, but damned if I know what's with those 15 terminations. Is that something to do with an API connection standard, some sort of multi-pin connector for modular components?

    Bear
     
  5. hollywood_steve

    hollywood_steve Active Member

    Is the 312 or 512 topology to be found on the web? (I hear the 212 is a bit thin sounding by comparison.)
    *********************************

    Where did the 212 criticism come from? I've never had the opportunity to A/B a 212 to a 312 or 512, but I have heard all three separately and did not notice any *big* difference with the 212. I'm curious whether the "thin" commment came from someone doing a direct comparison with another API model?

    Does anyone know if either of the mic preamp comparison CD's produced in the last few years contains different API models that can be compared against each other? (I guess that API is pretty confident of the 212 in that it is the only preamp available in their 6 figure Legacy Plus consoles - for whatever that's worth...)
     
  6. anonymous

    anonymous Guests

    Originally posted by Bear's Gone Fission:
    damned if I know what's with those 15 terminations. Is that something to do with an API connection standard, some sort of multi-pin connector for modular components?

    Yep. They almost all employ the same 15 pin connectors...I don't recall a mic pre using more than 9 of them [3 input, 3 output, 3 operating voltage, the +48vdc "phantom" runs off 'input XLR' pins 2&3].
     
  7. Bear's Gone Fission

    Bear's Gone Fission Active Member

    Where did the 212 criticism come from?

    rec.audio.pro, maybe just one or two posts said they sound thin, and I just believed it hook line and sinker without even hearing the evaluation criteria. Stupid of me, I know. At any rate, the 312 is the legendary circuit, so if I'm gonna do an API-esque pre, it's not a bad starting point, you'd figure, and I'm likely to find more info available on it.

    Unfortunately, my google searches aren't taking me much farther than that guy who'll sell you an MM-99 opamp, unlabeled Lundahl transformer, and a block schematic all at a huge mark up. Argh.

    Bear
     
  8. MPlancke

    MPlancke Member

    Originally posted by Bear's Gone Fission:

    Unfortunately, my google searches aren't taking me much farther than that guy who'll sell you an MM-99 opamp, unlabeled Lundahl transformer, and a block schematic all at a huge mark up. Argh.
    Bear


    Well the heart and soul of the API pre-amp is the 2520 opamp and of course the transformers, every thing else can be gotten from any electronics supplier. I have seen the trannies come up on Ebay from time to time. The opamps are probably available from an API dealer.

    My feeling is that once you buy all the parts and assemble them you might as well have just bought a raw 312 card and save yourself the hassle.
     
  9. Bear's Gone Fission

    Bear's Gone Fission Active Member

    I think I did see API transformer specs posted somewhere, so I could find substitutes from Jensen, Reichenbach or someone. Does API sell just the raw cards? I did find a name of someone at API who is supposed to be able to sell the 2520's, I guess I'll have to track him down and ask.

    As a further twist, I got an email from a pre-amp guru who said he sells his own opamp cards seperately, and will hook me up with schematics, too. Not in the same package, just on it's own card. Considering that, too. (I'll divulge who said guru is if he says he doesn't mind, as it was a private email and not on the forum. I'll just say his stuff is well known around here and kick ass.)

    As to why do it myself? First, my time isn't worth much on the open market, so the cost analysis of DIY isn't as foolish. Second, unless I get my hands into it, I'm not sure I'd ever know this stuff as well as I want to. And third, I have the notion of this being an evolving project to tweak and alter as my needs change. One thought for an eventual product is having all the pre's I need for a drum kit, in various flavors, in a package with a basic mixer and insert/ busing options so I can get a processed stereo pair (or mono) output. That's way down the road, but it'd be a nice sort of custom piece to tinker with and tweak that I doubt anyone is going to put on the market. Probably frivoulous, but this is my hobby, not my day job, so as long as it's fun and interesting, I'm into it.

    Bear
     

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