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So you want to build an API 312?

Discussion in 'Electronic Repair Modifications DIY' started by fallout, May 15, 2003.

  1. fallout

    fallout Guest

    The circuit is extremely simple, and all of the parts can be purchased... For the output transformer, you have 2 options. Either buy them for $50 from Dan Alexander Audio (dont know if he has any or how many he has), or get a group order together and get them for about $20 - $25 (from the original manufacturer)depending on shipping costs ect... So what I'm basically asking in this post is for everyone who is interested in a group order on the output mic transformers to PM me and let me know how many you'd want to buy. So far we have interest for 40 units, so we need another 60! smile.gif

  2. hmr

    hmr Guest


    i wonder how many have been spoken for....
  3. ArtCriminal

    ArtCriminal Active Member

    Approximately 70 have been spoken for. I created a page of info that I've rounded up over here. It's by no means complete and the web page is kinda hackish but I slammed that together in about 10-15 minutes. I wanna straighten it out some but for now it's fine.

    Fallout (Jay) and I have discussed the project some. We need to work on the board layout and PS. My personal preference is to have the front panel controls mounted to the pre-amp circuit board. That can be a bit of a pain though. We'll have to see how things go.

    The PS that I'm thinking about does +/- 15 volts via 7815/1915 regulators. That should give us an amp a side with should supply the pre's with more than enough juice as they consume 65ma at +30dBm output. I was thinking about using a LM317 for getting the +48 phantom supply and also doing a 7812 for a "control" supply for relays and LED's and such. That might be a bit excessive though.

    When I get a chance I'l build up a PS. Of course Kev's got a new PS under development that might be perfect for this and just about any other DIY opamp project.


  4. gus

    gus Guest

    I have question. Are the module type opamps that much different from a good IC OPA2604 etc? The NE5332 can often sound very good to this day. IIRC it is a mid to late 1970s design.

    Has anyone used the API transformers with an opamp circuit?
  5. fallout

    fallout Guest

    Gus is that you? The NJ guy? It's gotta be, not many other Gus' i know on these forums! :D We met at that class in New Brunswick a few years back and spoke a bit on Aron's stompbox page.. Dunno if you remember me.. LOL. Drop me a PM when ya get a chance...

  6. gus

    gus Guest

    Yes Jay its me. What was that a win98 or nt class??? I am still working at the same place also same email I gave you as well . I have been building/modding microphone circuits. I even have a nice little metal lathe now as well as a toy taig lathe.
  7. fallout

    fallout Guest

    Great to run into you again Gus.. Yeah it was a WinNT class if memory serves me right. I lost your email address but for some reason i remember it as YOURNAME@yahoo.com? Is that right? If not drop me a PM.. Catch you later..

  8. You may want to consider + and - 18vdc for your supply. (they can go as high as 20vdc) You can get the full 65db of gain with that. At 15 I think it is only 60db. They can actually run down at 12vdc but the only crank like 50db or so.

    I run my originals at 15.5vdc(thats what I had laying around) and they sound really good, but you may want the extra gain.

  9. Kev

    Kev Well-Known Member

    Yes these things can be considered to be different to a normal IC as we know it.
    There are made of discrete components and often potted, for secrecy AND for temperature stabilization.
    For us DIY'ers it can make for a potentialy simple high high quality Mic-pre. Most of the hard work is inside the pot.
    All we potentialy need to do is chose the larger components and assemble and provide power.

    Hopefully High Quality parts will equate to Great sound.

    stand back ... Shameless plug

    JLM Audio
    look for the JLM990 Discrete IC

    Some schematics to drule over
    http://www.jlmaudio.com/JLM990 Mic Pre.pdf
    http://www.jlmaudio.com/JLM990 Mic Pre Single Rail.pdf

    Two options here.
    Dual rail at 34 volts
    Single rail at 60 volts


    That should keep the BIG voltage boys happy.
  10. ssltech

    ssltech Guest

    JRE, the gain shouldn't be related to the rail voltage, surely? -Headroom, yes, but gain, no.

    Also, a halving of the rail voltage would reasonably be expected to equate to a halving of the maximum level, therefore a 6dB reduction. -I really don't see how the difference between 18V and 20V could possibly be 5dB... one-point-something perhaps, but not five... -and running at 12V compared to 20V resulting in a fifteen dB drop? ...??? That should be a 5dB drop I would have thought...

    Unless I'm on completely the wrong track with what you meant, those numbers can't possibly be anywhere near correct, surely?

  11. Flatpicker

    Flatpicker Guest

    I’m interested…where do I sign up? Also, what about the input tranny (Mfg? – Jensen?) and the 2520? Can’t we get them cheaper in quantities too?
  12. Flatpicker

    Flatpicker Guest

  13. ArtCriminal

    ArtCriminal Active Member

    Fallout is taking names of people that want output trannies. We've been talking about it. I've actually got the extra cash to front the whole run of a 100. We're pretty close to having that 100 accounted for. I'm still holding out for a few days in case we have more interest that way I'll just buy more transformers.

    As for input tranformers we're looking at using the Lundahl 1538XL. They are $57 a pop from K and K audio. I've got all the information I know of gathered at my site. Take a look around there and get some ideas. 8 channels of these guys are gonna run you between $1200 and $1400 depending on how fancy you get.


  14. The input transformer can be an API 2622 (pretty much impossible to find, but is the original part for these suckers). The Jensen JT-110 was used in some API consoles. The Lundahl 1538XL works as well, and I believe that is used in one version of the Old School Audio API style preamp. Also, for an even cleaner sound you could go with a Lundahl 1636 (amorphous core). I will likely build 2 with the 1538XL, and 2 with the 1636 (I like flavours).

    The circuit is super simple, and should reward good parts choices. I will likely stick with Forssell gain blocks, but would consider going for a grittier type block on the 1538XL based channels. Anyone know where to get original API 2520's?

    Anyone else notice that the output transformer is located in the feedback loop of the op-amp? Neat...I suppose that cancels out some of the distortion introduced on the primary side of the transformer.

    Regarding the voltage issue, I think +/- 15V is more than enough for these guys, there is not much to be gained in terms of headroom from a 4-6 more volts in the supply.


  15. Kev

    Kev Well-Known Member

  16. cjenrick

    cjenrick Active Member

    I needed something to go along side my AM16 in a 1U rack, and now this! What's approx price/availability on the JLM thingamajig?
    I know it's been around here for a while, but sometimes things just need a jump start to get going, ya know what I mean?

    Nice job James, and I love your chassis punches! Fallout, you are the man for handling this size order!

  17. Bear's Gone Fission

    Bear's Gone Fission Active Member

    Old School Audio is doing their own version of the 2520, right? Anybody know if they've made their opamps available to the public? Otherwise, for the 2520/990 footprint opamps I believe the ones available directly for sale are from John Hardy, Fred Forssell, Millennia Media, JLM, and possibly Summit? I know Summit has a 990, but I'm not sure if they sell it. Anything else?

  18. rascalseven

    rascalseven Guest

    The SPA690 from Inward Connections (they guys that made the Vac Rac stuff a few years back) also fit the 2520 footprint but can run on +/-24 volts. You can reach them through [url="((deadlink))[/url] .

    Inward Connections also sells custom I/O transformers made for them by Riechenbach, and they have a four channel kit you can purchase and build yourself. Good stuff.


  19. fallout

    fallout Guest

    No, James is the MAN! I'm just helping organize it.. silly.gif


    P.S. You're the man too.. Your T4B rocks! Now i just gotta get around to building the LA2A to use it in! lol....
  20. ArtCriminal

    ArtCriminal Active Member

    I think I tackled the OpAmp thingy awhile back. I s'pose that's in another thread. Anyway...The choices that I know about are:

    Hardy JH-990
    Millennia Media MM99
    Forssell Technologies JFET-990
    Avedis' 1122
    Joe Malone's JLM-990
    API 2520
    Jensen 990

    I believe that's about it. I'm sure there are others out there. Alot of those are obviously 990 variants. The 990, although pin compatible with the 2520, isn't a 2520. Each one of those opamps is gonna have a slightly different sound and interact with the surrounding circuitry a bit differently. Has anyone compared these different opamps? Perhaps I need to go out and take a look at these various offerings and pull their specs into a table.

    Anyway....For the purposes of creating an API 312 clone I would think that first up would be genuine 2520 opamps and second would probably be the 1122 opamps. I wouldn't know exactly though because I've never used them.

    So there's some stuff to marinate on. Have fun!



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