My API-312 from the original schemo

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raysolinski

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Jun 16, 2004
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329
Location
Cincinnati, OH
This may belong in the big API thread although it is not one of the projects from there...I just finished an exact clone of the 312 (from the walzing bear schematic) using a Melcor 1731 graciously sent to me bt Peter C. (arrived Thursday, MANY thanks)..Built on project board from ratshack and using a +/- 15 volt power supply I found at the electronics surplus store for 5$ (!.. 400ma)..and a Lundahl wired 1:7 for the in...after many trials and tribulations (one fried cap, wired the power supply diodes backwards...d'oh)..I fired it up and it sounds marvelous..a little twichy in the gain since I am using a 10k linear pot until I get a 22k reverse log or at least a 20k audio taper. This thing is SUPER quiet....Man, did I learn alot and read alot here last night... (my first raw build from a schematic)....I am running it unbalanced at the moment..I searched the threads for an idea on some output iron..Am I correct thinking that I need something that will take dc on the primary or is that only for projects using the 2520? Well, I am off to add phantom to this guy and do some listening tests.... :grin: :grin: :grin: :grin:

Ray
 
Am I correct thinking that I need something that will take dc on the primary or is that only for projects using the 2520? Well, I am off to add phantom to this guy and do some listening tests....

The original transformer, the 2503 does not have a gapped core, which is necessary for handling DC voltage.

Peter
 
You don't need an output transformer with a gapped core, because you do not need to put any DC through the transformer. This is just as true for the 2520 as it is with your Melcor. Gapped cores are for single-ended circuits that intentionally put the output transistor's DC current through the transformer primary, such as the Neve BA283AV. In theory there shouldn't be any DC on the output of the Melcor OR the 2520, but in practice there is a little bit - a few millivolts, maybe a few dozen. But you don't need to send that through the output transformer. If there's a lot of offset on the output of the amp you're using, then use an output coupling capacitor. Yes, I know there's no coupling cap in your original API 312 schematic. But there's no Melcor 1731 either. Slapping a cap in there is not a big deal.
If the offset is low, you probably don't need the coupling capacitor. As you can see in the schematic, the 312 had DC coupling to the transformer and it works fine, even though the 2520 is not a precision amplifier (meaning it does tend to have some offset).
I'm being kind of vague because I don't know exactly what the limit is in terms of how much offset you can have and still get away without a coupling cap. I converted a bunch of 325 cards into 312 circuits a while back, and I burned out a pair of 2520s while running them DC coupled to the output transformer. I was also running them on 20V rails, and I don't know which, if either, of these factors caused the amps to burn out. But that's one possibility for you. The other is that the modest DC bias on the transformer will reduce low-frequency headroom, causing an increase in distortion. This DC can also magnetize the transformer, causing a semi-permanent increase in distortion. Who knows, this could be why people like the sound of the API 312. 2nd-order distortion is known to be pleasing.
If you do decide to use a coupling capacitor, it should be a very good cap (Panasonic FM would be my choice), it should be large (2200 to 4700 uF), and should have the lowest voltage rating available - probably 6.3V. You might as well measure the offset and install the capacitor with the correct polarity, although a reverse-bias of less than a couple hundred millivolts is not going to ruin the capacitor. There is the theoretical distortion caused by the reverse-bias that occurs on the negative half of the waveform, and if you're worried about that then buy a bipolar capacitor instead. It also wouldn't hurt to bypass the electrolytic cap with a small (.01 or 0.1 uF) film capacitor - a nice polypropylene cap would be good here.
 
[quote author="ulysses"]
If you do decide to use a coupling capacitor, it should be a very good cap (Panasonic FM would be my choice), it should be large (2200 to 4700 uF), and should have the lowest voltage rating available - probably 6.3V. You might as well measure the offset and install the capacitor with the correct polarity, although a reverse-bias of less than a couple hundred millivolts is not going to ruin the capacitor. There is the theoretical distortion caused by the reverse-bias that occurs on the negative half of the waveform, and if you're worried about that then buy a bipolar capacitor instead. It also wouldn't hurt to bypass the electrolytic cap with a small (.01 or 0.1 uF) film capacitor - a nice polypropylene cap would be good here.[/quote]
FWIW, IIRIC I've also seen in some 'original' schematic (one of the...) that the mysterious combination of a 470uF elco & some resistor in // was used :roll:
 
Thanks fellas...I am running it unbalanced through a 1000 uf 25 volt cap right now (sprague)...I had always heard thru the grapevine about gapped tranny's for Neve and API style projects without fully investigating it (Thanks Justin) ...The reason I went with the verbatim build from the old schematic was that API originally used Melcor's before they started building their 2520's..it oscillates at low gain settings so I think I need to up the compensation cap from 47pf to 100pf (the schemo calls for 120..I will go to that if need be) and get (or build) a good 20-22k attenuator and run that in series with a good 200 ohm metal film resistor..I kinda pieced it together just to see if I could get it working (and was amazed that it did)..now that it does (!!) I will tidy it up for the final rack...Add one of Joe's go-betweens for phase and padding. I am going to keep the +/- 15 volts on the rails....I will post pics when it doesn't look like a bowl of cooked spaghetti :grin:

Ray

PS. And some sound files of this 312, my Spectrasonics 101's and for comparison my Wackie 1402 vlz's..
 

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