api 312 background noise?

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dramadisease

Well-known member
Joined
Jun 17, 2004
Messages
110
Location
portland - or
I've just finished up some api preamps based on the 312 circuit with a DI mod similair to Bo` Hansens (http://web.telia.com/~u31617586/API%20312%20DI%20mod..jpg) just minus the zeners and the protection diodes across the opamp inputs, and I seem to get a bit of noise when the gain is maxed out. Its a strange buzz that gets much louder when i switch it to di, but quickly dies back down where it was in the mic setting. I did use the 1M reference resistors, but mounted them on the MIC/DI switch (im going to try hardwiring them in the cards later)
it sounds like a wierd biasing thing with the opamp inputs. My opamp exp. is a bit on the limited side. Could someone lend a few ideas? also what exactly are the reasons for using the 12v zeners and the diodes across the opamp inputs? I figured the diodes were on the opamp as a rfi suppressant? Am I wrong?

thanks for the help!
-bryan
 
I have the diodes on and I get the same noise :S Bo told me that on his implementation there is no noise.... are you using my boards of you own?

cheers!
Fabio
 
im using modified 325 boards - keith's ssl psu +-18v and a ton of mogami wire! cinemag 75101A for the input trans with a 160k resistor to ground off the trans secondary
got your email Fabio, thank you much i'll put together an order and some other info - glad to here you're busy! it means you're doing a good job!

-bryan
 
i've been thinking about this alot, and I still think that the noise we are getting is related to the shift in ground reference on the input, if the 1m resistor was replaced with 160k and the opamp was always loaded down at the same resistance, would that work? I mean, would it load down the signal of the guitar too much? I'm not sure of how much of a buffering effect that coupling cap off the di input has.
-bryan
 
I was afraid it was just on my boards, I have changed that slightly over the revisions but still get some noise in v3 boards... will think about what you are saying about the load. I have tried without the 1M resistor, no luck too...

cheers!
FAbio
 
without the 1meg resistor (and the diodes) the di doesnt function, i assume because there is essentially no ground reference other than the impedance in the gutiar pickups.. im going to go fool around with this stuff right now...
ill post a bit more later

-bryan
 
Bryan, try this:

CDI1 =39pF
RDI1 = 3M3
RDI2 = 1M

Seems to work here for me...

cheers!
Fabio
 
didn't get a chance to update you the other day Fabio, the forum was down! I tried a couple different things - first was mounting the 1m resistor on the cards themselves, onto their ground instead of chassis and that helped alot. the second thing i found is that my channel 1 2520 is bad, when it starts to draw some real current, one of the output pairs in the opamp is clipping all strange... (atleast thats what i think is going on) so i replaced that. that wierd noise was gone, but i still had some buzz. I seperated my power grounding from my audio grounding on the cards themselves and my final little bit of buzz went away! ill have to email you a pic of the front panel, as im quite fond of it. (dont have pics handy right now)

thanks again for your help! ill see about getting an order together for some of your boards.

-bryan

BTW. i now have 2 bad 2520s 1 being a hunnington,ny 2520 and one being a later Datatronix 2520. the hunnington 2520 works up until right before max gain, then starts screwing up - the datatronix is just dead. Would it aid the group to have someone melt the epoxy off and figure the difference in parts between the 2? if anyone thinks so let me know pm or whatever works..
 
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