API 312 Thread!

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OK, my turn for a transformer question. I'm building a pair of Fabio's 312s using Cinemag 75101 input iron. Should I strap these for 10:1 or 5:1? I'm trying to cram the pair in a 1U with front panel Hi-Z jack, pad, pol, +48, and mic/line switches, so I don't really have room for another impedance switch. Currently I'm planning to use these with dynamics and fairly hot condensors, but if I were to want to use ribbons in the future would I be wanting that 150 ohm input? Any opinions/pointers appreciated...

A P
 
Thanks A P
I was looking into this at the weekend and that was my conclusion so it's nice you could confirm it, you've also asked my next question
 
hey AP,
just a thought, but would RLED1 being raised to 5K1 lower the 48V too much?
don't want to starve the PP to the mic (eventhough most mics nowdays have a pretty broad PP tolerance). hmmm, what exactly is RPP3 doing? it would be in-line with the PP so maybe adjust RPP3 to gain back some voltage if the RLED1 is bleeding too much of the PP. does this make sense?
I'll make you listen to me play through your Marshall next
week
Click to expand...
ha! i haven't turned that thing on in a while, it'll probably blow both of us up! :twisted:
-grant
 
Grant,

RLED1 is just a current limiter resistor for the PP LED (so it doesn't become a single use PP flash bulb). I wired it to the junction of RPP3 and the phantom switch which puts it in parallel with the PP fed to the mic. As long as the 48V supply can handle ~10mA for the mic + ~10mA for the LED it's not a problem. I didn't finish wiring everything up yet--still have to deal with the gain switch--so I haven't tested anything yet.

A P
 
Okay I know this is buried in this thread somewhere, but I can't find it. I have just finished my API from the original scans. I have oscillation, or I guess that is what is as I don't have a scope. Sounds like white noise. It only occurs at low volume. One of my opamps does it more than the other. I think I should change the 220pf cap to a 330 pf. Is that the correct solution? I am using the melcor 1731.
 
Look for the "Deconstructing the Melcor 1731" thread in the metas. It's a confusing thing, but near the end there are a lot of discussions about various component values. Make sure you built your 1731's using the correct values. What boards did you use, BTW? Also, Fabio recommends tacking an additional 100pF (or was it 120pF) cap to the back of the 1731 between out and -in. That way you don't have to mod your API board if you change to a different DOA. The 2520 only needs the 120pF that's on the API circuit, so it makes more sense to add the cap to the 1731 than the preamp itself.

I've built a couple of 1731s and bench tested them using a strapped together circuit like the API--no oscillation. Haven't finished my API boards yet, so can't say how they act in the real thing.

A P
 
Finally almost done with my 8 312's using Fabio's PCBs.. I have a quick question though. I remember reading about phantom power still being present on the mic input even when the switch for phantom is off.. Fabio mentioned using the Phantom LED to discharge the PP by raising the value of the RLED1 and connecting the LED to RPP1 and RPP2 instead of SW_VCC. Has anyone tried this? If so did it work out well? Any other work arounds? Thanks!

-Jay
 
Saw this amazing thead and would like to start also with a bunch of DIT aPI's 312. Are the designs Fabio made for the PCB's still available? I can not fin them on the other site :-(

Kind regards,


Harrie
 
Bump
[quote author="froombosch"]Saw this amazing thead and would like to start also with a bunch of DIT aPI's 312. Are the designs Fabio made for the PCB's still available? I can not fin them on the other site :-(
Kind regards, Harrie[/quote]

Yes, I'm affiliated to Harrie's Studio Froombosch.

@ Fabio, what's happened to your homepage (diypill etc)? If this is answered somewhere else, I missed it. Sorry. We'd really like to populate some (non-standard) boards. Are these A*P*I boards still available or their gerbers?

TIA,

Gerald
 
Hey guys,

What's the common P/S everyone is using for their A*P*I 312's here now? There's several options. I'd probably like to power 4-8 pres.

Thanks!
Siegfried
 
I'm curious about the same thing fallout (jay) is, that is using the phantom power led to drain the phantom current. I usually turn off phantom with a mic connected, but just in case, its a nice thing to add while doing the build. All my led's will be wired off board already, so it won't be a big deal for me.

Billy
 
Thanks guys. I've noticed that the cleaned PDF of the 2 boards show different revisions? It seems that the top image is a copy of the original scan that was posted here. I'm not sure what the image on the bottom is. Can someone clear this up, and which one I am to use?

Thanks!
Siegfried
 
A few things... If you are talking about the schem thats on danalexanderaudio.com, I dont see an R1.. So I'll assume this is probably the 22k Rev Log gain pot... The values of R4 and C5 are dependent on the input transformer you use. For example, Cinemag recommends a 150k when using the CM75101APC and NO cap for C5... Also, I dont believe C6 and C7 are needed with the input transformers you plan on using.. I hope this helps..

-Jay


[quote author="Siegfried Meier"]I'm also going to post this Parts List and perhaps someone can fill in the gaps for me. I'm missing R1 and a few voltages on some caps. Also, I think those are diodes and I'm pretty sure the transformers are correct (or can be at least).

Siegfried

http://www.siegfriedmeier.com/electronics/[/quote]
 
[quote author="enthalpystudios"]I'm curious about the same thing fallout (jay) is, that is using the phantom power led to drain the phantom current. I usually turn off phantom with a mic connected, but just in case, its a nice thing to add while doing the build. All my led's will be wired off board already, so it won't be a big deal for me.

Billy[/quote]

I've wired mine this way, but have stalled (no time, no motivation, other excuses :oops: ). When I get it up and running I will post here to let you all (ya'll) know how well the scheme works...

A P
 
sweet

i haven't started mine yet, but I plan to (hopefully) today

i'm going up to my local 'sign and trophy shop' to try and get plate engraved for each channel. they can't do 19inches, of course, but they can do smaller, so i'm thinking about doing cutouts in my frontpanel, with engraved back panels from the sign shop for each chan. I'll see what they say. Silly Sign peoples.

But hopefully I'll be ready to just plug in trannies when the group order comes through.

billy
 

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