I know the wiring is a bit sloppy but I've procrastinated making that a priority.
MountCyanide said:This OLD thread has been so helpful.
With this new addiction to DIY and my strengthening tolerance I had to up the dosage.
Ordering PCB's wasn't doing it for me. So I broke out the Ferric Chloride and learned to etch.
Thanks to the archive of this board and all of the brilliant and curious minds I pulled it off.
4 Channels of 312 preamps. And they sound awesome. Cinemag CMMI-8 in 2 and 75101 in the others.
Pictured above them in the rack is the Whistle Rock ML-12 which started this hunger for more API's.
What a satisfying feeling to make these things by hand. The etch really adds another layer of euphoria.
Not only the fumes of poison either.
JW said:There are 3 primary pins and 3 secondary pins. I believe in relation to the mixer schematic, that means the middle pin on both sides is the center tap, forcing me to series wiring.
I'm getting 12 ohms between the center tap primary and both the other windings on the primary. So, 24 ohms between the outside windings. The secondary has the bottom pin grounded to the case. Between the top pin of the secondary and the center tap I'm getting 1.3K
So question is, can I use this for the API circuit? And if so, what do I do with the center taps on either side?
This shunt arm resistor in series with the pot with rev.log taper, wired as a rheostat, set the amount of gain (NOT a volume pot) in correspodence with the feedback resistor. Max.gain is set with the pot dialed CW for zero ohms, so only the value of the fixed resistor (0 ohm + 37 ohm = 37 ohm) remains as the shunt arm resistor value.JW said:...I just measured 37ohm as the value I needed to obtain 'normal' 312 gain using these Yamaha parts. That is for the resistor in series with the volume pot.
with pot value 22K, this feedback resistor value would be same value 22K.So my values are:
120pF feedback cap
20K ohm feedback resistor
setting the max.voltage gain to 1+(22000/(0+37))=595.6 , this is log(595.6)*20=+55.5dB.37ohm for the above mentioned resistor in series with the electrolytic for vol. pot
setting the worst case HFP=max.gain to 1/(2*pi()*37*470E-6)=9.15Hz470uF for the lytic.
the gain setting pot (NOT a volume pot).22K rev audio pot.
the other components are the 22K rev.log pot.This is more in line with what I'm measuring (37ohm series resistor) for the gain I like, but then the other components 'conspire' to bring the gain down correct?
voltage gain of the input and output transformer is on top of the gain stage. With 1:7 in and 1:2 out ratio, this is +22.9dB on top of your 55.5dB gain stage.(corresponding to the 2622 input ratio and not the Yamaha transformer I'm using)
context? 'Closer to API' would have a gain stage maxing out at +40dB. When you dial in your pot to same value as the API312 (37 ohm fixed + 183 ohm pot setting = 220 ohm), your HPF with 470uF in the opamps shunt arm is set for 1.54Hz. A source that requires this amount of gain (+40dB + 22.9dB transformer gain = +62.9dB) most likely isn't a line device but a mic. As long as you don't want to investigate seismic figures with this mic, you probably want this HPF some octaves up in order to get rid of FI stage rumble. As a makeup gain stage for a passive summer or passive EQ, most likely a lot less gain is required wich in turn further decreases the HPF. IMHO I'd keep the 470uF. As always YMMV.JW said:I think I'll sub in a 1000uF cap for the 470uf to decrease the hpf to 4.3Hz, which is closer to API. Depending what kind of cap I use I suppose I could go even higher.
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