API 312 Thread!

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[quote author="AnalogPackrat"]

1) D2 is a double diode (two diodes in series, both with the same polarity so you get a 1.3V total drop across them)
2) C2 should connect to - in rather than the base of Q3. Simply pop out the leg closest to the - in pin and connect it to the stub of the pin that sticks out on the top side of the PCB. It might be easier to pull C2 and replace with a new on leaving one lead long enough to connect directly to the top of the - in pin (that's what I did).

A P[/quote]

Thanks A P

I'm gonna fix #1 and #2.

I've already made the C5 correction mentioned in #3. The 100pF cap is under the board.
 
benny said:
I've got either a selfbuilt Melcor, an original 2520 or 5534 ICs.
Click to expand...

While the Melcor may be a problem it is worrying that all 3 DIFFERENT op-amps have the same outcome, which leads me to think it is the main circuit, not the amp block.

I would ohm out your wires just to check you have connected it all correctly and also triple check your gain switch wiring.

Cheers

Matt
 
Missed that little tidbit, Matt. Whoops! You are right.

So I didn't see any description of your test setup, Benny. Can you describe what you've got hooked up here? What mic, what switch settings on the 312, what are you feeding the output into (soundcard, console, ???), etc. Maybe you're just overloading the input of whatever comes after the 312...

Another possible avenue is to pull the red/brown connections on the output iron and populate the board with the five components for unbalanced out (RUx and CUx parts) to see if that works. If it doesn't help, your problem is not the output iron.

Also, do you have any test equipment available (scope, meter, signal generator)?

A P
 
I tested the preamp with different microphones. All dynamic ones. Sennheiser, EVoice. I first had the line out connected to cheap active pc speakers but as the distortion occured I connected it to my mixer and the distoriton still was there. Output level does not seem to be too high. There's distortion and every switch position and combination and from lo to high gain. Even if I just whisper into the microphone. I think I'm gonna test around a bit with the output xfmr wires as they seem to be in a different order than on fabios boards or others I saw here.
 
Can you describe the distortion? Is it thin and raspy? Does the ouput signal lack bass? Maybe you've got the output transformer secondary windings wired out of phase. Someone else had this problem on a 1272 build a couple of months ago--you should be able to find that thread with a search.

Keep at it!

A P
 
I would say the distortion sounds like heavy clipping. Like speaking through a guitar amp with lots of gain. But it isn't thin or so. Just heavily distorted.
Unfortunately I don't have a scope or signal generator, just a multimeter.
Maybe soething could be wrong with the gain switch.
I connected the the middle pin of my switch to the pin left of number 1 in the picture. It's the one above the right pin of the gain pot when looking from component side. The other 12 positions each go to the resistors RG1 to RG12. I'm not using the gain pot, but do I have to connect the right pin of the gain pot to another point? The one leading to the wire jumper next to RG1? I don't have it connected yet.

http://www.thediypill.phx.com.br/forumfiles/R312parts_rev3.pdf
 
You can use a 25k rev log pot OR a gain switch--not both. The pole of the 12 pos switch can connect to any of the four holes on the trace to connects to C3+. Use whichever one you want. What kind of switch are you using here? Can you post a picture of the back of the switch so we can see how you've got it wired up? Also, what happened to your other photo?

A P
 
I'm using one deck of this switch. I had it at home so I used it. The pole is connected to one of the four holes you described. The other twelve pins are connected to the different gain resistors using ribbon cable. I think the gain switch is working right. I've got 12 steps from low to high gain, but it distorts even at low gain settings. I don't know about another photo? I thought I only uploaded one. :?

http://www.reichelt.de/?SID=28ORoI@H8AAAIAAB7fnkc1c592ce6d217b0d393b2b890ff6a053b;ACTION=3;LA=2;GROUPID=3283;ARTICLE=15294
 
OK, let's try to measure exactly what your gain switch is doing. Power off your unit and pull the op amp (just for extra insurance). You've got a meter--set it to read resistance. Clip a mini grabber or alligator clip to the end of R2 nearest the op amp pins (the end that's furthest from the gain switch). Clip another one to the pole of your gain switch. Be careful that nothing shorts agains the leads. Now turn the gain switch and tell us what resistance readings you get from min to max. You should see it vary from about 25k down to about 200R.

A P
 
All problems solved.

The 2503 on the output didn't work the way it should. I put in another one and now the 312 is working and sounds really nice. :green:
Just waiting for the 990 kits and the potted 2520 to make it all discrete!!! :thumb:
 
[quote author="Benny"]Just waiting for the 990 kits and the potted 2520 to make it all discrete!!! :thumb:[/quote]
but remember: don't use the HiZ input with 990s inside, they can't handle it
 
[quote author="[silent:arts]"][quote author="Benny"]Just waiting for the 990 kits and the potted 2520 to make it all discrete!!! :thumb:[/quote]
but remember: don't use the HiZ input with 990s inside, they can't handle it[/quote]
I didn't know that, will it damage the 990s ?
 
[quote author="mick"]
I didn't know that, will it damage the 990s ?[/quote]

Yep. That's what happened to the 990s Volker [silent:arts] used. The 990 died with nice stench of melted epoxy. :twisted: Correct me if I'm wrong but I think they cannot handle that high input impedance.
 
Thanks I'm in the middle of hooking up my 312's right now and one pair will have 990's, guess I can unhook the DI sockets then ! NOW !!
 
Hi, can somebody help me here please, I'm trying to implement the mod to allow the capacitor to drain through the phantom LED, is this the correct way to wire a SPST switch onto Fabio's board to allow this to work ???.

normal_PP.jpg


Thanks Mick
 
Ok, wired as above does NOT work, there seems to be about 1v which is always present at the mic in XLR neg and pos pins (phantom switched off), can anyone see what I've done wrong ??? when I power down and flip the PP switch (on) I can see the 1v drain down on my voltmeter.

Thanks Mick
 
So the way I interpreted this PP drain didn't work for me, would someone please tell me if this idea will work. I planning on putting the PP LED back where it belongs on Fabio's board, and the RLED resistor, then wiring a DPDT switch like below (I'd better add that this is not a PCB mount switch).

DPDT.GIF


My worry is when I switch off the power and the CCP1 drains will the RPP3 be enough (200r) or will I need to put another resistor on the switch as well ???

Mick
 

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