Hey guys, for anyone looking for complete build instructions, all in one place and with every step laid out, I made this build tutorial video. Could be a big help for someone new starting to put these together!

http://youtu.be/VsOgoayrr4s


Hi everyone. Just a quick question. Is the short burst of noise when you press the pad switch on the VP312 normal?

jsteiger

It is possible especially with phantom on. There will be a pop as well. This is fairly common.
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Just ordered two of these, will be my first preamp build.

Looking at the BOM:

http://www.classicapi.com/catalog/images/gallery/VP312/1_VP312-xxx-BOM.pdf

and the pcb:

http://www.classicapi.com/catalog/images/gallery/VP312/VP312-VPR-Rev-A-PCB.jpg

there is something I don`t understand. Where is R13? And there is no part for R14?

jsteiger

Just ordered two of these, will be my first preamp build.

Looking at the BOM:

http://www.classicapi.com/catalog/images/gallery/VP312/1_VP312-xxx-BOM.pdf

and the pcb:

http://www.classicapi.com/catalog/images/gallery/VP312/VP312-VPR-Rev-A-PCB.jpg

there is something I don`t understand. Where is R13? And there is no part for R14?
That's an early scan of the PCB. On the production boards, what you see here as R14 is now labeled R13. I don't recall what/when I caught and made that change. I don't even remember it, its been so long.
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CAPI is not affiliated with, endorsed by, or sponsored by API.

Ah that clears things up. Now just the wait for my kits to arrive  8)

My kits arrived earlier today, and I`ve just built my first pre-amp!

However, during the quick startup test, while probing my DOA-sockets, I get 0r between C and O. Have I shorted something somewhere?

All the other socket-probing turned out fine (over 200)

All transformer wiring is correct. I`m not sure about all the soldering, altough I gave it my best try  :P

Should i try it in the rack? Do I have to do the voltage check before I try it with an opamp? I don`t have the breakout-bracket that makes it easy, so I`m afraid to go poking in the rack while it`s on, afraid of not hitting the right pins straight away...
« Last Edit: February 25, 2015, 12:29:01 AM by fragletrollet »

jsteiger

It could be an auto-range function on your DMM. The resistance should be around 9 ohms so make sure your meter is set for less than 200 ohms.
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CAPI is not affiliated with, endorsed by, or sponsored by API.

Getting about 10ohms, should be good to go?

Ok fired it up in the rack, did the voltage testing and it checks out fine. +48V lights up when asked to. No funny smell.

Am I good to to put in the OpAmp ? Wow I might have gotten it right on the first try, fingers crossed!

Edit: So I plugged it in... and guess what... it works beautifully. Great sound! Can`t wait to play more with it! So happy!  ;D
« Last Edit: February 25, 2015, 01:00:21 AM by fragletrollet »


Made the second one today, much quicker this time around. Works right away!

So... do you think I`m ready for a couple of 553f`s? What is the "preferred" advancement through the capi line, in terms of build difficoulty? Something like (from easy to hard):

312/VP25/VP26 - 312di - VP28/553f - LC53A/VC528 ?


jsteiger

I would put the 553F's as a little easier than a VP28. You should be fine moving up to the next level. Many folks have built the 553F as their first project.
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CAPI is not affiliated with, endorsed by, or sponsored by API.

Cool, I`ll keep that in mind! Now im hoarding parts for 2x EZ1084, but like Scwharzenegger says....


I`LL BE BACK

Just wanted to chime in and say that I just built my two GAR2520. Both worked straight away! First took me a good few hours, but the second one was much faster.

I kinda bought them as training-kits, and was really preparing myself for the beginning of my DIY adventure to be alot of troubleshooting and maybe a few non-working projects to start with; but so far my rate has been 100% working. Kinda scary, hope I don`t get too discouraged when the inevitable hits!

Anyways, the GAR2520 sound great. A bit hard to to compare to my SL red dot`s, as I can only use my 312`s with a mic/my voice, that is hardly the same every time. Maybe when I get a DI, so I can run pre-recorded audio through them, a comparison might be easier.

Now I need to get something to stick my GAR`s in!!

Greg

After having spent years away from DIY and tech work...

I built 6 VP312. The build went fine. I didn't use the output attenuator; rather, I simply hard wired a DPDT to select between 1:2 (normal) and 1:1 (the extra secondary winding).

Thanks for doing the leg work on this one. Sorry for the poor quality of the picture.

Greg Stein
New Orleans, LA

I bought three VP312s from a guy, but funny enough, each one has some sort of issue. One's Grayhill step switch goes all the way around, one has a blown cap from what I think is a resistor wired the wrong way, and one is fine, but has all white buttons (the other two with full black buttons haha.) My plan is to make them all right again. Just a few quick questions though.

One, the steps, there should be 11 clicks right? On the one without the step pin, is there a way to fix that easily?

Two, I posted some photos of the one with a blown cap next to one without, I see one resistor with the orange stripe in a different spot, is that the problem? Check it out http://imgur.com/a/sMehZ This one also has a recessed LED, but I'm not too concerned.

Three, the pushbutton caps, are they easily removable? My money is on no. If not, are they easy to snap off and replace with ones of the right colour?

Thanks guys! Really stoked to use these pres all the time, just want them all nice and healthy first!!
« Last Edit: March 22, 2016, 12:48:28 PM by samguaiana »

I have a question regarding the output attenuator. In some designs the attenuator is placed after the output transformer, so you can drive the op tx for added saturation. Afaik in this 312 design it is before the TX, so all saturation by driving the input and backing off on the output is from driving the components in the circuit inbetween the two transformers.

Why is it designed this way, and what are the benefits/caveats of the two ways of doing this?

[silent:arts]

Afaik in this 312 design it is before the TX
Is it? No. Look at the schematic

jsteiger

I have a question regarding the output attenuator. In some designs the attenuator is placed after the output transformer, so you can drive the op tx for added saturation. Afaik in this 312 design it is before the TX, so all saturation by driving the input and backing off on the output is from driving the components in the circuit inbetween the two transformers.
As Volker stated, the t-pad attenuator comes AFTER the output transformer in all of my single stage preamps.
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CAPI is not affiliated with, endorsed by, or sponsored by API.

My bad then. I`m pretty new to reading schematics so it`s not so easy for me to see. What is the T-Pad marked as on the schem?


 

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