Classic API VP25 Build Thread (pics inside!)

[mrface2112]

So, I finished building a Classic API VP25 mic preamp a couple nights ago and thought I'd document the build process.

For those of you uninitiated, the Classic API VP25 is a 500-series compatible mic preamp which is firmly in the family of the old API 312 mic preamp. The Classic API site sells the kits, and there is a strong tie-in with the GroupDIY forum.

The build was super easy, with the opamp excepted. That required some tight soldering under a magnifying glass. I don't know what my pics can provide that the excellent build instructions that are included with the kit does not.....but these days I can't seem to build anything without taking pictures, so what the hell.....

First, we have the bare circuit board:


Then we add the resistors. I find it's easiest to use an online resource for mapping the color bands to resistance :


Add the diodes, being sure to get the polarity correct!


Add the capacitors and switches. Again, pay attention to the polarity where necessary.


Solder in the sockets for the opamp.


Add the transformers. Make sure to line measure your output transformer leads twice before cutting them, and be sure to account for stripping when cutting. Using heat shrink around the output transformer leads to group them will tidy things up.


Closeup on the 2503 output transformer:


Closeup on the "new style" T-Pad adapter board for attaching the attenuator. This is the one thing that's different than the instruction manual.


Add in the 48V Phantom Power Switch. Again, make sure you've got enough lead:


Mount the board in the chassis:


And attach the faceplate. If you followed the instructions properly, everything will line up perfectly:


Here's a shot of the finished mic preamp:


And a front shot with the knobs installed:


Here are two shots of the GAR2520 opamp. I didn't document that process as it's basically "stuff the board".




So.....how's it sound? In a word, fantastic. Big low end, aggressive/forward mids and a really nice, airy high end that isn't strident in the least. It's really nice on my vocals, and i haven't had a chance to put it on a drum kit yet, but i bet this thing would slay on kick and snare as well as guitar amp and powering a bass DI.

I've got another one enroute.

cheers,
wade

[Lt. Bob]

So, I finished building a Classic API VP25 mic preamp a couple nights ago and thought I'd document the build process.

can you use it standalone without the 500 series master box?
At that price I'd order one tomorrow if'n I don't have to also invest in the 500 module.

[mrface2112]

No, you've gotta have a 500 series rack to house it. You can either DIY one, or you can do what i did and pick up a used 6 space lunchbox for around $300. That comes out to about $50/channel for power and i/o connections, which isn't too bad considering the bang for the buck you get with the 500 series stuff.

[mshilarious]

Just buy the output transformer and stick it in an external box, and feed it with whatever preamp you have. That is likely most of the sound, no lunchbox required.

Those discrete opamps, I dunno, I look at that and think hey, lots of stray inductance . . .

[Blue Jinn]

Just buy the output transformer and stick it in an external box, and feed it with whatever preamp you have. That is likely most of the sound, no lunchbox required.

Those discrete opamps, I dunno, I look at that and think hey, lots of stray inductance . . .

I think the transformers are already on teh board. The card edge has power supply and I/O connectors AFAIK and the spec should be well documented. (Here are links: http://www.groupdiy.com/index.php?to...0389#msg530389 )

I don't see why you couldn't work around that some how, either with a card edge connecter of some sort, or drilling the card edge, somehow, You'd have to hack the board somewhat, but I don't see why you couldn't and then put it in a case of your own liking. Me, there is no way I could do that and make it pretty, but others may have better skills than me.

[mrface2112]

Just buy the output transformer and stick it in an external box, and feed it with whatever preamp you have. That is likely most of the sound, no lunchbox required.

yeah, probably, assuming you also include the input transformer in that too. as for the lunchbox, it gives you power and I/O and it conforms to an open standard, as Blue Jinn noted above, so there are lots of options out there from a lot of companies. sure you can diy something up to do something similar, but why? and would you really save any money? i don't know anything that comes even remotely close at the $300 per channel cost of these things.

Those discrete opamps, I dunno, I look at that and think hey, lots of stray inductance . . .

could be. but damn it sounds good.

I think the transformers are already on teh board.

no, they're not. you can see the places where the 2622 and 2523 are mounted in the top couple pics. and you can order the transformers separately from ClassicAPI, so if you wanted to kitbash something together, you certainly could.

cheers,
wade

[mshilarious]

yeah, probably, assuming you also include the input transformer in that too. as for the lunchbox, it gives you power and I/O and it conforms to an open standard, as Blue Jinn noted above, so there are lots of options out there from a lot of companies. sure you can diy something up to do something similar, but why? and would you really save any money? i don't know anything that comes even remotely close at the $300 per channel cost of these things.

If you don't have a preamp and want to build six of them to get the lunchbox cost down to your per channel figure, sure. But if you don't need another preamp and just want the transformer sound, it's much cheaper to just buy the transformer.

The input transformer will be far more linear than the output because it is typically seeing a much smaller signal level (and if it isn't, then you might have excess gain in the amp and have to either pad the input or bypass the input transformer). Usually with APIs people are trying to annihilate the output transformer to get their desired tone. The basic job of the input transformer is to unbalance the signal with good CMRR and improve input noise performance--functions that should already exist in another preamp.

Don't trust me, measure the input and output transformers with typical signal levels and see for yourself. Do a listening test with and without output transformer.

Anyway, you can't easily stick an input transformer in front of a preamp because that will drop the input impedance too low (unless you used a 1:1 transformer, which there wouldn't be any point)--and block phantom power too. You'd have to hack it into the internals of the preamp.

[mrface2112]

it's much cheaper to just buy the transformer.

Well, yeah....at $25 for the OT vs $225 for the preamp with OT.

functions that should already exist in another preamp.

should they? yes. do they always? no. at least, not with any consistency in the sub-$500/channel pricepoint. i've been through my share of "inexpensive" preamps and while most of them would probably benefit from hitting a transformer on the way out, simply adding an OT to any of them wouldn't make them sound like this thing does.

[mshilarious]

I mean CMRR and input noise are basic specs; if a preamp doesn't list them, don't buy it . . .

[mrface2112]

I mean CMRR and input noise are basic specs; if a preamp doesn't list them, don't buy it . . .

well, of course. i agree with you there. i was reading too much into your statement. that said, you also have to assume that the numbers aren't being fudged in some way, or even outright falsified. i don't trust some of those lower-end manufacturers in the least and don't believe a single number that their specs claim.

anyway, we've wandered quite a bit OT here.....i'm 2/3 through my 2nd VP25 kit. it's gone a LOT faster than the first one did, simply b/c i've been through the process once already (and i'm not stopping to take pics at every step of the way). It's taking about 1/2 the time the first one did. Looking forward to having a pair of these!


cheers,
wade