CAPI VP312 Preamp Build (and HP Filter Switch Mod)

CAPI is another source for Professional DIY audio kits. They make a bunch of 500 series and rack device kits. It is not by accident that [c]API look very similar to vintage products by API but CAPI has no affiliation with API.

I’ve built most of the components in my 500 rack as kits. As I started down that road, I also got myself pretty deep into microphone mods.

While you’re here, check out our custom microphone modifications and rebuilds. If you have a neglected MXL 990, MXL 910 or similar mic in your locker, we can very affordably turn it into a “go-to”.

For this kit, I decided to go simple and build the basic VP312 Preamp. This version has no DI input which greatly simplifies the build. I also chose to buy the Discrete Op Amp pre-assembled. When I built the Lola, I made the mistake of building the DOAs myself. This accounted for more than half of the build time and about 90% of the difficulty so I didn’t want to repeat that mistake.

I had the 312 done and working in less than three hours. It was by far the easiest kit I’ve done.

capi vp312 early in build
First few components installed

The array of resistors in the top left are for the 12-position stepped input gain switch. This is an upgrade option but you can get the kit with a variable input gain instead. I opted for the stepped version. That said, CAPI has been having a terrible time keeping the Grayhill switches in stock. I even had to pass on a couple of sales in my shop because I wouldn’t have been able to replace the kits if I sold them.


At the time I ordered the kit, they were out of stock of their own CA2622 Input Transformer. But they did have plenty of the Ed Anderson (EA)2622s in stock. So I ordered the “No Iron” option of the kit and then added the EA input transformer and upgraded to the Litz (red) output transformer.

I also ordered the “stepped” gain version which is a 12-position grayhill switch with an array of resistors.

vp312 assembled with front panel
Finished Kit on the bench

Not too many steps along the way and before you know it, the kit is done.

Although the kit was easy to build, this is the first kit where there was an error with the parts inventory. Fortuntely, it was just two missing 4-40 flat head screws. Unfortunately, my local hardware store did not have them so I’ve had to order some. Until they arrive, I can not do final assembly. It’s put together enough to install in the rack and use though.

In the rack with her siblings

Room for Mods

Besides being easy to build, an advantage of the low parts count of the CAPI is that there is a lot of room for modifications.

One thing that is missing from the Lola and the MP573 is a Lo Cut (hp filter) switch. These come in handy quite often and it is one reason why I bought the RND 511 preamp (which I don’t love). Adding this function to the CAPI looks to be very easy to do. There is plenty of room to mount an additional toggle switch and I manged to find some very cool mini toggles that have an illuminated handle!

Here’s the general idea: R3 is an optional Load resistor for secondary of the input transformer. I’m installing it and using it as the “R” in the R-C circuit. A trace is cut between transformer and R3 and I’m putting some capacitance across that gap with a switch to short it out. I found some cool illuminated toggle switches which will give me a cool indicator light when the filter is in.

The Switch will short out the cap disabling the filter. With the switch open, the other contact will close and light up the switch LED.

For fun, I also considered a two-position version of the filter whcih would use an on-off-on switch and two capacitors. This ended up being what I finally went with.

A couple variations on the HP Filter mod. The bottom one (two position) is what I went with
cut trace on capi vp312
This is the Trace to Cut
capi vp312 schematic
The CAPI vp312 schematic showing where the trace is cut
Drilling switch hole in Capi vp312
Drilling the pilot hole for the new switch

Can’t wait for the cool switches to arrive!


capi vp312 front panel with new switch hole
The pilot hole in the front panel

Adding the Mod

Well, my fancy illuminated switches were delivered today. That’s the good news. The bad news is I had them sent to the house I lived in six years ago. Oh well I hope the new owners enjoy them once they get over their initial confusion. Being impatient to get this done, I decided to skip the illuminated switch. The switches I had in stock were some SPDT On-Off-On switches. So I was able to do the two-position version of the filter. In hindsight, I’m really glad I didn’t get the illuminated switch. Having two steps for the Lo-Cut frequency is very nice.

Step 1 was to (ugh) drill a hole through the board to provide access for the wires. Next time I build one of these kits, I will think ahead and do the whole mod on the top side of the board which will eliminate the need for this hole. Not a big deal as there is plenty of open acreage on the board.

Step 2. The Two 0.01uF caps are wired in series across the cut circuit board trace.

Step 3 is to get some wires to the caps. I ended up doing two of the connection below the board and the other above. The pin on the input transformer made a nice big fat solder pad to hook one to. The other wire goes to the connection of the two series caps.

Two capacitors wired in

Step 4 is to wire the switch. I drew an arrow facing down so I could keep track of which way to install the switch. I want OFF to be with the switch handle down. The two blue wires go across the cut trace (shorting both caps). The red wire goes to where the caps are wired together (shorting one cap).

Switch Installed

And there it is with all the gadgets mounted to the panel again. Since this is the last time I hope to disassemble this, I put blue locktite on all the fasteners.

capi vp312 mod finished on bench
Final mod on the bench

The switch works as follows:

  • Down: Off
  • Middle: 220Hz
  • Up: 110Hz

I was a bit concerned that the extra wiring would pick up some hum or noise. They are between the input transformer and the high impedence input of the DOA. But upon testing, the amp is still just as silent as ever. Did I connect it to some expensive signal analyzer? Nope. But I did put on my sennheiser headphones and floor the amp and to my ears, it’s as quiet as it ever was (very). Operating the switch does not produce any clicks or glitches either. I didn’t expect it would as there is no DC anywhere near this part of the circuit.

In the down position, the switch is shorting both caps as if they were not there. In the middle position, both caps are in series producing a total capacitance of 0.005uF. This gives us a 225Hz corner frequency. In the Up position, one cap is shorted so we have 0.01uF which yields a 112Hz corner. I’m assuming the input impedence of the Discrete Op-Amp is much higher than R3’s value of 150K so the R in the RC circuit would be around ~140K.

The illuminated switch would have been cool, but having two settings of the filter is much more valuable. Now, I want to build another one!

And here it is installed in the rack and working.

capi vp312 added to rack
Final version installed in rack and working

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