HPF mod for Don Classic's NV 73 preamp

[Unit7]

This diary is aimed for DIY non experts like myself to help adding High pass filter to Don Classic's NV73.

Background: I often use a compressor while tracking, and for vocals, acoustic guitar etc I always use a HPF to get rid of LF peaks from plosives or 'thumps' that otherwise trigger the compressor in an ugly way. After some discussion and help from David I decided to go for a 70Hz/12dB per octave filter.

Of course I messed up doing a schematic even of this simple mod, so David was kind enough to help:

 

[Unit7]

The parts needed: DPDT switch, two 220n caps***, one 10k resistor and a 10k pot*** replacing the 5k pot in the original design
And here's a Mouser project: http://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=d78665a38d, including the same type switch as the 48V switch
Note 1: David told me that a 10k pot (instead of 5k) will work too in the original design, without the HPF.
Note 2: The 10k resistor is the same type as the 10k resistors used in the NV73.
Note 3: Wima film caps was recommended by David for maximum transparency. Could be replaced to anyone's taste, just keep 50V or higher.
Note 4: I've replaced the 10k pot because the shaft was too short for the ELMA vintage Neve style knobs I bought. New pot Mouser #652-3310Y-011-103L

Before I started soldering, to make sure I got everything right, David asked me to return a simple sketch of how I was going to connect the components. Well, I did mess up again, so here's the revised sketch:

Important note: The cable on the bottom side of the PCB connecting the P joints should not be used.

*** Edit April 2016: After some testing I thought the 220n caps cut too much and found I liked 470n better, so the Mouser cart is updated. Changed to yellow mustard caps for good Rupert karma. Also it seems David is now shipping the kits with 10k pots so probably no need to replace, so the pot in my cart won't be necessary if you don't prefer a 1/8" shaft.

 

[Unit7]

Here's a tip I got from a metal craftsman - drill at really low speed and the drill gets a grip immediately instead of slipping. Metal on both the L-bracket and faceplate was soft, so no issues here. Drill dimensions, big holes 6,5mm*, small hole on L bracket: 2mm. Distance from center to center of 48V switch and HPF switch: 15,5mm. If you use the same type HPF switch as I, don't place it closer or it will interfere with the Lo Z switch.
* I drilled 7mm which was almost too much, almost showing the edge of the hole outside the nut of the switch. Also, as you can see, as the drill slid a bit while drilling, I didn't manage to line up the holes perfectly on the vertical plane, but as there is a little play I managed to compensate when mounting the switches. So result was good enough for me, but next time I'll use a bench drill.

 

[Unit7]

Looking to do everything as simple as possible with as few extra cables as possible and no extra PCB I decided to solder the two caps and the resistor together in one solder joint. Tying the caps together made the job easy.

 

[dmp]

I did this last year to a EZ1290 build, but with a CLC filter (as in the 1073). I had to add some resistors for proper loading.

http://groupdiy.com/index.php?topic=51936.msg660666#msg660666

 

[Unit7]

I started by soldering the cable from the upper P mark at the PCB (above the NV73/serial number) to the HPF switch middle terminal. Then the cap/resistor package including the cable from the HPF switch to one of the lower poles marked P on the PCB. And then the rest in no particular order.

 

[Unit7]

Thanks David for all the help!