Build Log: Passive preamp selector box

General Discvssions & Theory/Vnderstanding
1

Intro
I have my chain split in two. One half is dacs straight to amps. The other uses a studio monitor controller to merge sources such as the pc, phono, xbox and route them to speaker amp or a headphone amp. While the controller has three inputs this is not always enough. I decided I needed a 3 x 3 switch box for music sources into the controller. Additionally the inputs would be coming in at differing volumes so ideally this would also be a passive preamp.

Parts List

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2 x RS25 4 poles 3-position selector Ø6mm Flat Axis
1 x VISHAY DALE Stereo Switching Potentiometer Shunt 23 Positions Notched Axis Ø6mm 10K Ohm
6 x ELECAUDIO ER-109 RCA Plated Plated Gold Black
6 x ELECAUDIO ER-109 RCA Plated Plated Gold Red

1 x HIFI 2000 Case ECO E401815 180x150x40
3 x aluminium knobs

1m Somer SC-Club Series MKII Highflex cable red
1m Somer SC-Club Series MKII Highflex cable black

One: Case Setup

I began by laying out the locations I would need to drill holes for the rca sockets on the rear panel and the rotary shafts on the front panel.

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Drilling was not ideal. I realised I need a new drill as mine is old, heavy and hard to control when working with metal. I’ve not used it for metal before and one of the reasons I decided to do this build was to test my processes before I move on to build a Whammy amp. However I managed to overcome the issues and seat all the elements in the front and rear panels.

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Two: Wiring

Next up I cut the cables into short sections, stripped the ends and tinned the wires. I decided to solder all the wires at the rotary selector and the potentiometer. This seemed to go well but as I discovered later I had a pair of wires on the right channel crossed on both the input and output. In hindsight I could have marked up the rear of the rotary selectors since they lack connection indicators.

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Next I located the front and rear panels in the case without fixing them and connected the wires to the rca sockets.

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I then tested the unit and discovered the issue with the crossed wires. Once I diagnosed and resolved it with the help of a multimeter I resoldered the wires correctly. At this point a made an open box test setup to confirm everything was now working correctly. Success!!

Conclusion

The finished box.

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The light scratches are much less visible in actual use and I can’t see the slightly out of line rca sockets. Other than that I am extremely happy with how it turned out. As far as I can tell it sounds transparent. I can detect no change in the sound when compared to the same signal with/without switch box. The volume pot has a really satisfying heavy click. It’s not intended to be changed often but is more a set and forget until needed. I’m very glad I spent more on the dials as the are satisfyingly heavy.

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14 Likes
2

a universal bane. good work on the inside though, where it counts.

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3

Thank you! It helps that I really enjoy organising cabling lol. My partner has banned me from organising cables when she is in the house :grin:

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4

Looking good so far! :slightly_smiling_face:

I actually need to buy a better drill and bits too. Mine was a hand me down from my father and it has seen better days.

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5

At the risk of lecturing on drilling holes, and as this is something I didn’t realize for years.
One mistake I see in drilling sheet metal over and over is you need to use a punch and drill a pilot hole.
The pilot hole needs to be JUST larger than the flat on the end of the intended final drill size.
By flat what I mean is drills don’t come to a point they taper to a small flat bit, for them to actually cut effectively, the angled parts have to contact the edge of the pilot hole.
If you don’t care about exact hole sizes a pilot bit and a set of step drills come in surprisingly handy.

The only complicated parts of the Whammy case work are lining the hole for the pot up, and cutting the hole for the IEC connector at the back. I made templates, but the job I did could have been better.

2 Likes
6

That’s great advice, thank you! I worked this most of the way through the build. I was using what I knew from woodworking and using a nail to punch a dent but it wasn’t enough for the larger bit to bite - which caused skidding. Guide holes would make things a lot easier.

I was considering getting a Dremel or equivalent and next time drilling holes a size too small and then filing them to the correct size with that.

For the Whammy I am looking at an enclosure with a pre fitted IEC connector to make my life easier!

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7

Quite impressive!!

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8

Dremel’s are a great tool to have, mine gets used pretty much exclusively for cutting, either using cutoff disks or the standard dremel multipurpose cutting bits.
If you were going to file a hole up to size, you have a lot more control with a handheld roundfile.
Generally I go the other way for thing like volume controls and switches and rely on the nut/washer to hold them in place.

Looks like a nice build though, I like you used a stepped attenuator.

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9

Sheet metal also comes with the fun characteristic of getting caught by a flute (is that the English word?) and whipping around to cut your fingers off.
Stepdrills avoid that. Good ones are pricey though.

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3 Likes