K and K Sound Preamp in a Road Case

The owner’s classical guitar was instrumented with a piezo pickup at the saddle and a condenser microphone inside the sound hole of her classical guitar. Both signals are sent out of the guitar on a TRS cable to this two-channel preamp.

The Unbroken String Crew was asked if we could house this in a box of some sort that could be transported from studio to live venue in a road case.  A rack case was way too big.  Could we help?  The Unbroken String Crew says YES!

02 effects proc

Along for the ride is this processor which provides reverb for one of the channels coming from the preamp.

03 spanner screw

To fasten the preamp to a rack shelf, these little spanner screws will need to be removed and reinstalled. No problem, with the right tool.  Perhaps K&K wished to keep prying eyes out, but this will not hinder the Unbroken String Crew!

04 inside view 1

WARNING – Tech Porn.  The inside of the preamp is clean, a single-sided circuit board with only a few insulated jumpers on the bottom.  No surface mount parts here.  The strong point of this package is that the entire thing is surrounded with iron, just the thing to keep interference out of the high-gain, low-level circuitry found here.

05 inside view 2

WARNING – More Tech Porn. The 4558 is a workhorse amplifier, often found in high fidelity circuits.

06 inside view 3

WARNING – Yet More Tech Porn. The TL074 is another mature, well-behaved amplifier.

07 rack shelf

This rack shelf was cut down and re-purposed for service as a method to facilitate mounting the preamp on a 1U space without treating it as a traditional half-space unit. The preamp will look better centered in the rack space.

08 trial fit

The trial fit looks pretty good!

09 no feet

The rubber mounting feet on the bottom of the preamp need to come off.  They are press-and-stick, so we can peel-and-discard them.

10 ground bolt

The ground stud in the middle of the circuit board goes all the way through the bottom of the case.  We can use this as one of the mounting points, securing the preamp to the shelf.

11 preamp ready

Everything is bolted down, including the two side ‘wings’ that keep fingers out of the electronics.

12 start stack

This is a mock-up of the units that will be fitted into the rack rails.

13 rails and fillers

This is the operator’s view of the front of the unit.  Everything, including some blank panels, are bolted to the 5U rack rails

14 bracket prep

The corners of the unit will be fabricated from this two inch aluminum channel.  Here, we’re tapping a hole for 1/4-20 hardware that will be used throughout.

15 rail assembly

The front and back rack rails on one side of the unit are screwed to the aluminum corners.

16 guts

Both end assemblies are bolted to the equipment.  Can you visualize how this comes together?

17 wood ends

Each end is covered with Baltic birch plywood.  These furniture screws bolt the plywood cover to the end assemblies.

18 with top

We’ve installed the top onto the end assemblies.  A 1U rack space was left above the preamp because the nomenclature for the control knobs is printed on the top of the preamp, which would be obscured by the top if the preamp was installed in the top slot or if a blank filler panel were installed above the preamp.  There is also a LINE/MIC control for one channel that is only accessible from the back, but the customer would almost never need that feature.

19 panel fab1

The back panel is fabricated from a 5U blank panel.  Holes for the electrical connectors and the power outlet are bored.

20 panel fab2

The mounting hardware screw holes are placed as required.

21 panel fab 3

Here you can see the IEC power socket.  The line-level outputs are XLR/TRS bulkhead connectors from Neutrik.

22 panel fab 4

Shielded twisted pair wire is attached to the back of the connectors and are strain-relieved.  The TRS input jack that accepts the cable from the guitar is located on the far right of this picture.

23 first test

Now we did a test run to verify functionality and then off to the studio for our first real music!  Powered speakers are the load.  There is a guitar here somewhere, but it is not in the picture…  email me if you find it.

24 straight edges

Now that we know this all works, it’s time to finish the raw edges.  We’ll sand them straight to remove the saw marks.  This emery paper-covered Stanley level is also used to level fret wires.

25 edge finish

Self-adhesive oak edging is used to cover the exposed edges of the birch plywood.

26 trim

The edging is trimmed flush with the plywood with a sharp Exact-o knife.

27 sealer drying

The wood parts are saturated with MinWax Wood Hardener, which is just a shellac wash.  Coat after coat is applied until the wood is saturated and sealed.

28 base fit

This bottom is fabricated to fit into the road case.  The road case is an off-the-shelf case for a Mesa Boogie Quad Rectifier amplifier head.

29 base

This is a good fit into the foam, but it is not tight.  The unit can be removed from the base of the road case as needed.

30 Final Assy

The partially-disassembled unit is lined up on the base and the holes are marked for the furniture screws that fasted the unit down to the base.

31 power cord

The power cord can be left in place during transport if I carve away this little bit of foam.  There is plenty of space left in the road case for cords and cables to be stored with the top of the case in place.

32 All Done

This is the assembled case.  The preamp will now be installed, the top of the road case latched down, and this guy is ready to travel the world!  We may come back later to paint the wood black, because the new owner complained that this unit distracted peoples’ attention from the musicians!  We can’t have that!

You can hear this box at work (and listen to an awesome ensemble as well) here: http://www.paranamusic.com/


Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE