Building a Portable DJ System into a Traditional Red Metal Toolbox Part 3: A Stereo Korg Nutube Pre-Amp
Adding a dual-channel “Nutube” pre-amp to the Red Tin to achieve a warm sound when playing digital music.
One of the things people often say about DJ’ing from a laptop, as opposed to playing vinyl, is that you don’t get the same “warm” sound quality.
I have a plan to get round that. I followed the blog about the creation of the Nutube pre-amp, witnessing some of the development first hand and designing and building the enclosure. I decided that two pre-amps, harnessed together for stereo, would be just what I needed. They are a nice compact size, so would slot into the toolbox nicely and would fit the slightly retro aesthetic I am aiming for, with the valves emitting a very pleasing bluish glow.
Getting all the bits together
The PCBs were ordered and arrived very quickly. I was particularly pleased that they were a nice red colour to match the tin. I compiled a list of components referring to the Bill Of Materials for the original guitar pre-amp project on GitHub:
- 2x RS Pro 6.35mm PCB Mount Jack Socket, Female, 50 V, 3A (044-9332)
- 5x Capacitor 100nF (538-1310)
- 3x Capacitor 1uF (711-2018)
- 4x Capacitor 100uF (711-1592)
- 1x 2.1 mm, 2.5 mm PCB Mount Right Angle Dual DC Socket (805-1699)
- 1x Inductor 4.7 uH (022-8135)
- 3x Semiconductor BS170 N-channel MOSFET (671-4736)
- 1x NuTube6P1 (144-9016)
- 3x 100 K ohm double Potentiometer (016-8386)
- 2x Trimmer Resistor (047-3477)
- 1x 1k resistor (707-7666)
- 7x Resistor 10K ohm (707-7745)
- 3x Resistor 150 ohm (707-7603)
- 2x Resistor 330 K ohm (707-8962)
- 2x Resistor 620 ohm (014-8455)
- 1x Diode (751-4843)
I have not done a huge amount of soldering so this was going to be good practice, if a little daunting. One handy tip I worked out early on in the process was to start soldering with the smallest components, otherwise when you flip the PCB over the large parts raise it off the surface you are working on and the small parts can shift about while you are trying to solder them, or even worse, drop out all together.
Using the picture from the 4th Nutube Pre-Amp blog post, the bill of materials and the schematic, I worked out where all the bits went and started soldering …
It soon began to take shape.
First the resistors went in, then the small capacitors.
I knew I needed to take care that the large capacitors, the LED, the diode and the transistors were all fitted the right way round. Getting the capacitors right was relatively easy, as the polarity is clearly marked on their casing and on the PCB. I had to do a bit of Internet searching and refer to the picture in the Nutube blog to work the others out.
All the “polarized” components, as I learned they were called, in their correct alignment.
Adding the Korg Nutube
Now it was time to add the Nutube itself. If you have not come across the NuTube before it is a miniature vacuum tube or valve, it has an anode grid filament structure, and operates exactly as a triode vacuum tube, so gives that traditional warm sound that I was after.
I then fitted the jack sockets, the power socket and, just for testing purposes, three single potentiometers. As I was using two of these amps, for stereo, the final version would be fitted with dual gang pots. Once the first board was assembled I tested it and found I had a resistor in the wrong place. That was soon put right, the Nutube emitted its gentle blue glow and with my ‘phone plugged in and playing music, I got a good level of sound out.
I went ahead and assembled the second board, fitting the double potentiometers and braiding the wires to keep them neat and tidy.
Now to fit it in to the Red Tin
I wanted to make the pre-amp a self-contained module that could easily be dropped into the tin and connected up. The main problem was going to be fitting everything into a fairly small space. From my past experience of designing and building enclosures, I knew that I needed to leave enough room for plugs and cables since they always seem to take up more space than you think. To help with this I made up a prototype enclosure top in cardboard. It’s very quick to cut out, it’s cheap and if you are designing something that involves bending acrylic, as I was, it can be folded.
I could see it was going to be a tight fit. I was going to have to raise the top slightly and some pancake style jack plugs like the ones used for chaining together effects pedals would help.
Even staggering the potentiometers to give a little extra space between them was not going to be enough to allow me to use the “chicken beak” knobs that I had planned on, so I was going to have to find something else suitably retro and space saving. The RS Pro Aluminium Potentiometer Knobs (049-8889) fitted the bill nicely
I opted for clear acrylic for the majority of the case to show off the tube and the red circuit board — it would be a shame to hide those. I added a small veneer section that harks back to my original design for the Nutube pre-amp case and fits in nicely with the aesthetic of the rest of the tin.
Tweaking the Sound
I tried the amp through my home hi-fi and initially the sound was a bit noisy – fine for a guitar pre-amp, but not what I wanted. I adjusted the bias slightly and got a sound I liked. At some point in the future, it might be a good idea to hook it up to an oscilloscope and do some more precise adjustment.
This has been well worth the effort, the amp sounds wonderful, and it looks great glowing in the top section of the Red Tin. I am prepared to bet I am the only DJ at the moment playing through a miniature tube pre-amp.