Project files: Little helpers – Alps PCBs III

What is it?
These are “little helper” boards for the Alps RK168xx series of motorised potentiometers. These pots are not quite as good quality as the standard Alps “Blue Velvet” RK27-series, but they are cheaper and smaller. They are also used in many commercial products, so they should work fine for many diy projects. The motor also means that the pots have a nice mechanical feel to them 🙂
There are two board versions, a 2-channel (for stereo with the RK16812) and a 4-channel (for balanced amps with the RK16814). Alps also makes a six-channel version of the pot and adjusting the footprint to fit these should be relatively easy, but I have no need for these now so I couldn’t be bothered 🙂

How big are the boards?
Both boards measure 1.85″ x 2.0″ (app. 47 x 51 mm) and the rear mounting holes are in the same place on both boards.

What is the status of the boards?
Both are version 1.0 since they are exactly as my prototypes.

Does it use any special/expensive/hard-to-find parts?
Mostly there’s only one real part on the board and that is the pot itself, so not really 😀

Anything else I need to know?

  • These are “preamp” style boards have a ground plane and a ground pad that can be used if you grounding scheme requires the shaft of the pot to be grounded. Use a piece of wire connected from the ground pad to either one of the screws on the back of the pot or soldered to a ring terminal wedged between the pot and the chassis. You can also use the grounding pad on the bottom instead.
  • The boards can also be used to make separate, passive preamps. In this case, a 10k potentiometer should be used.
  • The screw clamps are standard 5mm pin spacing types, but of course it is possible to solder bare wires to the boards as well.
  • The basic Eagle footprint for the RK168 was one I found in a diyaudio-thread, so I can’t take credit for that. All I have done is modify it to match the Quad-version as well.

Download design files here

Related information:
Note: Always read the “intro post” for additional important information about my designs.

As usual, please remember to consult the manufacturer’s datasheet as well.

Mini-stack (part 1)

Not exactly the most challenging builds ever undertaken, but still…

More or less since I originally built my passive preamps I have wanted to build more components in the same form factor. Here is the beginning of a “mini-stack”, namely a USB DAC and a passive pre, which should then be supplemented later by (at least) a power amp in the same small chassis (a GX143 from, so 124x40x73mm). That isn’t ready yet, but I do have a few ideas for what to do 😀 As for what is ready:

The passive pre is based on a Noble 10k pot that I happened to have lying around – not much more to say about that really 🙂

The DAC is based on an “off-shelf” module from ebay. It uses the SA9023 USB receiver/ES9023 DAC chipset so that it can accept up to 24/96 input signals. The module is powered from the USB connection. It is possible (and possibly recommended) to use an external supply instead, but since the preamp is passive I wanted to keep the design “cordless” except for the USB. Obviously that won’t work for the power amp… My only little piece of hacking here was to wire an “USB on” LED on the front panel instead of relying on the on-board SMD one.

The ES9023 DAC chip from ESStech is a highly respected performer and used in quite a few designs such as the AudioQuest DragonFly (albeit there with a more complicated/advanced USB front-end) and quite a few others. It is voltage output with a built-in charge pump to avoid capacitors in the signal path and it is actually capable of driving headphones directly, so the 10k load here isn’t really an issue.

The sound quality of the board is quite a bit ahead of the older PCM27xx-based designs as well. The sound from this little combo is actually quite good and the fact that no external power suppliers are required makes it a bit more interesting.

My idea for the power amp is to use a class D chip powered from an off-board switching supply. I have looked at both the TPA311x and the TDA749x chips but I am still exploring options here so nothing definite yet…let’s see what happens 🙂