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.

Downloads:
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.

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Project files: A PGA231x Volume Control

What is it?
A “digital” volume control based on the well-known PGA2310/PGA2311 ICs from TI. These aren’t actually digital, but just digitally-controlled resistor attenuators integrated on an IC – which doesn’t really matter anyway.

As is often the case, there are two board versions: One is a “normal” stereo version with a single PGA chip onboard, the other is a balanced/multichannel version that has two chips onboard (for balanced setups) and the possibility to daisy-chain more boards (for multi-channel systems). The boards are intended to be controlled from an Arduino or a similar microcontroller via an SPI-interface. I haven’t included any software in the download file, but there should be plenty of examples online that shows how to do this integration (especially for the Arduino).

As even the not so keen-eyed observer will notice, this isn’t a new layout. I recently realised I didn’t post these before and as the project they were meant for died somewhere along the way I am not sure I’ll ever pick these up again (at least not within the foreseeable future).

Note: I had some minor noise-issues with the boards. As I never got beyond bench-testing, the noise could be from any number of sources other than the layout (my shoddy test wiring, poor PSU, the poor USB supply that I used for the arduino etc.). This means that although the PCB layout is made according to (what I believe are) the TI recommendations for the PGA231x, I cannot guarantee that the finished board will perform 100% flawlessly without tweaks.

How big are the boards?
The stereo board measures 2.525” x 1.6” (app. 64 x 41 mm.)
The balanced board measures 2.525” x 2.65” (app. 64 x 67 mm.)

What is the status of the boards?
The boards are v1.0, meaning finished and technically working as I expected. Please do note the caveat above around noise though.

Does it use any special/expensive/hard-to-find parts?
None, really.

Anything else I need to know?

  • There are plenty of examples online for interfacing the PGA ICs with Arduino and other microcontrollers – do a search of diyaudio and the Arduino forums for a start.
  • The PGA chip wants to be driven from a low-impedance load and the input buffer sees to that – use whatever dual opamp you prefer here. If you don’t have a favourite already, I’d recommend the LME49720/LM4562 as long as they are available:) The PGA has a buffer of its own on the output and it is spec’ed to drive loads down to 600 ohms which should mean that all common configurations are catered for.

Downloads:
Download design files here

EDIT 23/4-2017: In response to the reader comments below, here is a basic BoM for the single board. As the dual-board is basically two sections of the same circuit, you should be able to work out the BoM for that yourself 🙂

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

Please also refer to the data sheet for the PGA2310 for information about controlling the chip etc.

Project files: Little helpers – Alps PCBs II

What is it?
Not exactly the most complex designs ever made 😉 A couple of PCBs for mounting Alps (or pin-compatible) potentiometers. Unlike the previous boards these ones have screw clamps for input and output, making them better suited to testing use and people with fat fingers (like yours truly) 😀 There are two versions, a 2-channel (for stereo) and a 4-channel (for balanced) amps.

How big are the boards?
The SE board measures 1.15″ x 1.975″ (app. 29 x 50 mm) and the balanced board measures 1.975″ x 1.975″ (app. 50 x 50 mm.)

What is the status of the boards?
I’ve called these version 1.5 since they are based on the previous boards of the same type, just modified with screw clamps for the input/output connections.

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 which can be a bit expensive, so yes, I guess so 😀

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.
  • The stereo board has a four-hole footprint to fit various pots that use that configuration, but the last set of pads isn’t connected to anything so will not work directly with a loudness tap.
  • 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.

Downloads:
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 if you use a different pot than the recommended type.

Project files: Relay-based attenuator

What is it?
This is the board files for my relay attenuator described here.

How big are the boards?
The board measures 3.85″ x 2.4″ (app. 98 x 61 mm.)

What is the status of the boards?
This board is version 1.0. It has been built and tested and I haven’t really found it necessary to make any changes based on the prototype.

Does it use any special/expensive/hard-to-find parts?
Not really 🙂

  • The relays must be the specified Omron G6K 5V version and the ULN and MCP ICs should be in the right packages (SO-IC). All these parts should be available from the usual sources, i.e. Reichelt in Europe and Mouser/Digikey in the US and worldwide.
  • The resistors should be 1% tolerance or better and some of the values are from the E96 range which might not be available in the shop around the corner but should be available from most of the mail order companies.

Anything else I need to know?

  • Refer to the original build article (link) for some additional information and helpful links to Arduino code samples etc.

Downloads:
Download design files here

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

relayattenuator-1

Click-click! A relay-based attenuator…

Yes, when you are not lucky enough to score a good deal on expensive pots (see my last post) then getting suitable attenuators for your project can be a bit of a pain 😀 If you are looking for a balanced attenuator there isn’t really much “middle ground” out there between an Alps RK27 costing app. €30 and a real stepped attenuator such as a DACT (which retails for more than €300).

The typical answer to this is simple – use relays. I was never really a fan of relay attenuators though, having always found the loud clicking noises whenever you even looked at the volume knob really annoying and not something I’d want to have to live with on a daily basis. However, relay attenuators do have a couple of advantages, among which that they can be built for a reasonable cost. A stepped attenuator also has very good channel matching, but whereas even the expensive DACT only gets you 24 steps, typically with 2dB between them, 128 or even 256 steps of 0.5dB each are quite easily achieved with relays. So, having thus abandoned my principles, I wanted to try one as well 🙂

Even a cursory look at the schematics would reveal this as a clone of TPAs “Joshua Tree” attenuator, however it is by way of another design. I actually started from the eagle files shared by diyaudio user MaxW in this thread. I have kept the circuit more or less intact, but removed the input selector that Max had and converted it to a (nearly) complete through-hole design.

I have decided to keep the I2C-controller and the attenuator itself onto a single PCB. A couple of reasons for this, mainly that it makes for easier wiring when the PCB is “self-contained” apart from the controller and PSU, but also that when the attenuator is used in a balanced or multichannel configuration each channel gets its own I2C-address, meaning you can easily control levels separately. The added parts cost is negligible to me. The attenuator stage itself uses Omron G6K 5V miniature relays and Vishay RN55 resistors. As far as I know there are no substitutes from other manufacturers for the relays (because of the uniform 2.5mm pin spacing), but Reichelt has them for a decent price so even that is manageable. The unit is intended to be controlled by an Arduino (or similar microcontroller) with I2C-capability and I have used MaxW’s sample code from the diyaudio thread as my starting point which seems to work very well.

Now, I have put together the first sample PCB and I am honestly a bit impressed. Obviously the clicking is still there, but it isn’t too loud with the tiny Omrons. The volume ramps very smoothly when you turn the encoder and I heard absolutely no audible clicking or noise in the headphones I used for testing. My next step is to build a pair of additional boards for a balanced setup and modify the Arduino code to support a balanced configuration as well 🙂

 

PS: If you need a different value for the relay attenuator, there are a few good pages out there with information and online calculators:

AMB’s “Delta 1” project. (also the AMB discussion forum is a good source of information)

Jos van Eijndhoven’s “Relaixed” preamp.

Twisted Pear Audio’s “Joshua Tree” and the matching controller.

Score!

I don’t normally post about all the audio-related stuff I buy (then this would not be a blog but a twitter-feed 🙂 ), but here’s an exception:

In a post coming very shortly I am going to complain about how expensive it can be to get decent quality volume control attenuators for DIY audio projects. I still think that I am right in saying that, but nevertheless – sometimes you have to be a little lucky 😉

A few days ago I was fortunate enough to find a sales listing on a local audio forum for three Penny & Giles RF15 stereo log pots. I saw the listing only minutes after it was posted and so managed to beat about a handful of others that wanted them.

I know that P&G pots are very highly regarded (and very expensive) but having never handled one before I didn’t really know why. Now that I’ve collected mine from the post office I can however see why. They are heavy, they seem very solidly built and the feel of the action is silky smooth. These are used and probably quite old, but I expect that they will work fine still. I already have a project lined up for one of them and the other two are going into storage until I find something else worthy of these beauties 🙂

And the price? Oh, about the same as you’d pay for a standard blue Alps pot when purchased from a reputable supplier… (I did say I was lucky to find these, right? 🙂 )

pandg-1