I'd really like a stereo input selector with relays, but those things are hard to find! (Apart from some of dubious quality on the *Bay)

Better make my own then. This one's got a custom footprint for an ESP32 devkit module, but it can also be controlled with a rotary switch.

I decided to use five Panasonic TQ-2 relays. The ESP32 module and relays are powered separately, power ground and signal ground are separated to avoid injecting any clicks, pops or noise into the audio channels.

The whole project is open source, free to use as you wish. Eagle project files, gerbers, the Eagle library for the ESP32 module and BoM are all available on the hackaday.io project page.

And, wouldn't you know, Hackaday presented this little side project on their blog!

I've been getting my new hobby room ready, and I knew I needed some kind of stereo system in there. Preferably one that didn't annoy others in the house, so I ended up ordering PCBs from Nwavguy's gerber files. Yup, I'm gonna use headphones. :)

The components that I didn't have at home were ordered from DigiKey, and I ended up doing quite a few component substitutions. The cabinet + front and back panels were ordered from Headnhifi.

I did spend a lot of time identifying the right components. There are a lot of different resistor values, and they have to be cross referenced with the BoM. Of course, it didn't help that I have a box with all E24 resistor values that aren't sorted in any way.

Once the sorting of components was done, it didn't actually take long to solder the thing. I'm really happy I got the recommended enclosure and panels. This thing looks really great.

Initial impressions are really good. My main problem (until the Sennheiser HD 600 headphones arrive) is that I only have an old pair of Koss PortaPro available. The foam around the earpieces is really old, and creates a cloud of tiny black dust...

Here's the finished Inverse RIAA. I haven't done extensive testing yet, but it works as promised AND it measures exactly the same on both channels. I'm really happy with the result.

Get the gerbers for this project here: invRIAA.zip

If it's going to be called accurate, you'd better get two identical channels. Here's how the invRIAA fares:

In order to construct this board, I built the vacuum pickup tool:

The lead free paste was applied using a stencil, which is extremely convenient. To do the actual soldering, I used this reflow heater bought cheap on eBay (Nope, I don't have space for a reflow oven...). The only negative with this heater is that the LCD back light is more on the front, so you need to tilt it to see what it says.

You really don't want to go with a normal heat gun for this. They push about 600 liters of air per minute, which will blow away all the components. This one does about 30 liters/minute, and it's adjustable both on the tool itself and by replacing the nozzles.

I even bought an Atmega based transistor tester as a kit, and it contained three SMD components. Here's how that turned out:

This is my first venture into SMD components, so I'm going to have some fun with tools, solder paste, stencils and hot air. :)

Get the gerbers for this side project here: invRIAA.zip

An inverse RIAA circuit has been on my wish list for a while, as it makes testing frequency response so much easier. But how, you might ask?

Well, the first problem you'll encounter when trying to test a phono stage is the fact that few signal generators produce a signal level low enough. If they do, they're often not very accurate at those levels.

The second, and most pressing problem, is that the phono stage applies equalization to your signal. Input a 50 Hz signal at 5 mV, and the output will be almost 5 mV. Input a 20 kHz signal at 5 mV, and the output will barely be measurable. You won't be able to see the frequency response from the readings, without doing a whole lot of conversions (and taking DMM/scope tolerances/misreadings into consideration).

What's needed is something that takes your signal and turns it into what you'd find on a record. That's your inverse RIAA equalizer.

Feeding your signal through an inverse RIAA equalizer, and then through a phono stage, will create a flat output at all frequencies. If this inverse RIAA equalizer is sufficiently accurate, it can be used to measure the accuracy of your phono stage. It's got the added benefit of bringing a higher input signal down to cartridge level.

I got Hagerman's inverse RIAA filter, and while it's a nice little device, I wanted to get one with better accuracy. That's achieved with a lot more components to even out their tolerances.

Not one of my designs, this is the Accurate Inverse RIAA from HIFISonix. I decided to make a stereo version. As I already have lots of screw terminals and DIP switches, I decided to use them too.

There's a lot of components in there, which is why it's SMD (the board size is approx. 8 x 5 cm). All SMD components are 1206 size, so it should be manageable to solder them in. I thought I'd use a hot air gun for the soldering.

I still have to get the boards manufactured, and I need components for it. Not sure how long it'll take, but I'll definitely let you know when it's done.

UPDATE (2017-03-09):

All components have arrived, and a few boards have been ordered. I also sprung for a cheap hot air soldering "pen" on eBay, and a stencil for the SMD-components on the board.

I managed to get the board size down to 84 mm (83.98 mm to be exact) x 51 mm. 84 mm width is what's needed for the board to fit into one of those B0905 enclosures. :)

UPDATE (2017-03-10):

Here's the final layout. The caps have all been changed from 1206 to 0805, since I only found them in that size at a decent tolerance of 2%.

The attenuation for MM is -44 dB, and for MC it's -68 dB.

I got a vacuum "tweezer" that was completely useless for these small components, so I ordered a fish tank air pump to make a real vacuum pickup tool based on this video:

UPDATE (2017-03-16):

The boards and stencil are here:

There are several ways to make your own Muffsy Phono Preamp, but the power supply has been missing. Until now. Below are both gerbers and PDFs that allows you to order or etch your own power supply boards.

There are a couple of differences between this board and the power supply kit:

• The board is single layer (bottom) with ground fill
• The AUX power has been left out

• The Gerbers are in 274X-format, view them in Gerbv to see how the PCB will look like
• Order as two layers to get through-hole soldering pads, and solder mask on both sides
• There is very little copper that needs to be etched away if you're etching yourself
• The PDFs have the bottom (copper) layer and the top (silk screen) layer, paper size is A4
  

• The bottom copper layer is oriented so that it can be toner transferred directly to the PCB

Download the files here:

• Muffsy Power Supply - Gerbers
  
• Muffsy Power Supply - PDFs
  

Hot on the heals of the official Muffsy PCB, here comes another option.

Over at theslowdiyer's blog, you can now find the project files for the Muffsy Clone.

Everything you need to build your own Muffsy Clone is available at his site. There's an impressive lot of information about his design goals and on how you should go about building the project.

Go here to see all of it, and build your own Muffsy Clone: 

Theslowdiyer - Muffsy Clone Project Files

If you are looking for a suitable power supply, theslowdiyer's got you covered as well with his GP-PSU v2.

We've seen some revisions of the Muffsy Phono Preamp, from green to red,  through blue and finally the black PP-3. Now get ready for a new one, only this time you can't buy it from here.

The question of a more generic circuit board that will accept other components than the ones included in the kit pops up now and then. Although there is nothing stopping you from designing a circuit board from the schematics, that might not always be a viable alternative.

As the Muffsy is based on the Audiokarma CNC phono stage, this is my way of giving something back. You will find a PDF for etching your own board and Gerbers for ordering it further down.

Here's some information about the board:

• The board is single layer (bottom) with ground fill
  
• There's space for larger capacitors (RIAA: 5/10 mm, Output: 10/15 mm pin spacing)
  
• There's space for wider resistors
  
• The width is 84 mm, it will still fit in a B0905 cabinet
  
• It has the standard Muffsy values for input impedance and RIAA equalization, all within the E24-series
  
• It has variable gain
  
• There's space for input capacitors
  
• Input, output and power in has 5 mm pin spacing for terminal blocks
  
• The Gerbers are in 274X-format, view them in Gerbv to see how the PCB will look like
  
• Order as two layers to get through-hole, and solder mask on both sides
• There is very little copper that needs to be etched away if you're etching yourself
• The PDFs have the bottom (copper) layer and the top (silk screen) layer, paper size is A4
  
• The bottom copper layer is oriented so that it can be toner transferred directly to the PCB

Here are the links to the MYOM (Make Your Own Muffsy):

• PDF - For etching your own Muffsy
  
• Gerbers - For having your own Muffsy manufactured