A very simple, but useful PCB. A power supply for the Muffsy Relay Input Selector (or anything else that can be powered by 5V DC/1A):

The MeanWell IRM-05-5 takes an AC input between 85 and 265 volts, or a DC input between 120 and 370 volts and converts it to 5 volts DC at a maximum of 1 ampere.

This open source PCB project created in Eagle lets you add input and output cabling, as well as providing mounting holes for the PSU. Download it here:

Github repository for the Eagle project files

The project contains the schematic, the board, and the gerber files (and also the .cam file to generate your own gerbers).

While there are lots of quality kits for preamplifiers, I have never really found a decent way of adding more inputs to a preamp. The existing ones are either of dubious quality, spec'ed like a spaceship, or both.

So I decided to make my own input selector. With relays. And an ESP-32 module! A year, and four revisions later, you can buy it on Tindie for $69.

The entire project is fully open source, no strings attached.

So what is it?

It is a programmable input selector for preamplifiers, the size of a Post-IT note, with:

• Four stereo inputs
• One stereo output
• Mute
• Mains power on/off through a solid state relay control
• Break-outs for all ESP-32 pins

It's controlled by a rotational encoder and IR remote, and you get the great build instructions that you've come to expect from a Muffsy kit.

The PCB, all onboard components, ESP-32, rotational encoder and IR receiver are all included in the kit. You also get the fully functional software for it, as well as the Eagle project files for the PCB.

The benefit of using an ESP-32 is that you can program it yourself, and use other functions like touch sensors, Wifi and Bluetooth.

Several people have been in touch lately with questions on how to modify the PCB into something it isn't right now. Popular requests are balanced inputs and/or balanced outputs, and DC servo on the outputs.

The PCB itself doesn't really lend itself to any of the modifications above, but you could always try doing add-on boards or roll your own from the open source freely available schematics.

An explanation of the Muffsy Phono Preamp PCB layout can come in handy for such alterations. And wouldn't you know, here it is:

Modifications you can do are changing the input impedance and altering the variable gain. I'm also happy to announce that the gain calculator has been updated to show the output voltage and voltage gain:

Finally, if you want to make major modifications, a way of simulating the circuit would really come in handy. Well, here's the Muffsy Phono Preamp LTSpice simulation file.

I just got a request from someone who wants to CNC a front panel for his Muffsy MC Head Amp, and he wanted to use the logo and the font that are on the printed circuit boards.

OSIFONT - The CAD Font

The font used on the PCB is the built-in vector font in Eagle, which is proprietary. The Eagle font is really similar to the "official" CAD font OSIFONT. Which is, I'm sorry to say, proprietary.

Luckily, Github user hikikomori82 has made an open source variant of OSIFONT:

The "Muffsyfont" - The OSIFONT to a T

I thought the lower case "t" looked wrong, so I changed it, and now it's the Muffsyfont.

Download the Muffsyfont here.

And Finally the Logo

The logo is a bit easier though, I can share that with anyone. And by virtue of placing it on this site, it becomes open source as well.

Download the large PNG with transparent background here.

Behold the new Muffsy Phono Preamp!

It'll be available on Tindie, very soon. Actually, it is available: https://www.tindie.com/products/skrodahl/muffsy-phono-preamp-kit/

What's New?

These are basically the same circuits as before. The focus has been on making the build experience even easier than before, and improve noise prevention.

• The op amps are OPA2134 and LM4562
• SNR improved by 2.9 dB, from 79.4 dB to 82.3 dB
  
• No screw terminals facing the side walls of the enclosure
  
• The AC power cables do not cross the input
  
• The DC power cables do not cross the output
• The AC and DC power connectors line up on the PP-4 and PSU boards
• The signal inputs and outputs line up with the RCA connectors on the back panel
• The grounding scheme is redesigned to avoid ground loops
• The input impedance switches are grouped together
• The gain switches are grouped together
• The back panel has a ground pad in one of the corners, to ground the cabinet
• The back panel now has gold pads and the golden "M" logo
• The power supply now has the golden "M" logo
• There's room for heat sinks on the power supply voltage regulators (if needed)
• The solder pads for the screw terminals are beefier
• The mounting holes have been increased from 2.8 to 3.0 mm
• The board width is approx 0.5 mm narrower for a better fit in the B0905 cabinet
• The board's corners are even more rounded to make it easier to insert into the B0905 cabinet

Built-in Grounding of the Enclosure

One prime example of the thought that's gone into this new version is the fact that the back panel connects to the enclosure for improved grounding and shielding from noise:

Crisp, Clear Pictures

I've put my new lightbox to good use, and produced some really nice pictures for the instructions.

Here's one of the new photos compared to the corresponding old phono. Judge for yourself:

Thanks

This new version of the Muffsy Phono Preamp couldn't have been created without the (without exceptions) enjoyably long and fruitful email conversations with so many people. Keep using the contact form.

A special thank you goes to the loyal customers who helped me test the prototypes and provided valuable feedback, you know who you are. ;)