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. ;)

It's become evident that I need a better solution for photographing my projects. So I went out and bought some surprisingly cheap LED panels:

But then I had no idea on how to build this into a suitable light box. So I spent the time buying suitably sized vinyl backdrops and some clamps from the UK based eBay seller photogeeks11. I can't thank the seller enough, small sized vinyl backdrops are very hard to find. They are also of excellent quality.

My first lightbox...

The first lightbox was three pieces of scrapped wooden planks. Just to hang the backdrops from. The two side LED panels leaned against the planks, and I managed to somehow balance the top LED panel long enough to take one picture. Then it all crumbled and fell to the floor. (Nothing broke, luckily.)

It did produce one good picture before the whole construction fell apart though. Keep in mind that this is taken using my Galaxy S7 Edge with default settings.

Rethinking the lightbox (the one that didn't fall apart)

Since I wasn't in the mood for having my (albeit cheap) LED panels crushed, I set out looking for alternatives to scrap wood. I finally found some nice square aluminum tubing and plastic joints.

With the building material in playce, I brought out my trusty old (and very dull) hacksaw and started construction. Here's the result:

The plastic joints had to be hammered into the tubing, and I was a bit worried that I would never be able to disassemble the light box (it's big, and I don't have that much space). Luckily, the plastic sort of shaved off. Using the hammer, I was able to take it apart.

While still a very snug fit - which is good, the lightbox is nice and stable - I'm able to pull the tubes apart so I can store the lightbox when it's not in use.

Pictures, or it didn't happen!

Well, I have only one picture. It's of my B&O IcePower module, on white vinyl backdrop.

This picture is also taken with my Galaxy S7 (the system camera is now on a tripod in front of the light box). I had to change exposure, and do some post-editing though. 

I think that's quite a beautiful result. Now that I have the system camera mounted, I've finally found my lightbox solution.

It wasn't exactly cheap though, it came in at just above US$ 300. It's tailor made to my needs, which is why we all do the DIY-thingie in the first place. Right?

Update: Picture taken with the system camera

Here's the Muffsy Back Panel photographed with my Sony Alpha 5000 with a Minolta MD Rokkor 50mm f/1.7:

...and the camera itself:

The Sony Alpha 5000, with its 20 megapixel APS-C sensor, is really great as a manual camera. It's got no viewfinder or fancy autofocus, so I wouldn't use it to shoot sports pics. Or outdoors, due to the relatively dim LCD monitor...

The range of vintage lenses that can be used with the Sony Alpha (E-mount) cameras is almost endless, since there are adapters available for most lenses. Check out the YouTube channel AdaptedAlpha to see lots of examples of older lenses being used with the Sony E-mount cameras.

While I've been sharing a lot of my work on muffsy.com, I've never really clarified how my view is on people using my work (or derivates of it). This is now changing.

I've decided to release all of my content (barring a few exceptions) under the Creative Commons Attribution 4.0 International license.

What does it mean?

It means that everything on muffsy.com, except where specifically stated, is open source. You have the right to:

If you're going to use any of the content on muffsy.com, you will have to adhere to these conditions:

• Attribution — You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.
  
• No additional restrictions — You may not apply legal terms or technological measures that legally restrict others from doing anything the license permits.
• You do not have to comply with the license for elements of the material in the public domain or where your use is permitted by an applicable exception or limitation.
  
• No warranties are given. The license may not give you all of the permissions necessary for your intended use. For example, other rights such as publicity, privacy, or moral rights may limit how you use the material.
  

There are also some conditions for muffsy.com to follow. Most notably is that the shared information must be in an open format. The only possible violation to this are my Autodesk Eagle design files. They can be used, but not always processed, by the free version of Autodesk Eagle. They can also be opened and edited (although not always perfectly) in the open source tool KiCad.

What are the exceptions?

The only exceptions to the Creative Commons on muffsy.com is for a few of my PCB designs, mainly variants of those that I'm currently selling. They are free for personal or non-commercial use, but you cannot use them commercially without my approval.

At the time of writing these are the only exceptions:

• https://www.muffsy.com/blogs/post/Make-Your-Own-Muffsy-PCBs
  
• https://www.muffsy.com/blogs/post/Make-Your-Own-Muffsy-Power-Supply
  

If not stated otherwise, everything is free to use as described above. That's right, text, pictures, schematics, design files. The lot!

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!

Check out this post for test setup and limitations.

ITU-R IMD is one way to measure intermodulation distortion. I am feeding the circuit with two test tones at -6 dBV, which will sum to 0 dBV. The test tones at 19 kHz and 20 kHz will produce a modulated signal at 1 kHz. The difference between the test tone and the modulated signal is the ITU-R IMD.

The ITU-R IMD for Burson V6 Classic is -118.1 dB / 0.000124%.

Click here for a high resolution image of the measurement.

How does it fare in comparison to the audio analyzer itself and, equally interesting, our OPA2134 reference IC op amp?

Well, the audio analyzer can measure down to -122.2 dB / 0.000078%, so the Burson measurements are well within the analyzer's capabilities.

Click here for a high resolution image of the measurement.

The Burson V6 Classic loses out to the OPA2134 once again. The OPA2134 measures -121.0 dB / 0.000089%.

Click here for a high resolution image of the measurement.

Following up on the THD measurement of the Burson V6 Classic (which also shows the test setup and limitations), here's its frequency response.

These measurements are showing up now because the QA401 is not linear. The measurements had to be compensated for that.

The frequency response is measured with an output of 0 dBV / 1V rms and a gain of 1.36 dB / 1.17X.

The Burson V6 Classic's gain is 0.1 dB lower than expected. It performs great otherwise, showing -0.2/-0.1 dB all the way from 10 Hz to 80 kHz. This is as high as I can go with 192 kHz sampling frequency. The graph uses a relative scale since it would otherwise disappear under the 0 dB horizontal line.

Click here for a high resolution image of the measurements.

But how high does it go? Time to whip out the Rigol DG1022 function generator and the Rigol DS1054Z oscilloscope. ~1.5 MHz is the answer (with the noise starting at about -20 dB below the signal). Go beyond that and the amplification still holds for a while, but the signal is no longer a sinus.

The signal is down -3 dB at approx. 3.7 MHz. By then it's a pure saw tooth signal.