Hifi Preamps for your Record Player

By: skrodahl | May 13, 2019

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):

Muffsy Input Relay Selector - 5V
Breakout Board for the MeanWell IRM-05-5 PSU

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:

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

By: skrodahl | May 02, 2019

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.

Muffsy Relay Input Selector - Kit
Muffsy Relay Input Selector - Kit Contents

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.

By: skrodahl | December 05, 2016

Here's another project with freely available Eagle project files.

There was a strange little thing that appeared in Danish magazine Ny Elektronik (New Electronics) in 1989. A very simple preamplifier that used two L63 tubes in Class A with no feedback and it operated pretty much badly out of spec. (It was supposed to though, that was the whole angle of the article.) It was called The Bastard, and gathered quite a following (it got the name because it was a hybrid. The phono stage used transistors for better SNR), and somebody suggested I should try it.

Once I had some suitable tubes (6S2S, NOS, shipped from Smolensk in Russia), I knew I could go ahead with the project. I decided to skip the phono stage, since I already have some of those, and did only the line stage. ;)

A few iterations of the PCB drawings later, and it was time to order some boards as well.

I called mine The BSTRD:

The original Bastard, while probably sounding very good, did not perform that well (it had up to 6% THD+N). Some other Danish guys took a second look at it, and made a couple of changes. First, they bumped the operational voltage from 37V to 80V, to get it into spec. Then they tamed the gain and improved the performance by adding some feedback.

The result was this circuit which has a THD+N of 0.185% and a gain of 2.3:

The unregulated PSU in the original article wouldn't work for this revised version, so I made my own. It's actually two power supplies, both regulated, that delivers 78V/0.7A and 6.3V/1.5A. The 6.3V is for the filament heater, so it doesn't need a lot of filtering as long as it can deliver the required current. The 78V features a voltage quadrupler and uses an RC filter for better smoothing.

Measuring the PSU with load shows 0.00 mV AC on both DC outputs. I'm satisfied with that, although I haven't checked out the noise on the scope.

The BSTRD has been built and tested, and it sounds pretty darn sweet.

Note that because of the voltage quadrupler on the 78V side, you ABSOLUTELY NEED TO USE TWO TRANSFORMERS.

Since the filament heater draws 300 mA of current (a total of 600 mA for two tubes), the LT1086 and its heat sink get really hot. I would recommend using 6 to 9V AC, anything higher than that would probably require moving the regulator off the board and fit a larger heat sink.