Hifi Preamps for your Record Player

By: skrodahl | March 15, 2017

The instructions give you three alternatives for powering the Muffsy Phono Kits. Here's another one:

Step-down power converter - Dual power supply
Step-down converter - Size comparison

This step-down power module is available on Tindie, and can provide up to two amperes of power. Use two of them and connect a 15V DC power supply at VIN/GND as shown in the picture above, and you will get a +/-12V output. This will provide sufficient headroom for the Muffsy Phono Kits.

The ripple is some 18-20 mV. This is waaaay higher than the Muffsy Power Supply, but should work reasonably well. Especially if you're strapped for space.

A small word of caution: The output voltage is set using very small surface mount resistors (included). You will have to do SMD soldering. It keeps the common ground though, so it should be able to work alongside other power supplies with all grounds connected.

No complaints on the price either, I paid $20 for five of these modules and international shipping is $3.

By: skrodahl | March 04, 2017

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.

RIAA curves, normal and inverse

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.

Accurate Inverse RIAA Equalizer - Stereo and SMD

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%.

Muffsy Accurate Inverse RIAA Equalizer - Final Version

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:

Inverse RIAA PCBs and stencil

By: skrodahl | January 27, 2017

I've been looking for a stamp to add resistor values, instead of writing them by hand. I did find some, but they weren't customizable and the shipping price was upwards to $100.

After some more searching, I found Itou-Kinzoku in Japan. They offer fully customizable rubber band stamps, so I thought I'd give it a go. I got four bands with the letters, numbers "1 2 3 4 5 6 7 8 9 0 M K". Itou-Kinzoku's representative made sure that I got the right height and width, and the right kind of rubber for my use.

Resistor values using a rubber band stamp

It all worked out as I had imagined. Not only does it look a whole lot better, but it took less than half the time it used to, to assemble the kit.

Category: Uncategorized 

Tags: stamp, packaging 

By: skrodahl | January 19, 2017

This project is freely available, and you can have the boards produced yourself. That's not for everyone though, so I've decided to sell the PCBs for the BSTRD on Tindie.

The Bill of Materials on hackaday.io has been greatly improved, and even contains a Mouser shopping cart for all parts except the tubes, tube sockets, LED resistors and transformers.


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.