Hifi Preamps for your Record Player

By: skrodahl | March 13, 2018

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.

Muffsy Stereo Relay Input 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.

Muffsy Relay Switch on Hackaday.com

By: skrodahl | October 16, 2017

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. :)

White o2 headphone amp PCBs

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.

O2 headphone amp components from DigiKey
Empty O2 cabinet and panels 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.

O2 headphone amp, soldering done
Completed O2 headphone amplifier

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...

Koss PortaPro headphones

By: skrodahl | April 19, 2017

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

Accurate Inverse RIAA completed

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

invRIAA Frequency Response - Both Channels

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

Vacuum pickup tool pump
Fish tank pump and tubing
Syrringe for vacuum pickup tool
A 3 mm hole in the syringe works really well
Sealing the vacuum pickup tool
Some silicone sealant to make the pickup tool airtight

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.

SMD reflow heat gun bought on eBay
SMD reflow heat gun bought on eBay

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:

Close-up of SMD components on the transistor tester kit
Full view of the transistor tester

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 | 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.