DIY headphone distribution amplifier

Sometime back in 2007 I was in need of a headphone distribution amp. I couldn't find a device for any reasonable price, so I had to make one myself.

The completed headphone amp in 2007

Then I happened to find the PAiA 9206K Headphone Distribution Amp. At the time I couldn't afford to buy it, but I was in luck as PAiA provides schematics for their devices. I love it when companies do that!

So I sourced the parts and built it myself. Now I owe PAiA one! I'll keep them in mind when it's time to start building my modular...

Insides with the op amps and pots soldered. Input and output jacks are still missing.

The schematic is very simple, and you can easily see how to add/remove channels if needed. The PAiA kit has 6 headphone outputs, but I needed only 4.

Use low-noise op amps that work with a supply of +6/-6 volts. Unfortunately I don't remember what op amps I used... They might have been TL072. Using a dual op amp makes for 1 op amp per stereo headphone channel.

The insides completed. Not very pretty wiring, but it works just fine!

Fitting it in, almost ready.

Now only missing some paint...

For the project case I used an old speaker. When painting it I tried to do some stencil work but that failed miserably as you can see...

The device works and is in use to this day. Thanks to PAiA!

Mesa Boogie F-50 footswitch schematic

I'm repairing a Mesa Boogie F-50 footswitch, and made a schematic of it.


Here it is if anyone needs it.

Schematic of the Mesa F-50 foot pedal

The pedal has three switches: channel, contour and reverb.

The connector is a 5-pin female DIN, similar to MIDI connectors.

Braun 1508 shaver battery replacement

This one might gross someone, but I don't give a f--k. If you're offended by imagery of decade-old greasy hairs - of dubious origin - go watch the disney channel.

I don't really use my Braun 1508 shaver that much, I prefer the 3-blade, but sometimes it's a lot of help when trimming the face in a hurry. I've had this shaver for over 10 years, and it's still working nicely for me - but the battery is now long gone. I've just used it plugged to the wall for like 6 years.

Finally, I decided to change the battery. It's just a 1.2V NiMH cell, you can get a new one from literally any grocery store. Here's a guide of how I did it.

First, TAKE OFF THE POWER CORD and keep it that way while working or you'll cease to exist. Then get the screen off and open the two screws on the side.

Open these screws...

After unscrewing them, you have to use some force to get the cover open. There's this white plastic locking mechanism with a spring in it, and that might break. My spring flew somewhere, and I couldn't find it, so in the end I just threw away the white piece of plastic and glued the shawer back together...

The 1580 opened up.

A view of the electronics.

This is the + side of the cell. Remember this. It's important.

...and the negative electrode is connected here.

I took away the cell with some cutters and a bit of force. It's glued in place.

Old cell ripped off the PCB, and the replacement cell.

Then I soldered some wires on the battery. Batteries shouldn't really be soldered on, since the heat will not do them good. But you probably don't have a spot welder at hand. Just use a soldering iron at maximum power and be fast, and it will be fine. The electrical connection doesn't have to be perfect, since it's a low-power circuit.

Wire soldered to the replacement Varta cell - negative electrode.

When soldering don't block the small vents on the positive electrode. They are there to release gas if the cell it overcharged. If they are blocked, the gas will build up and might result in an explosion when charging.

The positive side. Don't block the vents on the cap.

Then solder the wires to the PCB where the original cell was connected. Remember to solder it in the correct polarity, the same way as the original cell.

+ side

The - side.

That's it. Just put the thing back together and the shaver's like new! OK, maybe not brand new, but still, working.

It doesn't really make sense that you need to buy a new shaver when the battery dies. I quess it's partly a safety issue that they don't put a battery slot in the shaver, since it also has mains voltage inside... But mostly it must be the increased shaver sales they get due to dead batteries. So if you have any soldering skills, just fix the thing yourself. You'll prevent generating unnecessary e-waste and save some money from the greedy fingers of Braun.

F--k you Braun, I win.

LTSpice simulation of Chua's circuit

[EDIT 2012-03-25: fixed a stupid error on the sample rate]

From time to time I get spam from Farnell (a component distributor), and on their newsletter they're promoting a free webinar by Elektor magazine, titled "Let's Build a Chaos Generator!". The event is due 15th of December at 6:00 PM EET, so if you have nothing better to do, that's one good way to waste your time.

I had been reading on chaos generators in the past, and this e-mail got me interested again. Their design is somewhat complicated, with a total of 13 stages of 7 different circuit blocks. You can read all about it in the PDF they published.

Elektor's Chaos Generator won't fit in your pocket.

A much simpler chaos generator that can be built with a lot less components is called Chua's circuit. The downside with this simple circuit is that you need an inductor which you might have to wind up yourself. Also there should be much less variety than in the waveforms of the Elektor circuit. It has so many stages that can saturate to give different waveforms.

Still, Chua's circuit is very interesting to fiddle with. I haven't built the circuit yet, but I did some simulations with LTSpice IV and here's what I found out.

First, I googled for schematics. The first result was this great page giving me a nice schematic and details on the circuit. Another page I found had a quite similar circuit but with a little bit different component values. I built a circuit with LTSpice based those two pages.

My LTSpice version of Chua's circuit.

If you want to try simulating the circuit yourself, click here to download the .asc file.

The circuit is powered by two 9V sources, one positive and one negative. Would be nice to see a single-supply version by the way! There are two outputs, nodes V1 and V2. I chose the LT1351 op amp, since it should be quite near the common TL071 series op amps, and the model comes ready with the LTSpice installation.

The circuit can be tweaked by varying R6 from 0 to 2k. Somewhere in between lies chaos. I found 1.6kohms a good value.

A transient simulation to 1/10th of a second gives the following waveforms for V1 and V2:

From up here, voltage V2 (blue) seems to be bouncing around two different DC levels at the same time!

A closer zoom on the waveforms shows some detail:

And if we change the x-axis to be V2 and the y-axis to V1, we get the beautiful double scroll plot:

I also ran an FFT for the signals, to see what kind of frequencies are involved:

Hmm, seems like it's audible... and so I couldn't stop there. I wanted to hear what it sounds like, so I made a simulation which outputs .WAV files.

Since LTSpice IV clips WAV outputs with an amplitude over 1V, I had to attenuate the two signals with voltage dividers. Here's the LTSpice circuit I used:

LTSpice simulation with WAV output!

If you want to simulate this one yourself, download the LTSpice project here.

The simulation for 10 seconds of audio took some time with my settings, and generated 500MB of raw data. Looking at it now, the time step (0.1ms) is lower than the sampling time... You might want to simulate with approximately 1/44100 time step, but that will produce even more data.

After all this, I could finally listen to the horrible screeching noise of chaos.

And so can you: The signal V1 (.wav) is more high-pitched, while the V2 (.wav) has lower frequencies in it's spectrum (like you can see from the FFT).

What it sounds like is noise, and a very annoying kind of noise.

I don't know yet how changing the value of R6 realtime will affect the sound. For that I would need to build the circuit and try it. With low values of R6, the circuit oscillates with a constant frequency. It would be neat to hear the moments at the edge of chaos. So I'll be probably building this one sometime. If someone has already built it, go ahead and post a comment!

Portable speaker for MP3 player

I made this portable speaker in the summer. It mixes the stereo input into a mono signal, which is then amplified. It's powered by a six pack of AA batteries. The batteries are held in an external battery pack, so they are easy to change on the fly, without the need of a screwdriver.

Completed speaker pumpin' out some bass

I was going to a cabin with the guys to celebrate juhannus (midsummer) the next day, and there was going to be no electricity. Juhannus without music wasn't an option, and I'm not too impressed by the portable speakers available in stores (overpriced and shit sound). So I built my own blaster that evening.

The circuit is a modified version of the portable guitar amplifier I've built many times. I found the original circuit years ago from the great site Red circuit designs. The circuit is modified to have a simple stereo-to-mono mixer in the input, consisting of resistors R1 and R2. I chose 1k as their value, that should be enough to isolate the left and right channels from each other, since the headphone output on an MP3 player has a low impedance.

There is no volume control, instead I just use the volume control of the MP3 player. If someone wants to add one, just replace R3 with a potentiometer and connect "In+" of the TDA7052 to the middle pin of the potentiometer...

Schematic of the amplifier.

Going through my sizeable e-trash collection I found an old pairless computer speaker. It would get a new life soon.

The victim, a pairless computer speaker.

It was a left speaker, with just a speaker inside and an RCA jack on the back. I was going to use that as the power jack.

An RCA jack on the back was going to be the power input.

I took the metal leg off to access the screws and opened the case. Next I cut the wires off the speaker. There was some foam padding inside, which I removed for a while.

Case opened and wires cut.

I soldered all the amplifier circuit on a small piece of stripboard and then the speaker and the jacks. A single screw hole in the back of the case was used to fasten the thing there.

The amplifier circuit board is fastened to the wall with a screw.

I glued some mouse pad pieces to the most critical places to ensure there's no unwanted vibrations. To top it off I put back the original foam padding.

Some pieces of old mouse pads ensure the wires don't vibrate around.

I put also the original foam padding back to further eliminate vibrations.

Here is the backside of the finished speaker. I sticked an inspirational sticker on the back. The RCA jack on the left is for power and the 3.5mm jack is a stereo line input.

Connectors on the back.

I used some part of an old projector as a vise when gluing the battery pack together. Very handy. Another gem saved from the trash.

Gluing the battery pack together...

The power packs are also 100% recycled: the battery holders and the piece of RCA wire were also found in e-trash.

Finished test battery pack, with random AA's I had at hand.

The speaker was a hit in Juhannus and got very good feedback from it's bassy and clean sound. Just have enough batteries with you - it ate 3 six-packs of AAs during the 3-day trip and would have eaten even more. Of course, the speaker was on all the time. When the batteries are running low the sound starts to get distorted but turning down the volume a bit helps. This way you can also extend the battery life a bit.

That's all folks!

Links
Red free circuit designs -  Mini Portable Guitar Amplifier
My mini guitar amplifier build

Repairing a pitch bend wheel with style

These pics are from 2008. I had a Swissonic CK490 USB MIDI keyboard. It was a super-cheap controller with a lot of pots and 49 keys. The response was never very good with these things, especially with the velocity sensitivity, and it slowly deteriorated over time. It's enough for making some computer music, but I would never use this thing live.

Here is the keyboard before the repair. Or actually it's just a similar one. Notice the kitchen studio setup...

The Swissonic CK490. In a kitchen.

The keyboard was actually made by M-Audio, the former Midiman. The same keyboard has been marketed also under the Evolution brand. It's not as good as M-Audio products in terms of quality.

Since it was very light to carry, I used it for practicing quite a lot. At some point the pitch bend wheel stopped working, and the pitch of the whole keyboard became unstable. It was basically unplayable. The potentiometer had clearly given up.

So I decided to repair the wheel. After opening the thing and ripping the wheel out, I noticed it would be hard for me to find a similar linear 10k pot that would fit in. The two important properties were the physical size of the pot and how many degrees the thing rotated. Both would have to be exactly same with the replacement pot...

Don't pay any attention to the tablecloth. Please.

The modulation and pitch bend wheels ripped off from the CK490.

Closeup of the pitch bend wheel.

The pitch bend wheel in pieces.

Both the pitch bend and the modulation wheels used a similar 10k linear pot. After a bit of thinking I decided to use the mod wheel as the pitch bend wheel, and replace the mod wheel with just a normal pot. Since there was no spring in the mod wheel, I took that from the old pitch bend.

It worked great, and the new modulation pot didn't take a long time to get used to. It also looked great, see for yourself!

The result looks so f*cking ghetto!

Some time later, I sold the thing, as I had gathered some better keyboards. I wonder if it's still in use somewhere...

Mini guitar amplifier

[EDIT: also check out TDA7052 portable speaker for MP3 players I made.]

I've been building this circuit again and again since 2006, when I first built it.

It's an extremely simple mini amplifier. It has a single TDA7052 amplifier IC, a couple capacitors and one resistor... The circuit is designed and documented by RED free circuit designs. The TDA7052 datasheet also shows a very similar application example.

There's no volume control. You can use your guitar's volume knob. With full volume, there's usually some distortion, especially when the battery starts to drain up. But the distortion actually sounds pretty cool.

I've used this thing also as a speaker for my MP3 player. I just wired the left channel to the input of this thing. Works fine, but don't expect much from the sound quality...

My first amplifier build looks like this.

I had found some very cool old telephone routing / answering machines from Philips and decided to use one as the case. They're wooden and feature some very cryptic buttons...

Neat case!

You can see there's plenty of room left in the case... Note also the old film container I used as the battery holder.

The downside with the case is that it's far from airtight and doesn't give the best bass response.

Once a friend tried to use a wall wart to power one of these. He got the polarity wrong and blew the TDA7052, leaving a crater in the chip. I was called to the rescue. I had luckily used an IC socket, so I could replace the chip without any soldering.

Poor TDA7052...

Floppy disk drive motors...

...are some of the most beautiful things in the world.

Found these pics on my HD among other old stuff... I took them somewhere in 2006.