Category: Electronics

An update on my electronics projects (by )

One of the challenges of being a massive nerd - who loves learning complicated technical skills - is that it can sometimes be hard to find new skills to learn. Thankfully, analogue electronics has provided me with a rich vein to mine; it's full of things that take some work to master, yet offer the opportunity to create interesting and useful things.

So, after the success of the Polyp Mixer (in which I learn how to use op-amps and to manage impedances within a circuit) and the trickle charger (in which I gained experience with basic power electronics), I am currently working on a... thing. I don't quite know what to call it, but I'm going with the working title of "microphone pre-amp". And my goal with this (other than to end up with a working microphone pre-amp) is to master the class A bipolar transistor amplifier.

The Goal of the Microphone Pre-Amp

So, the Polyp Mixer sits on my desk, mixing the audio output from my two PCs, and with two spare channels - one of which I intend to connect to a radio transceiver in future, the other being spare for random laptops or listening into audio circuits I'm working on (such as the microphone pre-amp, as it happens). This means I can listen to all of these devices through my nice amplifier and speakers. I can even plug headphones into the amplifier if I want to.

But, when I talk to people in video calls, I am using the nasty little microphone built into my USB webcam - and I can't feed that audio to the other computer, nor to a transceiver or anything. Rather than have a plethora of microphones for everything, I'd much rather have a single microphone that just feeds microphone-level signals to all of my devices.

So, the first goal of the pre-amp is to have a box that has a microphone in socket on the front, so I can plug in a microphone, and a bunch (let's say 4: two computers, a transceiver, and a spare, matching the mixer) of microphone-level outputs on the back to feed into my devices' microphone inputs.

However, this is complicated by the fact that different microphones produce different signal levels (of the pile I've tested, I've seen signals from 5mV to 100mV peak-to-peak). Normally I'd deal with that by adjusting "mic gain" controls on the inputs to everything, but it would be simpler if I had a "mic gain" knob on the pre-amp to adjust for different microphones, and then all the downstream devices can have their mic gains set to a constant level.

But not every device has a mic gain knob, because some things are meant to be plugged into custom-made headsets rather than arbitrary microphones. So, as well as the front-facing "mic gain" knob to adjust to different microphones, I could also do with "mic level" output knobs next to each output on the back, so each can be tweaked to read the same volume.

Also, if I switch microphones, I'd like to be able to get the level right before talking into my devices - so it would be nice to have a mute control and a level meter. I can mute the outputs, talk to see the level meter deflect, and adjust the mic gain knob until it's just hitting the maximum and not going over it. (This is why vu meters always have a green bit and a red bit; you're supposed to adjust levels until they're using the green area nicely but not going into the red area). So, I need a mute switch and a level meter.

Some microphones have a really tinny frequency response, picking up high frequencies much better than low frequencies. It would be nice (and fun!) to be able to compensate somewhat for this. I can't quite stomach building full spectrum analyser, but a classic James-Baxandall tone control circuit would be a fun addition.

Some of my microphones have a push-to-talk button; it might be nice to be able to feed that into the pre-amplifier, and have it drive the mute control. So, the mute switch really needs three settings - audio on, audio muted, and do-whatever-the-input-push-to-talk-socket-asks-for. And an LED showing the current output status, so I can tell if it's working when in external-push-to-talk mode.

A radio transceiver needs to know when I push-to-talk so it can transmit, so there also needs to be a push-to-talk output socket on the back.

Oh, and finally - I have a few headsets, with a microphone and earphones, so it would be nice if the thing also contained a headphone amplifier with a line-level input and output, so it can be connected between the mixer and my amplifier, and save me from running a long cable down from the headphone socket on the amplifier. So stick a headphone output and a headphone volume knob on the front for that, too.

At first I wanted to put a number of different sockets on the front, so I can just plug headsets directly in - an Android TRRS socket has lines for a mono microphone and stereo headphones, and the Kenwood/Baofeng headset socket has mono microphone, mono headphone, and push-to-talk. But in the end I decided it would be simpler to just have separate headphone and microphone sockets like on a PC, and a 3.5mm mono push-to-talk socket, and buy or make adapters from other standards to those three jacks. I managed to find a TRRS->dual TRS splitter on eBay, and made my own Baofeng->headset, microphone, and PTT lead (using two five-ohm resisters as a passive mixer to mix the stereo headset signal into a mono one). Having these adapters also gives me the flexibility to connect Android or radio headsets to computers, or connect a radio headset to a smartphone.

In other words, this thing has accumulated some feature creep, through a bunch of weakly interconnected features! The most questionable of these is including the headphone amplifier - but it does feel convenient to be able to plug a headset into a single box, and as the mixer is conveniently situated on my desk while the amplifier proper is a bit more of a reach away, it would be messy to be running a headphone cable extension back; headset splitter adapters only have ten centimetres or so of lead, so the microphone and headphone sockets do need to be close.

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An anecdote from my distant past: The Tale of the Half-Working Network (by )

Ok, this is an interesting tale I've retold in person a few times. So I've decided to write it up for the world to see.

The scene: My student flat, 1998. Seven of us are living there, and we're all nerds so there's lots of computers. The world of networked multi-player games is exploding, and we want to play.

So, we buy some network cards, and go to Maplin for some coax cable, BNC connectors and terminators, and start to set up a 10BASE2 LAN.

But one of the computers can't talk to the others. What's going on? Read more »

Trickle charging spare batteries in the van (by )

So, my van is a former "welfare van"; originally the sort of thing that would pull up next to some roadworks, offering a space for the crew to shelter from the rain and have their lunch. The back has four seats (with belts, so people can travel in them, making it a seven-seater overall), a table and a bunch of storage compartments. But it also has a 200Ah deep-cycle battery pack and a bunch of auxiliary electrical accessories. Read more »

The Polyp Mixer (by )

So, on my desk I often have a desktop computer and a laptop. I've got a decent HDMI/USB KVM switch so I can flip my big monitor, keyboard and mouse between the two, and that's great.

However, I also have a hi-fi amplifier and speakers for audio output. This is hooked up to the desktop PC, and has selectable inputs, one of which is connected to a lead for the laptop - but I rarely plug the laptop in. This is because I can only select one input on the amplifier; and although I'm usually only listening to media from one device, I want to be able to hear notification pings from either. So I tend to leave the laptop on its own nasty little speakers and only have nice audio from the desktop PC.

Clearly, this sucks. Many years ago I had a cheapo mixing console that sat on my desk, with my CD player, minidisc player, and PC connected to the inputs, outputting into my amplifier; it was cool to be able to just hit play on anything and hear the result through my good speakers, and having all those knobs and sliders to play with was definitely gratifying. However, it was bulky, full of useless-to-me features like phono inputs and cross faders, and eventually died a death from being left switched on all the time.

Plus, I'd recently resolved to do more electronics, so there was only one thing to do: Make a mixer.

The Polyp Mixer

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Electronics Projects (by )

So, my electronics workbench is a mess.

This is abundantly clear in the picture from my blog post on redesigning my workspace; the awkward layout is certainly part of the problem, but a deeper problem is that I don't do many electronics projects. So this big workbench is rarely used for its intended purpose, and thus accumulates junk, and thus isn't very inviting to start projects at, which adds to the fact that I'm a bit edgy about STARTING electronics projects, and a vicious cycle has set in...

The only electronic projects I did lately were the 12 volt DC power distribution system for the van and a 9:1 impedance transformer, but those were mainly mechanical builds; the electronics were trivial.

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