Tuesday, 27 October 2020


 Through most of my life I have really enjoyed going to the science museum in London. My father said that he felt both honoured and decrepit when he saw an instrument he'd designed in there. Among my favourite exhibits were a huge steam engine by the door (it wasn't running on live steam but it was turning), demonstrations of bridge building and rocketry in the basement, a recreation of Babbage's difference engine, and a working 2 digit telephone exchange with dial telephones and Strowger switches. The aviation section was also very good.

The last time I went was a few years ago, and it will probably be the last time that I go. It was very crowded and all seemed a bit run down. The steam engine wasn't turning, there were no demonstrations in the basement, the telephone exchange had been taken away, and the café was expensive and slow.

I did however see one exhibit that really caught my imagination, but I've forgotten one of the key details about it. There was a set of very old sherry glasses that one of the pioneering microbiologists had used at home to grow cultures. I love the idea of groundbreaking scientific research being carried out at home in whatever glassware came out of the drinks cabinet. The problem is that I can't remember who's they were. Did I see Jenner's sherry glasses, Lister's? Pastuer's? somebody else's?

Richard "Sherry glass museum" B

Tuesday, 20 October 2020


 This story is third hand, so there are probably some inaccurate details – like all my other stories.

My brother recently changed jobs and he needed to buy himself a new laptop. He's a mechanical engineer, his old job was something to do with oil rigs, his new one swimming pools. His new job required CAD software so he had to buy a laptop with massive RAM and a huge GPU. I'm sure most of my readers can already see where this is going... it sounds like a gaming PC. That is exactly what he bought, and he now turns up to business meetings looking like he's a pro-gamer. I don't actually know anything about the laptop, but I'd like to think that it has a wild coloured (or super low albedo matt black) case, grotesque angular design, giant cooling vents and a brightly illuminated logo on the outside. Lets hope that the keyboard has individually programmable coloured LEDs, that the WASD keys are made from a different material, and that the membranes are certified for 100,000 cycles.

Richard "I hope he starts playing KSP" B

Wednesday, 14 October 2020


 This weekend I had houseguests from out of town. We went to admire a little corner of Dartmoor and I was reminded of something that happened in my childhood.

I think I was probably about 10 or 12 and I was swimming and playing in the river at Cadover Bridge. An unattended toddler fell from the river bank and ended up face down in the water. I recognised that this was important and that I needed to act quickly. I swam as strongly as I was able to the stricken toddler and even though I was nearly out of my depth I grabbed it and lifted it clear of the water. Not long afterwards the mother noticed what had happened, ran to the river's edge shouting, and grabbed her child from me. I was frightened and disturbed by the harsh way she acted and hurt that there wasn't even a word of thanks.

Looking back I can imagine that she was upset and that the welfare of her child was her main concern, but my overriding memory of the event is that I was cross and upset at how harshly I had been treated. Some time later, probably only a quarter of an hour the family approached my mother and I with effusive thanks and I was bought an ice cream. I'm pretty sure that I accepted it with bad grace.

Richard "One star. Would not rescue again." B

Sunday, 4 October 2020

Impedance mismatch

 There is a long running joke in product development that the software team will fix the hardware errors in software, and more rarely that the hardware team will fix the software errors in in hardware. In the distant past I did something similar, but it was a temporary workaround. The software guy was on leave, and we couldn't register any of these prototype devices with the PCs because (I seem to remember) the sense of an interrupt line was inverted. I wired in a switch and an inverter just to keep us working.

This weekend I did fix what I consider to be a software fault in hardware.

Back in the day, when I was working for my favourite boss I had to research some ORM solution. He also came from an electronics background and he warned me that I wad going to find the term "impedance mismatch" and that it was going to annoy me. He was right.

My mum is very forgetful, so her radio needs to behave exactly as the radio she had 20 years ago did. It's hard to get one with physical preset buttons, but not impossible. It needs to connect to the amplifier and speakers which she's already familiar with, but the one we found has a headphone output, not a line level output. Domestic line level is 1v peak to peak into something like 100kΩ. Headphones are normally a few hundred ohms. If you want to plug an ipod, mobile phone or suchlike into an amplifier, it's normally good enough to just turn the headphone volume to maximum and connect it to the input on the amplifier.

My mum's new radio doesn't have a real volume control, it's a dial which goes round and round and sets the volume in software. When the radio boots up it sets the volume to the last setting, unless the last setting was > 50%, in which case it uses 50%. This is far too quiet and meant that my mum would have had to wind the dial around every time she turned the radio on. It was a procedure she struggled with and didn't seem likely to accept.

I set about building a converter which would connect the headphone output (all amps and no volts) to the amplifier input (1 volt and no amps). It couldn't be something with a power supply because that would also need to be switched on and my mum would forget.

On stage you often need the opposite. Let say there's a keyboard with a normal line out, but you want to run that signal 10s of meters across a (electrically) noisy stage and connect it to a mixing desk which is expecting a microphone (µA of current in a push-pull circuit and no volts). The humble DI box does that exact conversion for you. DI stands for "Direct Injection" because you are injecting the signal straight into the desk without having to use a microphone.

These days DI boxes are usually active devices, taking the power needed to run a little op-amp from the 48v that the desk offers to the microphone in case its a condenser (capacitor) mic that wants to charge one of its plates. In the old days, or if you were cheap when you bought your desk, or if you've got some naughty mics that won't tolerate the phantom power, then you have to use a passive DI box. A passive DI box is nothing more that a matching transformer with a centre tap on the low impedance side in a sturdy box. Of course I own some passive DI boxes, and their ratings are surprisingly close to what I needed. 600Ω on the desk side, 50kΩ on the instrument side. I just need to connect a pair of them back-to-front between the radio and the amplifier.

I do sometimes use my lounge for sitting around and watching the tv, but here it is configured for electronics work:


Heatshrink to stop the cables from tearing apart:

Several layers of heatshrink so that the chuck type strain relief can grab the tiny little headphone cable:

This is the finished cable:
And this is the contraption:


To answer your questions:
Yes, it worked very well thank you.
No the microphone plugs aren't made by Neutrik, they're cheap copies cut off from a snake I own.
Yes, it is rather cumbersome.

Richard "XLR" B