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

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