Monday, November 26, 2012

MOTU 882 MK2 Mic Preamp Input

Today I disassembled a MOTU 882 MK2 Audio Interface that exhibited significantly increased noise on one of the microphone preamplifier inputs. While I had the unit open, I made this quick sketch of the input circuitry:

From chris' blog

From chris' blog

Interestingly the MOTU does not use a specialized microphone amplifier (such as the SSM2019 for example) but the standard instrumentation amplifier layout known since the dark ages, built from a NJM2068 dual opamp.

The next funny thing to notice: MOTU put a bridge rectifier on top of the SMD opamp (which has been removed in the photograph, it's normally sitting in the four holes on the corner of the small IC). It's protecting the amplifier from voltage swings out of the V- to V+ range. It's also missing from the hand-drawn schematic, it's on the positive inputs to the left opamp. In the end it turned out that this bridge rectifier was the cause of the increased noise. I replaced it with Z-diodes altogether, and now both channels (the unmodified one, and this one) show exactly the same (good) noise-spectra.

Monday, November 19, 2012

ligthdm: custom app on login screen

It's not rocket science, but it took me a while to collect all the pieces, so here's a short summary, mainly to serve as a reminder to myself: I need the IP address of a computer displayed on the login-screen (using lightdm), because every day it gets a different dynamic IP address and I want to login over the network, and logging in on the console just to type ifconfig (or ip addr) is cumbersome. No, it's not in DNS. No, using mdns/avahi doesn't work either in this particular instance. It's a laptop sitting just next to me on my desk.

So, without further adue:

  • install conky
  • in /etc/lightdm/lightdm.conf:
  • in /etc/lightdm/greeter-conky.conf
    xftfont Ubuntu Mono:size=18
    eth0: ${addr eth0}, eth1: ${addr eth1}

From chris' blog

Tuesday, November 13, 2012

RANT: complications of converting a picture to a pdf...

I just spent about one hour looking for a small tool (i.e. NOT starting libreoffice and putting a picture on a empty page, export as pdf) that will preserve resolution and geometrical size of a scanned-in bitmap and put it centered on a blank (A4-paper) page.

I assume that for every permutations of {'png','jpg','tiff','bmp','xpm',...},{'to','2'},'ps' there will be some tool in Ubuntu's repository, but they all seem to...
  • mess with the aspect ratio
  • either assume some fixed input resolution (ImageMagick's convert, for example) which gives you tiny, tiny, tiny output pdf-files (I could not reproduce the calculations it does) or correctly sized pages with the image enlarged until it fits border-to-border
  • re-sample the image so that the output file is huge, and image quality sucks.
But today, I finally found the small python script again I've written back in 2007, used through most of my university time (for scanning in travel invoices, notes, ...) and which still works \o/ . It's not a very pretty tool, but hey, at least it does what's advertized!
Have fun with it, in case you have to send in your Arbeitsunfähigkeitsbescheinigung like myself, being sick and getting MENTAL BECAUSE OF ALL THESE USELESS TOOLS one has to keep up with.

Wednesday, November 07, 2012


Found this ancient (well... almost, it's from about 1982) chip at work. It's a development version for applications where a mask-programmed 80C51 will ultimately be used, and you therefore don't have means to plug in separate ROM chips onto your circuit board:

As long as you are working on the software, you can burn it into (erasable, changeable) PROMs that get plugged into the piggyback-socket on top of the 80C51Px. Once you've finalized your code, you'll use OTP (one-time-programmable) or mask- (factory-)programmed chips that are, of course, much cheaper. The chip has pins in standard DIL-40 layout on the bottom, the piggyback sockets on top are also standard DIL-28 spacing.

From chris' blog

Interestingly, while this chip family is as old as rocks, software-compatible parts with much higher performance are still being produced, e.g. by Silabs.