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Well, to go through your letter point-by-point…

Things are generally okay.
I went to Trets for new year again which was good.
I’ve got a better job with my company and I have to roll out some of the
software I designed to all the depots on a national basis, so that’s kept me
quite busy.
I’ve got my website a bit better sorted to correct some of the fibs floating
around the internet and have started cleaning up some tracks with the
intention of getting all the stuff into the public domain. I spent £15 on a
little gizmo to stream audio (or video if I had any) on the webpage in a
little cd player kind of interface. I got the colour and style to match the
pretty basic and gloomy layout I’ve set. I’ll get better at this web page
stuff eventually as I’m going to get trained up in it at work as everything
is Oracle, asp and AJAX. Also, I’ve been interviewed by this weird guy in
Newfoundland about my band for a syndicated internet radio thing. Have a
look at http://www.crawlingchaos.co.uk It’s a pretty boring read as I just chuck
stuff onto the page as it pops in my head. It’s a bit like an editted and
revised blog as I keep changing it. I’ve heaps to do though yet.

Old 70’s mixer eh? What was it’s original cost/value and what sort of
specs does it have? It seems a bit naffish if it’s using din sockets
although it could be an early midi implementation.

There are loads of ways to add extra inputs etc. If every input is a
din then you’ve a lot of work. The pins should follow the standard din
pattern for 3,5 or 7 pins. I used to carry them round in my head, but
not now. Be aware, that if it’s following DIN standards then the DIN
standards used to be electrical standards as well, so may apply to the
input impedance and levels expected to be seen as well as the output
impedance and recommended loading. If I recall, the DIN input standard
was something like 100mV into 47k as opposed to the standard line
levels used in studios of 1mW into 600 ohms balanced. (this is 0dB on
the meters). I think it was a 10k out as well. Basiclly DIN was pretty
poor. If you replaced the din sockets with xlr connectors, physically
they’d be modern but electronically you’d have low headroom and a lot
of noise. You may have to change the pre-amps and if it’s really old,
everything, as quite often a modular contruction method wasn’t used in
those days. Without seeing it, I can’t say; I’m just thinking aloud!
The pots will need a damn good squirt with servisol and maybe
replacement if any crackly ones can’t be cleaned up. Direct outs can
be anywhere you choose. Ideally they should be set to provide line
levels. This could be done on a separate board with a load of op-amps.
The usual places are pre-fade, post-fade,post pre-amp but pre-tone
controls (i.e flat but amplified). You can have outs on the mixing
busses as well, just straight or preferably via separate level
controls (like a mini foldback or echo mix). When fiidling with a
circuit to find take-off points, BE VERY CAREFUL!. I used a very high
impedance crystal earpiece for signal tracing. It had an impedance of
about 2Meg and cost about 30p! So no loading to the circuit or my
pocket there! I cut the socket off and soldered a couple of probes,
one with a crocodile clip, to the two wires. All I had to do was clip
the crocodile to earth or 0 volts, say, and then prod around with the
other to see where the signals go. If there are discrete transistors
etc, be very careful to touch only one contact at a time on the board
as the probe can sometimes go across the gaps, which is a short, and
very very bad…You can easily pop a transistor and they are bastards
to unsolder and replace. If one goes, it can cascade that several will
pop as well. You can’t tell by looking or tracing. You need a high
impedance multimeter to test the PN junctions, usually the
emitter-base junction. When working this will be about 0.5-0.6V for
silicon transistors and 0.1V for germanium. Of course, you’ll need to
determine the transistor type, and find it’s pin-out diagram to check
all this, which could take a while. SO BE VERY CAREFUL NOT TO SHORT
CONNECTIONS ON THE CIRCUIT BOARD EITHER WHEN MAKING,TESTING,CHECKING
NEW OR OLD EQUIPMENT. (you can guess how I know this)

The best way to learn is with hobby books, technical journals and a
soldering iron! There’s loads online now. Your trouble is that you
probably don’t know the terminology and also the physics behind the
design decisions that have to be made.

As a start, consider these points:
In the analogue world, virtually everything is an amplifier! (I’m not
joking)
Anything that isn’t an amplifier is probably a filter of some sort!
Filters can be made without amplification (called passive filtering)
but most include some amplification (called active filtering)! The
best way to make sound quieter is paradoxically with an amplifier! (in
this case amplification is less than unity, or one, which is a flat
gain)

After years of fiddling and experimenting, engineers found that the
most convenient way to do all of the above is with a thing called an
“operational amplifier”, or op-amp for short. Here’s a link for a
start. There are much more technical things about…
http://sound.westhost.com/dwopa2.htm

Ops-amps are brilliant! They contain upwards of 30 transistors with a
balanced input and usually a darlington pair arrangement on the output
stage. Most are short-circuit proof (which can’t be said of discrete
transistor amplifiers!!!) They can be used as switches, comparators
and amplifiers in a myriad of scenarios. You can use cheap ones which
are a bit noisy, like the 741. However, now there are loads of op-amps
that have really low noise front-ends for critical instrumentation and
high quality audio applications. Something like the LM381 or better.
When you get into this you use the manufacturers data sheets to get
the best noise, slew rate, bandwidth, frequency response, flat
amplification(tracking) you can for your money. The LM381 is very old
now but it existed in several forms, the LM381AN being the best if I
recall. It will be the same now for the current crop of products. See
http://www.maplin.co.uk/search.aspx?MenuNo=12490&MenuName=Op-Amps&worldid=-2
&FromMenu=y&doy=4m7 Once you get feel for them, you can google and all sorts
of op-amps will turn up, much better than the maplin offerings.
Op-amps quite often come as several units in a single physical package, even
the high quality ones. This is good! The thing is, is that THEY ALL WORK
THE SAME WAY. The circuits shown in the link above will work with ANY
op-amp. All the user (you) has to decide is how much they are prepared to
pay for the quality and features they are after.

The basic attribute of an perfect op-amp is that it has two inputs of
infinite impedance and one output of zero impedance. The output can be
easily fed back into either (or both) inputs so that either negative
or positive feedback occurs. In combination with frequency dependant
components you can have a tone control or a radio (receiver or
transmitter)! The two inputs make op-amps ideal for making balanced
microphone amplifiers so that cable noise is minimised (this is the
bit you are interested in). Each input produces a different kind of
output at the output which is why they are called + and -; they are
180 degrees out of phase! Tranformers can be used in place of the
first resistor but they are expensive and need really good shielding
from interference (they are doing transformer amplification) as well
as the fact that they are inductors and thus frequency dependant
components. I never liked them because of these points. I mean, £100
for a little metal box and a few feet of thin copper wire wound on a
bit of ferrite! I’ve made them by hand you know; it doesn’t take long.
You can get the bits from Maplin. Here’s the wire..
http://www.maplin.co.uk/module.aspx?ITAG=SPEC&ModuleNo=44&doy=4m7#spec

Also, use the best passive components you can afford. That is; really
low noise resistors ESPECIALLY the FIRST one in the amplification
chain. This is where NEARLY ALL the noise comes from! (Boltzmann comes
into the thermal noise bit. See
http://www.sengpielaudio.com/calculator-noise.htm for a calculator.
You can see how keeping input impedance and temperature low keep the
noise to a minimum). However, if input impedance is too low then it
loads the mike so that the moving element in the mike hits the stops
causing distortion. This applies anywhere to any mike. Some mikes have
an amp in the module, but they are all amplified. Period. It’s just
not at the mixer that’s all. The same rules of physics apply whether
the amp is an inch from the capsule or a furlong.

You can pay quite a lot for a good quality resistor, normally they are
a penny! The same goes for capacitors. Look for polystyrene I think.
Tantalum bead and mylar are for compact power supply smoothing really.

To find out this stuff and how twiddling with components affects
things, you’ll need as well as some books, catalogues, etc; a
breadboard and some circuit board making stuff. It’s not expensive.
Make a beeper/phaser or a photoswitch from a 555. That’s a laugh and
will give you a bit of practice.

I don’t know what dealy boxes are!!!

Compressors/expanders/companders are volume and/or frequency dependant
negative and/or positive feedback amplifiers. See op-amps. Fast slew
rate, low noise and high linearity are essential.

Keep well. I’m off to bed now! Zzzz zzzz zzzz

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