70’s Mixer

Hi, hope all is well. What you been up to? Any music stuff? I am hopefully picking up an old 70s mixer. The inputs and outputs are 5 pin din connections. I want to add some xlrs and direct outs. What is A. the best way to do this? 2. How is the best way to learn about electronics? I purchased Designing Audio circuits, but I still really cant get my head around it. I would like to make some dealy boxes, the progress to transformer based pre-amps, EQ and Compressors. Any help, tips, information would be great. I would love to understand electronics for this. Favourite outboard would be Neve, API, UA 1776(vintage stuff) Thanks
http://myspace.com/paulrichards7

2 comments

  1. WOW, thanks for that. It will take me a while to get my hear around that. I will check out your site and all the links from home as Im at work right now and the internet reception sucks. I was going to send you the link for the mixer, but it seems the owner pulled it due to an error in the listing(I think that it was because no one had put a bid in yet!) Trying to get hold of an old reel to reel too. My Dad has an old valve one(quite basic( but I may have a fart around with it. Try out some mods or something. If I come across anything I think is imnteresting on ebay Ill forward you the link to see what you think. I am in the proces of redesigning my recording chain. From new software(Reaper cockos.com)much better than Pro Tools and only $40, then an 24bit 192kHz PCMCIA hardware 16in 16out. Keeping my bidding eye out for a Takamichi tape recorder, then trying to add some old vintage discrete electronics to the recording signal. Hence the vintage search and electronics understanding. Lets stay in contact. Maybe i could come down once I finish updating my mobile studio and have a play around with some found sounds.
    Take care for now

  2. You must be mad!

    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
    https://www.maplin.co.uk/search
    &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..
    https://www.maplin.co.uk/?doy=4m7&ITAG=SPEC&ModuleNo=44&#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
    https://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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