3 Responses to “70’s Mixer”

1. Strangely on July 5th, 2007 at 12:29 am

   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 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 Reply

2. paul on July 5th, 2007 at 3:01 am

   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 Reply

3. Probable DDOS attack Using SQL Injection on my Websites | Strangely Perfect on August 22nd, 2008 at 12:57 pm

   [...] If you click the link it won’t work now (read on for later ) but the correct link here does; http://strangelyperfect.tv/68/70s-mixer/ [...] Reply

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