{"id":13,"date":"1998-12-10T19:30:35","date_gmt":"1998-12-10T19:30:35","guid":{"rendered":"http:\/\/www.ljudmila.org\/~savskib\/?p=13"},"modified":"2024-09-13T19:19:29","modified_gmt":"2024-09-13T17:19:29","slug":"simplemixer","status":"publish","type":"post","link":"https:\/\/www.turborebop.net\/?p=13","title":{"rendered":"SimpleMixer"},"content":{"rendered":"

[Original at http:\/\/absurdevidence.radiostudent.si\/borut\/simplemixer.html<\/a>]<\/p>\n

A simple audio mixer<\/a><\/p>\n

A simple compressor\/limiter<\/a><\/p>\n

Advanced\u00a0 simple compressor\/limiter<\/a><\/p>\n

The simple song:<\/p>\n

It is organised as building blocks,<\/p>\n

combined according to personal taste and need,<\/p>\n

best if organised on a pre-drilled universal printed circuit board,<\/p>\n

with holes for the input, output sockets, potentiometers,<\/p>\n

all drilled into pcb,<\/p>\n

then screwed and fixed with bolts,<\/p>\n

with holes-conforming alu frontplate drilled in parallel,<\/p>\n

and later pushed over it,<\/p>\n

which would even act as Faraday cage,<\/p>\n

besides being nice & shinny;<\/p>\n

solution for the bottom?<\/p>\n

-> I leave totally to your aesthetic touch,<\/p>\n

but the mixer will be ‘elegantly’ slim,<\/p>\n

because we’ll use an outside universal power plug.<\/p>\n

This is what Monika wanted<\/p>\n

Simple mixer<\/a><\/p>\n

phones<\/p>\n

6,3 mm socket<\/p>\n

(two?)<\/p>\n

And here is a ‘physical’ layout<\/a><\/b>
\nconsisting of:<\/p>\n

-> one microphone preamp,<\/p>\n

-> one (passive) stereo line input,<\/p>\n

-> one channel output amplifier,<\/p>\n

-> stabilizer circuit<\/p>\n

… and its elements follow (with certain variations):<\/p>\n

but before the nice drawings download there is time for a brief ‘basics’
\ncourse!
\nThe main elements
\nused throughout are:
\n– resistor
\n– capacitor
\n– operational amplifier<\/p>\n

The first two are ‘passive’ elements and op-amp is an ‘active’ element.
\nAlso: the first two are ‘two-terminal’ elements and the op-amp is a ‘three-terminal’
\nelement.<\/p>\n

Resistor has the ability
\nto define the current that flows through it when a voltage is applied to
\nits terminals. Vice-versa: when a current flows through it a precise voltage
\ndrop appears on its terminals. The simple (most frequently used) formula
\nis:<\/p>\n

U<\/b> = R<\/b> x I<\/b> (also: R<\/b> = U<\/b> \/ I<\/b>
\nand I<\/b> = U<\/b> \/ R<\/b>)<\/p>\n

-> the first formula you read as: when a current of value I
\nflows through a resistor of value R it produces a voltage drop of
\nvaluer U accross its two terminals. In audio mixer we use resistors
\nto adapt the internal resistences (-> impedances) of op-amps. Also: to
\ndefine the necessary voltages by ‘dropping’ them with resistor combinations.<\/p>\n

Capacitor (in our use)
\nis an element that blocks the DC (direct current) voltages (-> supply voltages)
\nbut lets through the AC (alternate current) voltages (-> signals). We need
\nDC voltage to supply the energy to the active element and to define its
\noptimum working conditions). Also: capacitors are used to store the energy
\n(-> we use it as such in stabiliser circuit) or to filter out the undesired
\nAC voltages (-> we use it as such in ‘decoupling’ the DC voltage supply
\nlines).<\/p>\n

We combine resistors in series
\nor in parallel to achieve different values of resistence according to these
\ntwo formulas:<\/p>\n

– for two resistors in series -> R1 + R2 = R3<\/p>\n

– for two resistors in parallel -> 1\/R1 + 1\/R2 = 1\/R3<\/p>\n

Also we can combine capacitors
\nfor the same reason:<\/p>\n

– two capacitors in series -> 1\/C1 + 1\/ C2 = 1\/C3<\/p>\n

– two capacitors in parallel -> C1 + C2 = C3
\ntry to imagine all these as water-pipe sistem which fills bath-tubs:<\/p>\n

– generator\u00a0 takes care that the water pressure (-> voltage) in
\nthe system is high enough for the water (-> current) to flow.<\/p>\n

– pipes of different diameter allow more or less flux of water (->
\nresistence). Two pipes in parallel allow twice as water (-> current) to
\npass.<\/p>\n

– bath-tubs are filled with water according to their volume capacity
\n(-> capacitors). Two bath-tubs present double the water capacity.<\/p>\n

– what about series of two pipes, bath-tubs? It’s not that easy to
\nimagine anymore.<\/p>\n

Impedance is a term defining
\nthe ‘passive’ nature of elements (the one that ‘consumes’ the energy).
\nFor audio frequencies we can assume that resistor has only resistence and
\nthat capacitor has only capacitance. Active element also has both, but
\nbecause we use the most ‘perfect’ active element – operational amplifier,
\nwe can forget impedances and simply say:<\/p>\n

Operational amplifier
\nis a three-terminal (input terminal, output terminal and common terminal)
\nactive (ampifying) element with infinite input resistence, zero output
\nresistence and infinite amplification (->A). The basic formula is:<\/p>\n

U<\/b>out =<\/b> A<\/b> x (U<\/b>in+\u00a0 –<\/b>\u00a0 U<\/b>in-)<\/p>\n


\nDepending of configuration
\nop-amp can be inverting or non-inverting (-> it inverts the
\nphase of input signal, or the output is ‘in phase’ with the input). For
\nsuch reasons it has two inputs: inverting (-) and non-inverting (+). Configuration
\nis defined by passive elements in the ‘feedback loop’.<\/p>\n

Feedback loop means
\nconnecting output back to the input (-> taking part of the output signal
\nand bring it to one of the inputs). Two main configurations are:<\/p>\n

-> non-inverting amplifier (pretending that op-amp is ‘perfect’)<\/p>\n

U<\/b>out =<\/b> U<\/b>in x<\/b> (R<\/b>f +<\/b> R<\/b>s)
\n\/R<\/b>s<\/p>\n


\n-> inverting amplifier (again: simplified!)<\/p>\n

U<\/b>out =<\/b> U<\/b>in x<\/b> (1 +<\/b> R<\/b>f\/R<\/b>s)<\/p>\n

where Rs is not only the resistor but also the inner resistence of
\nthe source itself (-> microphone, whatever…)<\/p>\n


\nWe will use non-inverting
\namplifier for all stages because the input source does not define its amplification
\n(-> the source is ‘buffered’).<\/p>\n

Combining all three:
\n– first we define the optimum working conditions for op-amp:<\/p>\n

the output is (‘biased’) at middle voltage point (‘potential’) between
\npositive supply voltage and negative supply voltage (-> here: ground).
\nThis point we define at +<\/b> input of op-amp by bringing the half supply
\nvoltage to it (through Rin resistor). The –<\/b> input must also be ‘biased’
\nto half voltage but this is performed automatically from teh output (through
\nRf resistor) and by blocking the DC path through Rs with capacitor Cs (DC
\ncurrent cannot pass through capacitor – therefore infinite resistence for
\nDC but short-circuit for AC signals).<\/p>\n

– then we block all inputs and outputs of op-amp with capacitors, so
\nthat DC voltages that only serve for internal op-amp biasing do not worry
\nus anymore. The only thing about value of DC blocking capacitors is that
\nit defines the lowest frequency point of signal. A formula:<\/p>\n

f<\/b> =<\/b> 1\/<\/b>(2 x<\/b> 3.14 x<\/b> R<\/b>in xC<\/b>in)
\n, must give around 10Hz<\/p>\n

– when ‘mixing’ various inputs we must take care not to lose signals
\nfrom other sources because of the zero internal resistence of op-amps.
\nTherefore we put additional Ro resistors in series with op-amp outputs.
\nEach op-amp ‘sees’ his Ro and a parallel combination of all Ro’s from other
\ninputs. Study the above picture well – there is the basic principle.<\/p>\n

and now the nice drawings must have downloaded!<\/p>\n

<\/a><\/p>\n


\nall input signal grounds
\nshould go directly to stabilizer one-point ground, (however – they can
\nbe first connected connected all together)<\/p>\n

the alu frontplate should
\nbe connected only once to ground -> at a microphone input (the most sensitive
\none),<\/p>\n

capacitor at the input should be 1microfarad plastic foil, resistor
\n100Kohm going from the input socket to ground is to prevent a ‘thump’ when
\nconnecting an input cable;<\/p>\n

there is a possibility for
\na ‘phantom’ microphone supply – the red resistor; attention: there are
\nelectret microphones which have an additional ring\u00a0 -> like the stereo
\nmini jacks – these get their ‘phantom’ power at the ring (get a stereo
\ninput socket!) – connect the red resistor on the ring;<\/p>\n

the trimmer may serve for
\nsetting the input sensitivity but it can be also eliminated – together
\nwith the associated 10 microfarad electrolitic capacitor.<\/p>\n

<\/a><\/p>\n

<\/p>\n

Micro preamp is the simplest
\noperational amplifier non-inverting design, therefore a good op-amp is
\nneeded (TL 071, NE 5534, CA 3140); the design is for single voltage supply
\n(and op-amps are not), therefore half_the_supply voltage line is needed
\neverywhere.<\/p>\n

also: very close to every
\nop-amp a couple of ‘blocking’ capacitors (0,1microfarad 50V ceramic) are
\nneeded (to block the noise from the two voltage supply lines).<\/p>\n

<\/a><\/p>\n

For stereo (or mono) line
\ninputs no op-amp is used, just resistor from the input socket to the ground
\nand capacitor from the input socket to the next (mixing, prelistening,
\nsend) stage;<\/p>\n

<\/a><\/p>\n

however, adding a ‘buffer’
\nstage (-> a mic preamp with feedback resistor replaced by a wire) can make
\nthings easier with send potentiometers (-> cheaper and smaller!). Double
\nop-amps can be used (also 8 pins\/ same price! -> NE 5532, TL 072,…).<\/p>\n


\nthis is how a buffer (with
\ndouble op-amp) looks like:<\/p>\n

<\/a><\/p>\n

The place for pre-fader (pfl)
\nsend potentiometers is just before main mixer faders (one more SUM amp
\nis needed -> for mono send submix).<\/p>\n

The elements for send submix
\nare in yellow colour. If you don’t need send submix you can eliminate all
\nthe yellow elements. If you need more submix groups just replicate same
\n‘ yellow’ configurations.<\/p>\n

Double potentiometers are
\nnecessary because we don’t use ‘buffers’ for the line inputs (-> look up\u00a0<\/a>).<\/p>\n

<\/a><\/p>\n

Faders here are rotary logaritmic
\npotentiometers (instead of ‘sliders’ which are not cheap, or no good!),
\nhowever better (more expensive) pots will serve you better, double for
\nthe stereo input control, single for the mono\/ microphone control.<\/p>\n

After faders (through equal
\nvalue resistors) we connect (for the stereo inputs) all left to the left
\nsum common strip (bus), all right to the right sum common strip and for
\nthe mono\/ microphone to both (one resistor goes to left sum strip, one
\nresistor goes to right sum strip).<\/p>\n

<\/a><\/p>\n

Prelistening can be done
\neasiest by pushbuttons, taking the signal from before the fader – simplest
\nsolution uses just two-pole pushbutton (-> connected when pushed), more
\npractical solution needs a three-pole pushbutton (see the picture? soon!)
\n– one pole (the middle one in three-pole) goes to (common) prelisten line
\n(strip, bus) and further to the phones volume potentiometer.<\/p>\n

<\/a><\/p>\n

For the stereo\/ mono switch
\nof mixer output use a two-pole switch between the left sum strip and the
\nright sum strip.<\/p>\n

<\/p>\n


\n<\/a><\/p>\n

At this point we can add
\nanother double potentiometer (same as the above ones) for the master level
\n-> and now we come to summing amplifiers:<\/p>\n

<\/a><\/p>\n

Summing amplifiers amplify
\nthe sums of input signals and provide for the impedance transformation
\n(from high to low) – which serves best for feeding the output signal to
\namplifier, headphones, computer,…<\/p>\n

We need two\/ three kinds ->
\nfor the prelisten\/ phones, for the main mixer output, for the send submix
\noutput (if needed).<\/p>\n

Again they are build in the
\nexactly the same manner as the microphone preamps – except the feedback
\nresistor (going from pin 6 to 2) is about 10 times smaller -> 10Kohms.
\nBut for phones amplifier it should be larger -> 100Kohms. Experiment with
\nthis (feedback) resistor!<\/p>\n

All connections to grounds
\nshould go separately to common ground point in stabilizer (see above) and
\nall the voltage supply lines should also go separately to stabilizer (and
\nagain: blocking capacitors should be used with each op-amp).<\/p>\n

The best would be to think
\nof the ground connections which go to op-amps pins 4 as voltage supply
\nlines – complementary to the plus voltage supply line – these two leads
\nshould be ‘twisted’ together to prevent any ‘hum’ induction.<\/p>\n

Grounds which go from the
\npotentiometers are (same as input and output grounds) – ‘signal’ grounds.
\nActing in this manner you prevent any possibility of ‘buzz’ or ‘hum’.<\/p>\n


\n<\/a><\/p>\n

Capacitors of 10 microfarads
\nare electrolitic (polarized! take care!) and they should be rated for voltages
\nabove the supply voltage -> 25V will do. Largest electrolitic capacitor
\nis the 1000 microfarad in the voltage supply stabilizer part which should
\nhave 35 V. Just for the case!<\/p>\n

And the minus pole of this
\ncapacitor should act as the common ground point for all the blocks!<\/p>\n

We get power from an outside
\n‘adapter’ of 12V to 24V DC secondary voltage (100mA is OK), which lets
\nus play safely because we don’t deal with lethal tensions anymore! Just
\nget the right socket, connect its + socket via a forward biased diode and
\nit will be impossible to ruin the mixer with different adapters.<\/p>\n

The stabilizer circuit is
\nthe most common MC 78xx (xx = 12 or 18, which is the stabilized output
\nvoltage). To make use of the larger dynamic range (before clipping) of
\noutput signal – find an adapter of 18-24V DC and use MC 7818 (and MC 7809
\nas you’ll find in the next paragraph).<\/p>\n


\nThe op-amps need a split
\nvoltage supply, so we let them float exactly in the middle of stabilized
\nsupply voltage and ground. This is done by voltage divider resistors and
\na voltage follower (again an op-amp!) or we could use another MC 78yy (yy
\n= half the xx) just after the first one. The second one is a better solution
\nin some ways but it does not adapt if for any reason a change in main stabilised
\nsupply voltage occurs.<\/p>\n

And that’s all (for now).
\nIn future you may expact some image files for PCBs, because the building
\non the perforated board seems too much time & energy consuming.<\/p>\n

Borut<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

[Original at http:\/\/absurdevidence.radiostudent.si\/borut\/simplemixer.html] A simple audio mixer A simple compressor\/limiter Advanced\u00a0 simple compressor\/limiter The simple song: It is organised as building blocks, combined according to personal taste and need, best if organised on a pre-drilled universal printed circuit board, with holes for the input, output sockets, potentiometers, all drilled into pcb, then screwed and fixed […]<\/p>\n","protected":false},"author":1,"featured_media":795,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1,3],"tags":[],"class_list":["post-13","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-archive","category-tech"],"_links":{"self":[{"href":"https:\/\/www.turborebop.net\/index.php?rest_route=\/wp\/v2\/posts\/13","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.turborebop.net\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.turborebop.net\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.turborebop.net\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.turborebop.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=13"}],"version-history":[{"count":0,"href":"https:\/\/www.turborebop.net\/index.php?rest_route=\/wp\/v2\/posts\/13\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.turborebop.net\/index.php?rest_route=\/wp\/v2\/media\/795"}],"wp:attachment":[{"href":"https:\/\/www.turborebop.net\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=13"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.turborebop.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=13"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.turborebop.net\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=13"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}