STUDIO

Q root extractor or exponentiator. Add some nonlinearelements (a diode or two) and you have acircuit which will `solve' the complex andsimultaneous non -linear equations of nature inreal time.In audiospeak, multiplication or division of asignal are synonymous with gain or attenuation,or decibels being added or subtracted. It's ablessing for simplicity that practical VCAs canembrace both directions or 'Quadrants' to a lesseror greaterrextent. It's also handy that theacronym VCA can refer to both Amplification andAttenuation; though at least one VCAmanufacturer prefers to differentiate the lattermode, calling it 'VCAtt'.In its broadest sense, a VCA is a black box withthree ports (Fig 2). Two are everyday (audio)input and output. A voltage (or current) applied tothe third port, labelled 'CV' acts to alter theaudio through -gain and hence the output level. Tosave on brain -damage, it's convenient if a linearchange in control voltage (or current) causes anequally linear change in the dB level at theoutput, eg 10 dB per volt. Again, for many audioBrian BennetThe Shadow,Graham Broad & Steve LevineDo Not Erase ProductionsTony BrockRod StewartBill BrufordEarthworks. King Crimson, Anderson -Wakeman -Brutord -HoweAndy ButlerAnastasios CoroneosIndependent ProducerDennis DavisDB Studios Producer Alex Cooper ,Katrina 8 The WavesStevie WonderEarwhacksBoy George ProductionJohn EdenBrook House StudiosJulian ElkanIndependent ProducerZissis EvangelosIndependent ProducerAlan EvansIndependent Producer, ArrangerThomas Crooke Musical ServiceProdn 8 ProgrammingJo HammerJean Michelle JarreMichael HarrisonJanet JacksonJohn NedgecockIndependent ProducerIan HeronProducer. 125 StudiosStefan KaufmannAcceptJohn KeebleSpandau BalletRoly BerridgeA ck Astley. Gary Moore. Mary KanteGilson LavisSqueezeDave LawsonIn:dependeal ProducerEd MannFrank ZappaGraham MarplesThe Bunker StudiosNick MasonPik FloydTrevor MoralsHoward JonesIan MorrowIM ProductionsPat ReaganSecret WeaponTim RootJoe ZawinulPaul FishmanWorld Shock 90 Production CoAles Van HalenVan HalenPeter Van HookeMike 8 The Mechanics, Tarda Tikaram. MMC Recordsto you and all the othersTHANK YOU.MULTIPLIER,O- vIC DIVIDERAUDIO INCELL OR COREVOLTAGE TOCURRENT CONVERTORCy OvCI VC CURRENT TO VOLTAGECONVERTOR}- MULTIPLYDIVIDEFIG 2: VCA generic blockschematicOAUDIO OUTapplications, the linearity of this relationshipneeds to be maintained over a range of some3000x, or 70 dB. For others, 30 dB (30x) willsuffice. Then for the kind of accuracy andrepeatability that's desirable in up- marketequipment, the ratio between DC control voltageand audio gain /attenuation will need to be tightlydefined against the three Ts: temperature, timeand manufacturing tolerance.In the beginning, VCAs were built with tubes.The first solid -state circuits appeared in the early'60s. Using diodes and later FETs, they wereSIMMóNSSDXsometimes called 'vario- lossers'. Flushed with thesuccess of the first IC op -amps in the late '60s, ICmakers specialising in analogue, notably Burr -Brown, the aptly named Analog Devices andNational Semiconductor, began to develop at firsthybrid, and then monolithic `analoguemultiplier /divider' chips, using bipolar transistors.Within a few years -by the mid '70s -some oftheir products were approaching the kind ofperformance that would be acceptable to pro -audio, but prices were high. Around the sametime, a handful of specialist audio VCA makersarrived. Each appeared waving a patent whichannexed one of the bridges that lay between amultiplier suited to monitoring processes in asausage factory, and one that was good for fadingaudio. Today, the world of Audio VCAs revolvesaround two kinds of circuit topology, produced involume by five US manufacturers. Both kinds aretraceable to techniques that were first figured outfor analogue computation about half a centuryago.TransconductanceThe most rudimentary VCA one can build withtransistors is just a simple development of thefamiliar differential amp, or long -tailed pair'.Looking into Fig 3, gain control is achieved byarranging for a voltage across 'CV' to vary theamount of (constant) current pulled through theamplifier transistors, TRi and 2. The change inoperating current directly affects theirtransconductance (current divided by voltage, ormA per volt =gm), hence the voltage gain, Grecovered at Al's output (G= gm.RL). This kind ofVCA and others whose operation relies onchanging the ratio of voltage -to- current transfer ofactive devices, are loosely known as OTAs(Operational Transconductance Amplifiers), or astransconductance VCAs (or multipliers). NationalSemiconductor's LM 3080 is a well knownexample of the genre.As it stands, the circuit has a limited range. ForDC, a linear change in transconductance isadvertised over 3 decades (60 dB), but for audiowhere the need for dynamic range enters thepicture, non -linearity sets in early. For less than1% distortion, the audio input is limited to a fewmillivolts, essentially because variations intransconductance can only produce changes involtage gain if the circuit is operated open-loop, iewithout NFB. Mechanisms include a progressiveincrease in the VBE in TR.1 and 2 for tailcurrents above lmA. The effect is only partiallyself -cancelling, hence the non -linearity. Distortioncan be reduced for operation at normal line levelsby passively attenuating the incoming audio (byaround 40 dB), then recovering the level with anoutput amplifier. SNR suffers commensurately.Then, with high attenuation, slew rate becomesincreasingly embarrassing as the active devicesare starved of current. Instrumentation engineersrefer to this circuit as a 'two quadrant multiplier'.This is to say that the audio or 'X input' has abipolar capability (ie the signal can swingsymmetrically), while the CV or 'Y input' isrestricted to control voltages that are alwayspositive (relative to the negative rail).A more workable technique, employed in NatSemi's LM 13700 (Fig 4), involves introducing a(nearly) constant bias current into each inputnode with a pair of diodes (D1,D2). Provided thediodes' geometry and temperature are similar tothe devices in the OTA cell (Q4,5), theirrespective non -linearities are complementary, andpartially cancel (Fig 5). With this predistortionmethod, drive levels can be raised by up to 15 dB D