The One Knob Squeezer

Transcription

1 The circits within this applicatin nte featre THAT4301 Analg Engine t prvie the essential elements f vltage-cntrlle amplifier (VCA) an rms-level etectr (RMS). Since writing this nte, THAT has intrce several new mels f Analg Engines, as well as new VCAs. With minr mificatins, these newer ICs are generally applicable t the esigns shwn herein, an may ffer avantages in perfrmance, cst, pwer cnsmptin, etc., epening n the esign reqirements. As well, a stanalne RMS is available t cmplement r stanalne VCAs. We encrage reaers t cnsier the fllwing alternatives in aitin t the 4301: Lw spply vltage an pwer cnsmptin: 4320 Lw cst, spply vltage, an pwer cnsmptin: 4315 Lw cst an pwer cnsmptin: 4305 High-perfrmance (VCA nly): 2180-series, 2181-series Dal (VCA nly): 2162 RMS (stanalne): 2252 Fr mre infrmatin abt making these sbstittins, please cntact THAT Crpratin's technical spprt grp at apps_spprt@thatcrp.cm. 45 Smner St, Milfr, MA USA; inf@thatcrp.cm Cpyright by THAT Crpratin; All rights reserve. Dcment Revisin 02

2 Electrnic cmpressrs are wiely se thrght the ai instry fr ai leveling, limiting, as an "effect", an fr ynamic range preservatin. The typical cmpressr seen in the pr ai instry can be an intimiating evice t many en sers hwever, with their assrte knbs an switches t cntrl parameters sch as gain, threshl, cmpressin rati, attack an release, stere linking, an the like. With this many cntrls, it is ifficlt fr even an experience ser t get the perfrmance they esire, especially since these cntrls ften interact with each ther. These interactins can be istracting an cnfsing, making it ifficlt t iscern the tre effect f a given ajstment. An alternative t this high level f cmplexity is a circit we call the "One Knb Sqeezer", the tpic f this Design Nte. The circit allws fr the cntins ajstment f gain, threshl an cmpressin rati with a single ptentimeter, frming the basis fr cmpressr esigns which are mre intitively accessible t a wier range f sers. Cnventinal cmpressr sie-chains Develping an intitive nerstaning this new circit's avantages is easiest when cmpare with mre cnventinal cmpressr esigns. In this paper we cntrast the "One Knb Sqeezer" esign with a generic cmpressr t illstrate the frmer's avantages. Cmpressrs can be f the feefrwar r the feeback variety. While each tplgy has its merits, this applicatin ses the feefrwar tplgy, in part becase there is n interactin between the cmpressin rati an the rms-level etectr's timing characteristics. Inpt C R1 28k8 Threshl VR1 R5 2M0 U1B 1 IN OUT 4 2 RMS IT CT 5 R16 R11 + C4 C7 47p D2 U1D OA1 18 R2 D1 VR2 k Gain VR3 R3 R15 6 R12 C5 0n U1C 8 OA2 7 t Cntrl Prt Figre 1. Basic cmp/limiter siechain with inepenent gain an threshl A cmpressr may r may nt incle ne r mre threshl levels. Cmpressrs witht threshls are typically se in nise rectin systems an in sme gain leveling applicatins. While there have been a few cmpressrs with mltiple threshls, these evices, as ne might expect, are ifficlt t tne, an all bt the mst experience sers are left t rely n the previs ser's settings r whatever factry presets are prvie. Mst cmpressrs have a single threshl, abve which cmpressin takes place, an belw which the inpt is passe witht mificatin (except fr, in sme cases, cnstant gain). The abve threshl cmpressr is a first rer apprximatin f gain riing in the sense that a recring engineer cl be expecte t rece gain at higher levels t avi clipping, bt wl be nlikely t raise gain in qieter passages which might exaggerate the by THAT Crpratin Page 2 f 9

3 nise flr. Still, while Still, while a single threshl makes setp easier, having a single cmpressin rati can be cnstraining, especially if that cmpressin rati is : Figre 1 shws the siechain f jst sch a t limiting cmpressr. This siechain has bth a 0 threshl an a cmpressin rati ajstment, B the tw primary cntrls fr an abvethreshl cmpressr. Aitinally, there is an - ajstment fr makep gain. Figre 2 shws hw the tpt varies as the threshl ajstment -20 is swept frm ne en t the ther in apprximately B steps. B in The mst bvis effect is that, as the threshl is lwere, the tpt level fr signals abve threshl is lwere as well. This Figre 2. Typical har knee threshl characteristic interactin between threshl an tpt level necessitates the aitin f a makep gain 30 ajstment. The effect f varying the makep 20 gain (with a fixe threshl) in B steps can be seen in Figre 3. 0 In rer t hear the effect f changing the t threshl, the cmpressr mst be ajste - B while the signal is abve threshl. T avi -20 having a change in threshl alter the verall -30 tpt level, the ser f a generic cmpressr mst cmpensate by simltanesly ajsting the makep gain cntrl In aitin t the gain/threshl interactin, there is als an interactin between B in cmpressin rati an tpt level. The effect f changing the cmpressin rati can be seen Figre 3. Typical makep gain characteristic in Figre 4. Ntice that the tpt level shift with change in cmpressin rati is prprtinal 30 t the amnt that the signal is abve 20 threshl. Figre 5 shws a slight variatin f the t 0 siechain circitry shwn in Figre 1 t help eal with changes in tpt level. This circit - B cmbines the gain an threshl cntrl, an -20 applies a gain change when the threshl is -30 ajste. The effect f this change can be seen in Figre Referring again t Figre 2 an the circit f Figre 1, we see that recing the threshl B in by B in a generic cmpressr reslts in a rectin in tpt level f B. Fr lack f a better term, we ll refer t this as the Figre 4. The effect f changing cmpressin gain/threshl interactin rati. With the mificatin f Figre 5, the egree f interactin by THAT Crpratin Page 3 f 9

4 Threshl VR1 R16 C7 47p D2 R2 R15 R12 C5 0n t Cntrl Prt Inpt C R1 28k8 R5 2M0 1 IN 2 IT U1B R11 U1D D1 OUT RMS CT 5 20 OA1 R3 U1C 6 + VR2 C4 7 k 8 OA2 Figre 5. Cmp/limiter siechain with cmplementary gain an threshl is actally rece by a factr eqal t the cmpressin rati. Ths, as is shwn in Figre 6, a cmpressin rati f :1 reslts in an tpt level variatin f 1/, r zer. Fr lwer cmpressin ratis, hwever, the sitatin is imprve ver that f the generic cmpressr, bt it is nt cmpletely slve by the circit f Figre 5. Fr example, a cmpressin rati f 4:1 reslts in a gain/threshl interactin rati f 1/4 r 25%. With a cmpressin rati f 2:1, the gain/threshl interactin rati is 1/2 r 50%, an s a B change in threshl reslts in a 5 B change in tpt level. S, in rer t ensre a cnstant tpt level at lwer cmpressin ratis, anther makep gain ajstment mst be ae, an at cmpressin ratis ther than :1, the peratr mst still ajst bth threshl an gain simltanesly. The "sft knee" cmpressr Abve an beyn level interactin, the cmpressrs iscsse s far have very sharp threshls where the cmpressin rati shifts abrptly frm 1:1 t as mch as :1. This sen transitin can be aible an istracting whereas, in mst ses, the en reslt f cmpressin shl be incnspics t the listener. Lwer cmpressin ratis will generally reslt in a less cnspics snic signatre, bt can be inaeqate t prvie verla prtectin. High cmpressin ratis prvie aeqate prtectin, 0 bt can sn nnatral. While verly cmpresse material is, ner mst circmstances, preferable t clippe r heavily t - istrte material, it can still be snically -20 B nesirable. This prperty limits the se f higher cmpressin ratis ver a large prtin -30 f the inpt signal's ynamic range. Mltiple, ajstable threshls wl aress this isse, since ne cl prgram ifferent cmpressin ratis ver ifferent parts f the initial ynamic range, an graally sqeeze the signal harer an harer as the verall level increase, bt witht the abrpt B in Figre 6. Cntrl characteristic with cmplementary gain an threshl by THAT Crpratin Page 4 f 9

5 Threshl VR1 Inpt C6 22 R16 R6 240k U1B R1 1 IN OUT 4 28k8 2 RMS 5 IT CT R5 2M0 R4 D2 R11 C4 R OA1 U1D D1 VR2 k Gain VR3 R15 R6 R3 6 8 R12 C5 0n 7 OA2 U1C t Cntrl Prt change in cmpressin assciate with a single threshl. Hwever, this apprach is nt generally se e t the cst an cmplexity f the esign. A practical alternative is the "sft knee" threshl shwn in Figre 7. The har knee "peratinal rectifier" se in the preceing circits acts, t a egree, like an ieal ie with n frwar rp, an this behavir is essentially limite nly by the nn-ieal characteristics f the p-amp se t implement the circit. In a sft knee apprach, feeback is n lnger se t iealize the threshl ie's behavir, an a secn ie is se t generate a temperatre-cmpensate ffset t cancel the threshl ie's frwar rp at a single frwar crrent. It can be shwn that the cmpressin rati fr a feefrwar cmpressr is C.R. = B In B (Eqatin 1) Ot = 1 1 where β is the net gain thrgh the siechain. We can see by inspectin that a net siechain gain f ne reslts in a cmpressin rati f :1, an that a net effective siechain gain f ne-half reslts in a cmpressin rati f tw. Neglecting parasitic resistance, the small-signal impeance f a ie at rm temperatre at any given crrent is Z = V T I Figre 7. Sft knee cmp/limiter with cmplementary gain an threshl = 26mV I (Eqatin 2) This impeance can be se t make a first-rer apprximatin f the cmpressin rati reslting frm a given ie crrent. Here, ne can see by inspectin that the impeance eqatin is a hyperblic fnctin, an that as the ie crrent increases, the reslting impeance gets qite small which, when factre int the gain f the sie-chain, reslts in an ltimate cmpressin rati which asympttically appraches :1. Likewise, when the ie crrent is very small r nnexistent, the ie becmes an effective pen circit, an the cmpressin rati appraches 1:1. As an example, let's arbitrarily assme that the crrent thrgh D1 is 3 μa. The ie's small-signal impeance becmes Z = 26mV 3 A = 8.7k by THAT Crpratin Page 5 f 9

6 an the reslting sie-chain gain will be A S.C. = 5.9k 5.9k +8.7k = 0.58 We can calclate the cmpressin rati t be C.R. = 1 1 = = 2.4 r 2.4:1. Next, assme that the crrent thrgh D1 is 20 μa. Then Z = 26mV 20 A = 1.3k an the sie-chain gain is A S.C. = 5.9k 5.9k +1.3k = 0.9 giving a cmpressin rati f C.R. = 1 1 = = r :1. It shl be nte that these calclatins are, at best, a first rer apprximatin f the behavir f the sft-knee threshl. Nevertheless, ne cl calclate cmpressin ratis at ther ie crrents t verify that the cmpressin rati increases smthly as the ie crrent increases, eventally appraching :1. The actal sltin mst take int accnt bth small an large signal effect f the ies, as well as parasitic resistances, thgh the effects f these secn-rer terms reslt in nly minr changes t the simplifie calclatin. Figre 8 shws the reslt f a SPICE simlatin f a sft knee cmpressr (with the sie chain shwn in Figre 7). The esirable characteristic is evient, as the signal is initially cmpresse nly slightly, bt becmes mre heavily cmpresse as the verall signal level increases. This is, in sme sense, the eqivalent f having an infinite nmber f threshls an cmpressin ratis which initially cmpress incnspicsly, bt ltimately prvie the prtectin f a limiter B in Figre 8. Sft knee threshl characteristic Still, this circit has a threshl, gain an cmpressin rati ajstment, an thgh it might by snically sperir t the circit in Figres 1 an 4, it still sffers frm the same interactins an cnseqent ifficlties in ser ajstment. The "One Knb Sqeezer" There were tw primary gals in esigning the "One Knb Sqeezer" circit. The first was, bvisly, that it sn g in thse applicatins where cmpressrs are sally emplye. 0 - t -20 B by THAT Crpratin Page 6 f 9

7 The secn was that it prvie an interface simple engh fr even a nvice ser t tne. A thir (implicit) gal was that the circit be inexpensive an practical. The first gal was achieve by carefl listening an tweaking f the circit. The thir was by achieve by sing a single-chip ynamics prcessr, the THAT 4301 Analg Engine, an being iligent abt minimizing the parts cnt. Tning simplicity was achieve by an innvative esign wherein threshl, cmpressin rati, an makep gain were implemente with a single ajstment by taking avantage f the sft-knee threshl's signal-epenent cmpressin rati characteristic. As was iscsse previsly, any cmpressin rati frm 1:1 t near :1 is reaily achievable with the sft-knee threshl circit. The cmpressin rati that will reslt at any given inpt level is epenent n the threshl setting, an as sch, the cmpressin rati ajstment can be mitte. T accnt fr the changing tpt signal level, a variatin f the cmplementary gain an threshl siechain was esigne. After aitining a nmber f variatins, the best apprach was fn t be a make-p gain ajstment that cmplie with the sft-knee crve. The reslting transfer characteristic is shwn in Figre 9. This series f crves shws inpt verss tpt while the "Mre" ptentimeter is swept frm flly CCW t flly CW. One can see that the tpt level remains cnstant arn the - B level, the intene nminal listening level. As the "Mre" ptentimeter is swept twars flly CW (which lwers the threshl while simltanesly increasing the makep gain), the cmpressin rati increases an the crves pivt abt the - B pint. As this happens, a larger prtin f the inpt signal's ynamic range is mre heavily cmpresse. Figre 9. Transfer characteristic f the One Knb Sqeezer Dring nrmal peratin, the ser shl ajst the gain f the signal chain ahea f the cmpressr s that the average signal level is at the esire nminal listening level (in this case, - B). Once this level is set, the ser can easily a cmpressin with a single ajstment, backing ff when pmping r sibilants becme nticeable r the signal sns verly cmpresse, all witht the istractin r cnfsin case by changes in tpt level. by THAT Crpratin Page 7 f 9

8 As a trbleshting an esign tl, Figre shws the tpt f the cntrl prt bffer (U1C). The prtin f the crves which are hrizntal crrespn t regins where the signal is nt cmpresse. Figre 11 shws the final versin f the "One Knb Sqeezer". The VCA prtin f the circit is ientical t the recmmene applicatins circit in the ata sheet, except that C6 has been increase t 22 μf. The rms-level etectr is cnfigre with 7.5 μa f timing crrent an a μf timing capacitr, which reslts in a 34 ms time cnstant an, with a 28.8 kω inpt resistr, a - B zer B reference level. Sme circit "tricks" were se t minimize the parts cnt in the siechain. Unlike earlier versins, the sft-knee threshl is implemente with a pair f matche NPN transistrs. Q1 frms the threshl. The frwar rp f its base-emitter jnctin is cancele by Q2 which is biase by R14. R14 effectively acts as a crrent srce, biasing Q2 at apprximately 9 μa. The threshl is ajste by smming in the "Mre" signal with the rms-level etectr tpt at inverting inpt f U1D. This ffsets the etectr's tpt n the "sft knee" crve. A cmparable circit frms the "sft-knee" prtin f the make-p gain circit. A pair f PNP transistrs, Q3 an Q4, an the resistr R13, are cnfigre in a manner similar t the Figre. Otpt f the cntrl prt bffer threshl circit t prvie "shaping" fr the gain cmpnent f the "Mre" cntrl, reslting in an apprpriately attenate versin f the "Mre" cntrl. A cnventinal make-p gain ajstment is prvie fr sitatins where - B is nt an apprpriate pivt pint. This make-p gain ajstment is smme with the "sft-knee" prtin f the make-p gain an the tpt f the "sft-knee" threshl int the inverting inpt f U1C, which then rives the cntrl prt. While this cnstittes a secn cntrl n the "One Knb Sqeezer", it's an ajstment that nly nees t be mae nce when the eqipment is initially cnfigre, an is left ntche thereafter. All tning is still perfrme with the "Mre" knb. by THAT Crpratin Page 8 f 9

9 Clsing thghts The "One Knb Sqeezer" can perfrm amirably in a variety f cnventinal cmpressr applicatins, an especially where the en prct will be ajste qickly r by relatively inexperience sers. G examples are gitar an bass amplifiers, where a plethra f cmpressr cntrls wl be t f place, bt where a single-knb cmpressr cntrl wl be entirely welcme. Fr certain specific applicatins, mificatins can be mae t the circit f Figre 11. All cmpressrs exhibit sme LF (lw freqency) istrtin reslting frm nfiltere ripple n the cntrl prt. A fixe vale fr the timing capacitr (C4) is a cmprmise between LF istrtin an fast respnse. A "Nn-Linear Capacitr" circit, escribe in THAT Crpratin's Design Nte DN03 (frmerly Applicatin Nte AN3), eliminates the nee fr this traeff. The Nn-Linear Capacitr respns qickly t rapily changing levels, bt heavily filters static r slwly changing signals in the cntrl path. This enhancement is perfect fr a bass amplifier with a bilt-in cmpressr. Stere applicatins (bvisly) reqire tw channels. Unfrtnately, tw inepenently cmpresse channels can reslt in the stere image "wanering" as the cmpressrs react t the iffering infrmatin in their wn signal path. Stere linking, which is escribe in Design Nte DN116, aresses this prblem. When tw channels nee t be linke, the ngrne sie f their respective timing capacitrs are shrte tgether reslting in what is ften referre t as "tre pwer smming", an bth VCA cntrl prts shl be riven frm the reslting single sie-chain signal. Inpt R5 2M00 C1 47 R1 28k0 The One Knb U1B 1 IN OUT 4 2 RMS IT CT 5 Mre VR1 0k Less cw R11 6k04 C4 R15 R16 R OA1 U1D R4-15 R3 U2C (Q3) U2D (Q4) R13 1M6 +15 R14 1M6 cw VR2 R17 k Optinal Make-p Gain U2A (Q1) U2B (Q2) C5 0n R12 6k OA2 U1C Sym R C2 47 cw Nte: U2 = THAT340 Matche Transistr Array R7 300k 5% R6 51R 5% C EC+ 11 SYM R IN OUT 20k0 GND EC- U1A 16 9 C3 47p NPO R8 20k0 12 OA3 Otpt Figre 11. by THAT Crpratin Page 9 f 9