NEWSi HAM LOGARITHMIC COMPRESSOR

Transcription

1 NEWSi l HAM ' L". (k LOGARITHMIC COMPRESSOR Aids in Peventing Ovemodulation While Inceasing Signal Effectiveness t 4: \. 1 Fig. 1. The Logaithmic Compesso eady to plug into you pesent micophone jack. Contols ae, left to ight, in -out switch, compession contol, output contol and a -c on -off switch. FEATURES- Povides 10 db inceased effectiveness Uses self-contained speech - ange filte Thee tubes, Including ectifie Small size-space saving CONTENTS Logaithmic Compesso pages 1-5 Sweeping the Spectum page 6 Ticks and Topics (VFO Hl -LO Switch; Wie Coveing; Pope Cae of Cystal Micophones; Micophone Cove) pages 7-8

2 TRANSFER CHARACTERISTIC OF BACK-TO-BACK COPPER OXIDE INSTRUMENT RECTIFIER TRANSFER CHARACTERISTIC OF USUAL DIODE CLIPPER CIRCUIT 47 K 3V 1 OUTPUT 0.06 V SINE WAVE -INPUT- A "COMPRESSED OUTPUT WAVE SINE WIVE FOR COMPARISON SINE WAVE -INPUT- "CLIPPEO" OUTPUT WAVE B /NC WAVE FOR COMPARISONFO Fig. 2. A compaison between the output wavefom of a Logaithmic Compesso and a diode clippe. Evey phone man, at some time in his QRM-idden life, has wished that he had a small switch available which would pemit him magically to incease his powe tenfold. This would be Utopia-fom one kilowatt to ten kilowatts by pessing a button. This button is now available, and it is mounted on the font of the Logaithmic Compesso. This unit will give an effective signal gain which is adjustable fom a few db up to as much as ten db (ten to one in powe). The Logaithmic Compesso is an audio amplifie device which is inseted between you micophone and you pesent speech amplifie. Its function is to push up the aveage modulation level, with the esult that high pecentage modulation is assued at all times, egadless of the sound level eaching the micophone. COMPRESSION VS. CLIPPING Those familia with clippes o clipping cicuits can see that the Logaithmic Compesso is intended to do the same sot of job as a clippe. Thee is, howeve, an impotant diffeence between logaithmic compession as used in the Logaithmic Compesso and clipping. Fig. 2 compaes the chaacteistics of the two diffeent systems. In eithe case the input wave suffes distotion, but the distotion caused by the clipping action of the odinay diode type clippe (Fig. 2B) is wose fo a given amount of signal compession than that caused by the logaithmic compession of a coppe -oxide instument ectifie (Fig. 2A). Distotion pesent in eithe cicuit will add "hashness" to speech signals and without futhe teatment would esult in excessively boad signals. Theefoe, any distoting type cicuit should be followed by a suitable filte to pevent the high fequency poducts poduced by this distotion fom eaching the modulated stage. With such a filte much of the "hashness" will still be pesent but the adio -fequency signal need not be boad. The hashness esults fom coss modulation (distotion) poducts that lie within the pass band of the filte. 2 The advantage of the logaithmic compession system is that the distotion is less sevee (fo a given amount of compession) than the clippe type, and this makes possible the use of a vastly simple filte aangement. Thee "stages" of R -C type filteing used in the Logaithmic Compesso ae as effective as moe elaboate shap -cutoff types of L -C filte vitually necessay with the clippe type of cicuit. Futhe, the tansient esponse of the R -C type filte is such that no oveshoot of signal peaks can occu. This is not the case with shap -cutoff L -C filtes. This means that the logaithmic compesso cicuit with a popely designed R -C filte is supeio to the odinay clippe cicuit followed by a shap L -C filte. Repeated tests confim this statement. CIRCUIT DETAILS With efeence to Fig. 3 it will be seen that the fist 12AT7 acts as a two stage audio amplifie to bing the signal fom the micophone to a sufficient level so that the compession cicuit itself opeates at the pope level. Resisto R, in the fist stage has been added as a pecaution against adio -fequency feedback. Special cae has been taken to attenuate low audio fequencies pio to compession. Doing this gives a well balanced speech esponse as well as minimizing much of the distotion caused by coss -modulation between the low speech fequencies and the intelligence -beaing high speech fequencies. The values of condenses C2, C3 and C4 ae chosen to attenuate the low fequencies adequately befoe speech compession. Condenses C7 and Cq seve the same pupose afte compession has taken place. Resisto R4, by vaying the signal input to the second section of the fist 12AT7, enables contol of the amount of compession. The audio tansfome, T1, is necessay because the limiting cicuit must be fed by a low -impedance, low -esistance souce. Using the cente -tap on this tansfome accomplishes this function. The actual limiting o compession cicuit consists

3 MIKE II AT7 II II 1Mal6 C: C4 ELECTRICAL CIRCUIT ---==-_ CI R RI 2 R4 R5 R6 IUSED NT TI C5 C6 MI. RI3 IN 'I SOUT 115 V AC SI T2 J 6X5 5 x V CIO, R16 R17 CII OUTPUT TO MIKE JACK ON SPEECH AMP. Fig. 3. Cicuit diagam of the Logaithmic Compesso. CIRCUIT CONSTANTS (All esistos and capacitos t 20% toleance unless specified othewise) CI, CH 0.01 mf 400 volt pape o mica R9 C2, C7 1.0 mf pape (see text) R12 C mmf mica RI, C mf 400 volt pape RI6 C, 1000 mmf *10% mica RI6 C6 100 mmf *10% mica Rl C mf *10% mica R16 C mf 450 volt electolytic SI S2 T1 (see text) C mf 450 volt electolytic RI, R64 10,000 ohm, % watt R2 10 megohm, % watt R3, R6, 470 ohm, % watt R4 0.5 megohm potentiomete R; 0.1 megohm, 1 watt R6, Rll 47,000 ohm, 1 watt R7 47,000 ohm, % watt R6 56,000 ohm, *10%, % watt T2 W 0.56 megohm, *10%, % watt 0.1 megohm potentiomete 0 47 megohm, % watt 470 ohm, 2 watt 2200 ohm, 2 watt 1000 ohm, 1 watt 4700 ohm, 1 watt SPST toggle switch SPDT toggle switch Push-pull plates to voice coil audio tansfome, UTC R -38A (see text) Powe tansfome, volts at 50 mils, 6.3 volts at 2 ampees UTC R-54 Coppe -oxide instument ectifie (see text) only of R7 and W, the latte being two sections of a coppe -oxide instument ectifie. Resistos R6 and R,, togethe with condenses C. and C6 act as a two - section R -C filte. The output of this filte feeds the second 12AT7 diectly. Resisto R12 acts as an output contol so that the output level fom the speech compesso may be made to match the output level of the micophone. Thus when the speech compesso is switched out of the cicuit no othe adjustment need be made. The output tube is equied fo two easons. It is necessay to pesent the pope load to the two R -C filtes and, secondly, to pemit a thid R -C stage to be utilized. Inasmuch as the second section of the 12AT7 tube is not used this may seem like wasting pat of the tube, but the use of a high -mu tiode was dictated and the 12AT7 fills this equiement nicely. Note that the heate of the unused section need not be enegized. Many uses fo this exta tube section will undoubtedly suggest themselves. The in -out switch, S2, allows the unit to be switched in and out of the cicuit easily. Note that shielded wie is specified fo the connections to this switch. The output itself is caied by a shielded lead which plugs into the mike jack of any speech amplifie designed to handle a high impedance dynamic o cystal micophone. The powe supply is conventional in all espects. Because of the low cuent dain on the powe supply a esisto -capacito filte is employed. Resisto R16 and condense C11 povide decoupling and additional filteing fo the fist 12AT7 section plate voltage. The connections indicated by the heavy black lines in the powe supply section should all be made to one gound point. This will pevent the chassis fom caying the ciculating capacito cuent and help to keep the unit hum -fee. CONSTRUCTIONAL DETAILS As may be seen fom the photogaphs, the entie unit, including powe supply, is mounted on a 5 by 7 by 2 inch chassis. While the layout is not citical, it is advisable to keep the powe supply potion of the cicuit as fa away fom the est of the cicuit as possible. The layout shown is quite satisfactoy. With efeence to Fig. 1, the font panel layout, fom left to ight, is: mike jack, output lead, in -out 3

4 I _ I `. F - 4a l _.-i., Np N -- 1, n-., 1.-*~ }. b. Y _ ' 1. :,- f , i Ca Fig. 4. Unde -chassis view of the Logaithmic Compesso. Thee is ample oom fo all component pats. switch, compession contol, output level contol, a -c on -off switch and a -c cod. The tubes ae, left to ight, input 12AT7, output 12AT7 and 6X5 ectifie. Note that the two 12AT7 tubes ae shielded. Fig. 4 gives the details of the wiing. Nothing hee is citical if nomal wiing pocedue is followed. Note that R. is placed as close to the gid pin as possible. The wiing can be made simple if the unused leads fom the powe tansfome ae pulled inside the tansfome case and secuely taped to avoid shots. This was done with the 2.5 volt and the 5.0 volt windings. The unit pictued uses a bottom cove plate fo the chassis. This is ecommended to avoid -f feedback. Any sot of thin metal will seve fo this pupose, if you chassis comes without a bottom plate. COMPONENT PARTS While no extemely citical values ae equied, it is ecommended that the specified values be used in all cases. Fo example, C2 and C ae specified as 1.0 mf condenses. If lowe values wee to be used, the fequency esponse would suffe, and if highe values wee used, the esult would be insufficient low -fequency attenuation. Condense C.., is about the only component which could be changed. Hee a mf condense could be used, with one of the 20 mf section on eithe side of Bp.. Almost any sot of push-pull plates to voice coil tansfome will seve as T.. Wattage ating of this tansfome is not impotant. If possible, linea tape potentiometes should be used at R4 and R12. This sot of tape will give a smoothe action than othe types of tape. Cae must be taken in puchasing the limite ectifie, W, because instument ectifies come in seveal diffeent styles. Basically, of couse, they ae 4 used to make a -c metes out of d -c metes. Howeve, they can be puchased as half -wave units, double units, full -wave units and bidge units. Two sepaate half -wave units, connected as shown, will wok, and the bidge -ectifie style will wok if the pope leads ae used. The "full -wave" unit will not seve because the two diode sections ae connected impopely. In the double type ectifie the two diode units ae connected as shown in Figs. 2 and 3 and theefoe this type of instument ectifie would be the best to use. COMPRESSION ADJUSTMENT The adjustment of the Logaithmic Compesso is done vey easily. Plug in a mike and place the in - out switch, S2, in the "out" position so that the micophone is connected diectly to you speech amplifie, then follow these thee steps: 1. Adjust the audio gain contol on the tansmitte fo nomal modulation as seen on an oscilloscope (the best method) o some othe instument wothy of tust. 2. Put the output contol on the unit to zeo and set the compession zontol so that it is about half open. Switch the compesso to "in" and advance the output contol while speaking into the micophone until the peak modulation is the same as in step 1. While an oscilloscope is not absolutely necessay in ode to make this adjustment, it is stongly ecommended. 3. Adjust the compession contol so that the aveage plate cuent in the modulato stage on a sustained " " is, say, not ove twice that obtained with the compesso out. Then ty compesso "in" and "out" on a few QSO's to find the best opeating point of the compession contol fo the micophone you ae using and the eceiving condiditions pevalent at the othe fellow's QTH.

5 40 SI. '~. t, R Jt.. Fig. 5. Top view of the Logaithmic Compesso. The audio section is on the ight and the powe supply section is on the left. USE OF THE COMPRESSOR With the Logaithmic Compesso in use the modulato tubes ae equied to handle much moe aveage powe than usual. In fact, it is possible that you modulato stage will not be capable of handling the exta aveage powe equied. Caeful checking with an oscilloscope will detemine if this is the case. As a geneal ule, if you modulato can handle a sine wave signal at 100% modulation, then the aveage powe capability of you modulato is adequate fo use with the Logaithmic Compesso. (Afte all, this ten db gain has to come fom some place!) This means that, fo a kilowatt ig, you modulato should be capable of continuous opeation at 500 watts output at 1000 cycles. Fo lowe powes the same atio holds. In opeation the compesso must be used with judgment-good judgment that is. Too much compession may make an othewise acceptable signal almost intoleable. With a judicious amount of compession one can expect to add fom 6 db (4 to 1 in powe) to 10 db (10 to 1 in powe) in the effectiveness of his signal povided conditions at the eceiving point ae such that undestandability without the compesso is impaied by QRM o high backgound noise. RESULTS WITH THE COMPRESSOR In many months of test at W2KUJ the following infomation has been obtained. Neaby stations, o stations not expeiencing QRM, pefe that the compesso not be used. Stations eceiving a weak signal o listening though sevee QRM pefe that the compesso be used. Repots fom the latte stations ange fom eight to ten db jump in effective signal stength when the compesso is switched in. Repots fom neaby stations ae that the signal is loude, but somewhat less eadable with the compesso in use than without it. In no case has a epot been given that the signal was boade when the compesso was used, even when this question was asked of neaby stations. Tests made at W2RYT's shack indicate that diffeent micophones give somewhat diffeent esults when used with the compesso. Fo example, an Electo -Voice Model 605 dynamic mike (pictued in Fig. 1) and an Electo -Voice Model 915 cystal mike seemed to have identical speech chaacteistics (although the dynamic mike had less output) when used without the compesso. When used with the compesso, the dynamic mike was found to have a speech quality which was less hash than that of the cystal mike. Futhe, it was found advisable to advance the compession contol with the dynamic mike. The foegoing is not intended as a ecommendation fo dynamic mikes, no is it intended as an authoitative compaison between two Electo -Voice micophones. The compaison has been made to emphasize the impotance of testing you compesso caefully with each micophone you may use with it. In summay, one can expect to boost the effectiveness of his signal when it is needed most by use of the compesso (it fequently means the diffeence between making a contact o not) with some decease in ease of eading the signal whee the compesso is not needed. Bea in mind that the compesso can be misused (to you disadvantage). Seek honestly to find the opeating points which best exploit its use. In many cases it is best to not use the compesso. But in those cases whee it is needed, the Logaithmic Compesso can eally do a job fo you. 5

6 s I1 il I have eceived inquiies and lettes on all sots of stationey, in all kinds of envelopes (some without stamps on), fom a good many counties, but seveal months ago W4IKC pulled the best stunt of all. At eleven o'clock on a egula woking day my telephone ang. The voice identified himself as a local ham-w2pgv. He told me to QRX, and shotly, via ten mete phone and a phone patch Ivan of W4IKC asked if he was talking to Lighthouse Lay. Although somewhat taken aback, I answeed in the affimative. Ivan then went on to ask if I would send him seveal cetain copies of the Ham News. I assued Ivan that I would be moe than happy to oblige. W2PGV cleaed the phone patch and went on with his QSO, and that was that. I must say that this phone patch deal is vey effective. I have used one myself many times, but this was the fist time I was eve on the eceiving end. Needless to say, Ivan got his Ham News issues post haste. Now and then I am ecognized on the ai. The last time was duing the ARRL DX Contest. I got in contact with VP9KK, and Dean came back to me, gave me my numbe, and then asked if I ween't the fellow who had peviously sent him some copies of the Ham News. I pleaded guilty, afte which we had a vey nice, but shot, QSO. Any day now I expect to get a equest fo the Ham News by caie pigeon. Speaking of the Ham News, hee's an open offe to all of you. If you build a piece of equipment descibed in the Ham News, and find that as a esult you copy of Ham News has become bedaggled, spotted with flux, buned with a soldeing ion, o othewise disfigued, let me know and I'll be happy to see that you get a fesh, clean copy. This assumes, of couse, that the back copy you equest is still available. All of this is anothe way of saying that I am inteested in how you make out when you build Ham News units. Those of you that have touble usually let me know. How about heaing fom some of you that don't have any touble? On the fist of Mach I eceived a publicity elease fom the Receive Division of the Geneal Electic Company. I think it will inteest you. It says, in pat: "Radiation fom FCC appoved diathemy and industial equipment, and intenational shot-wave stations, does not affect eithe the image o the audio in Geneal Electic's cuent line of television eceives, the Company's distibutos have been told in a lette mailed today by the G -E eceive division. "G.E. stands pactically alone today in being able to offe this demonstatable advantage because the Company's TV sets now opeate in the 'elatively quiet' megacycle band, the distibutos have been infomed. "In its epot to distibutos, G.E. enclosed a lette fom the Liebel-Flashein Company of Cincinnati congatulating G.E. 'on being the fist majo poduce of television eceives to adopt an intemediate fequency that is not sensitive to the megacycle signals geneated by FCC appoved industial, medical and scientific appaatus.' "The Cincinnati company explained that it had placed one of G.E.'s eceives alongside its shot-wave diathemy unit and 'afte polonged tests thee was no sign of TVI,' it said. "The company pointed out that 'the ealie 21 to 26 megacycle IF standad used by most television eceives has been the cause of consideable television intefeence when sets ae opeated in the vicinity of FCC appoved diathemy appaatus.' " I don't believe much comment is equied. This change will make a lot of hams happy and ít will cetainly mean that thee will be many happy G -E television eceive customes. Of couse, if that TV set you ae messing up is not a new G -E model, thee is always the Hamonike (Ham News, Vol. 4, No. 6)! -1'i9lt ltosae.2a444/ 6

7 TRICKS AND TOPICS How did you solve that last poblem that almost had you stumped? Be it about tubes, antennas, cicuits, etc., Lighthouse Lay would like to tell the est of the hams about it. Send it in! Fo each "tick" published you win $10 woth of G -E Electonic Tubes. No enties etuned. Mak you lette "Enty fo Ticks and Topics" and send to Lighthouse Lay, Tube Division, Bldg. 269, Geneal Electic Company, Schenectady, New Yok, o in Canada, to Canadian Geneal Electic Company, Ltd., Toonto, Ontaio. TUNE Fig. 6. VFO HI -LO SWITCH PAD TO GRID CI LOW SETT C2 ki_ HIGH Details of the HI -LO Switch descibed by W2FZW. C3 The idea of poviding a panel switch on VFO's aanged to alte the VFO output fequency by a small amount (5 kc-10 kc) highe. o lowe than the "set" fequency has had much appeal both to DX opeatos and to taffic opeatos. In both of these activities the ability to switch 5 kc-10 kc eithe side of the nominal VFO fequency adds a flexibility to opeating pocedue which effectively inceases the opeating efficiency of the station many fold. The difficulty in poviding such an aangement fo the VFO lies in the small fequency shift desied. Fo example, at 28 mc a shift of 5 kc epesents a fequency change of one pat in 5600, o appoximately 0.02%. In ode to achieve this esult the necessay capacito change should be appoximately 0.04% o one pat in Assuming the VFO tank capacito to be 500 mmf, the indicated change would be oughly 0.2 mmf. Many seies -tuned (o Clapp) VFO's have tank capacitos one tenth o less that of this example, and would equie changes of 0.02 mmf o less. These capacity changes ae vey small compaed tc the stay capacities associated with the best switches available fo poviding the cicuit changes. In addition, capacitos of these small values ae not commecially available and attempts to constuct small timmes fo the pupose ae likely to be disappointing because of mechanical difficulties and additional stay capacity effects. A solution fo these difficulties may be eadily found if one ecalls the once popula seies bandspead scheme which was used by many home constuctos in yeas past, and which is still used commecially in the National HRO seies eceives. The poblem is identical; that of making the elatively lage tuning capacito behave as if it wee a athe small one. To adapt it to ou VFO poblem we need only make the cicuit look like Fig. 6. This figue shows the "business end" of a Clapp oscillato tank with a typical switch aangement. The tank (o pad) capacito might be 50 mmf. Let us also assume that we wish to have appoximately 5 kc fequency changes between switch steps at 28 mc. This, as was noted above, epesents fequency changes of 0.02% pe switch step, o changes in effective capacity of 0.04% pe switch step. A change of 0.04% in capacity would amount to 50 X0.0004, o 0.02 mmf. In othe wods, C1 and C2 in seies should have an effective capacity of C1 minus 0.02 mmf, while C1 and C3 in seies should have an effective capacity of C1 minus 0.04 mmf. If C1 is chosen as 1 mmf, then Cs should be appoximately 50 mmf, and C3 should be appoximately 25 mmf. These capacities ae eadily available in the nomal ange of timme values (o small fixed ceamic units if these ae pefeed) and will also be easonably lage compaed to the switch capacity and othe wiing stay capacities. The same stunt will wok on all bands, with C1 usually chosen lage on the lowe fequency bands (whee a lage pecentage fequency shift is desied) o whee highe C VFO cicuits ae used.-w2fzw. WIRE COVERING Having two paallel loose wies going to a phone plug, I wanted a coveing ove them to keep them fom sepaating and to fom a smooth flexible cod. At the local dime stoe I puchased a pai of 40 inch shoe laces. The ends fom one lace wee emoved which left me a hollow cotton tube. (Most shoe laces ae hollow.) The ends of the wies wee wapped with a small piece of scotch tape. The wies wee then vey easily pushed though the hollow shoe lace as the lace tends to incease slightly in diamete and slightly decease in length like shield wie. Afte the wies wee in the lace it was pulled out to its oiginal length by a pulling action along the lace while holding one end and the wies fimly. This makes a smooth snug fit. A dop of cement was added to the ends of the shoe lace and then wapped with a shot piece of thead. This method can also be used fo thee o fou wies and makes a pofessional looking cable to you exact specifications.-w2fen. PROPER CARE OF CRYSTAL MICROPHONES Cystal micophones ae likely to lose thei sensitivity and fequency esponse duing hot humid weathe. Although micophones ae sealed against moistue, moistue may ceep in and damage the cystal element. Fo sometime I have noticed the modulation pecentage in my tansmitte gadually go down until I could baely modulate 50% with the audio gain wide open. A caeful check of the speech amplifie and modulatos indicated no touble thee. The micophone was then suspected because of the unusually damp weathe that has existed hee all summe. I was about tc discad the mike o send it to the factoy fo epais, but I decided to ty a little stunt as a last esot. The micophone was placed in a clean, dy, aitight can with one pound of fesh silica gel 7

8 1 Obtained fom a local adio and efigeation supply house. In twenty-fou hous the mike woked as good as new; in fact, it woks bette now than it has in two yeas. I would ecommend that this tick be tied on all cystal micophones which have been subjected to excessive moistue o humidity befoe discading. In fact, I believe it would be good pactice to stoe the mike in this manne when not in use paticulaly duing the hot humid months of summe.-w4aee (Ed.'s note-the following complete stoy on the pope cae of cystal micophones appeaed in a ecent Electo -Voice elease-lighthouse Lay.) "What causes cystal micophones to lose sensitivity? To answe this let's fist see what the cystal is made fom. "Rochelle salt cystals ae fomed synthetically. When the ochelle salt cystal is gown it takes fou molecules of wate of cystallization fo evey molecule of sodium potassium tatate. In humid climates, the cystal has the popety of absobing moistue and the esult is leakage esistance o low output. Placing the micophone o catidge in a silica gel desiccato will educe this excess moistue but thee is the possibility that if left in too long the moistue content of the cystal might be loweed to the point of damage. "If the humidity is less than 23% fo a long peiod you will get dehydation o dying out. If the tempeatue ises to 127 F. the sodium potassium tatate dissolves into the wate of cystallization. In othe wods, if a cystal micophone gets too hot, too dy o too wet, it won't wok. "These statements seem to un down devices using ochelle salt cystals. This is not tue. If the humidity is between 23% and 86%, except fo bief peiods, cystals will stand up fine. If the tempeatue is below 127 F., no touble will esult. That's why the manufactue puts a guaantee on cystal devices and caution is given about heat. Occasionally cystals ae damaged when the limits mentioned above ae exceeded. "Rochelle salt cystals ae teated to pevent damage fom moistue. E -V has given an additional potection fo added life on cystal catidges. The case is completely filled with silicone to pevent moistue fom getting to the cystal. The leads ae a bad spot fo moistue to ente and the E -V silicone teatment eliminates this fault. E -V cystal devices ae all thooughly moistue inhibited, the esult of intensive moistue -poofing eseach." (Electo -Voice "Repot to the Distibuto," August 15, Repinted by pemission of Electo -Voice, Inc.) MICROPHONE COVERS To the hams that use cystal mikes which ae subject to moistue conditions, just snoop in the kitchen fo cellophane dish coves, and secue the pope size cellophane cove fo you micophone. It makes a much nice cove fo a mike than a dish. Ty it, it also keeps out dust and othe foeign paticles.-- W DWF. Cyp HAM NEWS FROM: