Low-Cost High-Performance Volume Compressor S. S. ANDREWS More than 60 db of compression at less than $15, with fast attack, slow decay, and low distortion. The heart of this scheme is a photoconductor. Fe tae aa Lae pgupeatey ere ‘Tho Neod for Compression Why compress? The amplitude range of live musical presentations is often greater than many reeorders will handle ficiently. Therefore, continual “gain viding” must be done. However most people cannot reset in 10 milliseoonds ‘and the end result is overlond resulting in distortion rocumt 3 Fig. 1. Series lamp compressor scheme, Fig. 2, Rayslstor compressor scheme. AUDIO. © DECEMBER, 1964 Fig, 2. Schematic of tis obvious that a need for automatic ‘compression is very desirable, Most eom- ‘mercial reeording studios and broad- casting stations tse some type of eom- pressing deviee, This narrows the range ‘of amplitude variations to controlled Timits. Also, in backgroud musie ap- plications such as for restaurants, hotels, Gepartment stores, and co on, wide faniplitade variations are not desired. ‘Various designs have evolved over a period of years for compressing and/or Timiting the audio signal for recording. ‘The simplest of these methods, no doubt, ‘yas using a. pilot lamp in series with the ‘output eirenit (Pig. 1). The charactor- istic of pilot lamps is to inerense in re- sistance as they become brighter (more Signal), and decrease in resistance as they dim. Therefore, high amplitude levels will eauze the resistance of the Inmp fo inerease and less signal will arrive at the outpat than was fed to the Inmp. Conversely, at low levels, the Jnmp will have less resistance and’ pro- portionstaly more signal will reach the ‘output, The advantage of this system is comprossor-amplifir. obvious; the cost is extremely low. The disadvantages, however, are numero First, it uses a great deal of power; ond, impedance matehing is dificult; third, jt is most effective when the lamp is glowing rather brightly; fourth, it Jnas an extremely small dynamie range. ‘Another commonly used method is to rectify the audio signal and use this po- ‘ential to bias a pontode electron tube snd feed the signal through the sawe fleetron tube. As the audio signal changes amplitude, the reetiied biasing potential changes and therefore controls the output of this tube. This method, of course, has a great deal of merit over tho Imp iret, Tt docs not consume power from the output eirenitry. The amount of compression ean be controlled rather accurately and, by means of an RC network, the attaek time may be varied over a large range. Tt_also hs disadvantages. First, the cirenit is rather complex and involves numerous components, including a power supply, which makes the cost rather consider able. Seeond, the audio levels presented 19Fig. 4. A. C. VIVM schematic, to the pentode should operute over a suall dynamic range 60 a8 not to over- Toad the tube, When this is done, the problem of hum becomes extremely troublesome, Special techniques must be employed with regard to power supply and layout. Third, if higher amplitude audio levels are used at a higher level ppoint in the circuit, then the tube is not operating over & linear portion of its curve and distortion may become excessive. There are many more methods more or loss sophistiented than the above mentioned, However, these exam- ples are two of the more eommonly found, A Now Device A new approach is to use a Raysistor. ‘The Raysistor is a Raytheon trade name Zor an opto-lectronie component de- ‘signed for applications in a variety of control functions, providing eompara- tively noise free control of ac. or de. signals over a wide dynamie range, ‘without transients or contact chatter and ‘vith & high insulation and electrical io- Iation between the signal and control cireuit. It eonsists of s light souree and a photo-esistive clement assembled in a Tight tight ease. ‘The characteristics of the Raysistor which made it very appealing for use in compression are as follows: 1. Fast “ON” action (10 ma); 2. Slow “OFF? action (800 ms); 3. Tamp filament slow response provides an averaging ef fect; 4. Noise free control; 5, Auto- atic control; 6. Wide dynamie range. Placing the photocell portion (a vari- ‘able resistance clement) in the grid cir- cenit of an audio amplifer (Fig. 2), makes it possible to easily regulate the signal to the grid eireut sineo the laaap, which is across the output terminals of the amplifier, glows brighter daring loud passages and dimmer or off during soft passages. The photocell has a high resistance when the lamp is off (10° 20 ohms) and a low resistance when the lamp is on (50 ohms). Design Considerations ‘There are curtain design considera- tions whieh must be taken in account. T have ehusen the CK1103 which has a control lamp with a yolt filament drawing 175 ma, Most amplifers are ea- able of produeing voltages whiel will far exceod this although it is true that as the amplifier iuereases in output the input is doereasing by the aetion of the photocell. However, when low-eficieney Speaker systems are utilized, of the bias of the photocell is set to a point where the output would be an aeceptable level, then tho voltage will exceed lamp a fleations. $0 it is apparent that some ‘means has to be provided to decrease the voltage to the lamp. ‘A reasonable solution is to place a THOURAL CONTROL VONAGE ED variable resistor plas a lamp of approx imately the same voltage and current characteristies as the one in the Raysis- tor in series with the Raysistor. The se ‘es lamp would allow the voltage level to be determined and also could be used to szive indication of the smount of com- pression taking place at any instant ‘The series potentiometer eould be varied to wive the predetermined level, The eirenit ean be made as simple or as complicated as desired. In my testing procedure T wanted to lave greater eon ‘rol so that I might study some of the clfects of the compressor, T decided to place an a. voltmeter across the lamp cireuit 50 as to have minimum effeet of cireuit operation. T used a VEVM iz cuit. This eireuit, when ineorporated in he unit, will give a continuous indica. tion of the voltage across the lamp and give a continuous observation during operation. The VEVM cireuit is by no means necessary, The Circuit About the cirenit: The first half of the triode in the amplifier section is a standard voltage amplifier to compensate for eireuit losses. The output of the next stage is less than unity. The next stage is s conventional eathode follower which is used for impedance matching par ‘poses and to Keep the impedance low s0 ‘that the high-frequeney entoft is beyond the audible rango if cable lengths are fairly long. ‘The outpnt of the amplifier is fed to the eontrol portion of the Raysistor 20 that as the output beeomes greater the resistence becomes less, thus shunting ‘the grid and reducing the signal and the ‘output, Tt takes only 10 ms for this to (Continued on page 71) 1a TO Fig. 5. Typical curve of resistance versus control voltage. i AUDIO. © DECEMBER, 1964COMPRESSOR {from page 20) happen. Naturally sinee the output has hheen redueed, the control light: becomes dimmer and’ allows the signal to be greater. However, the light has « slow ‘recovery tine, 800 ms. Therefore, a gen- eral tapering off oecurs. A switeh ean be placed in the lamp eitenit to disable the lamp when it is desired to have the eir- cuit fanetion normally without compres- ‘The a.c, VEVM eireuit is mainly con- ventional, however no provision for eal bration i incorporated ond a control is placed in the input eirenit so aoeuraey is minimal, The VTVM is not used for accurate measurements but rather as an Indicating dovieo so that relative volt- fages in the lamp cirenit may be ob- served. The VIVA is used in a manner Similar to a VU meter Installation of the Compressor If the system in which the compressor is to he incorporated as a program amplifier and a preamp, the ideal method fof connecting would be to place the “sig nal” portion (photocell) between the preamp aud the program amplifier and ‘connecting the “eoutral” portion (lamp) ‘to the outpat. ‘This circuit could slz0 be placed at low level inputs. However, are should be taken with regard to shielding so that Jum and noise will not be at an objec- tionable level. Before making connoetions to the am- pif, the following should be observed: First the amplifier should have no sig- ‘nal present; second connect the input section fist; third the eontrol potenti ometer should he adjusted to short eir- fit the lamp; then a signal showla be introduced into the amplifier to estab: lish the proper operating level; then in- crease the signal slightly and adjust the control potentiometer until the signal is restored to the proper level by action of the compressor. The grid control poten- tiometer and the lamp control potenti- ometer interact so repeated adjustments ‘will bo necessary to establish the desired ‘operating parameters. Tt is moat im- pportant that the indicating series light be observed carefully during all adjust- ments so that the filament specfiestions will not be exceeded. Using a Reysistor, or equivalent de- viee, for the heart of the system the compressor ean be made as elaborate fr simple as the eonstruetor desires, The Raysistor alone connected to the output and input eireuit gives you 60 db of ‘compression for less than fifteen dollars zx