光电音量控制器.doc

Home Products o Optoelectronicso Solderable Photodiodeso Photodiodes in Hermetic Packageso Photodiodes on Ceramic Substrateo Photodiode Arrayso CdS/CdSe Photocellso Transmissive & Reflective Assemblieso Audiohmo Optocouplerso Audio Characteristicso Principles of Operationo Product Datasheetso Datasheet Index Capabilities o Microengineering Capabilitieso Concept, Design and Prototypeo Validation and Testingo Manufacturing and Distributiono Application Samples Technical Reference o Audiohm Application Noteso Compressoro Limitero Switcho Level Controlo Expander/Noise Gateo Animated Optocoupler Presentation About Silonex o Company Profileo Careers Sales & Support o Main Sales & Supporto Head Officeo Representatives & Product Distributorso Regional Sales Representativeso Product DistributorsTechnical ReferenceAudio level control with resistive optocouplersoptocoupler comparison table attenuator configurations defined attenuation law crossfader Controlling the level of an audio signal by means of an applied voltage or current has always been problematical but often desirable, particularly when it is necessary to control a circuit from a computer. This application note deals with voltage or current controlled attenuators using optocouplers, primarily in relatively static situations (as opposed to the dynamic control that occurs in compressors, limiters and noise gates where the gain is continuously being changed in relation to the signal). Although the circuit configurations are broadly similar to those used when switching audio signals, the demands on the optocoupler are rather different, as it is being used as a variable resistor rather than being hard ON or OFF. Audiohm optocouplers offer a unique set of features for controlling audio signal levels.The main features are: 1. Very high isolation between control port and audio circuit. 2. Relatively simple drive circuit. 3. Relatively low drive requirement ( good distortion performance. 6. Large signal voltage range: up to 500 V. To compare their performance with other methods: DeviceSignal voltageIsolation voltage/feedthroughDrive complexityPower consumption #DistortionSpeedCostJunction FETa few V0/mediumhigh2-50 mWmedium *fastlowAudiohm coupler60-500 V1 KV/v.goodmedium2-100 mWmed-goodmedmediumIC VCA15 V0/goodlow-medium50-100 mWmed-goodfastmediumVCA Module 15 V0/v.goodlow200-500 mWgoodfasthighMotor driven potentiometer50-200 V1 kv/goodhigh1 W+v.goodslowhigh#including support circuitry *depends on drive circuit Attenuator configurationsResistive optocouplers can be used in either series (Figure 1), shunt (Figure 2) or series/shunt (Figure 3) configurations.The hybrid series/shunt (Figure 4) is a cross between the shunt and series/shunt and offers some advantages.In most normal solid state audio circuits, the optimum source and oad resistances are going to fall somewhere in between the Ron and Roffof the couplers. There is likely to be a significant asymmetry between the up (decreasing attenuation) and down (increasing attenuation) times of the series and shunt configurations. If this a problem, it is best to use a coupler with a relatively fast response, for example the NSL-32SR3. This device also shows the best distortion performance of any of the NSL-32 series.This is an important consideration when being used as a linear attenuator, as it will not be driven hard ON or OFF as in a switching circuit. It is better to drive the coupler LED from a constant current source, to minimize the effects of variations in LED forward voltage from device to device and temperature. A simple circuit that gives 1mA per Volt input is shown in Figure 5.The measurements of the various circuits were taken with an input level of +10 dBu at 1 KHz, and the distortion (THD+N) measured with a 30 KHz bandwidth. Although some of the distortion figures may seem less than ideal, three factors should be kept in mind:1. These measurements were taken at fairly high levels, typically corresponding to -6 dBFS on professional equipment. They will decrease rapidly with input level typically being 3 to 4 times less at 0 dBu, and virtually unmeasurable at -20 dBu. 2. The distortion products produced by an Audiohm coupler are all low order, second and third harmonics, which are less objectionable than high order harmonics produced by crossover artifacts. 3. In the series/shunt and hybrid series/shunt attenuator configurations the worst distortion occurs at high attenuations. The output signal level is so low that the distortion is unlikely to be noticeable. Series AttenuatorFigure 6shows the attenuation and distortion vs. control voltage of the circuit in Figure 1, driven by the constant current source. Please note that the horizontal scale is in mA, for example 300 m = 300 microamps. Input signal was 1 KHz, +10 dBu. The high dark resistance of the NSL-32SR3 gives quite good attenuation (typically about 80 dB at 1 KHz). However the circuits usefulness is limited by the non-linear relationship between LED current and attenuation, and high distortion levels at any attenuation level below -10 dB. This is because the voltage across the non-linear cell resistance increases as the coupler turns off. Time constants are approx 0.5 and 12 msec for increasing and decreasing gain respectively. At frequencies above a few hundred Hz, the maximum attenuation is mostly determined by the parasitic capacitance and decreases with frequency.Shunt attenuatorIn this configuration (Figure 2) a coupler with low RON, for example the NSL-32SR2, provides the best OFF attenuation for a given LED current. With Rs = 47 K0 and ILED = 10 mA, 60 dB can be readily obtained. However, the time constant for increasing the gain will be approximately 300 msec which will cause a noticeable lag in response. To get an equivalent attenuation from the faster NSL-32SR3 requires an Rs of 100 KOhms, which may give noise and interference pickup problems. The output needs to feed into a high impedance buffer amplifier to minimize insertion loss. A FET input device is ideal.The attenuation and distortion response of this circuit is shown in Figure 7, note the peak in distortion at 2 to 3 dB attenuation. At very low currents the cell resistance is so high that the non-linear resistance of the cell has little effect, even though it has the full signal voltage across it. At high currents the cell resistance is low with most of the signal dropped across Rs. Since the cell non-linearity is proportional to the cell voltage, distortion is low. It is at currents where the cell resistance is a little less than Rs but still has a large proportion of the signal across it that the worst distortion occurs. Also note that the rising THD+N below 25 dB attenuation is NOT due to distortion, but inherent circuit and measurement system noise. If measured with a FFT analyser, the distortion continues to fall with attenuation. Alternatively, a two stage attenuator as shown in the Figure 8 can be used. This has the advantage of not taking any more drive current than a single stage, as the LEDs are in series almost doubling the attenuation range for a given Rs. Figure 9 shows the response. Although the control law is not linear, it is gentle enough to give a smooth response if the control voltage is derived from a potentiometer with a reverse logarithmic law.Series/shunt attenuator.This configuration (Figure 3) achieves better OFF attenuation and symmetrical time constants, at the expense of an additional coupler. The circuit could be driven by complementary current sources, but the simple arrangement shown in Figure 10 saves complexity and current consumption, and also provides a reasonable dB/linear control law over a good proportion of the range. With the values shown, the circuit imitates a 5 KOhm potentiometer, although there is some overall variation in total resistance with attenuation as it should be fed from a low impedance source. C1 sets the up and down time constants at about 20 msec, which achieves a fast but smooth audible response. The attenuation and distortion curves are shown in Figure 11.Note the distortion null at 6 dB attenuation where both couplers have the same resistance.The hybrid series/shunt arrangement shown in Figure 12 gives an improvement in distortion performance, Figure 13, by using resistor R3 to drop a proportion of the signal at low to medium levels of attenuation. The bent attenuation law is useful, giving greater resolution in the normal working area of the control, and a sharper response towards cut-off. To further improve on this distortion performance there are two possibilities:1. Use a coupler with a higher voltage rating, for example the NSL-37V51. 2. Use two couplers in series to reduce the voltage across each cell. The temperature dependence of the coupler LED and diode forward voltage drops make the attenuation of these circuits sensitive to variations in ambient. These dependencies can be compensated by running the circuit from a 5V supply that tracks the variation. A suitable circuit is shown in Figure 14. The LED should be an AlGaAs type. The control voltage will also need to be referenced to this.Defined Attenuation law: All the above circuits have a control law, which is determined by the inherent characteristics of the couplers and supporting devices. Although consequently non-linear, it is useable for many simple applications. If a true linear response is desired the circuit of Figure 15 can be used. A shunt attenuator is placed in the feedback loop of IC1, which sets the current through the coupler OP1 so that the feedback voltage on IC1 pin 3 is equal to the control voltage applied to R1. Matched coupler OP2 has the same current flowing through the LED as OP1, forming a shunt attenuator linearly controlled by V_CONTROL. OP1 should feed a high load impedance for best linearity and insertion loss. C1 provides frequency compensation around the loop for stability. The output attenuator is fully floating and can be run hundreds of volts away from the control circuit. This feedback control could equally be applied to a series/shunt circuit giving a floating linear potentiometer output.Level Controls and Crossfader:Frequently in audio mixing it is desirable to fade between one source and another. A standard slider potentiometer can be used for this purpose. Unless the highest quality conductive plastic elements are used, the track rapidly degrades with use resulting in contact noise as the wiper is moved. The circuit in Figure 16 helps reduce this by using the potentiometer (designed for DC use) to generate control voltages. These in turn control two sets of Audiohm couplers that determine the relative gains of the audio channels. Capacitor C1 smooths the wiper voltage of the potentiometer, which is buffered by IC1a to reduce wiper current, thereby reducing contact noise. The complementary control voltages are slightly bent by the networks around LD1 and LD2 to offer the smoothest fade law, and then fed to IC2a and IC2b which have OP1 and OP2 in their feedback loops, in the same way as the circuit in Figure 15. As shown with matched NSL-32SR3S couplers the maximum attenuation is typically 60 dB and the insertion loss less than 0.2 dB. For a greater attenuation range two stage attenuators could be used. Alternatively, the A and B signals from the emitters of Q1 and Q2 could be used to fire muting switches when the potentiometer is at either extreme.Silonex offers these documents in support of the product only and does not guarantee the performance of any application designed from the information. It is the customers responsibility to ensure the end product is manufactured correctly.ProductsAudiohm OptocouplersSilonex Audiohm optocouplers are ideally suited for high performance audio circuits. Most design-ins have demonstrated proven performance benefits over silicon based audio control devices such as VCAs and FETs. The Audiohm optocoupler is a very cost-effective way to improve the performance of your audio equipment.Audiohm optocouplers are manufactured using a photocell as the output and an LED as the input. By varying the chemistry of the photocell layer and the electrode pattern, Silonex makes photocells with various characteristics. By matching the LED and photocell characteristics, couplers are produced with different breakdown voltages, resistance and response times.Silonex supplies a number of standard parts. We can also fine tune (by selection) the individual performance of a device for a specific application.The correct selection of an optocoupler depends on the application parameters that the designer requires.In determining the most appropriate product, one must include the following in the analysis: Digital or Analog Mode of Operation Is distortion an issue? New design or replacement device Ideal performance specification in terms of speed and resistance range The Technical Reference section of this website shows how these design factors are applied in actual applications. The platform package allows easy insertion into a through hole PCB. They are supplied in tubes for manual insertion. The axial package allows the user the option of installing the device in any orientation, with any lead form or between boards. Product Datasheet (.pdf file)LED Current (mA)Resistance Max. on (Ohms)Resistance Min. Dark (Ohms)Maximum Photocell RatingsVoltage (V)Power Dissipation (mW)NSL-2616200010M120200NSL-281640010M10050NSL-28AA1625001M10050NSL-3216500500K6050NSL-32AA1610K10M6050NSL-32B-1011750500K6050NSL-32B-1021960500K6050NSL-32B-10311650500K6050NSL-32B-10412800500K6050NSL-32H-1011750500K6050NSL-32H-1021960500K6050NSL-32H-10311650500K6050NSL-32H-10412800500K6050NSL-32SR220401M6050NSL-32SR2S2040 typ1M6050NSL-32SR3206025M6050NSL-32SR3S2060 typ25M6050Notes:Maximum LED IF = 40mA. Dark Resistance measured 10 seconds after removal of LED Current. Power Dissipation measured with Power rating at 25 C derated linearly to 75 C. Dual element components matched to 25% Fax Data SheetsData sheets describing individual products or families of related products are available. The copy on this WWW site is as up-to-date as the fax version. If you want a faxed copy of a data sheet please contact your local Silonex office. :13 AM #1 georgehifi diyAudio MemberJoin Date: Sep 2003Location: Manly (Gods Country)Lightspeed Attenuator a new passive preamp Ive played with every concievable passive pot, Bournes, Alps Blue Velvet, Alps Black Beauty, Penny&Giles, Dact Switched resistors, and transformers. Nothing has beat the sound of a series/ shunt LDR arrangment in the attachement, it has no contacts in the signal path, I believe that is the secret. The only stipulation that all these passive controls including the series/shunt LDR need I believe is, 1: Low source impedence (cd player) 50k. Cheers George Attached Images _Avatar : Production Lightspeed A Last edited by georgehifi; 23rd August 2009 at 01:51 AM. 25th May 2006, 08:27 AM #2 gfinlayson diyAudio MemberJoin Date: Jan 2006Location: Aylesford, Kent, EnglandGeorge,Im looking to build a high-quality passive preamp in the near future. I like the LDR idea. Can you provide any more information on the design and components used?Thanks,Graeme 25th May 2006, 08:50 AM #3 georgehifi diyAudio MemberJoin Date: Sep 2003Location: Manly (Gods Country)Use a dual 100k log or linear pot to supply the voltage to the leds, one gang of the pot increases the series ldrs led voltage while the other gang in reverse lowers the voltage on the shunt ldrs led, this way you have a constant input and output impedence of the passive, regardless of where the voltage control setting is. I believe its the perfect passive volume control.You can put an LRD in one end of a tube and a Led in the other end x 4, you have to sort through a few ldrs to get 4 that track together for good channel balance, the last thing you want is to have a balance control, but even thats possible if you use two single 100k led voltage pots instead of a dual.Cheers George _Avatar : Production Lightspeed A 25th May 2006, 02:34 PM #4 st2_998 diyAudio MemberJoin Date: May 2006Quote:Originally posted by georgehifi The only stipulation that all these passive controls including the series/shunt LDR need I believe is, 1: Low source impedence (cd player) 100k. Thanks George, Ima very bad at electronics, so Ill try to sort it out. For example, I dont remember if LDR increases or lowers resistance with increasing light.This type of control perhaps should be put after a buffer (attenuating buffers self-noise) but I understand that impedance matching would be wrong, isnt it? _Luca 25th May 2006, 10:42 PM #5 georgehifi diyAudio MemberJoin Date: Sep 2003Location: Manly (Gods Country)You should look for a LD