Symetrix 420_ug 电路图.pdf

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1、420 420 Stereo Power Amplifier Users Guide RadioFans.CN 收音机爱 好者资料库 420 Table of Contents Chapter 1Introduction1 Chapter 2Operator Safety Summary2 Chapter 3Fast Setup3 Chapter 4Front there is no voltage across the load. Leaving the output connec- tions alone and inverting the polarity of one amplifie

2、r channel causes one end of the load to swing positively, the other end swings negatively. Thus, the voltage across the load is twice what it would be if the load had one side grounded, which results in four times the power (since the power increases in proportion to the square of the voltage). As f

3、ar as the amplifier is concerned, each channel drives an equivalent load equal to one-half of the load impedance. An 8 ohm load, connected across the two amplifier channels results in each amplifier channel seeing the equivalent of a 4-ohm load. Few users recognize the fact that neither side of the

4、load is grounded when you mono-bridge a stereo amplifier. This means that if you use TS (tip-sleeve) phone plugs for speaker connectors (and lots of folks do), the shell of the plug, which is usually grounded, is not grounded. If the plug should come in contact with a microphone (perhaps via its sta

5、nd.) it is quite possible for the entire sound system to immediately oscillate, usually at full power, which is not healthy for loudspeakers and living things. If you operate any amplifier in mono-bridged mode, take steps to ensure that neither of the output connections can come in contact with grou

6、nd, themselves, or anything else. Driving Loudspeakers Considering the modest output power of the 420, there isnt much to say here. If you are driving multiple speakers (per channel), ensure that the combined impedance of all speakers is 4 ohms or greater (per channel). For multiple speakers, parall

7、el connection is preferable to series connection. Loudspeakers connected in series do not perform the same as the same loudspeakers connected in parallel1. Do it if you must, but only if you must. A Word About Wire Were not going to debate the pros and cons of deuterium-enhanced, 99.999+% purity sat

8、urnian copper litz wire here (wider soundstage, glows in the dark, out-of-this-world high end, $2x106 per meter). Instead, how about one paragraphs worth of common sense? Even for a twenty watt amplifier, your speaker wire should be reasonably large. At a minimum, 18- 420 9 gauge zip cord works just

9、 fine. If you can, 14-gauge wire is even better. Use godzilla-cable if you want. The biggest single thing you can do for your loudspeakers when you pick a cable for them is to minimize the wire resistance. Aside from using silver (which has higher conductivity than copper), sheer physical size is th

10、e way to fly. Remember: the longer the length of speaker wire, the more important the wire size becomes. Driving Headphones Driving headphones is no great feat. Getting them loud can be accomplished two ways: money and science. The money method is simple: buy a big amplifier. If it isnt loud enough,

11、 buy a bigger amplifier. Repeat as long as there is money. The scientific method starts with a volume requirement, and works backward towards the amplifier. To be truly successful (without an unlimited budget), you need to consider the sensitivity of your headphones before you buy them. Usually, wha

12、t most folks want is LOUD. The way you get LOUD is to increase the amount of power delivered to the headphones. There are two fundamentally different types of headphones: high impedance and low impedance. For the purposes of this discussion, high impedance is defined as 600 ohms or higher, and low i

13、mpedance is defined as 200 ohms or lower. Fortunately, there arent many headphones (that are popular in studios) in the in-between region. Now you should realize that although most of these phones say “8 ohms” somewhere or another, what they really mean is “suitable for 8-ohm outputs,” or “these hea

14、dphones are sensitive enough to be driven from an 8-ohm output that normally drives loudspeakers.” The Great Gain Control Mystery The gain/volume controls found on amplifiers are another object of misunderstanding. For the record, the gain control on an amplifier inserts an adjustable amount of loss

15、 (attenuation) into the signal path. Given a constant input signal, varying this loss varies the overall gain, which varies the output signal level, which amounts to changing the output power, which changes the volume (yes!). Note the phrase, “Given a constant input signal.” This means that a given

16、amount of loss, intro- duced via the gain control, can be overcome by increasing the input signal by the same amount. This destroys the myth that reducing the gain setting somehow reduces the power output capabil- ity of the amplifier. The gain control reduces the gain of the amplifier or reduces th

17、e input sensitiv- ity of the amplifier. The maximum possible output level is the same, regardless of the setting of the control, as long as some signal gets through. The setting of an amplifiers gain control affects only the amplification factor of the amplifier and has no bearing or effect on the a

18、mplifiers maximum output capability. 1 When two loudspeakers are connected in series, each speaker “sees” the impedance of the other speaker as its source impedance. Since damping factor is the load impedance divided by the source impedance, in effect the damping factor seen by each speaker is unity

19、. The fact that a loudspeakers impedance is anything but resistive only complicates matters. 420 10 Using the 420Chapter 6 This section is intended for more advanced users. If you are a first-time user, we recommend that you start out by using the procedure found in “Fast Setup.” Block Diagram On th

20、e following page you can find the block diagram for the 420. Please take a moment and take note of the following: The balanced input amplifiers are ahead of the GAIN controls. The TRS and XLR connectors are paralleled with each other. The SPEAKER MUTE switch does not affect the front-panel STEREO HE

21、ADPHONES jack. The front-panel MONO switch is ahead of the two GAIN controls. The mono-bridge mode switch is located internally (refer to Appendix A). Installation The 420 may be installed freestanding or rack mounted. Multiple amplifier installations should allow breathing space between each amplif

22、ier. Single amplifier installations should strive to keep the vent holes in the amplifiers top and bottom clear of obstructions by at least one rack-space (1.75 inches). Installation Requirements MechanicalOne rack space (1.75 inches) required, 12.5 inches depth (including connector allowance). Rear

23、 chassis support recommended for road applications. Allow at least one empty rack-space above and below the unit for ventilation. Electrical117V AC nominal, 60 Hz, 100 Watts maximum. 230V AC nominal, 50 Hz, 100 Watts maximum. ConnectorsXLR-3 female for inputs. Pin 2 of the XLR connectors is “Hot.” T

24、RS female connectors are also provided. The XLR and TRS connectors are paralleled. Operating Modes The 420 has several different operating modes allowing changes in how the GAIN controls operate, mono operation, and mono-bridged operation. Gain Control Options The gain controls may be operated two d

25、ifferent ways: DUAL TRACKING and INDEPENDENT. These modes differ as follows: DUAL TRACKINGThe gain of both amplifier channels is controlled simultaneously by the CHANNEL 1 GAIN control. INDEPENDENTThe gain of each amplifier channel is controlled independently by the CHANNEL 1 and CHANNEL 2 GAIN cont

26、rols. Mono and Stereo The MONO/STEREO MODE switch, located on the front panel, selects between mono and stereo operation. In STEREO mode, the 420 operates as two independent amplifiers. In mono mode, the two input channels are mixed and the mix routed to both channels. The proportions of the mix are

27、 determined by the input signal levels (not the two gain controls). The two GAIN controls determine the output level of their respective outputs. The MONO/STEREO switch is isolated from the input jacks (its position has no effect on signals at the jacks). You can use the mono/stereo switch three way

28、s: To mono-sum a stereo input signal to check for mono compatibility. To mono-sum two unrelated signals (like paging and music). To drive both amplifier channels with the same input signal. 420 11 SSERP PRESS CHANNEL 1 GAIN CHANNEL 2 GAIN CLIP CLIP GAIN CONTROLS OFFSET NULL SERVO OFFSET NULL SERVO S

29、PEAKER MUTE MONO BRIDGE SWITCH DUAL TRACKING (SHOWN MUTED) 100 ohms 100 ohms (4 ohms MINIMUM) REV-B INDEPENDENT INTERNAL ON PCB (SHOWN AS STEREO) MODE SWITCH CHANNEL 1 + OUTPUT CHANNEL 2 - OUTPUT CHANNEL 1 - OUTPUT CHANNEL 2 + OUTPUT STEREO HEADPHONES (SHOWN STEREO) TIP=CHANNEL 1 RING=CHANNEL 2 INPU

30、T 1 BALANCED INPUT 2 BALANCED INPUT 1 UNBALANCED/ BALANCED INPUT 2 UNBALANCED/ BALANCED Figure 6-1. Overall block diagram 420 12 Mono-Bridge Mode As described in Chapter 5, mono-bridge mode uses both amplifier channels to deliver 40 watts into a single 8 ohm load. Some important things to remember:

31、Both sides of the load (loudspeaker) are driven. There is no “ground” as far as the loudspeaker is concerned. Any connectors used between the amplifier and the speaker(s) should be insulated so that neither side of the connector can accidentally contact ground, the other conductor, or any other cond

32、uctor. Ordinary phone plugs are especially problematic in this application (since the design of the plug exposes both conductors and the two conductors are momentarily shorted together when the plug is inserted or withdrawn). To avoid inadvertent operation, the mono-bridge switch is located internal

33、ly; the top cover must be removed to alter the switch setting. Refer to Appendix A for setting instructions. In mono-bridge mode, use the CHANNEL 1 INPUT, and connect the speaker across the two plus (+) output terminals. The channel 1 plus output terminal is the in-phase or non-inverting output conn

34、ection (connect it to the plus side of the speaker). Only the CHANNEL 1 GAIN control is functional. If you connect a pair of headphones to the front panel STEREO HEADPHONES jack, they will receive out-of-phase signals (not harmful to anything except phase or polarity fanatics). In mono-bridge mode,

35、the 420 delivers 40 watts into an 8 ohm load. The minimum load impedance is 8 ohms. 420 13 ApplicationsChapter 7 Here are a few applications that the 420 lends itself to. Recording Studios This is probably the primary use for the 420. Use it to drive the Auratones or the small home-stereo reference

36、speakers. The 420 probably isnt powerful enough to drive most near-field monitors except in very small studios. You can also use the 420 to drive your headphone distribution system (see below). Commercial Sound In commercial sound installations, use the 420 to operate a local system monitor speaker,

37、 or to operate highly-divided zone paging systems. In a church, the 420 can drive a wired hard-of-hearing system or simultaneous translation system. Broadcast Use the 420 to drive the booth monitors, or to ensure that the headphones are loud enough to draw blood. No Fooling Distribution Amplifier In

38、 applications requiring driving extremely long lines, or an extremely large number of outputs, the 420 performs well. In bridged mono mode, the 420 delivers at least +27 dBm into a 600 ohm balanced load. For distribution amplifier applications, refer to Figure 7-1. REV-B Figure 7-1. Distribution amp

39、lifier wiring Ensure that the unit is operating in bridged mono mode. You can drive 37 isolated output connec- tions this way, each to a level of approximately +23.5 dBm. Shorting any one output has no effect on the remainder. For driving long lines, put the unit into bridged mono mode. Drive the li

40、ne (balanced, of course) from the two plus output connections. Tie the ground from the balanced line to either of the minus output connections. At the other end of the line, you may need to terminate the line with a resistor. It may also be necessary to build-out the output impedance of the unit at

41、the sending end. This depends on the nature of the line itself, and whether or not there are transformers or repeat coils involved. 420 14 Headphone Distribution Systems The 420 is ideally suited to headphone distribution systems. There are two basic strategies involved in making a loud headphone sy

42、stem: 1.If you have low impedance phones, then you need high current capability to make them loud. This translates to a modest 8 ohm power rating, like 20 watts. 2.If you have high impedance phones, then you need high voltage capability to make them loud. This translates to a moderate 8 ohm power ra

43、ting, like 100 watts. Although the low impedance phones generally get louder, you cant hang as many (as the high impedance guys) on your headphone amplifier because sooner or later you arrive at the amps minimum load impedance. This probably isnt too serious a limitation unless you are into recordin

44、g orchestras or other large groups live, using one mix. Figure 7-2 shows a typical distribution box. In a typical studio, there might be four or five of these boxes sprinkled hither and yon throughout the studio. Each box has four headphone jacks on it (youre welcome to add more) and interconnects t

45、o the headphone system using mic cables. Figure 7-2. A simple headphone distribution box 420 15 RIGHT(+) LEFT(+) COMMON(-) 321213 COMMON(-) LEFT(+) RIGHT(+) ADDITIONAL CONNECTIONS AS REQUIRED REV-A We use mic cables for this application because there are usually more mic cables hanging around the av

46、erage studio than anything else. TRS plugs or stereo phone plugs short their tip and ring connections momentarily as they are mated or unmated. This can be harmful to the 420 (most other amplifiers, too). Figure 7-3 shows the connections needed at the amplifier end. Notice that all connectors are si

47、mply wired in parallel. Take special pains to ensure that pin 1 of the connectors remains isolated from the system ground (except via the 420). Figure 7-3. Amplifier connections for headphone distribution system The resistors shown in Figure 7-2 isolate the jacks from each other, protect the phones

48、somewhat, and (most importantly) protect the amplifier from shorted cables and channel-to-channel shorts (which occur whenever you plug in or unplug a set of phones). Pick the resistor value according to the impedance of the headphones found in your studio. The maximum number of headphones that can

49、be driven at once depends on the headphone impedance and the amplifiers minimum load impedance. The table below lists maximum quantities for the headphones shown. You can add volume controls and channel switching to the box if you want, but those details are beyond the scope of this (short) discussion. HeadphonesMaximum #Max SPLMaximum #Max SPL 4 Ohm total load4 Ohm total load8 Ohm total load8 Ohm total load K24017512287125 Pro 4X4113520138 HD424525118212121 MDR-75062113410137 420 16 Technical Tutorial This section discusses a multitude of things, all related to getting signals in a