NelsonPass-FirstWatt-5-pwr-sch维修电路原理图.pdf

First Watt model F5 Operation and Service Manual So far, First Watt has made a few different amplifi ers: Very different amplifi ers. Quite a few people have asked me for a regular sort of amplifi er, you know the kind you plug like any other, with some voltage gain and a real damping factor. Amplifi ers that have low distortion and noise, and will drive a 4 ohm load. The last time people asked for that they got the Aleph J, which satisfi ed most of those requirements. Single-ended Class A, the Aleph J is an easy-going design which is happy driving 8 ohm loads with a warm, relaxed presentation. By way of contrast, I present the F5 (taa-daa!), a push-pull Class A amplifi er, utilizing JFETs and MOSFETs in a very simple two stage complementary circuit a little bit like a complementary version of the Aleph J. But like all the other First Watt amps so far this one is different. In many ways, its an ordinary topology - the basic circuit is found in numerous preamp circuits and the odd power amplifi er (Check out the Profet amp from Selectronics). But the F5 is the product of numerous decisions that set it apart. It has very wide bandwidth, DC to 500 KHz. No capacitors anywhere in the circuit. (except in the power supply, of course!) It has a high input impedance 100 Kohms, and a high damping factor (60) The distortion is very low, between .001% and .005% at 1 watt. Its very quiet, about 60 microvolts or so. It will drive a 2 ohm load without burping, and 1 ohm without misbehaving. Did I mention that it sounds terrifi c? So enjoy. In addition to the normal owners manual information, I have appended the original DIY F5 article which appeared in AudioXpress. Nelson Pass 5/24/08 RadioFans.CN 收音机爱 好者资料库 Setup The initial setup of the amplifi er is very straight-forward. Place the amplifi er in a well-ventilated location, as it draws about 180 watts during operation and requires as much opportunity to cool itself as possible. You should be able to put your hands on the heat sink during operation. If you cant do this for 5 seconds or so, they need more ventilation. On the front panel there are two blue LED lights, one for each channel, indicating power to the channel. On the rear panel you will fi nd a pair of RCA inputs, speaker outputs, a fuse holder, an AC power receptacle, and on/off switch. The label will indicate a serial number and also what AC line voltage the amplifi er is set for. If the voltage is 120 VAC, then the fuse value will be a 3AG slow blow fuse rated at 2.5 amps. If the voltage is 240 VAC, then the fuse will be rated at 1.25 amps. Do not substitute a larger value fuse. Contact First Watt if you have any questions. Im assuming that you know how to attach the speaker cables to the 5 way output connectors provided. Please make all the connections with the amplifi er power switch in the OFF position. For two channel operation, input signal is connected to the RCA inputs. The output connections to the loudspeakers are made through the gold plated brass 5 way connectors. The red (top) connection is positive and the black (bottom) is negative. In this amplifi er the black banded output connectors are connected directly to signal ground. A caveat is in order here this is a very wide band amplifi er with a high input impedance. In order to prevent the output voltage from bleeding back to the input at very high frequencies (thus making a fi ne power oscillator), keep the input and output cables separate, and dont externally connect the speaker ground to the input ground. Good ground shielding on the input cables is important, and caution is called for in using Litz and other specially low inductance / high capacitance cables. I have not seen a specifi c example of a problem, but historically it is to be expected when an amplifi ers bandwidth exceeds 200 KHz. If the amp makes funny noises, runs extra hot, or blows fuses, this might be an indicator of such an issue. If you have any questions, just drop me an email: With everything connected up and the source equipment powered up fi rst, you can proceed to turn on the power switch to the amplifi er. Turn-on and turn-off thumps and noise are small in this amplifi er, and should not present any hazard to delicate drivers. At this point you should be able to listen to music. This amplifi er has less gain than most (15 dB), but at 25 watts, its not likely to need it. If you need to turn the gain up on your preamp, then do so. If you cant get enough gain, then you are probably using either the wrong speaker or the wrong amplifi er. Talk to your dealer if this is the case. The power supply of the amplifi er is isolated from the chassis and AC earth ground by a thermistor which connects the circuit ground to the chassis and earth ground. This helps to prevent ground loops, but the thermistor stands by to conduct AC line voltage to ground until the fuse blows in case of transformer or other such failure. The input impedance is 100 Kohms, and the input capacitance is very low, so you should fi nd it easy to drive with tube equipment if you like. The amplifi er is largely indifferent to the source impedance of your preamp, so a high source impedance is not a problem. The amplifi er requires about 1 hour of operation to reach normal operating temperature, and this warm-up time is appropriate for the most critical listening, but is not otherwise an issue. The amplifi ers fi nal adjustments were made after a 2 hours, and the performance difference between that and cold operation is signifi cant. I do not personally see a reason to run the amplifi er all the time, but you can do that if you want to. The power supply RadioFans.CN 收音机爱 好者资料库 capacitors are likely to last about 15 years or so, and while they will slowly dry out just sitting there, they will have a shorter life span with the amplifi er running constantly. Also, at 180 watts it makes economic sense to shut the amplifi er off if you arent planning on using it for the rest of the day. Again, the heat sinks on this amplifi er run fairly hot, and you want to make sure that they get adequate ventilation. They will run at around 25 degrees C. above the ambient temperature, which puts them around 50 degrees in the average listening room. At this temperature you should be able to put your hand on them for about 5 to 10 seconds or so. Now the following is for your protection Do not defeat the AC line Earth ground connection on the amplifi er power cord. It provides an extra barrier to prevent potential shock hazard. Do not replace the fuse with a type other than specifi ed. Do not operate the amplifi er outside in the weather, or in and around water or anything resembling water. If you spill a drink in the amplifi er or if your dog/cat/child urinates on it, turn it off immediately, unplug it, and do not operate it until cleaned by a qualifi ed technician. If something gets loose or rattles around inside or smells funny, or if you cant touch the heat sinks for 5 seconds or so, then turn it off, unplug it from the wall, and contact First Watt. There are no user serviceable parts inside. Do not open the amplifi er, and if you do anyway, dont operate it with the cover off. There are hazardous voltages inside. If you need to change the operating AC voltage, contact First Watt. If you have a problem, contact First Watt. We are much happier helping you solve problems so that we can be certain that its done properly. If you are far away and dont want to ship the product for repair, we will assist your technician with information and parts. Contact:www.F Summary of the nominal specifi cations: Measured at 120 V AC with a 25 ohm source and an 8 ohm load: Distortion 1 watt.001% to .005% 1 KHz Input Impedance101 Kohm Damping Factor60 Output power stereo 8 ohms25 watts 1% THD, 1KHz Voltage Gain15.3 dB Maximum unclipped output+/-20 Volts Maximum output current10 amps Frequency response- .0 dB dB, -1 500 KHz Noise60 uV unweighted, 20-20 KHz Power consumption180 watts Fuse3AG slow blow type, 2.5 Amp for 120VAC 1.25 Amp for 240 VAC Warranty: Parts and labor for 3 years, not covering shipping costs or any sort of consequential damages. Warranty coverage is voided for modifi ed products, and repair of modifi ed products is at our discretion. Copyright 2008 General Amplifi er General Amplifi er Inc. PO BOX 7607 RENO NV 89510-7607 www.fi F5 Power Amplifi er (As printed in AudioXpress, May 08) Nelson Pass 1/31/08 Intro As many of you may know, First Watt is dedicated to exploring the performance quality of small simple power amplifi ers. Over the past four years, fi ve such amplifi ers have been designed as concept pieces and produced in limited quantities. The F1 and F2 explored the possibilities of current source operation with single-stage Class A circuits and no feedback. The Aleph J used JFET devices for the front end of a two stage single-ended Class A amplifi er. The F3 achieved very low distortion using power JFETs in a single-stage, single-ended Class A circuit. The F4 demonstrated that an amplifi er did not necessarily require voltage gain to be useful. You can follow the progression at www.fi Here is the F5. We want to further push the performance boundaries of simple little amplifi ers with a FET two-stage Class A push-pull design. A Quickie Tutorial on FET Amplifi ers One of the aims of these articles is to get people to build amplifi ers, so here is some tutorial material to get beginners going. I have written up some of this material before (“The A75”, Audio Amateur 4/1992), but that was 15 years ago, and maybe it will be helpful to repeat bits of it. All 31 years worth can be found at and related links. Im assuming that you understand the concepts of voltage, current, and resistance. If you already know how a FET works, you can skip ahead. Figure 1 shows an N channel FET, a quantum mechanical black box with three connections. This device is meant to function as a valve, a little bit like a water faucet. In this picture, the Drain (D) of the FET is attached to an electrical power supply, analogous to the pressurized water supply on the other side of the faucet. You can imagine the pipe as wire, and the tank of water as a battery or even a charged-up capacitor. To continue the metaphor, the voltage of the supply is the water pressure, and the water fl owing from the supply is the electrical current. The Source (S) connection of the FET is the output of the faucet. The Gate (G) of the FET is the control pin, and like the handle on the faucet, it controls the amount of electrical current through the FET from the Drain to the Source. For the FET, this control is a function of the relative voltage between the Gate and Source pins. For an N channel FET, raising the Gate positively with respect to Source increases the current fl ow. Yes, I know some of you are thinking that maybe the Source should be on top and the Drain on the bottom, but theyre not. You might not want to call a quantum mechanic if your plumbing stops up. The idea that the current going through FET transistor is controlled by the voltage between the Gate and the Source pins remains the key idea, and if you have that fi rmly fi xed, we can leave the waterworks metaphor behind. FETS come in different types. There are two polarities, N channel and P channel. There are different voltage, current, and power ratings, and different semiconductor processes resulting in JFETs and MOSFETs. In all of them, the current from the Drain to Source is controlled by the voltage between Gate and Source. A FET is a three pin device, and there are three ways to amplify with them. Figure 2 illustrates these with an N channel FET: Common-Source (CS) is the connection which can give us both voltage and current gain in a circuit. The input voltage (shown as a little graphical sine wave) goes to the Gate, and the output is taken from the Drain and appears across a resistor between the Drain and the supply voltage. The Source is grounded, and doesnt show a signal voltage, and thats why its called Common-Source. Note that the output voltage is inverted in phase from the input voltage. Common-Drain (CD) gives current gain only, and is also known as Source follower, because the output voltage across the Source resistor is nearly identical to the input voltage at the Gate. While the Drain is usually attached to a DC voltage value, the AC voltage is ideally zero, and so it is called Common-Drain. Common-Gate gives non-inverted voltage gain only, with the input signal going into the Source and coming out the Drain. The Gate is grounded. Figure 2 only shows what happens to AC signals, but it doesnt illustrate the DC voltage and current values that the FETs need in order to operate. These DC values are often referred to as the bias of the device, and you will hear that word a lot with respect to amplifi ers. The optimum bias values vary by the device and the needs of the circuit. In general, to function as an amplifi er, the FET needs to have a least a few volts between the Drain and the Source. If the FET is an N channel type, the Drain must be at least a few volts positive with respect to Source. If its a P channel type, the drain must be negative with respect to Source. In addition, the Gate of the FET must be placed at a DC value relative to the Source so that the current and voltage of the FET is positioned in the linear region between the extremes of voltage and current somewhere between all the way on and all the way off. It is in the middle ground where the distortion is low. Generally for N channel JFETs, it is with the Gate voltage at 0 or slightly negative with respect to the Source, and for N channel MOSFETs the gate voltage is a couple volts positive. One of the important functions of a circuit is to set up the DC conditions of gain stages so that the devices have stable operation in this region. For every amplifying circuit, there will be a “sweet spot” of voltage and current which will give the best overall performance. Figure three shows some examples of real circuits that illustrate simple Common-Source amplifi ers and the bias voltages and currents that would be typical for them. You can build both of these circuits and they will work. On the left you see an example of a simple preamplifi er stage with a gain of roughly 10 times (20 dB). The JFET is self biased in this circuit: With 5 mA going through the JFET, the gate needs to be at about -.25V with respect to the Source. We put 47 ohms in series with the Source, raising its voltage to +.25V and this conveniently allows us to bias the Gate at 0V DC, or Ground. On the right, we have a power MOSFET set up as a simple power amplifi er, intended to deliver about 1 watt into an 8 ohm loudspeaker. The 16 ohm power resistor performs as a current source to the circuit, and the two 10 Kohm resistors on the input set the DC value for the Gate to about 4 volts get it to conduct a 1 amp bias current. For a