FirstWatt-F1-pwr-sm维修电路原理图.pdf
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1、FIRST WATT F1 POWER AMPLIFIER SERVICE MANUAL RadioFans.CN 收音机爱 好者资料库 Introduction This is the service manual for the First Watt F1 power amplifier. There were only 101 amplifiers built of this design and made available for sale. As none have broken in the field yet, this document is most likely to a
2、ppeal immediately to the audio DIY enthusiasts, and is so structured. Related reading would be the F1 owners manual and the two articles about current source amplifiers posted at and also published in AudioXpress Magazine. The following comments are directly from the owners manual: The F1 is a two c
3、hannel transconductance power amplifier, which is a fancy name for an active current source. A given input voltage results in a particular output current. Ordinary amplifiers are voltage amplifiers an input voltage translates into an output voltage. This is not that kind of amplifier, and as of this
4、 writing, the F1 is the only such amplifier available for audio use. Being a current source, the F1 operates differently with a loudspeaker. A regular audio amplifier supplies an arbitrary output voltage, and the speaker draws current according to its complex characteristic. As such, the current thr
5、ough the loudspeaker is not exactly proportional to the voltage in either amplitude or time. A current source amplifier delivers a precise current to the voice coil of the loudspeaker driver, ignoring the series impedance elements in the circuit, including the wire, the inductance of the voice coil,
6、 the resistance of the voice coil versus temperature all that stuff. This is potentially valuable in that the current passing through the magnetic field of the loudspeaker produces the force and acceleration on the voice coil and cone, and this translates directly into sound pressure. A current sour
7、ce is simply the most direct way of controlling the acceleration of the voice coil. Since most loudspeaker systems are designed around a voltage source, only a small subset of products can take advantage of this effect. In general, these are high-efficiency drivers (ones that produce more than 90 Ra
8、dioFans.CN 收音机爱 好者资料库 dB per watt). Of greatest interest is the performance offered to high- efficiency full-range drivers, where not only is the loudspeaker very efficient, but also covers (or tries to cover) the full audio spectrum through a single radiating surface. Often this translates to delic
9、ate single cone drivers such as the products from Lowther or Fostex, with big motor assemblies coupled to light fragile paper cones. These are the speakers that often dont sound good with “high quality” solid-state amplifiers, most often because the two are mismatched in impedance and wattage. This
10、is a very unusual amplifier that will not sound good with about 98% of the loudspeakers on the market. It requires careful attention to loudspeaker loading to get the best performance. The accompanying white paper “Current Source Amplifiers and Full-Range High-Efficiency Drivers” is required reading
11、. This is a tinkerers amplifier, and experimental tool. If you just want to be a consumer, then buy something else, otherwise be prepared to patiently experiment with your system to get the best performance. It does not often work well with ordinary passive crossover networks the components and thei
12、r locations tend to be reversed. It wont break, but probably wont sound good. If you have an electronic crossover, youre in better shape, at least until I can finish a white paper describing passive crossovers for use with a current source. If being a current source amplifier isnt different enough,
13、the F1 is special in other ways. It uses no feedback to reduce distortion, flatten frequency response or create a low output impedance, and it ignores the voltages that appear across the speaker terminals. It also ignores the wire and the quality of your speaker connectors and so on. The F1 has only
14、 one gain stage, not 2 or 3 or 4 or 9. It operates in pure class A mode, which is the very best, but at a cost of constantly drawing 100 watts per channel and only giving 10 watts to the speaker. This is a very quiet amplifier, with a typical figure of about 100 pico-watts noise. A pico-watt is a tr
15、illionth of a watt. As Class A amplifiers go, this one is referred to as a “balanced single- ended Class A” device, in which a single “differential pair” of transistors are biased by three constant current sources. RadioFans.CN 收音机爱 好者资料库 You can use either the RCA inputs for single-ended input oper
16、ation or the balanced XLR input for operation from a source with balanced outputs. If you use the RCA inputs, then be sure to use the included gold plated U shorting jumpers between pins 1 and 3 of the XLR connectors as shown here, or the amp wont work properly. Some of you will lose these jumpers,
17、but I can send a replacement. Note that pin1 of the XLR is ground, and pin 2 is the + input and pin 3 is the - input. Pin 1 of the XLR is exactly the same as RCA ground, and pin 2 is exactly the same as RCA hot. The outputs of the amplifier are balanced, and operate at a 13.8 Volt DC potential above
18、 ground, just like your car battery. Dont worry, the speaker sees only the difference between them, and not the DC, but dont expect the amplifier to operate properly if you attach either output to ground. The fuse is a 3AG slow blow type, 4 amps for 120 VAC and 2 amps for 240 VAC. No substitutions.
19、Contact First Watt if you have any questions. The power supplies of each channel are isolated from each other electrically, except for a thermistor on each which connects the circuit ground to the chassis and earth ground. In this way ground loops are prevented, as the channels will typically share
20、ground only at the source, but the thermistors stand by to conduct AC line voltage to ground until the fuse blows, in case of transformer or other such failure. The amplifier requires about 1/2 hour of operation to reach normal operating temperature, and this warm-up time is appropriate for the most
21、 critical listening, but is not otherwise an issue, as the performance difference between 5 minutes and 60 minutes is very marginal. I do not personally see a reason to run the amplifier all the time. RadioFans.CN 收音机爱 好者资料库 Here is a simplified schematic of the amplifier: The principle is straightf
22、orward. The circuit is a simple differential pair of gain Mosfets commonly biased by a constant current source attached to their Source pins, with the input signal impressed upon their gates. The voltage difference between the plus and minus input signals causes current to flow in opposition between
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