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Summary of Contents for First Watt F1
- Page 1 FIRST WATT F1 POWER AMPLIFIER SERVICE MANUAL...
- Page 2 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 writing, the F1 is the only such amplifier available for audio use.
- Page 3 The F1 has only 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.
- Page 4 The fuse is a 3AG slow blow type, 4 amps for 120 VAC and 2 amps for 240 VAC. No substitutions. 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.
- Page 5 Here is a simplified schematic of the amplifier: The principle is straightforward. 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 the two Mosfets, and this creates voltage and current at the Drains of the devices (the outputs), where the loudspeaker is...
- Page 6 Here is the schematic of one channel of the real amplifier.
- Page 7 Making reference to the channel schematic, we note the following typical voltages: P1 is adjusted so that the Drains of Q1-Q4 operate at 13.8 Volts DC relative to ground. This value is readjusted after a 1 hour warmup, with the heat sinks operating at approximately 50 deg.
- Page 8 This is the printed circuit board layout for the left and right channels. Note that they are mirror images, right and left.
- Page 9 Here is the schematic for the power supply for both channels...
- Page 10 RCA input ground to the preamp and back to the other channel’s RCA input, and also the F1’s AC line ground back to another component’s AC line ground. Each amplifier is tested for proper ground / chassis resistance and high voltage tested to withstand 2200 volts primary to secondary.
- Page 11 Here is the PC Board Layout for the power supply board. Note the placement of thermistors for AC voltage on the left hand edge. (120V – 120V and 240V). All four mounting posts go to chassis ground.
- Page 12 Summary of the nominal specifications: Input Impedance, balanced 100 Kohms, 8 ohm load Input Impedance, unbalanced 50 Kohms, 8 ohm load Output impedance 80 ohms Output power 8 ohms 10 watts @ 5% THD 1KHz Output power 4 ohms 6 watts @ 5% THD 1KHz Output power 16 ohms 9 watts @ 5% THD 1KHz Gain...
- Page 13 Typical performance curves:...
- Page 14 More Curves:...
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