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Introduce the principle of high-fidelity power amplifier

2015-01-03 14:24  
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This article is to introduce the principle hi-fi amplifier. The principle of combining text comprehension is a good suggestion. The circuit also adds the ability to adapt to the quiescent current, and the transistor amplifier will provide 100 Designation of w into 8 ohms, the maximum voltage of / -42 v. ? It is very easy to get supplies from the 30-0-30V  transformer . Connection provides a SIM (voice damage monitoring), there is a resistor (R17) added to allow bridging. This resistor is connected to the output of the other amplifier (master). Use to 4 ohms (including bridging into 8 ohm load), does not exceed / -35 v (from 25-0-25V transformer). ? Most applications will be satisfied with a lower voltage and reliability assurance speaker with almost any load. ? In bridge mode, this amp will happily produce 200 w into 8 ohms, and will do so reliably even continuous high power levels. ? Never attempt to operate the amplifier into 4 ohms in bridged mode, because it represents the equivalent of a 2-ohm load to each speaker. VR1 is used to set the quiescent current, under normal circumstances it would be about 100 ma. ? Amps will happily work at low currents, but the distortion began to be noticed (in a distortion meter) less than about 40 ma. ? Driver (Q4) of Grade A as a constant current load bootstrap circuit R9, R10 and C5. ? Stability is made C4, this should not diminish the value of the hat. With fast output transistors as specified (2 and 2 sa1302 sc3281), power bandwidth will be good to 30 kHz.

 

 

Figure 1
Figure 1 – Amplifier Schematic

 

Gain27dB
Input Impedance24k
Input Sensitivity1.22V for 100W (8 ohms)
Frequency response110Hz to 30kHz (-1dB) typical
Distortion (THD)0.04% typical at 1W to 80W
Power (42V supplies, 8 ohm load)2….90W
Power (35V supplies, 8 ohm load)370W
Power (35V supplies, 4 ohm load)120W
Hum and Noise-73 dBV unweighted
DC Offset< 100mV

Notes

The frequency response is dependent on the value for the input and feedback capacitors, and the above is typical of that when the specified values are used.? The high frequency response is fixed by C4, and this should not be changed.Operation into 4 ohm loads is not recommended with the 42V supplies.? Peak dissipation will exceed 110W in each output transistor, leaving no safety margin with typical inductive loads.? All supply voltages are at full power – your transformer may not be capable of maintaining regulation, so power may be slightly less than shown.This figure is typical, and is dependent on the regulation of the power supply (as in 2, above).? Worst case power with 8 ohm loads is about 50W, but the supply needs to be fairly ratty for the power to drop this low.

 

 If the temperature continues to increase, the heatsink is too small.? This condition will (not might – will) lead to the destruction of the amp.? Remove power, and get a bigger heatsink before continuing.? Note also that although the power transistors are mounted to the board, never operate the amp without a heatsink – even for testing, even for a short period.? The output transistors will overheat and will be damaged.

 

Before describing a power supply, I must issue this …

WARNING:Mains wiring must be done using mains rated cable, which should be separated from all DC and signal wiring.? All mains connections must be protected using heatshrink tubing to prevent accidental contact.? Mains wiring must be performed by a qualified electrician – Do not attempt the power supply unless suitably qualified.? Faulty or incorrect mains wiring may result in death or serious injury.

A simple supply using a 30-0-30 transformer will give a peak power of about 100W into 8 ohms, or 90W or so continuous.? This is influenced by a great many things, such as the regulation of the transformer, amount of capacitance, etc.? For a pair of amps, a 300VA transformer will be enough.? Feel free to increase the capacitance, but anything above 10,000uF brings the law of diminishing returns down upon you.? The performance gain is simply not worth the extra investment.

Figure 2
Figure 2 – Power Supply

For the standard power supply, as noted above I suggest a 300VA transformer.? For 115V countries, the fuse should be 6A, and in all cases a slow blow fuse is required because of the inrush current of the transformer.C1 must be rated for 240V AC (or 120V AC) operation – do not use standard 250V DC caps under any circumstance, as they will fail, and R1 will explode!? This is not intended as humour – this is fact!? C1 and R1 may be omitted in most cases, and if you cannot get a mains rated capacitor I suggest that you don’t install these components.The supply voltage can be expected to be higher than that quoted at no load, and less at full load. This is entirely normal, and is due to the regulation of the transformer. In some cases, it will not be possible to obtain the rated power if the transformer is not adequately rated.The bridge rectifier should be a 35A type, and filter capacitors must be rated at a minimum of 50V.? Wiring needs to be heavy gauge, and the DC must be taken from the capacitors? – not from the bridge rectifier.

 


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