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发表于 2004-4-13 23:16:55
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原文的东西。。。。。。。。。。。。
HD83 Circuit Description by Gary Devon
The HD83 headphone amplifier is extremely simple. It uses a pair of ECL83 triode/output pentode valves, one per channel. In each envelope is a signal triode and a small power pentode, this was done for economy as you only needed one valve plus a rectifier to make an amplifier, mono of course! the triode/output pentode valve was very popular in TV sets, radios and record players in the 50s and 60s, the Dansette had one inside. The HD83 circuit is extraordinarily simple, with only one amplification stage before the output stage. In our application the pentode section is connected as a triode, with it`s screen grid connected to it`s anode via a 100 ohm resistor. This was done because we do not need the extra power output which the straight pentode can achieve, and we wanted the sonic purity of the triode output stage. The triode connection still allows around 1 watt of power, which gives enormous headroom on headphones. The input stage is the triode section of the ECL83, this is used conventionally apart from the feedback arrangement which uses the bass compensation scheme where overall feedback is increased with decreasing frequency by virtue of the capacitor, C4 and C7 in the triode cathode, this gives a very strong and solid bass quality. The output transformer and the derivation of overall feedback are a little different to the norm. Here a special tertiary (techspeak for third) winding is incorporated into the output transformer.
The most obvious reason for this is the various connections which are possible with the four separate secondary windings. There are four combinations of these windings making it possible to match headphones from 16 ohm to over 300 ohm. If the feedback were taken directly from the secondary this would mean changing feedback components each time, but with the tertiary winding there is no need. The coupling of the tertiary to the secondaries is very good but not perfect, this shows itself as a leakage inductance which here is very useful. It helps filter RF, stopping it from entering the amp's feedback loop, and in a similar way to the essential inductor on the output of solid state amplifiers it helps keep stability with reactive especially capacitive loads. The power supply is again very straight forward with solid state rectifiers and capacitor input filters, with RC decoupling. |
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发表于 2004-4-13 23:16:13
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