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Posts: 1002
Location: Texas
Joined: December 6, 2009
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"Another method of sorting out if a coupling cap exists is to examine the DC capabilities of the circuit. If a coupling cap is present the circuit will have a timing constant that will prevent DC response. We don't see that in the Circlotron- the output is capable of holding DC levels indefinitely, in fact requires a DC offset control system due to the lack of a coupling cap. So I've always seen this as a 'cake and eat it too' sort of thing; if you have a coupling cap you don't have DC response. This is of course a pragmatic viewpoint, I am sure stricter definitions exist!" I think an important point in the discussion is the relative timescales of the period of the audio signal, on the one hand, versus the timescale of the "pumping up" of the smoothing capacitor every 1/120 of a second. Or, in other words, the relative frequency of the audio signal under discussion versus the mains frequency. But first, it should be emphasised that regardless of the relative frequencies, the audio signal *will* go through the power supply, since there is no other route from the anodes of the output tubes than through the power supplies. This is just basic conservation of charge. The only matter for debate could be how much passes through the smoothing capacitor and how much through the rectifiers and power-transformer secondary. If we consider an audio signal with a frequency that is large compared to 60Hz, then there will be many oscillations of the audio signal between the periodic replenishment of the charge on the smoothing capacitors, which happens every 1/120 of a second. During much of the time between the replenishments, the instantaneous power-transformer secondary voltage will be less than the voltage across the capacitor, and so the rectifier diodes are non-conducting and the *only* route for the audio signal to follow is through the capacitor. It is in series with the loudspeaker. In this regime, the audio path is clearly through the smoothing capacitor, and it is playing a role much like any other coupling capacitor. At the other extreme, if we consider an "audio" signal that is very low in frequency compared to the 60Hz mains (and the DC response you mentioned is the limiting case of this), then there will be many replenishments of the charge on the capacitor during one cycle of the very low "audio" frequency. In this regime, the power supply with its smoothing capacitor is behaving rather differently from a pure coupling capacitor, since the recharging process from the power transformer is now playing a big role during the course of a single very low frequency "audio" cycle. That "replenishment" process doesn't occur with an ordinary coupling capacitor. In practice, the various parameters like the current-supplying capability of the transformer, and the value of the smoothing capacitor, are chosen so that there is very little "sag" of the power supply DC voltage under the load demanded by the amplifier, and there is very little ripple on the DC output level from the supply. Under these conditions, it is therefore the case that the rectifier diodes are non-conducting for the majority of the mains cycle, and that therefore the power transformer is completely "out of circuit" for the majority of the time. Since the fluctuating current (the "audio signal") passing through the output tubes must necessarily pass through the power supply, it follows that it is necessarily passing overwhelmingly though the smoothing capacitors, and not through the rectifier diodes and power transformer secondary winding. (This is especially clear cut for the higher audio frequencies. One could debate a bit more about the "near to DC" extreme case.) Chris
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