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2nd e-mail from Grant
Subject: hybrid amps article
The hybrid amps article in Tube CAD Journal, April-May 2001, inspires me to ask you about the topology of a current amplifying (current output linearly proportional to voltage input) hybrid amplifier of the tube input, solid state output type (T>SS). It is probably too late to influence your next hybrid amps article, but you may be interested in addressing this issue in your e-mail section.
I ask about this because I have seen some discussion of how the speaker driver, particularly the dominant moving coil type, is more like an electric motor than a resistor. There is merit in driving this with a power amplifier (power out linearly proportional to voltage in), but failing this goal, a current amplifier also has merits compared to the standard voltage amplifier.
I am keen to experiment with a hybrid current amplifier, but being somewhat of a novice, I need some guidance. Your Tube CAD Journal tutorials are the most understandable and practical that I have seen.
Ideally, I would ask you to comment on the following:--
1) A hybrid T>SS power amp, as defined above (voltage to power) for driving typical speakers.
2) A hybrid T>SS current amp (voltage to current) for driving typical speakers. If #1 above is not feasible, I would appreciate any detail on a
topology that might work here.
3) A hybrid T>SS current amp (current input to current output) for driving typical speakers. The current input is of interest because many DACs have current outputs, so maybe there is potential for tapping directly to the DAC output.
I believe your next hybrid amp article will address single ended T>SS topologies, hopefully with choke loads that are not unobtainable, or
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with an alternative to choke loads. It is the single-ended application that interests me most, and I hope that your response, if you have one, caters for single ended topologies.
Grant S
Adelaide, Australia
Grant, you are no beginner, not if you can frame your questions as well as you do. You obviously understand the big overview, which is much more important than the particulars.
The prevailing model has been that an amplifier should function as a voltage source, i.e. zero output impedance and a perfect step-up ratio between input and output; In other words, a voltage-to-voltage amplifier.
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But as you point out, three other models are equally possible. Voltage-to-power is your first alternative. Here the assumption is that the load has a complex impedance; In other words, it is not a pure resistance. For if the load is a pure resistance, the common voltage-to-voltage (voltage source) amplifier will also serve as a voltage-to-power amplifier. But when the load impedance is as squirrelly as is a normal loudspeaker, the voltage-to-power amplifier will break the fixed ratio between input and output voltage. For example, when the loudspeaker hits its box resonance, the impedance can spike fourfold, which would prompt the voltage-to-power amplifier to increase its output voltage by twofold. (W = V²R; thus 1W and 2R = 2V)
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