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Apart from a very low input impedance, what other design elements are needed for "current drive"? I would imagine you need a first gain stage with high Gm, as opposed to high mu, but...
Thanks.
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Hey,
Essentially what is needed is a current amplifier or an I/V converter and a voltage amp. The general thought is to load the cartridge with a dead short to turn it into a current generator rather than a voltage generator and then amplify the current it outputs.
dave
such as I, the jargon can get fairly confusing. For example, a very LOMC cartridge with a very low internal impedance, is it not per se primarily a current generator? So, the downstream circuit does not attempt to turn it into a voltage generator by placing a resistance in parallel with its output. Is that the deal? Thanks.I am guessing that the RIAA correction is done downstream from the current-amplifying stage.
Edits: 05/10/16
The term is unfamiliar enough to have several plausible meanings. Do you have a circuit, or even a description? Does the term in this case mean a current output or a current input?
Transconductance (gm) is output current divided by input voltage; transresistance is output voltage divided by input current.
"Current drive" or "current driven" is a term coined by some makers of phono stages, lately. I've read about several such devices. I agree, the term is ambiguous. If it is unfamiliar to you, as it is to me, then I would tend to think it's jargon concocted to infer that one particular design has taken a novel approach, in this case with respect to receiving the output from a low output moving coil cartridge with a low internal resistance/output impedance. Some of these same phono stages are said to "automatically" adjust themselves to the "correct" input impedance to suit the LOMC used to drive them. Thus one need not be concerned about phono loading, or so it is said.
You asked, "Does the term in this case mean a current output or a current input?" Input from the cartridge. Obviously, for LOMC cartridges. Hence, such a phono stage could be said to be "current driven".
Thanks, that clears up the situation a lot.
You are probably correct that it's "jargon concocted to infer that one particular design has taken a novel approach" - I blush to confess I have done the same myself :^)
This can easily be done with feedback, or approximated by driving the cathode (tubes) or the emitter (transistors); it's often done in VHF radio receivers, with a cascode circuit. Usually it's necessary to have a large series capacitor between the cartridge and the circuit. I recall seeing circuits that do this 30 or 40 years ago - nothing new under the sun I guess.
There is a widespread theory that a low impedance load on a LOMC increases the electromechanical damping in a cartridge, helping reduce the high frequency resonance of tip mass and vinyl compliance. With a voltage input, the low resistance also reduces the output of the cartridge, so there's a tradeoff. This approach would get around that problem, and offer the most damping possible.
Your last paragraph is reminiscent of some of the claims I have seen for this type of phono input. Unfortunately, most companies won't show a schematic, so I don't know whether or not they use a series capacitance. That might be a case of the cure being worse than the disease.
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