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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: thanks! posted by MikeCh on March 13, 2002 at 13:55:38:
In addition, I assume a transformer can't drive a load (preamp/amp/interconnect cable) as well as an opamp, which generally has very low output impedance.Not necessarily, no. While an opamp in and of itself typically has an output impedance orders of magnitude lower than any transformer, you typically don't find opamps driving the line directly. Typically it's driven through a series output resistor often around 100-300 ohms. It's there basically as a current limiter to reduce the current draw from the opamp at higher frequencies where the reactance of the line's capacitance becomes less and less.
So if you're using a proper output transformer such as the JT-11-DMCF whose primary and secondary DC resistances are only 40 ohms each, you'll only be adding 80 ohms to the output impedance which isn't terribly significant. And of course if you just lower the value of the buildout resistor on the opamp the same amount, it's not of any significance and you break even.
However if you go and use an input transformer such as the JT-11P-1 as an output transformer, that's another matter. Now you've got a transformer with much greater primary and secondary DC resistances and your output impedance, even if you were driving it directly from the opamp, will be over 2k ohms. Considerably worse technically, though no worse than a passive "preamp" using say a 10k attenuator.
Anyway, these are just the technicalities. What one ultimately chooses to use will depend on what their goals are.
se
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- Re: thanks! - Steve Eddy 14:43:13 03/13/02 (0)
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