On Wed, 7 Apr 2004 00:20:01 +0200 (CEST), Nomen Nescio
wrote:

rec.audio.tubes
What output impedance should tube tuners, phono preamps, and such line out equipment have?

Robert Casey writes:
What level of impedance is tubed equipment like tuners,
phono
preamps, tuned CD players, and such line outputs are expected to
have? Somewhere around 10K to 50K?

The 10K-50K you mention is the input impedance to the power amp.
Be careful at the low end, as many CD players expect to see at least 10K.

On the output of the pre one megohm is classic, half a meg common,
250K a minimum except under special circumstances.
The resistor used in setting the output impedance forms a filter with
the common protection cap in series with the output (it should be
tuned to 1.6Hz). You can therefore not just select this resistor in
isolation, because it should be high as you want the cap to be small,
and you should further consider that the resistor to ground forms a
voltage divider with the output impedance of the previous stage, so
that too low a value will cut into the gain. The argument and math is
the same as for a grid leak on a small signal stage.

Still pretending you understand basic circuitry, Andre?
The resistor to ground after the output capacitor, in a pre-amp, has
virtually nothing to do with "setting the output impedance"!

In the case of a cathode follower output, you could use a 1 meg
resistor here, and still have an output impedance well under 1k.

In the case of a common cathode output stage, the output impedance is
determined by the Ra of the triode in parallel with its load resistor.
The resistor you mention should still be the least significant term in
the equation.

The rule of thumb is that we want output impedances to be as low as
possible, and input impedances to be as high as possible. 50K input
impedance is common in squalid-state equipment, while 500k is commonly
found in vintage tube equipment.

Preamps with common cathode outputs do not like to see power amp input
impedances down in the 10k-50k range.

In European equipment using DIN connectors, nominal signal voltages
around 250mV were common, whereas 775mV was more common in American
equipment using RCA connectors.

-peter