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Old 8th April 2010, 11:49 AM   #1
ksporry is offline ksporry  China
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Default determining input/output impedance

Guys,
Can anyone tell me how I determine the input impedance of my (power) amp, and teh output impedance of my pre-amp?
cheers,
K
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Old 8th April 2010, 02:23 PM   #2
DougL is offline DougL  United States
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Usually, the input impedance of a tube amp is the grid resistor. It is chosen to be lower than the grid capacitance while being as high as possible to avoid loading the preamp.
Typical values are between 50K and 100k for tube power amps, and 10K or more for solid state. Some tube amps with a pentode input were 250K to avoid loading the preamp.

HTH

Doug
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Old 8th April 2010, 06:05 PM   #3
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Yes. The input of a power amp will either have a grid ground-referencing resistor between the signal hot and ground, or the track of a volume potentiometer (the grid being connected to the wiper) in the same position. The value of this resistor/pot is the input impedence of the circuit, close as makes any difference.

There are mathematical expressions with which to determine the approximate output impedence of the preamp, but the simplest way to determine is to test. Put a largish (470K or so) resistor across the signal output hot and ground, and a signal on the input. Adjust the volume control for, say, 0.5 or 0.1 volts AC on the output. Now replace the 470K resistor with a 10K or 20K pot, and dial down the resistance until the signal voltage is half the original value. Measure the resistance of the load; this is the Zout of the circuit. You can also do this by putting smaller and smaller value resistors across there, but it's laborious. How do I know this, hm?

When you disconnect and connect the loads, turn off the circuit power.

Aloha,

Poinz
AudioTropic

Last edited by Poindexter; 8th April 2010 at 06:07 PM.
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Old 8th April 2010, 06:27 PM   #4
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Default output impedence

If the output of a tube device is not transformer coupled, then you can first order determine the output impedance by looking at the size of the plate resistor. For a transformer output coupled tube, you need to measure the impedence of the primary winding, which is not the same as the DC resistance.
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