Amplifier output impedance measurement

[tube fanatic]

When it comes to tube amps, many cite their typically high output impedance as the reason they can't possibly sound good (even when they have never heard one).  This really excellent video explains two ways of measuring it which are quite accurate.  It's a bit technical, but will help to clarify what it's all about.

 

https://www.youtube.com/watch?v=ieAhBejHe2M

 

Maynard

[Guest thesloth]

Here is an other way.

 

This method will use two 10 ohm resistors that are easily found at radioshack cheaply and your average multimeter.

 

The equation is: (V1-V2)/((V2/R2)-(V1/R1))

 

V1 is 1V and R1 is the 10 ohm resistor. V2 is the measured voltage and R2 is 5 ohms (two 10 ohm resistors in parallel)

 

 

Place the 10 ohm resistor at the amps output and input a 60Hz sine wave (the meter will be most accurate at 60Hz).

 

Increase volume until you reach 1V output across the 10 ohm load. Now place the other 10 ohm resistor in parallel effectively halving the load resistance and note the voltage, Lets say we measured .8V

 

Plug the numbers into the formula: (1-.8)/((.8/5)-(1/10))= 3.33

[Scott Grammer]

Push-pull tube amps that use negative feedback, such as, for instance, the Dynaco Stereo 70, can have fairly low output impedances. It's single-ended amps with little or no NFB that tend to have high output impedances, but this is not necessarily a problem, especially with speakers designed with higher source impedances in mind. Some speakers really open up in the bass with a higher source impedance. Unfortunately, some speakers have a problem with these amps, or more accurately, some of these amps have a problem with speakers that have a highly reactive load, as their output will tend to rise and fall with the speaker's impedance.