Calculating or measuring output impedance

[Thijs]

Hi all,

I've recently completed a Stereomour II build and this thing sounds terrific. I'm using the 8 ohms tap with 8 ohm speakers. I'm looking into custom designed loudspeaker enclosures and sometimes I come across topics about the output impedance of an amplifier when not referring to the tap rating and how this relates to the design of the enclosures and driver selection. Is there a way to measure/calculate the output impedance of the amplifier? I tried a search on the forum but couldn't find anything regarding this topic. (Please correct me if I'm wrong.) Does the output impedance vary or is it fixed?

I hope the above makes some sense.

Thanks!

Best,
Thijs

[Paul Birkeland]

There's a thread that you might find helpful here:

https://bottlehead.com/smf/index.php?topic=7288.15

It's very easy to measure the output impedance of a preamp by just using a pot and a 60Hz tone to measure the unloaded voltage output, then load the preamp down till it is putting out half of what you started with.  The value of the resistor that causes the output to drop by half is your output impedance.

In a transformer coupled amplifier, things get a little more vague.  In general terms, we aim to load a triode in a single ended amp with about 3X the tube's plate impedance.  For the Stereomour, the plate impedance is about 900 ohms at the operating point (I think, I don't remember the exact value) and the transformer is a 4K primary, so we are just a little more than 3.  For the purposes of designing a speaker to work well with zero feedback single ended amps, assuming the DF is 3 is not a bad idea.

[Thijs]

There's a thread that you might find helpful here:

https://bottlehead.com/smf/index.php?topic=7288.15

It's very easy to measure the output impedance of a preamp by just using a pot and a 60Hz tone to measure the unloaded voltage output, then load the preamp down till it is putting out half of what you started with.  The value of the resistor that causes the output to drop by half is your output impedance.

In a transformer coupled amplifier, things get a little more vague.  In general terms, we aim to load a triode in a single ended amp with about 3X the tube's plate impedance.  For the Stereomour, the plate impedance is about 900 ohms at the operating point (I think, I don't remember the exact value) and the transformer is a 4K primary, so we are just a little more than 3.  For the purposes of designing a speaker to work well with zero feedback single ended amps, assuming the DF is 3 is not a bad idea.

Paul,

This is great! Thank you so much! As luck would have it the topic you posted happens to discuss the exact same driver that I’m currently considering to pair with the SII. Awesome!

Thanks again!

Regards
Thijs