Great post Scott. I’ve always used the FR loaded/unloaded and a pile of calculations to determine output impedance and TBH, it rarely lines up with manufacturer specs.
Most impressed, and it seems you’ve basically removed as much uncertainty as you can with short connections right on the outputs.
Hi @VAR, yes, great post indeed!
Hi @VAR. You’ve done a great job. For a few months now, I too have been using the method of taking the signal to measure the DF directly at the amplifier output terminals to prevent the connection wires between the amplifier and the load from heavily influencing the result (especially for high DF) and I can confirm your results. Below is the DF measurement of the same amplifier (a single-ended class A amplifier) taken by sampling the output on the load (a value heavily influenced by the connection cables) and then the same measurement taken by sampling the signal directly at the amplifier output. As you can see, the difference is remarkable. I must say that I then simulated the DF value of the amplifier with Spice (LTSpice) and the results are in reasonable agreement with the second graph (signal taken directly from the amplifier terminals). I wanted to add that the QA403 is a great product.
@Claudio - Thanks for sharing your measurements and the "direct’ method sure makes a difference with the amp- quite a large damping factor for it!
@matt Thanks for a great product and allowing the software to plot damping factor!
@restorer-john- getting a compliment from you made my day. It means a bit more work for my testing, but I’m retired so no big deal, though I am considering just keeping that setup connected to my QA402 to simplify things. It is nice to contribute to the forum when possible.
Many thanks to @restorer-john who found an error in my schematic shown earlier:
The switch needs to be in series with the 8R resistor not in parallel.
Your schematic means the 8R load is always connected and actually shorted out when you throw the switch. I know that’s not what you meant to show, but some people might not get it and short out their amps…
