Inverse RIAA revisited


When building RIAA pre amps it is handy to have a tool to check the RIAA response of your brewed phono amps (or any other phono pre amp).

In this thread came along some networks that generate an inverse RIAA response to check the response. However in practice it is not so easy to get this networks more accurate than 0.1 dB, even with 0.1 % resistors and 1% capacitors. Besides that, the network is extremely sensitive to the impedance of the driving generator, giving errors of over 0.5 dB.

With modern audio software and a soundcard, life becomes much easier. I am using ARTA analysis software together with a M-audio Audiophile 192 soundcard. In ARTA you can use a frequency compensation file (.mic file) to compensate for response errors.

The trick is this:

Generate an accurate inverse RIAA response file with Excel. Load this file as an compensation file into ARTA. Now measure the response of your inverse RIAA network. The resulting response is a deviation from the ideal inverse RIAA including deviations from the soundcard itself. Export this response as an ASCII.txt file and rename to a .mic file. Now use this generated response in ARTA as a frequency compensation file. Done...!

Together with the inverse RIAA network you can now measure the deviation of the RIAA curve of phono amps up to 0.01 dB accurate. Even if you use 1% resistors for the inverse network.

Cheers and a Happy New Year ;)