Grado3 MI Cartridge Compensator

[Craig Maier]

D.J. Bohn, who is a member of this forum asked if we could provide a compensator to produce maximally flat response over the audio spectrum (20 Hz to 20 kHz) for his Grado3 MI phono cartridge. He created a 48 kHz sampled file at 16 bit resolution through an RIAA preamp made by P.S. Audio which has an input termination resistance of 47 kOhms. The source recording was a test record made by Hi-Fi news (Analogue Test LP). He used Pink Noise at -20 dB both left and right channels. I found a bit of a kink in his file near the top octave of the spectrum and corrected it using the Paragraphic EQ. Here are some screen shots showing the raw file and the corrected file as well as the preset. Note that the top graph is the response of the raw file and the lower graph is of the compensated file. (note: that small residual spectral spike around 16 or so kHz I suspect is the self resonance of the cartridge coil inductance and its distributed capacitance).

Craig

[Craig Maier]

Here is the preset to obtain the compensated response curve.

[Marc Hildebrant]

If the output of the Grado cartridge was passed to the RIAA preamp and then sent as the output, could there have been some distortion from the preamp in the picture ?

Marc

[Craig Maier]

Yes, the preamp transfer function is not known in the presentation above. If it was meeting its original performance specification, it was supposed to be within the RIAA curve with an accuracy of +/- 0.5 dB. But, one would have to measure it to know for sure.

[Marc Hildebrant]

D.J. Bohn and Craig

Could you try the same test with a flat preamp (Diamond Cut Productions works well). I have that cartridge from Grado and could use the results.

Marc

[Craig Maier]

DJ's flat preamp is the Tracer unit which is incorrectly terminated on it's input. It has 24 kOhms effective termination resistance rather than 47 kOhms. How do I know? I was provided with one and measured it with a 1 kHz Generator and an oscilloscope and a variable series input resistance. I started with the variable resistance set for zero Ohms and noted the signal amplitude on the scope. Then, I tuned the input resistance until I hit the -6 dB point on the output of the preamp as seen on the scope. Then, I measured the variable resistance value with an ohmmeter which came up to be 24 kOhms (it should have measured 47 kOhms). To confirm, I opened the box and reverse engineered the circuit and sure enough, the equivalent input resistance value calculated out to be 24 kOhms (the Thevenin resistance reflected to the input terminals of the preamp (proving that Ohms law still works).

Craig