I’m interested in the spectrum, and to measure noise from rms level requires eliminating some of the interference spurs which I don’t think I can easily, the inductors are basically efficient very-long-wave antennae.
In the days of vinyl dominance phono stages were tested connected to a Shure M75 cartridge placed in a well-screened enclosure. Shure’s were the dominant cartridge in the day (and claimed to have the best tracking ability), either way it was a ‘standard’ and at least allowed different phono stages to be meaningfully compared. I test my phono stages with the inputs shorted - this allows me a common baseline to compare but better still, lets me compare MM and MC stages.
In your tests adding the inductors boosts the high frequency noise floor as expected, but does this affect the frequency response? I would expect the noise to increase from the combination of input resistance and cartridge/inductor’s varying impedance as you pointed out - but the change is so distinct and large its seems as if the RIAA correction is being affected?
The best place for a Shure M75 is in a diecast box being used as an input load for a phono stage…
Diecast box isn’t going to do much with 50 or 60Hz interference…
Let me share a little of my experiece with phono preamps (been designing them for 50 years). The LF hum will be a major problem if the source and analyzer share a ground. Ideally they are both isolated and connected to power either with very low leakage transformers (no power line filters to ground!!) or a battery on the source. Using a differential input on the analyzer helps but you can still have leakage induced hum and noise. For perspective, if the loopthrough resistance on the grounds is 1 Ohm and the leakage is 1 microAmp you get 1 microVolt added to the ground’s zero reference. A 40 dB phono stage amplifies that 1 uV to 1 mV (60 dB at 20 Hz) which is only 40 dB below 100 mV signal. Any other noise across the grounds will show up this way. Only very careful setup and maybe a floating source (Boonton 1120) or transformer source (Audio Precision) makes avoiding this easier.
Next, use an attenuator on your source. Digital attenuation is nice but 40 or 60 dB of attenuation makes your source into an 8 bit DAC. Use resistors and operate the source closer to its max. If using a digitally corrected source (QA403) you need to plan for the 40 dB difference between 20 Hz and 20 KHz. A passive network fixes this BUT won’t have the same absolute precision. Try to get the atteuator as low resistance as is practical. Its source resistance will define the noise floor. 100 Ohms is 1.2 nV/rtHz. Or for 20 KHz bandwidth .18 uV. With a zero noise 40 dB preamp that will be 18 uV (not correcting for RIAA). or possibly significantly more with the 20 dB boost at low frequencies. For moving coil input I made an attenuator with 100 Ohms total and .1 Ohm output. This is a little extreme but its the general idea.
Source impedance is important and it will interact with the resistive and capacitive load. If you can get an old Shure/Stanton/Grado its enough to see what happens.There is often a resonanace at the top end between the inductance and the input C with moving magnet cartridges. Its actually used to help extend the response.
In the 70’s through the 2000’s the Japanese were really fastidious on their RIAA. Its a parameter that can be measured so they worked hard to get it right. Not so much with American HiFi companies. I was unusual for using custom Teflon caps and Vishay foil resistors.to get the response within +/- .1dB. And even then I did not have the tools to confirm the accuracy then. John Curl had the same challenges for the Levinson products…
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Hello All,
Lately I have been testing Moving Magnet type RIAA pre-amplifiers largely with shorted inputs.
Currently I have a 16Ga steel electrical junction box test bed that has a switched input with shorted inputs, 60R resistor shorted inputs and a Audio Technica AT91 MM cartridge for a reference input.
After I get the test RIAA Box up and operating with the Audio Precision Analyzer I will connect it to the QA403 and post the results.
Tom
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