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RIAA, Questions only please

I have closed the previous thread on RIAA and concluded that very few indeed understand the curves or the purpose. Here is my closing statement from that thread. For those who want to understand and have valid well stated questions I am happy to answer. 

Not wanting to leave the party without a clear and accurate statement I will say the following:

The answer to the question concerning noise reduction is that the simple filter that RIAA decided upon was to raise the high frequencies gradually by about 12 dB starting below 500 Hz, being up 3 dB at the 500 Hz pole. The circuit then cancells the pole with a zero at 2,200 Hz and there is then 3 dB of boosting left as one goes to 20 Khz. It is all done very gently with just two resistors and two capacitors.

By reversing this process on playback we get to enjoy 12 dB less noise above 500 Hz.

The RIAA part of things is the same for all cartridges. However we are accustomed to seeing RIAA combined with the 6 dB/octave compensation for a velocity cartridge. That takes off 12 dB, and along with two things that happen at the very ends of the response, brings the total EQ for a velocity cartridge to 40 dB. Next time you look at an RIAA curve ask yourself why there is that flat bench between 500 and 2,200 Hz.

An amplitude cartridge needs only the RIAA EQ of 12 dB. Which also speaks to the fact that the majority of the spectrum of a record is cut at constant amplitude. When you put a sewing needle in a paper cup and play the record you are getting amplitude playback not velocity.

I study these things because they interest me. Anyone can look up the parts values to make an RIAA filter or inverse RIAA. What interests me is that some manufacturers still get it wrong.

128x128ramtubes

Showing 1 response by johnss

johnss
291 posts
 
with all the discussion on the RIAA curve, would suggest real vinyl philes consider the following; test your LP playback set up and see if the playback response is flat according to the RIAA spec.

To do this test, you will need a NAB RIAA playback disc or similar, and a dbm meter or volt meter.

play each freq on the disc from 30 hrz to 18khz. Write down the  DBU or voltage output on your phono stage for each test frequency. you may have to repeat the test until you get the hang of it.

 Transfer or copy the voltage or DBU data into excel and generate a plot.
Scale the Y axis accordingly.

You will quickly see what the true response is for your phono cart, phono stage and turntable set up.

Learned this from an old broadcaster engr friend.

Have measured over a dozen phono/table combinations; some are close but none I have measured were completely flat. Rising top end among MC carts is quite common. This is also a much better way to establish MC cart loading than doing it by ear. Once you have two measurement points with the known cart loading resistor values, you can run a simple calc to estimate what the desired R value is to bring your MC cart into a flat range. 

if you can tweek the values to get the response within 1-2 db across the whole freq range, you will be shocked at how much better your records will sound. Much more like high speed tape....

best

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