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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
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@ct0517 The thread was closed by the author. 
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rauliruegas
9,123 posts
 
Dear @ramtubes : I did not avoid  digital/vinyl but the other way around because first than all I’m a MUSIC LOVER, it’s with I’m " married " not with source media technology or system hardware.

R.
clio09
2,978 posts
 
@uberwaltz: I said there were 3 experts, but I never said I agreed with any of them or that their opinions were the only valid ones. I just happen to feel these 3 people have the most to offer to the discussion and they are people whose knowledge I respect, perhaps more will join in.

Furthermore, as you noted that was my opinion, if it’s a free forum as you say then my opinion is as good as anyone else’s, and as you further state, as long as it complies with forum guidelines, which I believe I have.

Now back to the discussion:

@solypsa mentioned cartridge design. I would like to learn more about the design of a strain gauge cartridge as it seems to me eliminating the EQ is a good thing.
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almarg
8,673 posts
 

Ramtubes 3-1-2019

Here is something to consider. What if we didnt use RIAA or any EQ in making a record. If we recorded a constant voltage sweep from 20 to 20Khz and looked at it under a microscope what might we see? When we played the final product what would a voltmeter connected to the speaker (or elsewhere) indicate from 20 to 20 KHz.

Choose your cutter wisely and state if it is amplitude or Velocity. Even better do both.

In modern times, at least, cutter heads and the vast majority of cartridges are nominally considered to behave as velocity transducers. I believe that you (Roger) attested to that in the deleted thread.

**If** both transducers were to conform to a theoretically ideal model of a velocity transducer, the response of the cutter head (in terms of groove excursion) would fall at 6 db/octave, the response of the cartridge (in terms of output voltage) would rise at 6 db/octave, and the falling response of the cutter head would be cancelled out by the rising response of the cartridge. So in that hypothetical situation, with no equalization having been applied either in the making of the record or in playing it, the voltmeter you referred to would indicate constant voltage from 20 Hz to 20 kHz. And the excursions of the groove on the record, as viewed under a microscope, would decrease in amplitude as frequency increases.

However, Ralph stated toward the end of the deleted thread that the behavior of both transducers differs considerably from those idealized models, and I certainly have no reason to doubt that. I am not in a position to comment, though, on the manner or the degree to which those deviations from idealized behavior would change the results I’ve described for idealized models.

Best regards,
-- Al
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atmasphere
7,068 posts
 
@solypsa mentioned cartridge design. I would like to learn more about the design of a strain gauge cartridge as it seems to me eliminating the EQ is a good thing.
A strain gauge usually employs a power supply which puts a bit of voltage across a sensor usually piezo in nature- similar to a crystal. The power supply voltage is imposed across the sensor, whose resistance changes with the strain imposed on it (in this case by the stylus). That change in resistance with a voltage across it means that the voltage is thus the audio signal.

To say that it does not need EQ is inaccurate. It is more correct to say that will only be slightly off of the required RIAA compensation. It would need less EQ than a magnetic cartridge and with the EQ will still have plenty of output so no phono preamp would be needed.
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