Re: MC PHONO PREAMP

Patrick Turner wrote:

Ian Bell wrote:
Patrick Turner wrote:
John Byrns wrote:
In article <gp8rn5$vsl$1@xxxxxxxxxxxxxxxxxxxxx>,
Ian Bell <ruffrecords@xxxxxxxxx> wrote:

John Byrns wrote:
In article <gp61p5$uas$1@xxxxxxxxxxxxxxxxxxxxx>,
Ian Bell <ruffrecords@xxxxxxxxx> wrote:

John Byrns wrote:
In article <gp5gil$p22$1@xxxxxxxxxxxxxxxxxxxxx>,
Ian Bell <ruffrecords@xxxxxxxxx> wrote:

Patrick Turner wrote:
But RIAA reduces noise. It allows bass signals to "fit" onto the record
to give use 20 minutes a side from LP.
No it does not. As I said, in amplitude terms, what is recorded on disc
is nearly constant amplitude versus frequency.
That depends on how you define "nearly constant amplitude versus
frequency"? During the recording process the RIAA curve attenuates the
frequencies above approximately 2 kHz by more than 12 dB, not my idea of
"nearly constant".
Mine neither. You are saying that above 2KHz the amplitude of signals on
the disc is attenuated by 12dB or more. That makes no sense to me. Do
you have a reference for this?
Hi Ian,

I don't have a reference handy, although this issue was discussed in
another group a few years back and someone actually found a reference on
the web.

Let me explain, as you said "in amplitude terms, what is recorded on
disc is nearly constant amplitude versus frequency." The operative bit
here is that your stated reference is "constant amplitude", not the more
commonly used constant velocity reference. To change the reference from
constant velocity to constant amplitude we must apply a correction
factor to the "RIAA" recording curve that rolls off across the entire
audio band at a rate of 6 dB per octave with increasing frequency. When
the conversion is made from a constant velocity to a constant amplitude
reference we are left with a recording curve with a pole at 500 Hz and a
zero at 2,122 Hz, ignoring the 50 Hz and ultra sonic time constants.
What this says is that on an amplitude basis the response starts rolling
off at 500 Hz at a rate of 6 dB per octave, and then flattens out again
at 2,122 Hz. Between 500 and 2,122 Hz the attenuation gradually
increases to approximately 12 dB above 2,122 Hz. In other words, the
response recorded on the LP is shelved down approx. 12 dB above 2,122 Hz
relative to the response below 500 Hz, on the basis of recorded groove
amplitude basis.
Got you. It was clear to me there was a kink in the amplitude response
due to the separation of the two mid time constants between which the
response would fall at 6dB/octave. I had not realised they were two
octaves apart. So to summarise, the recorded amplitude response is flat
from 50Hz to 500Hz, then rolls off at 6dB per octave to 2.12KHz where it
is about 12dB down and thereafter is flat out to 20KHz.
Precisely!
Without being able to view relevant diagrams and response curves of real
world hardware, I am precisely confused why John is so precisely in
agreement.

Patrick Turner.
OK, Here goes.

The (amplitude) response of the cutter head falls by 6dB/octave.

OK, so as F rises, there is less amplitude in the grooves, no?.


Yup.

So the bass grooves are the biggest, so if you cut bass and boost treble
somewhere near 6dB/octave, then the groves of any F will all become
constant amplitude.


Yup


The
inverse RIAA curve, which is used on record starts, with a rising curve
from 50Hz to 500Hz at 6dB/octave. The combination of these two means a
constant amplitude gets recorded on the disc between these two frequencies.
From 500Hz to 2.12Khz the RIAA curve is flat, so because of the head
response, the amplitude recorded on the disc falls by 6dB/octave for
about 2 octaves.

But the RIAA has about 5dB difference in eq between 500 and 2,112Hz.....


Yes, I was using the Bode straight line approximations - in practice the real things are curves. In practice 500Hz and 2.12KHz are the 3dB point so if you add 2*3dB to the 5dB you see, you get 11dB. However, there is still 12dB amplitude difference on disc between frequencies removed from the 3dB points.

There is NO flat part on the RIAA recording or playback curves between
say 20Hz and 22kHz.

In practice no but again I was using Bode approximations - see above.

From 2KHz upwards the RIAA record curve again slopes upward at
6dB/octave which, when combined with the head response of -6dB/octave
means the recorded amplitude is constant versus frequency.

So the slight change inthe rate of RIAA attenuation/boost between about
500Hz and 2,112Hz means that there is a change in amplitude between F
ranges below 500 and above 2,112?


Yes


On playback the mag cartridge has a response which rise at 6dB/octave.

The RIAA playback curve falls at 6dB/octave from 50Hz to 500Hz. So the
constant amplitude on disc plus the rising characteristic of the mag
cart means the pre-amp input signal rises at 6dB/octave between these
frequencies and is compensated for by the falling characteristic of the
RIAA replay curve.

Between 500Hz and 2.12Khz the signal from the disc falls at 6dB/octave
which combined with the 6dB/octave rising characteristic of the mag cart
means the signal at the pre-amp input is constant between these two
frequencies and the flat part of the RIAA replay curve between these
frequencies leaves the input signal unchanged.

But preamp input isn't constant between the two F.....

No, but see above.

From 2.12KHz upwards the constant amplitude from the disc combined with
the 6dB/octave rising characteristic of the cart means the signal at the
pre-amp input rises at 6dB/octave which is compensated for by the
6dB/octave falling characteristic of the RIAA replay curve.

Hope that helps.

I hear what you are saying, and basically the faster the cutter has to
move side to side to make a groove then the less it travels.

And the faster the cartridge moves side to side to track gooves, the
higher the amount of voltage output and its all a fairly linear system.

And to avoid having high bass groove swings which would take up too much
room on the record they cut the bass signal to the cutting head and to
avoid treble grooves having too small an amplitude they boost the treble
signal to the cutter. The groove amplitude is thus more or less flat
across the whole range.


That's is basically it.

But the cart then puts out little bass and a huge amount of treble
because its response is linear with F and so the RIAA playback curve
reduces treble and boost bass in reference to 1kHz.


That's right

Disc Noise is
reduced at HF above 1kHz at least, and amplifier noise somewhat more
methinks.



Amplifier noise certainly, disc noise I am not so sure about.

Cheers

Ian

Patrick Turner.



Cheers

Ian
--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/
.

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