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Platenspeler.Com |
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After building three phono equalizers (tube amps of-course) I realized some weeks ago that the platenspeler.com (translated: turntable.com) site did not yet contain much information on RIAA equalization.
And even now, I'm sure there are somewhere on the net much better sources of information, but at least I could not find them, therefore I created these pages which contains the following information:
Page 2: Passive RIAA filters (used in my tube amps)
Page 3: Active RIAA filtering (used in my OpAmp based phono amps)
Well, anyway I hope the information on this page is at least useful to someone else. It better be for me, since I'm planning to build yet another phono equalizer based on either ECC83 or 12SL7GT tubes that will be based on experience with Tweety, yet will contain a filter setup as in Scratch.
Coding/Recording lower frequencies require most space on Vinyl, since the lower the frequency the wider the groove. This could result in difficulties for the stylus to follow the groove without jumping out and at the same way it reduces the recording time on the record (which is not fixed but dependent on the number of grooves that fit on one side). The solution was to reduce the amount of lower frequencies during recording and reverse this process (amplify lower frequencies) during playback.
Higher frequencies are amplified before putting them on the record, and corrected during playback. Since noise is for a large part present in the higher frequencies, so this process makes the signal to noise ratio a lot better.
In the 50's, there was no standard for the correction process mainly because record companies had not standardized the recording equalization. Amplifiers in that time therefore allowed their owner to adjust the equalization parameters. In the 60's the RIAA curve became the standard for recordings.
The picture below is the very well know RIAA reproduction curve, which is found
everywhere on the web. Although it is easy to find this figure, it is less easy
to find how the RIAA curve is defined and how you can check your own designs
against the definition. On the RIAA plot it is easy to see that the curve bends
down at the 50Hz point, goes up at 500Hz and down again at 2122Hz. At 1000Hz
there is the 0 dB point which is the reference for all other frequencies: Relative
to this point, lower frequencies are amplified (> 0dB) and higher frequencies
are attenuated (<0 dB). The mid-band, between 500-2122 Hz is critical since
voices are all in this band and our ears are sensitive to these frequencies.
Of-course, when building a RIAA filter the notion of "amplification of
low frequencies" is nonsense. The filter in between two amplifier stages
will not amplify AT ALL, it will merely filter higher frequencies stronger than
the lower ones. It doesn't matter also, since we assume that the 1000Hz point
is our 0dB point and the "amplification"-factor of other frequencies
are relative to this 0-dB point.
In the spreadsheet output above, clearly is shown the RIAA curve in blue and the asymptotic lines that are responsible for the curve: At 50 Hz, 6dB per octave , between 500 and 2122Hz a flat response and after 2122Hz the curve falls again with 6 dB per Octave.
The standard RIAA reproduction filter has one zero (t3) and two poles (t1& t2):
With the formulas given above it is very easy to plot the RIAA correction for
every frequency desired or for a range of frequencies. Of-course I did so, and
the results are available in an Excel
file for those interested, or comparison of your own projects against the
RIAA standard.
Please keep in mind that for this calculation the 1000 Hz point should be at 0dB and therefore you will have to substract the value for 1000Hz from every value you calculate with the formula. Since the calculated value for 1kHz is -19.11dB, substract this value from all other calculated RIAA values and you will reproduce the perfect RIAA curve.
Years after publishing the standard RIAA reproduction curve described above, an enhanced version was proposed. Reason for this proposal was (according to literature) that amplifier equipment at that time was more and more using DC-coupled which meant that low frequencies (turntable rumble) would be reproduced better than before.
The solution was simple: define a 4th timeconstant on the curve and define it as a hi-pass filter. This would make the curve ramp up from 20 Hz and have it's maximum value around 31Hz instead of 20 Hz for the standard curve.
The following figure contains the plot for the IEC RIAA curve for 20-20kHz.
Clear is to see how the lowest frequencies are filtered compared to the standard
curve.
Still, the IEC version has not become very popular, as most manufacturers seem to think that turntables, arm and cartridge combinations should be of such high-quality that the problem should not arise in the first place.
But there is a better way for improving the good'ol RIAA curve. Today, there are many people that deliberately add a second zero timeconstant to the RIAA curve. This is because the cutter of the records will beyond a certain point not be able to get higher frequencies on the record with an extra amplification of 6dB/octave. Record cutter manuals seem to specify 50kHz as a roll-off value. As a result, there must be a fourth timeconstant at 50 kHz that defines how the RIAA curve should flatten out.
For comparing our own phono amplifiers against this "enhanced" reproduction curve, it would be so nice if our spreadsheet would enable comparison against both the standard reproduction curve and the enhanced one.
OK, let's see what happens if we put this formula in a spreadsheet and plot the resulting transfer function in a chart. It is too early to tell what the effect of such a fourth time constant is on the sound of my phono preamps. In the designs for Lookie, Jerry, PhonoClone and GainPre I have included a fourth time constant in the design, but I did not stick to the definition but made the amps behave in between the standard and the enhanced RIAA curve.
As shown in the chart (in purple line), around 10kHz the curve will slowly "bend"
making sure that higher frequencies are no longer filtered 6dB/octave. Hopefully,
implementing this enhanced curve will reveal details in your recordings that
did not appear with your "old" phono preamplifier.
According to literature for a transfer function phase shift is defined as the arctangent of the real part devided by the imaginary part of the equation.
The phase of Loekie, PhonoClone is similar to this ideal phase curve. For filter designs based on the standard RIAA curve without the 4th time constant the phase curve differs for the higher frequencies.
Above you find the resulting phase plot of the enhanced RIAA curve in an Excel
chart. For each of the time constants the phase contribution is plotted separately
and the total phase shift in plotted in bold (purple).
In order to reproduce the signals from our turntable as meant during recording, we have to use a reverse RIAA filter (=RIAA reproduction filter) on the signal. A filter is according to Webster defined as:
A filter is a device that passes electric signals at certain frequencies or frequency ranges while preventing the passage of others.
There are basically two ways for constructing a RIAA filter:
Nowadays, active filtering is amongst audiophiles often considered an inferior way of filtering. However, when working with integrated components such as opamps, there is always feedback involved and therefore in my opinion it is a matter of preference and design. Therefore, use whatever
On the next two pages filter construction is explained for both types of filtering.
Page 2 describes the passive filtering
design and Page 3 describes active filtering
which is often used in opamp designs.
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Page 2: Passive RIAA Filtering >> |
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