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RIAA calculation help needed
RIAA calculation help needed
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Old 12th November 2019, 05:11 PM   #51
Mark Tillotson is offline Mark Tillotson
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BTW if each pole and zero is created with a separate RC section, the calculations are easy, but the circuit is more convoluted:
Click the image to open in full size.
The input opamp has 3.75k || 20nF, which is the 75s pole. The impedances are halved to reduce noise, 7k5 || 10nF would also work.


The second opamp is a gain block only. ( sidenote: I synthesize 110F bipolar electrolytic coupling caps - not sure how good that is, the idea is to keep them formed by biasing the midpoint through 22M resistor )


The third opamp stage has (10k + 15k + 6k8) || 10nF, which is 318s zero, and (56k + 56k + 47k) || 20nF, which is 3.18ms pole.

All standard E12 values, only one filter capacitor value, and accuracy of response depends only on the tolerance of the passives. (Assuming one can ignore the high frequency limits of the opamp and compensation caps, and low-freq roll-off of the "bipolar" 110F caps)


This circuit was done as an exercise in what can be done, not claiming its sensible approach (other than to avoid algebra), and it does work in real life. Note the need to protect the opamp outputs from capacitive load / ensure stability, using the 470R / 33pF networks.

The technique does allow isolated variable first-order zeros and poles if the resistors are made variable, which will be useful for something somewhere (probably).
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Old 13th November 2019, 01:23 PM   #52
Bonsai is offline Bonsai  Europe
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Here is an article on Phono amp design and a spread sheet tool that uses the a Lipshitz’s methodology for all active stages (best for lowest noise and overload with opamp based designs).

I’ve had exceedingly good results with this.

RIAA Equalizer Amplifier Design

Here is another one on ‘inverse RIAA testing’

An Accurate Inverse RIAA Network

Last edited by Bonsai; 13th November 2019 at 01:25 PM.
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Old 13th November 2019, 11:32 PM   #53
Mark Tillotson is offline Mark Tillotson
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Douglas Self's "Electronics for Vinyl" covers many examples and topologies of feedback network to optimize capacitor values and size for this approach, highly recommended.


Note that doing everything in one stage is convenient, tends to work quite well, but does push the opamp hard - for instance there's much less open-loop gain available with a single high-gain stage, so its quite likely my step-by-step approach is more tolerant of opamp shortcomings - so long as you boost up out of the noise in the first stage and keep impedances sensible, a multi-stage approach throws more feedback into the mix improving linearity and bandwidth (and thus reducing IM from any ultrasonic junk that's floating around). But its a lot more components - perhaps more suited to a reference circuit than practical, its trivial to invert it by swapping pole and zero sections around and make an inverse RIAA circuit, or by replacing parallel RC by series RC in each section.
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Old 14th November 2019, 06:35 PM   #54
Bonsai is offline Bonsai  Europe
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A good modern opamp has an open loop gain of 10^7 at LF - more than enough to get HF distortion on an RIAA down to low ppm on a single stage design.

I get 50 ppm on an NE5532 which IMV for the 30 or 40c you pay is a fine bargain in terms of noise, distortion and overload capability.

Fact is for opamps, a single all active stage gives you the best of all worlds:- lowest noise and highest overload
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Old 14th November 2019, 06:59 PM   #55
Mark Tillotson is offline Mark Tillotson
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True, especially at these signal levels, but high gain in a single stage should beg the question "are two stages better here?".
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Old 14th November 2019, 10:30 PM   #56
Bonsai is offline Bonsai  Europe
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Distortion is a non issue for any competently designed electronics since the vinyl+cart+arm distortion and noise are 2 orders of magnitude worse than the electronics.


Multi-stage EQ's are definitely easier to design but half the challenge of a single all-active stage is calculating the values (I use a spread sheet) and then tweeking it on LTspice.
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Old 15th November 2019, 05:27 AM   #57
PRR is offline PRR  United States
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Quote:
Originally Posted by Mark Tillotson View Post
.... doing everything in one stage is convenient, tends to work quite well, but does push the opamp hard...
The opamp's open loop gain "echos" the RIAA curve only higher.

For people who think a NFB amp's OLG should be flat across the audio band, this is "perfect" because the gain-margin is very nearly flat all the way to the extremes.

For target 34dB@1kHz, a old '741 opamp has real-slim 5:1 excess gain above 2kHz, which is tons better than the bass gain-shortage of any simple discrete, but not great. And not necessary today. A '5532 has plenty of excess gain all across the audio band. There are other choices which may be better today.

That's not to detract from your very elegant step-by-step design. It sure simplifies part-ratio-matching and ordering. It does need more parts that I like to see (I'm lazy).
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Old 15th November 2019, 03:04 PM   #58
Mark Tillotson is offline Mark Tillotson
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Which is also the reason integrators (and thus state-variable filters) are fairly tolerant of low-spec opamps. RIAA is a somewhat modified integrate step after all.
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