DESIGNING A PHONO PREAMPLIFIER
Part IV: RIAA
In the concept development phase we explained our reasons to go with passive RIAA compensation. We are talking about passive filter circuit with a carefully tailored output characteristic. There is almost an infinite number of R, L and C combinations that can give us the required response. Two most common (and simplest) configurations are shown below.
Even though both circuits will perform well, there are some minute differences. Circuit "A" will have capacitors C1 and C2 working in series at the high end of the frequency bandwidth. That means that their parasitic inductances and resistances will add up. Everything else being equal, circuit "B" does not have that drawback, having only C1 connected directly across the filter's output and C2 in series with R2, making C2 parasitics negligible in comparison the required high value for R2. So, we will go with circuit "B".
The filter circuit is really simple, indeed. Filter response is relatively easy to mathematically derive, however, equations in their closed form are pretty long (unlike those defining gain of the op-amp circuits). Luckily, should you decide to experiment with your own design, there are few good online calculators that work with pretty good.
Again, there is a large number of possible combinations (and online calculators make it easy to try many of them), but final choice will depend on circuits that are being connected at the input and output of the RIAA filter. Ideally, signals brough to the input should come from a source with a very low impedance. The filter's output should, ideally, be connected to an infinite impedance. Inevitable realities of practical circuits (non-zero input and non-infinity output impedance) will modify filter's response, it is our challenge to minimize the effects (or to qualify and compensate for them).
In our case, the filter will see very low impedance the op-amp output (few ohms) and filter output will be connected to the op-amp input (few hundred million, or even a billion ohms). Selecting R1 value to be about thousand times higher than op-amp's output impedance is a good compromise between filter accuracy and noise (high resistor values would help accuracy, but they will also have higher thermal noise, so it is on designer to pick his poison.... or how he mixes them).
Other than that, we need to be aware that calculations will give us results to few decimal points. Component vendors need to be more practical, though, so they make components with standardized values. We need to select and calculate values that are close to what is available on the market, or close to a combination of standard values (by adding components in series or parallel) and, if still not perfect, to understand and qualify effects.
Examining these effects, along with effects of component tolerances are the next steps we need to take.
Starting with assumptions outlined above, we ended up with the following ideal filter:
Voltage source V10 is an ideal source with zero output resistance and RL2 is our load, which is going to be very high.
Resistor R10 and C20 came out right on the standard values, but R20 and C10 were a bit different. Let's see how practical considerations affect this circuit:
Our voltage source is less than perfect, with 50 ohm output impedance (by proper selection of op-amp we can go lower than that, for now let's play safe). R20 got replaced with two standard resistors connected in series and C10 became C1 with standard value of 220nF (0.22uF).
Now seems to be the right time to fire up the circuit simulation software and speed up our analysis.
Perfect RIAA has a perfect response, which is our reference (the pictures below may not be great, so I'll provide links for PDF versions and clicking on the pictures will have the same effect):
We are examining very wide frequency range (10Hz-100kHz) just to make sure there are no surprises once we start looking at real-world imperfections.
Considering that we had to use standard component values instead of mathematically correct ones, we can expect that there will be deviation in the RIAA response. We expect (hope?) differences to be very small. The picture below shows deviation of the practical circuit from the ideal RIAA curve (link to PDF):
Assuming standardized component values, maximum deviation from the perfect RIAA curve will be 0.07dB at about 200Hz. Looks pretty good, doesn't it? Our wish list had the accuracy within 0.2dB, looks like our wishes are coming true!!!!
Well.....not so fast..... I don't feel like exercising my marketing skills at this time, there is more work to be done....
Designed, assembled and tested in the U.S.A.
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