Software riaa eq

First we add a voltage source to provide an AC signal source that can be made to sweep from 10Hz to kHz. Second we add the network itself; and, finally, we add a voltage meter to the output to see the results. The graph follows the desired curve closely, as can be seen below.

Still as was mentioned earlier, comparing several tens of points to the table is tedious and error prone. We can place the RIAA inverse transfer function, which we have already covered earlier in this article, in series with the output and look for the deviations from a flat output. Now, if we look at just the second voltage meter's output, we will see how close the network matches the desired equalization curve.

Not bad. If you think otherwise, be sure to check the units used on the two axes, 10Hz to kHz, not kHz, and milli-decibels one-thousandth of a decibel or one-tenth of a mB , not dbs decibels. If we change the Y-axis increments to dBs, then the overwhelming flatness is apparent. Decibels differ from ohms and watts and coulombs and meters and kilos in that they do refer to any fix quantity, but rather they represent a relationship between two fixed units, such as volts and watts.

Unfortunately, this equalization network seldom used with an infinite load impedance or a zero-ohm source impedance. Well, two FET-input op-amps do come close, but even in this case a coupling capacitor and ground resistor must be added to prevent the first op-amplifier's DC offset from being amplified. How much influence will the added capacitor and resistor make to the equalization curve? Why didn't we add the actual op-amp models to the circuit?

If you examine the actual models provided by the op-amps' manufacturers, you might be shocked to find that the models don't look anything like the op-amps' actual schematics, with the models consisting of only a few transistors and a few resistors. What is going on here? Where are all the cascoded input stages and current mirrors? The answer is that the model only has to give the same results as the actual op-amp, not define all of its hundreds of parts, which would only slow down or even possibly halt the SPICE engine's execution, particularly if the schematic held hundreds of op-amps.

The preamp's results are not too bad, being only 1dB off at 10Hz. By the way, if you think the capacitor is the culprit here, your blaming the wrong part. No, the real culprit is the 1M resistor, as it effectively reduces the 75k resistor's value to 69k, which throws the network's time constants off. Overcoming this problem requires using a source impedance of 6k or adding the 6k to the 75k to yield a 81k resistor. In the case of the op-amps, the output impedance is usually well under ohms, so the latter method works better, but with tube circuits, the source impedance might be even higher than 6k, which would require using a grid resistor even lower than 1M or reducing resistor R 1 's value.

For example, a 12AX7 used in a grounded-cathode configuration with a k plate-load resistor results in an output impedance of roughly 44k 62k in parallel with k.

If we reduce resistor R 1 's value to 37k, the output returns to close to flat, but with a slight bump in low bass. What would be the optimal value for resistor R 1? Here we can enter a beginning value for R 1 and incrementally add ohms to it until 39k is reached. After we run the test, we look for the flattest graph plot. In this case, the bold green line looks about as good as we can hope for. This line represents the 14 th sweep, so the value of R 1 must be 14 times ohms plus 36k We can increase the accuracy of our circuit analyses by adding the grid-stopper resistor and the Miller-effect capacitance.

Because the grid-stopper resistor's value is so low compared to R 1 's value, this resistor's effect is minor at audio frequencies. And the Miller-effect capacitance just adds to C 2 's value, so using a C 2 value that accounts for the added capacitance is a good idea.

Of course, some readers are going to want more than just the modeling of the equalization network; they will want the whole circuit. Well, here it is. Shown above is a complete 12AX7-based phono preamp that uses passive equalization. The plotted "Gain" line is custom defined in the "Edit Signal" dialog box. Here we specify that Gain equals the signal leaving the pre-emphasis transfer function divided by the input signal's magnitude and then this result has 20dB subtracted from it to reference Gain to 1kHz.

Why not just plot and display the raw output signal instead? Furthermore, the graph displays the Y-axis in relation to a 1-volt output signal, and as the input signal was only 0. For example, in the graph below taken from the second voltage meter's output , the gain at 1kHz appears to be dB down, which it is relative to 1Vac, but not the actual 1mV input signal.

The old pros would quickly work out in their heads that 0. The two circuits below embody the desired functions. The first circuit defines the shelving function that starts flat at DC and begins to fall off at 50Hz until Hz is reached, where thereafter it returns to flat but attenuated by dB. This makes sense, as at DC, capacitor C 1 represents an open circuit and there is no attenuation of the DC signal. And at infinitely high frequencies, the capacitor represents a dead short, which makes a voltage divider out of resistors R 1 and R 2 , with the signal reduced to one tenth of its starting value.

Notice how all these time constants match the ones we saw in the complete passive RIAA equalization circuit. How well do these two networks define the composite RIAA equalization curve?

The following circuit sets out to test the accuracy of cascading the two sub-circuits to create a complete RIAA equalization curve. A custom SPICE modeled unity-gain function could be defined to mimic a perfect buffer, but the BUF04 is more than good enough in this test, as can be seen below.

Now that we know that the two circuits will work, how do we go about implementing them? The order that they appear in the circuit does not matter to the curve realization, but it might matter to the preamp's overload and noise characteristics.

This is useful if you want to use a 'flat' phono preamp, but it is also valuable when recording 78s, or other older pressings which don't use a standard curve. In such cases, the RIAA equalisation applied by your phono preamp will be incorrect. VinylStudio can 'reverse' the equalisation applied by your preamp and then apply the correct curve. You don't need to do anything special to accomplish this, just enter the correct settings.

One straightforward test method would be to measure the normal frequency response of the preamp, then compare it to the RIAA standard at multiple frequencies. This approach is complicated by the fact that the RIAA curve does not have a simple constant slope, making it very difficult to see deviations by eye. Another approach would be to provide the preamp under test with an input signal that replicates the raw phonograph output of a recording of white flat spectrum noise The RIAA equalization of the preamp under test should then result in a flat measured response spectrum.

Deviations are then easy to spot by eye, and production tests using Spectrum Limits can use simpler horizontal Max and Min limits. Now go to the Generator controls dialog. Load the Default. GEN setup just to be sure you have a "clean slate". Enter 1M to request 1 MHz, which will be automatically be limited to half of the current sample rate. You should be using the default Hz rate for this test, so the maximum will be Hz. CRV to. Leave Window at the Hann default.

Now toggle the Generator on and set the desired signal level with the F9 volume controls. In Spectrum mode you'll see a noisy but rising spectrum. Use Spectrum Averaging to smooth out the noise for a better view of the curve. Use Exponential mode with a Frames Request of 32 for preliminary work, just to familiarize yourself with what's going on.

For final testing use Linear with a Frames Request of Click on the LO button just below it to apply that curve to the current spectrum.