We've seen how the RIAA filter behaves with low and very low capacitor values. (See all simulation articles here) Now let's have a look at what happens if the capacitor values are higher than the recommended ones.

This is the Muffsy Phono Preamp with capacitor values that are 2.5% higher than specified.

As with the simulation with lower than recommended capacitors, this shows that we're still following the RIAA curve quite nicely even though the capacitor values are slightly wrong.

Now what happens if we bump the caps up to 10% higher than the recommended values?

The lowest level is -0.535 dB, and the highest is +0.186 dB. The average deviation is still only +/- 0.36 dB. There will be a slight attenuation of mid bass and the higher frequencies though, something that will become apparent if we really try to exaggerate by changing the capacitor values to +25%:

All the mids are attenuated and so is the treble, while the low bass is amplified a bit. +0.44/-1.16 dB is not something you'd want in your phono stage under normal circumstances.

A Conclusion...

The best scenario is to use the specified capacitor values, or within +/-10% of these. 

Deviations that are extremely low will give you a slight "loudness" effect, while extremely high capacitor values will attenuate the mids and the treble. Luckily, these extreme values won't enter your phono stage unless you do it on purpose.

These are also all good examples of what hides between the RIAA compliance numbers, that vendors seldom show in detail.

I wanted to know what happened to the Muffsy RIAA compliance if the capacitors were out of spec, and started with a plausible scenario where the capacitors were 3% below their stated values.

This time, lets see what happens if the capacitor values are extremely low. Let's say they're 10% below what it says on the capacitors. Like last time, the simulation goes from 20-20.000 Hz.

Clearly, the graph is a little more wiggly. But how bad is it? As it turns out, not too shabby at all.

The lowest deviation from the RIAA curve is -0.16 dB, which is all the way down at 20 Hz. The highest deviation is +0.55 dB, at around 19-20 kHz. Even though this could be read as if though the bass is lacking, notice that it is spot on again at 30 Hz.

What this simulation tells us is that you'll get a small boost in the bass and treble regions, an ever so slightly, very small loudness effect. Although this scenario isn't that probable, it could still happen as most of my measured capacitors are on the low side in the tolerance range.

So what happens if we take this to extreme levels? Pretty much more of the same. Below is a picture of the RIAA equalization circuit that is used in the Muffsy Phono Preamp.

These are the correct values:

•  C1: 136nF (2x 68nF)
  
• C2: 47nF

The results of the -10% capacitor values are expressed even more. You will get a huge boost in both the bass and treble frequencies.

The level is -1 dB at 20 Hz and gets compliant at 45 Hz. It increases to +1.85 dB at 200 Hz, down to -0.2 dB at 1.5 kHz, before it goes up to +1.55 dB at 20 kHz. 

You may very well find that you like this "loudness-RIAA", but it shouldn't be done in the phono stage. Nevertheless, this really illustrates what happens with extremely low capacitor values. Although it is way out of compliance, the result will be a fairly pleasant one.

For the next installment of these simulation, we'll look at what happens when the values are higher than what it says on the capacitors.

The RIAA equalization added to vinyl records can and do vary wildly, and you might rightfully say that a phono stage's RIAA compliance doesn't matter too much.

It's still a standard, and it should be one of the considerations when choosing a phono stage.

The Muffsy Phono Preamp uses Wima MKS2 capacitors with 10% tolerance. I have measured a couple of hundred of them, and find that they actually have much better tolerance. They tend to measure about 1 nF below the stated value, so I did some simulations to see what effects that deviation actually has.

This first graph shows a quite plausible scenario, where the values are off by -3%:

The lowest deviation from the RIAA curve is -0.01 dB, and the highest is +0.16 dB. 

This is realistically what you can expect from the Muffsy Phono Preamp kit in terms of RIAA compliance. Here's the measured RIAA compliance of a Muffsy Phono Preamp with no matched components:

These measurements show +0.21/-0.16 dB deviance. Just a smidge more than what we simulated above.

More simulations with really bad capacitor values will come, so stay tuned.