Several people have been in touch lately with questions on how to modify the PCB into something it isn't right now. Popular requests are balanced inputs and/or balanced outputs, and DC servo on the outputs.

The PCB itself doesn't really lend itself to any of the modifications above, but you could always try doing add-on boards or roll your own from the open source freely available schematics.

An explanation of the Muffsy Phono Preamp PCB layout can come in handy for such alterations. And wouldn't you know, here it is:

Modifications you can do are changing the input impedance and altering the variable gain. I'm also happy to announce that the gain calculator has been updated to show the output voltage and voltage gain:

Finally, if you want to make major modifications, a way of simulating the circuit would really come in handy. Well, here's the Muffsy Phono Preamp LTSpice simulation file.

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.

Measuring the actual Muffsy Phono stage reveals a very good RIAA compliance, with a little extra amplification of the bass.

THIS ARTICLE HAS BEEN UPDATED WITH IMPROVED MEASUREMENTS.

The specifications for the Muffsy Phono Preamp PP-3 states that the RIAA compliance is a miniscule +/-0.025 dB. That's of course with filter resistors and capacitors measuring exactly as they are supposed to. Problem is, they almost never do. 

The resistors have a tolerance of +/- 1%, which means that a 16k resistor can measure between 15840 and 16160 ohms at extremes. The capacitors have a tolerance of 5%, which allows for an even greater variance.

So what happens then, if you just solder in the components in the kit? I did just that, and measured the RIAA compliance from 20 to 40.000 Hz. Here's the result: 

What happened is that the circuit measures +0.21/-0.16 dB across the audio band. At 40 kHz it's still only 0.21 dB below the ideal value.

Not exactly the +/-0.025 dB we expected, but that would only have been the case with precisely measured and matched components. The result is very good though, and on par with, or better than the best commercially available phono stages.

It's also worth noting that most vinyl records aren't following the RIAA curve exactly anyway. The RIAA equalization standard is what we have to aim for though. 

Your amplifier and (especially) your phono cartridge and loudspeakers can be expected to perform way worse than this, as they are specified at +/- 3 dB across the audio band. This range happens to be the extreme values of the graph above. As you can see, the response is very close to being centered at the 0 dB value in the middle.