Paralleling class 1 & class 2 MLCCs for audio?

[Zoios]

Hi everyone,

I've been looking for any information on using X7R and C0G capacitors in parallel for passing audio signals. I've seen some examples of this technique used for DC blocking in mixer schematics - would using a 0.1uF C0G cap in parallel with a 22uF X7R, for example, be better than just using the 22uF cap?

I've already ruled out using electrolytics for their size and tantalums for their bad reputation, so this leaves me with ceramics and the aforementioned question. Thanks in advance for any input!

 

[Newmarket]

Film caps too large ?

 

[Zoios]

There are very limited options when it comes to getting film caps larger than 1uF where I live, and I'd like to stick to SMD if possible.

 

[JohnRoberts]

This sounds like they are resurrecting old audio myths, about paralleling small high quality capacitors to fix large low quality capacitors.

Back in the 70s I bench tested this theory and found that the small high quality capacitor needed to be at least 10% of the value of the larger so-so capacitor to make a measurable improvement. To make your 22uF X7R behave you would need a 2.2uF COG. Good luck with that.

JR

 

[john12ax7]

Is X7R better or worse than a bi-polar electrolytic in the audio path?

 

[JohnRoberts]

Is X7R better or worse than a bi-polar electrolytic in the audio path?

Different horses for different courses... Back in the 70s my bench testing was looking to improve the ESL of a 22uF electrolytic capacitor in the gain leg of simple RIAA phono preamp. I was measuring (not hearing) tens of degrees of phase shift at 20kHz. In the course of that investigation I also discovered that some 22uF tantalums I had on hand had much lower ESL and reduced the phase shift significantly.

I would expect a ceramic X7R capacitor to have lower ESL than an electrolytic but would have to look at data sheets of both to compare voltage coefficient that I would expect to dominate distortion. I'm not sure I have ever seen voltage coefficient specs for electrolytic capacitors (but maybe they should).

JR

 

[merlin]

would using a 0.1uF C0G cap in parallel with a 22uF X7R, for example, be better than just using the 22uF cap?

No, because the 0.1uF has 220 times higher impedance than the 22uF.* So only 0.5% of the current is gonna flow though the 0.1u cap.

*Ok this ratio will change a bit at high frequencies, but probably above the audio range.

 

[JohnRoberts]

No, because the 0.1uF has 220 times higher impedance than the 22uF.* So only 0.5% of the current is gonna flow though the 0.1u cap.

*Ok this ratio will change a bit at high frequencies, but probably above the audio range.

Nonlinearities above the audio band can generate IMD products that are down in the audio passband. Of course it depends on the specific application.

Note: modern electrolytic capacitors are a much improved from what I had on my bench a few decades ago.

JR

 

[ccaudle]

would using a 0.1uF C0G cap in parallel with a 22uF X7R, for example, be better than just using the 22uF cap?

Possibly marginally, but the difference would likely be academic, i.e. you may be able to measure a difference, but unlikely to be a noticeable improvement.
Merlin argued from a view of current division between the two capacitors, but another way to think about it is that the voltage coefficient of the X7R capacitor is going to dominate the distortion performance. Look at the voltage change across the capacitor (DC bias - voltage at negative signal peak, vs. DC bias + voltage at positive signal peak) and see how much the capacitance of the X7R changes with that voltage change.
Then see how much the capacitance of the parallel combination of X7R and C0G changes with that voltage change. The C0G cap has no voltage coefficient to speak of, so it will be a small constant capacitance in parallel to a larger variable capacitor. The larger the C0G cap, the less that the combination capacitor varies with voltage, but you would likely need to get the C0G up to a significant fraction of the X7R capacity (of course depending on what performance you are willing to accept).

Is X7R better or worse than a bi-polar electrolytic in the audio path?

Worse. Not only does X7R have a noticeable voltage coefficient, unless you have a super small size limit you should be able to get an electrolytic in higher capacitance, which reduces the AC voltage across the capacitor, further reducing any effect on signal quality.

If you really have such a tight size limit that you cannot fit electrolytic, then consider increasing the load impedance and including the capacitor in the feedback loop if you have to use X7R, or if you can increase the load impedance enough perhaps you can use parallel C0G capacitors to get the value you need.

 

[Newmarket]

I would expect a ceramic X7R capacitor to have lower ESL than an electrolytic but would have to look at data sheets of both to compare voltage coefficient that I would expect to dominate distortion. I'm not sure I have ever seen voltage coefficient specs for electrolytic capacitors (but maybe they should).

Me neither. But for a 'lytic I'm usually well exceeding the voltage it will see and be required to work at - to greatly increase the expected lifetime. But - yeah - might be interesting from an audio pov.

 

[JohnRoberts]

Me neither. But for a 'lytic I'm usually well exceeding the voltage it will see and be required to work at - to greatly increase the expected lifetime. But - yeah - might be interesting from an audio pov.

I would expect voltage coefficient to only be important for in-band audio frequency filters that generate significant AC terminal voltage across the capacitor. Most electrolytic capacitors are used in DC blocking applications with extremely low audio voltage across the two terminals.

Of course passive loudspeaker crossovers using electrolytic capacitors are a concern. It can be a significant cost increase to use film in place of electrolytic in passive crossover filters.

JR

 

[Newmarket]

I would expect voltage coefficient to only be important for in-band audio frequency filters that generate significant AC terminal voltage across the capacitor. Most electrolytic capacitors are used in DC blocking applications with extremely low audio voltage across the two terminals.

Of course passive loudspeaker crossovers using electrolytic capacitors are a concern. It can be a significant cost increase to use film in place of electrolytic in passive crossover filters.

JR

Yes. I'm not familiar with LS crossover design but the cost difference film vs 'lytic can be large both in cost and size. And tbh I'm not sure how the polarity aspect works out in LS crossovers... Bipolar ? "Back to Back" ?