Interesting capacitance vs voltage effect

[Prehistoricman]

Just watched a video about this effect that MLC capacitors demonstrate where they have less capacitance as the voltage increases.

https://www.youtube.com/watch?v=HMNwBKOIGfs

I believe there is an EEVblog video on this too.
I wondered if anybody's experimented with this effect in audio and if it could be used to make a limited parts-count filter.
I'll have a play around right now to see if I can get something out of it.

[EBK]

I've wondered about this characteristic of ceramic caps before.  Could get some interesting (or terrible) harmonic distortions out of a circuit that exploits this.

There is a similar effect with junction capacitance of reverse-biased P-N junctions.

[Rob Strand]

Posted some stuff about this a few years ago, see reply #5,
https://www.diystompboxes.com/smfforum/index.php?topic=111219.0

One interesting thing is people don't give much thought to analog circuits which connect caps to Vref and Ground.  For example, on the first opamp the 4k7 + 100n is connected to Vref,

http://www.generalguitargadgets.com/wp-content/uploads/ts-808.jpg

However in this case the same parts are connected to ground,

https://elektrotanya.com/PREVIEWS/45612216/23432455/hangszer/ibanez/ibanez_ts-9_tube-screamer_sch.pdf_1.png

For film caps there wouldn't be much difference but for SMD caps the first case the cap has no DC where in the second case it sees Vcc/2.   In the second case the capacitance would decrease.

Having said that.  In another example, which I found in a product, the SMD cap was something like 20% *over* the rated capacitance with 0VDC.   IIRC, SMD caps are measured at 1Vrms.  So imagine the DC capacitance at 1V to 1.4V being somewhat less than the rated capacitance, basically following the voltage dependency profiles.   Then in this case the drop in capacitance was large enough that the AC test signal is seeing capacitance a value higher than the rating at 0V and less capacitance at 1.4V but producing an average capacitance which matches the rating at 1Vrms - basically a very non-linear capacitor!

[Prehistoricman]

I've wondered about this characteristic of ceramic caps before.  Could get some interesting (or terrible) harmonic distortions out of a circuit that exploits this.

There is a similar effect with junction capacitance of reverse-biased P-N junctions.

Is there about an order of magnitude change in junction capacitance? I always assumed it was a minimal change.

Having said that.  In another example, which I found in a product, the SMD cap was something like 20% *over* the rated capacitance with 0VDC.   IIRC, SMD caps are measured at 1Vrms.  So imagine the DC capacitance at 1V to 1.4V being somewhat less than the rated capacitance, basically following the voltage dependency profiles.   Then in this case the drop in capacitance was large enough that the AC test signal is seeing capacitance a value higher than the rating at 0V and less capacitance at 1.4V but producing an average capacitance which matches the rating at 1Vrms - basically a very non-linear capacitor!

Ah that's a very useful thing to know regarding SMD cap ratings.


I gave this idea a spin with a function gen at low speed and DC offset (to bias the opamp).



It was okay... This circuit is a bit too crappy and I'm not sure that modulating the bass is a good idea. Do modulated filters usually target the highs?

[PRR]

https://www.edn.com/Pdf/ViewPdf?contentItemId=4416466

Note that -60dB THD is 0.1%, "nothing" in guitar land.

[Prehistoricman]

I found the crappiest 1µF capacitor I could - an X5R SMD 0402. It's also a muRata brand so here's the DC bias characteristic curve:



Here's a pic of it:



Look at this tiny bitch! Fun to solder 0402s... in my experience, it doesn't usually matter which side you put your iron on because both ends heat up anyway.
So here's a little demo of how it sounds in the circuit I posted previously:

I'm not too disappointed by that (I also added 100 more ohms to the output filter). It could do with another high-pass afterwards to get rid of any woofing sounds, and maybe a better opamp. I'm running this at 18V to get more headroom but it doesn't help the lower output limit.

[Rob Strand]

So here's a little demo of how it sounds in the circuit I posted previously:

Extremely cool idea.   It actually sounds pretty good.

I'm not too disappointed by that (I also added 100 more ohms to the output filter). It could do with another high-pass afterwards to get rid of any woofing sounds, and maybe a better opamp. I'm running this at 18V to get more headroom but it doesn't help the lower output limit.

You might be able to use two caps and a differential method to stop the LFO feeding through.  Look up Sam Hay's differential diode tremollo circuit.

What next?  maybe a Magnatone circuit or phaser?