Microphonic Film caps - Page 1
Forum section: electronics / projects

author: ubbut
author_rank: Regular Contributor
timestamp: November 02, 2021, 09:57:50 am
content: Hi,in a high impedance Audio circuit we initially used 56n ceramic caps, but due to to them being microphonic and noise issues with that, we decided to switch to (large and more expensive) film caps instead.To my surprise the caps are still microphonic! I can tap on the cap with a non conductive tool and can hear/see the effect clearly. Tapping beside the cap produces no signal. It is certainly much better then before, but I am surprised the effect is still there with film caps.We used Panasonic ECH-U1C563JX5We would use electrolytic caps if they existed in 56n ...Anything we could use or try instead?thank you

author: ConKbot
author_rank: Super Contributor
timestamp: November 02, 2021, 11:46:16 am
content: Were your original caps X5R/X7R? There the bending of the substrate/dielectric causes the microphonics. C0G is generally just alumina, and doesn't suffer from this. Though both C0G and film can suffer from the effects of the plates being moved closer and pulled apart by board stress if they are SMT. Even more so if leadless SMT. Leaded devices would be an improvement. Even more so if it's through-hole with adequate lead length for isolation of stress, and the package secured from flopping around with neutral cure RTV silicone, or some other flexible adhesive.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 02, 2021, 01:09:34 pm
content: See DaveBest not to use ceramic caps for audio sigal chain if yoy can avoid it.

author: ubbut
author_rank: Regular Contributor
timestamp: November 02, 2021, 01:10:55 pm
content: Quote from: Terry Bites on November 02, 2021, 01:09:34 pmPlease see the original post. This is about film caps showing the same behavior

author: magic
author_rank: Super Contributor
timestamp: November 02, 2021, 04:52:53 pm
content: Quote from: ubbut on November 02, 2021, 09:57:50 amSounds like a nonissue to be honest.Probably the capacitor deforms internally under pressure and its capacitance changes slightly.Quote from: ubbut on November 02, 2021, 09:57:50 amCheaper film capacitors, as PPS is somewhat exotic.Or C0G/NP0 that has been mentioned, but it may not be very cheap nor small at 56nF.

author: Gyro
author_rank: Super Contributor
timestamp: November 02, 2021, 07:39:22 pm
content: SMD PPS capacitors are still a layered construction, with nothing mechanically clamping the layers together (I have even seen brand new unsoldered ones delaminating). Any capacitor will be microphonic if you change the distance between the electrodes and compression of the dielectric. Through hole parts with tightly wound construction are far less susceptible, this is probably unhelpful in your current situation though.The best you can do is reduce stresses on the smd parts. PCB slots, mechanical damping etc.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 02, 2021, 08:54:11 pm
content: Same advice. PCB slots to reduce transmission of vibrations to the caps.Alternatively, you can redesign to limit the use of caps on the signal chain, but if you need analog filtering, that's hard to avoid.

author: graybeard
author_rank: Frequent Contributor
timestamp: November 03, 2021, 03:30:07 am
content: If there is a DC voltage across the capacitor and vibration changes the capacitance at all will a current will result. That is how condenser microphones work.Q=CVdq/dt = I = C dV/dt + V dC/dtif the voltage is constant then I = V dC/dtthus the change in capacitance from vibration is the source of the signal.In most circuits the voltage will vary due to the induced current, but it is still the induced current from the capacitance change that is the source.

author: KE5FX
author_rank: Super Contributor
timestamp: November 03, 2021, 03:47:35 am
content: Try back-to-back tantalums.

author: graybeard
author_rank: Frequent Contributor
timestamp: November 03, 2021, 03:50:10 am
content: Quote from: KE5FX on November 03, 2021, 03:47:35 amA different film capacitor might work too. I just looked up the capacitor used and it is a stacked construction. Perhaps a wound capacitor would be less susceptible to microphonics.Another possibility is that the sound is propagating though the board to another component and it is not the suspect capacitor being tapped on.