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samicksha
samicksha
11/8/2013 3:41:05 AM
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Artist
Re: Film Capacitor
Yes RedDrek, one of the plus here is direct contact to the electrodes on both ends of the winding, this keeps all current paths to the entire electrode very short.

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RedDerek
RedDerek
11/6/2013 7:15:02 PM
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Master
Re: Film Capacitor
@Samicksha - I agree that a film cap would have been a much better choice in the application. Film caps do have low dissipation and they have other advantages. However, they do not have the capacitance volume as an electrolytic or tantalum. Film caps are also good for applications where low drift is required. Another option would be to use a glass cap.

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Hughston
Hughston
10/28/2013 11:11:18 AM
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Newbie
Re: Simulation models
The opposite can actually be true. A telephone does sound natural and the frequency response is shaped to reduce the low frequency by the telephone. If the response was flat, it would sound unnatural.  Perhaps the reason for that depends on the frequency response for the rest of the system. You could add more high frequency bandwidth and it would not hurt anything or sound unnatural. At low frequencies you hear the difference.

Some microphones are designed to be close talking and some are not. In a phone or aircraft system, the microphones are close talking.  In a noisy environment you have to get close to the microphone. A singer does not get very close to the microphone in most circumstances because it increases the low frequencies. But they will get close if they want to make their voice sound lower.

Many speech systems will be in a noisy environment; especially today where the person can be talking from a football stadium, a moving car with the top down or a Hong Kong street. Or they are trying to hear in that environment. The weak links in the quality of that system are not the S/N of the converters but the acoustics and noise at each end. Extra bandwidth does not help; it just adds to the noise.

On the microphone side, the system can use noise reduction with signal processing, microphone arrays and automatic gain control.  On the receive side you can make the response louder or shape it for the environment. For example, in a cell phone, the earpiece does not fit well to the ear and that affects the low frequency response into the ear. What a cellphone does is assume a leaky seal to the ear and boost the low frequency to compensate. I don't think it helps much. In the old telephones, the earcup fit well against the ear and that gave good low frequency reception.

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yalanand
yalanand
10/27/2013 7:09:58 AM
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Master
Re : Plug That leak: Look out for Capacitor Leakage!
In current digital circuits, utmost capacitors are used to even the power amount and lessen circuit noise. When capacitors are used for frequency generation or pulse width modulation they commonly have a flexible resistor or crystal to set the control. When capacitors are used to de bounce a switch or hold open a transistor, the exact clench time is frequently not critical. 

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WKetel
WKetel
10/26/2013 8:55:03 PM
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Artist
Re: Simulation models
I wish that folks would abandon that concept that only a narrow bandwidth is neeeded for speech. In some communication systems it may make some sense to reduce the bandwidth a lot, but the result is unpleasant to hear and quite unnatural sounding. It is used a bit in amateur radio with speech processing and while it may provide better understanding of speech in high noise conditions, it is not enjoyable to hear.

The simplest way to reduce capacitor leakage is to have the same voltage on both sides of the capacitor. But that is seldom an option.

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liteman0
liteman0
10/25/2013 10:21:33 AM
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Teacher
RE: Coupling Capacitor
Ideally, a coupling device would not store a charge, it would only transfer a signal.

The problem I see in this circuit is the capacitor needs a discharge path which would reduce leakage and improve the response time. Also, without a discharge path, the input impedance of this circuit can vary drastically depending on the input signal.  I would try placing a 1M trim pot across the mic and trim it to the lowest possible value.

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jcaldwell
jcaldwell
10/24/2013 12:37:14 PM
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Newbie
Re: Simulation models
Hi Bill,

Thanks for your comment. I really enjoyed your article on high-performance balanced audio interfaces. The solution you propose is eventually what I convinced the customer to do, he was already using a JFET input op amp (TL082) so increasing the input resistance to allow for smaller capacitors of different dielectric types was not a problem. 

You bring up a good point about experimentation. Many of the comments here have stated "couldn't you just simulate this?". Well, yes, if you took the time to build a capacitor model which includes the leakage characteristics. But many people just plop down the standard capcitor model in SPICE, and when it doesn't work in real life: blame the op amp!

After all, a resistor is a resistor, a capacitor is a capacitor, an inductor is an inductor right?! ;-)

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Guru of Grounding
Guru of Grounding
10/23/2013 10:12:47 PM
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Teacher
Re: Simulation models
Anyone who's actually experimented with capacitors (do engineering students do that kind of thing any more?) is very familiar with the rather high leakage currents of electrolytic capacitors ... reading a data sheet completely will help a lot, too. Since the "film" (actually aluminum oxide) actually forms with applied voltage, leakage is also a function of how long voltage has been applied ... it will reduce considerably over minutes or hours. Also the reason that any long-stored (years) equipment should be powered up slowly because the leakage current may be high enough to overheat the capacitor and make the (wet) electrolytic paste steam and the capacitor explode (ever wonder why there are either little rubber plugs or criss-cross indentations in the aluminum can?).  Anyway, the solution for this circuit is to choose an op-amp with extremely low bias current (FET input, for example), raise the value of R2 to perhaps 10 Megohm, and use a plastic film capacitor of 0.47 uF to get the same time constant (-3 dB frequency). That being said, electret mics have their own low cutoff frequency due to the internal capacitance between the electret film and the internal FET (the "output" terminal is simply the drain of that FET).  While I'm here, someone offered that the cutoff frequency for audio should be set at 20 Hz. I would disagree with that! If you care about phase distortion (properly measured as deviation from linear phase) at low audio frequencies (so a kick drum still sounds like a kick drum after passing through a signal chain), you should set the cutoff frequency to about 0.5 to 1.0 Hz. These phase distortions are cumulative and no amount of "aligning them" will reduce the problem. While cascades of low-pass filters can be aligned to some extent (as a multi-pole Bessel filter, for example) to flatten high-frequency phase deviation, there is no counterpart at low frequencies (i.e., no Bessel high-pass filter). Extended LF response at each stage is the only cure (short of a FIR filter if the chain uses DSP). - Bill Whitlock, Jensen Transformers, AES Life Fellow & IEEE Life Senior

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Hughston
Hughston
10/21/2013 6:26:20 PM
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Newbie
Re: Simulation models
I have seen electret mics used that way before. It was used to detect a door opening via pressure changes for a security system. I would try a larger electret transducer for that application. When people use electret mics for audio, they often don't realize the mic can pick up some very low frequencies if you don't filter them out. I saw hoses on a TV program used as part of a sensor system for picking up low frequency vocalizations of elephants.  That was the reason for the elephant reference.  But this brings up a relevant point. Limit your bandwidth or else you will saturate your amplifiers with unwanted low frequency noise. If you saturate your amplifier with low frequency signals, then that's all you'll hear.

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jcaldwell
jcaldwell
10/21/2013 6:16:09 PM
User Rank
Newbie
Re: Simulation models
Hughston,

Thank you for your comment. This actually was not for an audio system, at least not in the standard 20Hz to 20kHz sense. Microphones are really only a tranducer that converts changes in air pressure to an electrical signal, most commonly for audio uses, but occasionally otherwise. In this case, the customer was attempting to use this microphone to measure extremely low frequency changes in air pressure. 

 

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