Effects of series capacitors.
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URL: https://forum.speakerplans.com/forum_posts.asp?TID=87564
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Topic: Effects of series capacitors.
Posted By: Teunos
Subject: Effects of series capacitors.
Date Posted: 08 July 2014 at 6:17pm
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This thread is a recap and discussion forum about the effects of capacitors in series with a driver. From my topic about my new build tops, several members got into a discussion about these effects, when a capacitor is not used for the crossover itself but instead for i.e. DC blocking. Please provide your opinion, measurements and whatnot so that hopefully we can not only clarify this for now, but also enlarge the knowledge available to the great community that speakerplans is. I have reserverd the first 4 posts for measurements. This way we can append detailed measurement (series), or theoretical approximations, models etc. to the front page, making the topic more clear. Please contact me via this topic or PM to append certain text or images to these first posts. RC Circuit Theory basics An RC circuit is an electrical filter circuit composed of an electric resistance R and a capacitance C. The total impedance of this circuit is a complex system, in the name that it has a real resistive part Zreal = R and imaginary capactive part Zcomplex= 1/((k+iw)*C). Here
The special quality a capacitor has (in series with a resistor) is that the current through the capacitor is equal to a constant multiplied by the change in the voltage, or mathmatically
Basically this equation states that the current through the capacitor (left hand side) is equal to the current through the resistor (right hand side). Off course in a closed circuit lacking a current or voltage source, the currents have to be equal and of opposite sign as any current will be dissipated through the resistance. This equation is basically a first order differential equation, because it states that a change in voltage over time multiplied by a constant is equal to the voltage itself. We can solve this equation for the voltage with the knowledge that the only equation satisfying this condition is an exponential decay function, where the behavior of the decay is controlled by the multiplication factor. Rewriting the equation we get:
Integrating the left and right side we get
Where B is an arbitrary constant. Rewriting again by taking the exponent left and right we get
or
When we call Exp(B) the voltage V0 across the capacitor at time t=0, we get the voltage
Although this is purely mathmetical, we can see something very important. This is that it takes time for a capacitor to react to a certain voltage applied to it. This reaction is controlled completely by the constant RC, the decay constant. When the value of RC is large, the voltage over the capacitor will stabilise slower than when the RC constant is lower. To explain why a capacitor in series with a resistor can act like a high-pass filter, we can look at the transfer function, the comparison between the voltage applied across the RC circuit, and the voltage actually presented to the resistor. ---To be updated. Without further mathmatical reroute's, it can be found on wikipedia anyway... (link below) http://en.wikipedia.org/wiki/RC_circuit" rel="nofollow - http://en.wikipedia.org/wiki/RC_circuit The transfer function of the RC circuit over the reistor is again a complex value. It equals: Hr(s)=RCs/(1+RCs) where s again is equal to iw. We can plot the transfer function of this system in a frequency-gain window and a frequency-phase window. This is generally called a BODE plot. it always has a characteristic shape as shown below.  What we can see from this is that a simple RC circuit gives a -6dB/octave decay in the frequency domain, and a phase twist of maximally 90 degrees at frequency zero. The phase shift is always 45 degrees at the -3dB point of the filter, also known as the knee point or F3. We can do a similar analysis for a low pass network where the capacitor is switched out for an inductor ( a coil) as well as for higher order filter networks. The basics may however (hopefully) be clear. As the frequency increases, the relative impedance of the capacitor compared to the resistor decreases and the effects the capacitor has on the signal become small. The phase shift of the filter is said to asymptotically approach zero as the frequency goes to infinity, however the differences become rather small pretty quickly in practice. The entire signal however is still passing through the capacitor, even though the ''measurable differences are small in in practice. The contradiction. For many years, it has been known that different capacitors give slighly different representations of sound. Qualitative measurements about the differences in how a capacitor handles ripple current, equivalent series resitance of a capacitor, memory effects have all been researched. Most effects however only become noticeable when the frequency of a sinoid becomes way higher than any audio loudspeaker will ever be able to reproduce. The discussion came to live when i could not hear a clear difference between having a series capacitor in place, whilst others reported the contrary. After several measurements, which will be included below, i came to my own conclusion that there is no (noticeable) effect a capacitor has on the sound quality. From a theoretical point of view, it is however completely frustrating and bonkers that we can not comprehend to explain which quantity would be responsible for the (significant) audible difference in sound quality. Hence the case for this discussion: What is it, that makes the sound quality deteriorate, especially when using larger capacitors?? ------------- Best regards, Teun. |
Replies:
Posted By: Teunos
Date Posted: 08 July 2014 at 6:18pm
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Reserved post ------------- Best regards, Teun. |
Posted By: Teunos
Date Posted: 08 July 2014 at 6:19pm
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Reserved post 2 ------------- Best regards, Teun. |
Posted By: Teunos
Date Posted: 08 July 2014 at 6:19pm
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Reserved post 3 ------------- Best regards, Teun. |
Posted By: Teunos
Date Posted: 08 July 2014 at 6:19pm
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Reserved post 4 ------------- Best regards, Teun. |
Posted By: Teunos
Date Posted: 10 July 2014 at 9:59pm
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Just found this at Prosoundweb, JBL uses a series capacitor in their studio monitor series: So they are using a 6dB/oct electrical HP filter on the compression driver, like in the S9800 ! Does the M2 include a protection cap on the compression driver?  Originally Posted by  If is is excursion related then a 6dB/oct filter is a dangerous game to play... The excursion will increase at a 12dB/oct rate (x4 each octave down), so a 6dB/oct electrical filter will only cut half of that (x2 each octave down remaining...). The sealed volume of the back chamber will somewhat limit that (adding a 12dB/oct filter) but the frequency at which it starts to "operate" will probably be quite low (dependent on the volume of the chamber). Thinking that some "purist" audiophiles do not hesitate to let the back chamber of their expensive driver opened, and then use first order filters with very low cutoff frequencies... JBL is using first order electrical filters on the S9800 (Truextent-like mylar surround) and M2 (ring radiator). Maybe this is for a reason?... I think Earl Geddes is also using first order filters in the Summa. The M2 has an acoustic 6th order L-R crossover at 800Hz (or so). To do this the woofer needed a 6th order active filter and the tweeter needed a 2nd order filter. The tweeter’s second order filter is divided in two. One first order section is active (the one in the DSP tuning) the other is passive and is in the loudspeaker enclosure. Yes, there is a capacitor on the tweeter! It’s there to protect the tweeter from turn-on pops and accidental miss-wiring of the speaker. There is also an passive attenuator on the tweeter. Its there to reduce amplifier hiss and noise. This is in line with G.T.'s advice to always but some kind of passive pad, something on the order of 6 dB, on the compression driver to reduce amplifier hiss and noise when bi-amping. And we can see that the D2/waveguide combination has a 4th order roll-off somewhere around 800 Hz coupled with a 1st order passive filter and a 1st order active filter. Remember how the 4430/4435 used a passive 1st order along with an active 1st order on the 2425/2344 combination in bi-amp mode? http://www.audioheritage.org/vbulletin/showthread.php?33972-JBL-Master-Reference-Monitor/page20 ------------- Best regards, Teun. |
Posted By: soundspud
Date Posted: 12 July 2014 at 7:20pm
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Epic tome on different capacitor dielectrics and their distortion: http://www.scribd.com/doc/2610442/Capacitor-Sound
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Posted By: firehazard
Date Posted: 13 July 2014 at 12:05am
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Hi Teunos, thanks for posting all that information. The difficulty of proving or disproving any assertion about the quality of a sound has been well known for a long time. More than fifty years ago a wise man wrote.... "In the end it will be judged by the mind. So the mind is really the ultimate measuring instrument. But unfortunately it is a most unsatisfactory instrument for arriving at definite conclusions." I wish you luck in what you're up to and it's great to see you doing all the cool measurements. I'm sure you are aware that a sine wave is not complex enough to provide much useful data about qualities like musicality or intelligibility. Cool speakers by the way.... |
Posted By: ArthurG
Date Posted: 13 July 2014 at 12:28am
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more water to the river: http://www.humblehomemadehifi.com/Cap.html" rel="nofollow - http://www.humblehomemadehifi.com/Cap.html I have tested some of these caps and I agree with the guy edit:  |
Posted By: firehazard
Date Posted: 14 August 2014 at 11:11pm
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Hi A lot of power amp schematics show the output connected to the ground with a capacitor. This capacitor is in series with a speaker connected to the output. I am only aware of one amp design that uses mosfets and output transformers. If series capacitors are a problem what amps should I buy? Thanks |
Posted By: RobinMatrix
Date Posted: 27 August 2014 at 11:56am
I'd put money on it that in a blind listening test, providing the capacitors where good quality film caps and the values were well matched, he'd be unable to tell one from another. If he is hearing any differences, which in many cases I would doubt, it will be due to minor variations in capacitor values as part of the natural 5% tolerance. As soon as someone writes: "Spatiality is one of the main strengths of this capacitor" or "They sound a bit dull at first because they take a while to burn-in." Triggers the BS detectors, well into the red. I sometimes think hi-fi guys were simply put on the planet to give the pro-audio guys a good laugh. Remind me to commission another batch of gold plated fuses ... and don't forget to turn them around every 6 months ... We use Solen or ClarityCap in all our crossovers, good quality film caps, take a bit of a pasting and never had a an in-service failure, can't say fairer than that. ------------- Matrix Pro Audio :: http://matrixproaudio.com/" rel="nofollow - http://matrixproaudio.com/" |
ArthurG wrote:Posted By: mikesrv
Date Posted: 25 September 2014 at 5:02pm
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teunos, this is an
excellent read and you have explained in detail about the high-pass filtering
capacity of a capacitor. The diagram that you have added is perhaps the easiest
bit of this post of understand which makes it easier for others to understand.
The quality of the sound is something that we can measure quantitatively. So it
just basically varies from person to person. A capacitor has many other
properties besides capacitance that have an effect on the quality of the sound
which makes people jump to conclusions without researching. |
Posted By: heliok
Date Posted: 05 March 2023 at 11:15am
| where I can get articles about capacitors for beginners https://electronicshacks.com/electronic-parts/capacitors/ - https://electronicshacks.com/electronic-parts/capacitors/ - https://electronicshacks.com/electronic-parts/capacitors/ |