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#1
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X7R ceramics for DC blocking ?
Everyone knows that high dialectic constant ceramic capacitors (X7R,
Z5U, Y5V) exhibit voltage sensitivity. This apparently causes distortion when any "significant" AC voltage appears across the capacitor. In a filter this would be the -3dB corner frequency. Does anyone have any idea how much distortion it actually produces ? Due to limited PCB space I want to use ceramics for DC blocking at the input of some FET op-amps. I see that high voltage X7R caps have the lowest voltage sensitivity but I don't have a THD analyzer to get any meaningful numbers. For example, would a high pass made with 50V X7R 100nF and a 1M ohm resistor give me THD = 0.02% at 20Hz ? Adam |
#2
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
Adam S not.valid@nosuchaddress wrote in
u: Everyone knows that high dialectic constant ceramic capacitors (X7R, Z5U, Y5V) exhibit voltage sensitivity. This apparently causes distortion when any "significant" AC voltage appears across the capacitor. In a filter this would be the -3dB corner frequency. Does anyone have any idea how much distortion it actually produces ? Due to limited PCB space I want to use ceramics for DC blocking at the input of some FET op-amps. I see that high voltage X7R caps have the lowest voltage sensitivity but I don't have a THD analyzer to get any meaningful numbers. For example, would a high pass made with 50V X7R 100nF and a 1M ohm resistor give me THD = 0.02% at 20Hz ? Adam AVX Capacitors offers a comparison between ceramic and tantalum that includes some data on hte problem http://www.avxcorp.com/docs/techinfo/mlc-tant.pdf If you keep the DC Bias high wrt the AC component there isn't a problem. -- Bob Quintal PA is y I've altered my email address. -- Posted via a free Usenet account from http://www.teranews.com |
#3
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
Adam S wrote:
Everyone knows that high dialectic constant ceramic capacitors (X7R, Z5U, Y5V) exhibit voltage sensitivity. This apparently causes distortion when any "significant" AC voltage appears across the capacitor. In a filter this would be the -3dB corner frequency. Does anyone have any idea how much distortion it actually produces ? Due to limited PCB space I want to use ceramics for DC blocking at the input of some FET op-amps. I see that high voltage X7R caps have the lowest voltage sensitivity but I don't have a THD analyzer to get any meaningful numbers. For example, would a high pass made with 50V X7R 100nF and a 1M ohm resistor give me THD = 0.02% at 20Hz ? Adam Some time ago I did some testing on this but with my A1 set that has a bottom THD reading at 0.002% I could not find any distortion, whatever capacitor and at any voltage or frequency. Someone with an AP who can measure a decade lower ?? Andre |
#4
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
Adam S wrote: Everyone knows that high dialectic constant ceramic capacitors (X7R, Z5U, Y5V) exhibit voltage sensitivity. This apparently causes distortion when any "significant" AC voltage appears across the capacitor. In a filter this would be the -3dB corner frequency. Does anyone have any idea how much distortion it actually produces ? Due to limited PCB space I want to use ceramics for DC blocking at the input of some FET op-amps. I see that high voltage X7R caps have the lowest voltage sensitivity but I don't have a THD analyzer to get any meaningful numbers. For example, would a high pass made with 50V X7R 100nF and a 1M ohm resistor give me THD = 0.02% at 20Hz ? Use an electrolytic. It'll way outperform a ceramic. Graham |
#5
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
Bob Woodward wrote:
Some time ago I did some testing on this but with my A1 set that has a bottom THD reading at 0.002% I could not find any distortion, whatever capacitor and at any voltage or frequency. Do you remember the component values you tested were ? I assume they were not NPO or COG ceramics as these behave essentially like a film capacitors and should be "distortionless". |
#6
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
Adam S wrote:
Bob Woodward wrote: Some time ago I did some testing on this but with my A1 set that has a bottom THD reading at 0.002% I could not find any distortion, whatever capacitor and at any voltage or frequency. Do you remember the component values you tested were ? I assume they were not NPO or COG ceramics as these behave essentially like a film capacitors and should be "distortionless". It was a X7R, 100n 0805 from PHILIPS ( today BC components ) Since then I have noticed that almost everyone uses conventional, wired, film-capacitors for audio-signals. That includes major pro-audio manufacturers and electronic magazines. When no space on board-surface let an electrolytic fly above the other components,. Andre |
#7
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
In article , Adam S not.valid@nosuchaddress wrote:
Everyone knows that high dialectic constant ceramic capacitors (X7R, Z5U, Y5V) exhibit voltage sensitivity. This apparently causes distortion when any "significant" AC voltage appears across the capacitor. In a filter this would be the -3dB corner frequency. Does anyone have any idea how much distortion it actually produces ? Due to limited PCB space I want to use ceramics for DC blocking at the input of some FET op-amps. I see that high voltage X7R caps have the lowest voltage sensitivity but I don't have a THD analyzer to get any meaningful numbers. For example, would a high pass made with 50V X7R 100nF and a 1M ohm resistor give me THD = 0.02% at 20Hz ? Adam The Z after the cap will have the most effect. greg |
#8
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
Bob Woodward wrote:
Do you remember the component values you tested were ? I assume they were not NPO or COG ceramics as these behave essentially like a film capacitors and should be "distortionless". As blameless as polypropylene, according to extensive tests published in Elektor by Cyril Bateman. From memory, other ceramics range around -60 to -80dB distortion with 1V at 1kHz across them. Common polar electrolytics are better, but poor examples not by much. Films ranged from around -90 to being unmeasurable with a floor at -120dB. Order was as you might expect, with polypropylene the lowest. Contrary to what others here have said, distortion increased, in general, both with signal *and* with bias. I notice Bob Quintal says "If you keep the DC Bias high wrt the AC component there isn't a problem.", which sounds like a contrary view, although not necessarily. I wonder what the evidence is? cheers, Ian |
#9
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
See the PDF document on John Hardy's 990 preamp
where there is a section at the end about his use of COG/ NPO ceramics in the signal path. www.johnhardyco.com |
#10
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
In message , Adam S
writes Everyone knows that high dialectic constant ceramic capacitors (X7R, Z5U, Y5V) exhibit voltage sensitivity. This apparently causes distortion when any "significant" AC voltage appears across the capacitor. In a filter this would be the -3dB corner frequency. Does anyone have any idea how much distortion it actually produces ? Due to limited PCB space I want to use ceramics for DC blocking at the input of some FET op-amps. I see that high voltage X7R caps have the lowest voltage sensitivity but I don't have a THD analyzer to get any meaningful numbers. For example, would a high pass made with 50V X7R 100nF and a 1M ohm resistor give me THD = 0.02% at 20Hz ? Adam X7R dielectric isn't too bad from the capacitance versus dc bias point of view. Z5U and Y5V are absolutely dreadful. If you use RMAA to test certain soundcards you'll find some that have horrendous IM distortion when using an LF and HF signal, when driving into relatively low load impedances. The reason for this is that the small value of the output cap means that there is a large-ish signal appearing across the output cap at the low frequencies, thus making its capacitance value alter with the LF portion of the signal. Result is poor LF distortion, but ok HF distortion, and also poor intermodulation distortion. Replace the Y5V output caps with X7R (if you have the space) and the performance is much improved. -- Chris Morriss |
#11
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
Ian Iveson wrote:
Bob Woodward wrote: Contrary to what others here have said, distortion increased, in general, both with signal *and* with bias. I notice Bob Quintal says "If you keep the DC Bias high wrt the AC component there isn't a problem.", which sounds like a contrary view, although not necessarily. I wonder what the evidence is? cheers, Ian Was that related to film capacitors or electrolytics ? When electrolytics, applying DC-bias would avoid having the cap inversely polarized for half periods of the audio signal. Andre |
#12
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
Contrary to what others here have said, distortion increased, in general, both with signal *and* with bias. I notice Bob Quintal says "If you keep the DC Bias high wrt the AC component there isn't a problem.", which sounds like a contrary view, although not necessarily. I wonder what the evidence is? Was that related to film capacitors or electrolytics ? When electrolytics, applying DC-bias would avoid having the cap inversely polarized for half periods of the audio signal. Particularly with electrolytics as I remember. I wish I could find the article...actually Electronics World rather than Elektor. At the time I read it I was not aware of some of the myths he claimed to explode. I remember wishing that he had included some lower frequency tests because he said that electrolytic action is too slow to have an appreciable effect at audio frequencies. Non-polar electrolytics, used with zero bias, tested well, as did back-to-back polar electrolytics. The article may be available from EW, and I believe Cyril produced a CDROM of his work with capacitors. Not for free though. cheers, Ian |
#13
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
Ian Iveson wrote:
Contrary to what others here have said, distortion increased, in general, both with signal *and* with bias. I notice Bob Quintal says "If you keep the DC Bias high wrt the AC component there isn't a problem.", which sounds like a contrary view, although not necessarily. I wonder what the evidence is? Was that related to film capacitors or electrolytics ? When electrolytics, applying DC-bias would avoid having the cap inversely polarized for half periods of the audio signal. Particularly with electrolytics as I remember. I wish I could find the article...actually Electronics World rather than Elektor. At the time I read it I was not aware of some of the myths he claimed to explode. I remember wishing that he had included some lower frequency tests because he said that electrolytic action is too slow to have an appreciable effect at audio frequencies. Non-polar electrolytics, used with zero bias, tested well, as did back-to-back polar electrolytics. The article may be available from EW, and I believe Cyril produced a CDROM of his work with capacitors. Not for free though. cheers, Ian I just found a 1980 article "Picking Capacitors" by Walter Jung and Richard March. Made scans and packed them in a zip. http://www.tubesworld.com/inf/Picking_Capacitors.zip Andre |
#14
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
"Bob Woodward" "Bob wrote in message
Ian Iveson wrote: Contrary to what others here have said, distortion increased, in general, both with signal *and* with bias. I notice Bob Quintal says "If you keep the DC Bias high wrt the AC component there isn't a problem.", which sounds like a contrary view, although not necessarily. I wonder what the evidence is? Was that related to film capacitors or electrolytics ? When electrolytics, applying DC-bias would avoid having the cap inversely polarized for half periods of the audio signal. Particularly with electrolytics as I remember. I wish I could find the article...actually Electronics World rather than Elektor. At the time I read it I was not aware of some of the myths he claimed to explode. I remember wishing that he had included some lower frequency tests because he said that electrolytic action is too slow to have an appreciable effect at audio frequencies. Non-polar electrolytics, used with zero bias, tested well, as did back-to-back polar electrolytics. The article may be available from EW, and I believe Cyril produced a CDROM of his work with capacitors. Not for free though. I just found a 1980 article "Picking Capacitors" by Walter Jung and Richard March. Made scans and packed them in a zip. http://www.tubesworld.com/inf/Picking_Capacitors.zip Too bad you didn't search for the article using google. Here's one of many posts of it: http://www.reliablecapacitors.com/pickcap.htm BTW, the article is not entirely composed of generally agreed-upon truth. |
#15
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
Ian Iveson wrote:
Contrary to what others here have said, distortion increased, in general, both with signal *and* with bias. I notice Bob Quintal says "If you keep the DC Bias high wrt the AC component there isn't a problem.", which sounds like a contrary view, although not necessarily. I wonder what the evidence is? Was that related to film capacitors or electrolytics ? When electrolytics, applying DC-bias would avoid having the cap inversely polarized for half periods of the audio signal. Particularly with electrolytics as I remember. I wish I could find the article...actually Electronics World rather than Elektor. At the time I read it I was not aware of some of the myths he claimed to explode. I remember wishing that he had included some lower frequency tests because he said that electrolytic action is too slow to have an appreciable effect at audio frequencies. I know that on Doug Self's designs, he's got electrolytics all over the place to block DC. There's no biasing to keep them from becoming reverse-biased by the AC signal. He never mentions how and why that's okay. |
#16
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
"Ian Iveson" wrote in
. uk: Bob Woodward wrote: Do you remember the component values you tested were ? I assume they were not NPO or COG ceramics as these behave essentially like a film capacitors and should be "distortionless". As blameless as polypropylene, according to extensive tests published in Elektor by Cyril Bateman. From memory, other ceramics range around -60 to -80dB distortion with 1V at 1kHz across them. Common polar electrolytics are better, but poor examples not by much. Films ranged from around -90 to being unmeasurable with a floor at -120dB. Order was as you might expect, with polypropylene the lowest. Contrary to what others here have said, distortion increased, in general, both with signal *and* with bias. I notice Bob Quintal says "If you keep the DC Bias high wrt the AC component there isn't a problem.", which sounds like a contrary view, although not necessarily. I wonder what the evidence is? cheers, Ian From what I understand of the distortion creation mechanism, the increase in voltage causes a change in the capacitance. This change is most pronounced at low voltages especially for Y5V caps. Since without bias, the change in voltage is equal to the peak-peak voltage, that represents a large change in the impedance through the cap. The bias reduces the percentage change significantly. Refer to figure 2 on page 3 of the link I posted, http://www.avxcorp.com/docs/techinfo/mlc-tant.pdf -- Bob Quintal PA is y I've altered my email address. -- Posted via a free Usenet account from http://www.teranews.com |
#17
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
dizzy wrote: Ian Iveson wrote: Contrary to what others here have said, distortion increased, in general, both with signal *and* with bias. I notice Bob Quintal says "If you keep the DC Bias high wrt the AC component there isn't a problem.", which sounds like a contrary view, although not necessarily. I wonder what the evidence is? Was that related to film capacitors or electrolytics ? When electrolytics, applying DC-bias would avoid having the cap inversely polarized for half periods of the audio signal. Particularly with electrolytics as I remember. I wish I could find the article...actually Electronics World rather than Elektor. At the time I read it I was not aware of some of the myths he claimed to explode. I remember wishing that he had included some lower frequency tests because he said that electrolytic action is too slow to have an appreciable effect at audio frequencies. I know that on Doug Self's designs, he's got electrolytics all over the place to block DC. There's no biasing to keep them from becoming reverse-biased by the AC signal. He never mentions how and why that's okay. That's simple. Electrolytics don't become non-linear until the reverse voltage exceeds ~ 100mV. Since coupling caps don't have very much AC voltage across them it's easy to ensure they operate in this situation. The non-linearity seems to be the result of the cap becoming an electrolytic recitifier AIUI and you have to exceed the 'forward voltage' i.e 100mV for that to occur. To make sure simply this never happens use large value electrolytics, say 10 -22 uF for inputs and 100- 200 uF for outputs. It works a treat. It also ensures very low phase shift at LF as a bonus. Graham |
#18
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
Bob Quintal wrote: From what I understand of the distortion creation mechanism, the increase in voltage causes a change in the capacitance. The dielectric constant isn't errrr... constant ! This change is most pronounced at low voltages especially for Y5V caps. Since without bias, the change in voltage is equal to the peak-peak voltage, that represents a large change in the impedance through the cap. The bias reduces the percentage change significantly. No. Not so. Coupling caps *do not* see the signal voltage across them. They wouldn't be doing their job if they did. Graham |
#19
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
On Tue, 17 Jul 2007 23:56:16 GMT, dizzy wrote:
Ian Iveson wrote: Contrary to what others here have said, distortion increased, in general, both with signal *and* with bias. I notice Bob Quintal says "If you keep the DC Bias high wrt the AC component there isn't a problem.", which sounds like a contrary view, although not necessarily. I wonder what the evidence is? Was that related to film capacitors or electrolytics ? When electrolytics, applying DC-bias would avoid having the cap inversely polarized for half periods of the audio signal. Particularly with electrolytics as I remember. I wish I could find the article...actually Electronics World rather than Elektor. At the time I read it I was not aware of some of the myths he claimed to explode. I remember wishing that he had included some lower frequency tests because he said that electrolytic action is too slow to have an appreciable effect at audio frequencies. I know that on Doug Self's designs, he's got electrolytics all over the place to block DC. There's no biasing to keep them from becoming reverse-biased by the AC signal. He never mentions how and why that's okay. Coupling caps do not become reverse biased by AC signals. Whatever voltage is across them when the equipment is switched on remains across them in the presence of an audio signal. This business of the being reverse biased during negative-going half cycles (or whatever) is a myth. Just examine how a circuit operates. Of course extremely low frequency signals will reverse bias small coupling caps, but that isn't what we are talking about here, is it? d -- Pearce Consulting http://www.pearce.uk.com |
#20
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
Arny Krueger wrote:
"Bob Woodward" "Bob wrote in message Ian Iveson wrote: Contrary to what others here have said, distortion increased, in general, both with signal *and* with bias. I notice Bob Quintal says "If you keep the DC Bias high wrt the AC component there isn't a problem.", which sounds like a contrary view, although not necessarily. I wonder what the evidence is? Was that related to film capacitors or electrolytics ? When electrolytics, applying DC-bias would avoid having the cap inversely polarized for half periods of the audio signal. Particularly with electrolytics as I remember. I wish I could find the article...actually Electronics World rather than Elektor. At the time I read it I was not aware of some of the myths he claimed to explode. I remember wishing that he had included some lower frequency tests because he said that electrolytic action is too slow to have an appreciable effect at audio frequencies. Non-polar electrolytics, used with zero bias, tested well, as did back-to-back polar electrolytics. The article may be available from EW, and I believe Cyril produced a CDROM of his work with capacitors. Not for free though. I just found a 1980 article "Picking Capacitors" by Walter Jung and Richard March. Made scans and packed them in a zip. http://www.tubesworld.com/inf/Picking_Capacitors.zip Too bad you didn't search for the article using google. Here's one of many posts of it: uumph ... http://www.reliablecapacitors.com/pickcap.htm BTW, the article is not entirely composed of generally agreed-upon truth. My reflex reaction was caused by the fact that this article was what made me start measuring caps. When discussing the influence of caps, keep in mind that most musicproductions, recordings or live performance, are made using large mixing-consoles and outboard equipment that is loaded with caps in the signal-path. Mostly electrolytics, no bias. Once heard Rupert Neve during a lecturer claim he did apply about 1V bias to make all electrolytics beeing right polarized for most of the signal ( amplitude ). Then I did a complete overhaul of a large console ( DDA DCM232 ) ( http://www.ddaconsoles.com/pdf/schem...schematics.pdf ) that was 12 years old. measuring frequency response and distortion of the single channel-strips showed a disaster. Still it has been in use till days before it was taken to my works. Roll-off starting at 200Hz down giving -10dB at 20Hz, or Distortion over 1%, or High-cut at 6kHz, or combination hereof. All these faults originated from electrolytics. Dried, leaked, exploded. So finally decided to recap the whole console. My conclusion: unless special precautions have been taken, it is wise to check behavior every 5 years and when these faults occur, recap. In new design; go DC, use servo circuit to correct offset in DC, use film capacitors, use ( good quality ) electrolytics with bias, use electrolytics and replace them every 1000 days of use. So to the OP. how much DC must you block ? is there space on another spot of the pcb? Do you have an unused op-amp in a dual or quad package to make a servo-circuit? Or can you accept the DC in the first stage and block it somewhere further in the circuit? Andre |
#21
Posted to rec.audio.tech
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X7R ceramics for DC blocking ?
Bob Woodward wrote: Once heard Rupert Neve during a lecturer claim he did apply about 1V bias to make all electrolytics beeing right polarized for most of the signal ( amplitude ). Never seen any Neve gear that did that. All the dual supply op-amp kit they've made that I've seen (quite a bit) uses zero bias electrolytic coupling caps. Their single supply stuff if course DC biased. There is one company I know of that used that technique. It was Midas (SR consoles) in the 70s / early 80s and they did it because they were using tantalum bead electrolytics that rectify more easily AIUI than aluminium ones. Graham |
#22
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X7R ceramics for DC blocking ?
Bob Woodward wrote: Then I did a complete overhaul of a large console ( DDA DCM232 ) ( http://www.ddaconsoles.com/pdf/schem...schematics.pdf ) that was 12 years old. measuring frequency response and distortion of the single channel-strips showed a disaster. Still it has been in use till days before it was taken to my works. Roll-off starting at 200Hz down giving -10dB at 20Hz, or Distortion over 1%, or High-cut at 6kHz, or combination hereof. All these faults originated from electrolytics. Dried, leaked, exploded. So finally decided to recap the whole console. That's because they dried out with age. The absence of DC bias isn't a factor there AIUI. Graham |
#23
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X7R ceramics for DC blocking ?
Bob Woodward wrote: In new design; go DC, use servo circuit to correct offset in DC, DC servos everywhere ? You have to be joking. use film capacitors, use ( good quality ) electrolytics with bias, use electrolytics and replace them every 1000 days of use. Why not just use 105C electrolytics ? They'll take 4 times as long (compared to standard 85C types) typically to dry out from elevated temperatures. It does get warm in busy channel strips. Neve V series was a case in point. A 48 channel console took about 3kW of DC power. Graham |
#24
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X7R ceramics for DC blocking ?
"Bob Woodward" "Bob wrote in message
Arny Krueger wrote: http://www.reliablecapacitors.com/pickcap.htm BTW, the article is not entirely composed of generally agreed-upon truth. My reflex reaction was caused by the fact that this article was what made me start measuring caps. I measured a few caps after reading it, once I stopped laughing. More to the point, I actually listened to some capacitors, and developed more evidence for laughing. When discussing the influence of caps, keep in mind that most musicproductions, recordings or live performance, are made using large mixing-consoles and outboard equipment that is loaded with caps in the signal-path. Mostly electrolytics, no bias. That's because despite all the hubub, most equipment that uses caps is properly designed. Once heard Rupert Neve during a lecturer claim he did apply about 1V bias to make all electrolytics beeing right polarized for most of the signal ( amplitude ). Ole Rupert has said a lot of things, some ordinary, some wonderful, and some pretty strange. Then I did a complete overhaul of a large console ( DDA DCM232 ) ( http://www.ddaconsoles.com/pdf/schem...schematics.pdf ) that was 12 years old. measuring frequency response and distortion of the single channel-strips showed a disaster. Still it has been in use till days before it was taken to my works. Roll-off starting at 200Hz down giving -10dB at 20Hz, or Distortion over 1%, or High-cut at 6kHz, or combination hereof. All these faults originated from electrolytics. Dried, leaked, exploded. So finally decided to recap the whole console. A well-known exposure with older equipment, one that has zilch to do with the topics in the Marsh-Jung article you cited. My conclusion: unless special precautions have been taken, it is wise to check behavior every 5 years and when these faults occur, recap. That makes as little sense as recommending a complete overhaul of every car engine after 3-5 years. Most people don't do that. They first check the condition of the engine to see if its performance warrants the overhaul. |
#25
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X7R ceramics for DC blocking ?
Eeyore wrote in
: Bob Quintal wrote: From what I understand of the distortion creation mechanism, the increase in voltage causes a change in the capacitance. The dielectric constant isn't errrr... constant ! This change is most pronounced at low voltages especially for Y5V caps. Since without bias, the change in voltage is equal to the peak-peak voltage, that represents a large change in the impedance through the cap. The bias reduces the percentage change significantly. No. Not so. Coupling caps *do not* see the signal voltage across them. They wouldn't be doing their job if they did. Graham Every capacitor has some series impedance. In a properly designed circuit, a coupling cap should be chosen to have a very low impedance compared to the rest of the circuit, In the case of a ceramic cap, the impedance may be 1% of the load impedance it sees, but if the capacitance changes, that impedance will change. All I'm saying is that if it changes from 1% when the signal is at negative peak to 2%, when the signal is at positive peak, that will introduce a certain amount of distortion. -- Bob Quintal PA is y I've altered my email address. -- Posted via a free Usenet account from http://www.teranews.com |
#26
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X7R ceramics for DC blocking ?
Bob Quintal wrote: Eeyore wrote Bob Quintal wrote: From what I understand of the distortion creation mechanism, the increase in voltage causes a change in the capacitance. The dielectric constant isn't errrr... constant ! This change is most pronounced at low voltages especially for Y5V caps. Since without bias, the change in voltage is equal to the peak-peak voltage, that represents a large change in the impedance through the cap. The bias reduces the percentage change significantly. No. Not so. Coupling caps *do not* see the signal voltage across them. They wouldn't be doing their job if they did. Every capacitor has some series impedance. In a properly designed circuit, a coupling cap should be chosen to have a very low impedance compared to the rest of the circuit, In the case of a ceramic cap, the impedance may be 1% of the load impedance it sees, but if the capacitance changes, that impedance will change. All I'm saying is that if it changes from 1% when the signal is at negative peak to 2%, when the signal is at positive peak, that will introduce a certain amount of distortion. Let's suppose the signal voltage is 1V. 1% of that (per your example) of 1V is 10mV. That's the kind of signal that'll be *across* the cap. Your statement "the change in voltage is equal to the peak-peak voltage" is simply incorrect. Perhaps you could simply have accepted that without seeming to obfuscate ? FYI I typically use a 10uF (electrolytic) coupling cap for inputs. That has an impedance of -j795 ohms @ 20 Hz. With a 1V rms signal and a typical 10k input impedance the voltage *across* the cap in those conditions is ~ 30mV. In this case that 30mV is well below the threshold at which electrolytic rectification takes place and the signal is not distorted. I have performed measurements to confirm this btw. Graham |
#27
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X7R ceramics for DC blocking ?
Eeyore wrote in
: Bob Quintal wrote: Eeyore wrote Bob Quintal wrote: From what I understand of the distortion creation mechanism, the increase in voltage causes a change in the capacitance. The dielectric constant isn't errrr... constant ! This change is most pronounced at low voltages especially for Y5V caps. Since without bias, the change in voltage is equal to the peak-peak voltage, that represents a large change in the impedance through the cap. The bias reduces the percentage change significantly. No. Not so. Coupling caps *do not* see the signal voltage across them. They wouldn't be doing their job if they did. Every capacitor has some series impedance. In a properly designed circuit, a coupling cap should be chosen to have a very low impedance compared to the rest of the circuit, In the case of a ceramic cap, the impedance may be 1% of the load impedance it sees, but if the capacitance changes, that impedance will change. All I'm saying is that if it changes from 1% when the signal is at negative peak to 2%, when the signal is at positive peak, that will introduce a certain amount of distortion. Let's suppose the signal voltage is 1V. 1% of that (per your example) of 1V is 10mV. That's the kind of signal that'll be *across* the cap. Your statement "the change in voltage is equal to the peak-peak voltage" is simply incorrect. Perhaps you could simply have accepted that without seeming to obfuscate ? FYI I typically use a 10uF (electrolytic) coupling cap for inputs. That has an impedance of -j795 ohms @ 20 Hz. With a 1V rms signal and a typical 10k input impedance the voltage *across* the cap in those conditions is ~ 30mV. In this case that 30mV is well below the threshold at which electrolytic rectification takes place and the signal is not distorted. I have performed measurements to confirm this btw. Graham Graham, The thread is about ceramic capacitors used for coupling. I usually use a 6.8mF cap for coupling a microphone input,and 22mF for interstage . It's a fact that ceramic caps do have a capacitance/voltage sensitivity. If the capacitance changes then the series resistance (the real part of impedance) will change too. It doesn't happen with electrolytics, so you don't have to worry unless you change your designs. The Original Poster was asking about cercaps because of their smaller physical dimensions per unit capacitance. Using those as coupling caps requires additional considerations be entertained over yours. Not everybody remains stuck with a 1960s mentality. -- Bob Quintal PA is y I've altered my email address. -- Posted via a free Usenet account from http://www.teranews.com |
#28
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X7R ceramics for DC blocking ?
Bob Quintal wrote: Graham, The thread is about ceramic capacitors used for coupling. Actually it's about coupling caps and whether ceramics are suitable. I think they're not suitable (in the types available for the required value) and I'm recommending an alumimium electrolytic instead. I usually use a 6.8mF cap for coupling a microphone input, For a low impedance mic input ???? m means milli btw. I assume you don't mean 6800uF? and 22mF for interstage . A larger value for interstage ? That's curious. It's a fact that ceramic caps do have a capacitance/voltage sensitivity. Medium and high K ceramic dielectrics do. Low K ceramics don't exhibit this problem. If the capacitance changes then the series resistance (the real part of impedance) will change too. No, the complex part changes (on account of K changing). It doesn't happen with electrolytics, That's exactly my point. Electrolytics exhibit a different 'problem' that's very easily completely avoided. so you don't have to worry unless you change your designs. The Original Poster was asking about cercaps because of their smaller physical dimensions per unit capacitance. It's got to be pretty desperate if there's no room for a 10uF electrolytic. Using those as coupling caps requires additional considerations be entertained over yours. Not everybody remains stuck with a 1960s mentality. What 1960s mentality ? Graham |
#29
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X7R ceramics for DC blocking ?
"Arny Krueger" wrote in message ... My conclusion: unless special precautions have been taken, it is wise to check behavior every 5 years and when these faults occur, recap. That makes as little sense as recommending a complete overhaul of every car engine after 3-5 years. Most people don't do that. They first check the condition of the engine to see if its performance warrants the overhaul. And I agree with you both. As he said "check behaviour" and "when faults occur, recap" Similar to doing a compression test on a car engine and when crook, rebuild. However in both cases it is usually obvious something is wrong, so you do more thorough checking whenever necessary, not at any particular time interval IMO. MrT. |
#30
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X7R ceramics for DC blocking ?
Mr.T wrote:
"Arny Krueger" wrote in message ... My conclusion: unless special precautions have been taken, it is wise to check behavior every 5 years and when these faults occur, recap. That makes as little sense as recommending a complete overhaul of every car engine after 3-5 years. Most people don't do that. They first check the condition of the engine to see if its performance warrants the overhaul. And I agree with you both. As he said "check behaviour" and "when faults occur, recap" Similar to doing a compression test on a car engine and when crook, rebuild. However in both cases it is usually obvious something is wrong, so you do more thorough checking whenever necessary, not at any particular time interval IMO. MrT. Very capable and respected producers have been working on that console for twelve years. Wear of caps, and so sound becoming worse goes slow. In one channel that looses the low frequencies you plug in a microphone for vocals and it appears to be ok. Eventually they find one or two channels to be "defective" and don't use these for a while, stick some tape over the fader and carry on working. This was at a radio and tv station. The producers and programtechnicians haven't bought the console, they don't own it, they don't care. I care, I checked it and proposed a complete overhaul. And so did. Found that half of all electrolytics needed to be replaced. Concluded that the other half would be in that state within a few years and so replaced all electrolytics. Did replace electrolytics,... no, let's say; _needed_ to replace electrolytics in various other apparatus like distribution amplifiers ( 1-in 6-out ) and all kind of outboard. Therefore I propose to check every now and then. When _MY_ car makes a strange noise or has a bit less compression, I'll notice. Andre |
#31
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X7R ceramics for DC blocking ?
"Bob Woodward" "Bob wrote in message ... Very capable and respected producers have been working on that console for twelve years. Yes, I always find that a laugh too, unless they complained, nothing was done, and they simply gave up. That's all too common IME. Wear of caps, and so sound becoming worse goes slow. And these capable producers have NEVER listened to anything else in all that time? I'll bet most could tell something was wrong. In one channel that looses the low frequencies you plug in a microphone for vocals and it appears to be ok. Eventually they find one or two channels to be "defective" and don't use these for a while, stick some tape over the fader and carry on working. Well if you can accept the loss of those channels, it's not a problem then. And they obviously DID notice a problem if they went to the trouble of marking it (for the technician to fix later). Of course they carried on working for that session, I do exactly the same if I can still work around the problem with spare channels. This was at a radio and tv station. The producers and programtechnicians haven't bought the console, they don't own it, they don't care. I would have thought the technicians were being paid to care. Kick them up the arse! I care, I checked it and proposed a complete overhaul. And so did. Found that half of all electrolytics needed to be replaced. Concluded that the other half would be in that state within a few years and so replaced all electrolytics. A reasonable course of action. Therefore I propose to check every now and then. Won't hurt. When _MY_ car makes a strange noise or has a bit less compression, I'll notice. And you don't think you will notice the audio problems you mention? In fact I'm more likely to notice a small loss of bass or treble than a small loss of compression myself. If the problems are big enough, anyone will notice in either case. MrT. |
#32
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X7R ceramics for DC blocking ?
Eeyore wrote:
It doesn't happen with electrolytics, That's exactly my point. Electrolytics exhibit a different 'problem' that's very easily completely avoided. so you don't have to worry unless you change your designs. The Original Poster was asking about cercaps because of their smaller physical dimensions per unit capacitance. It's got to be pretty desperate if there's no room for a 10uF electrolytic. And where will you put a 10uF electro in a portable MP3 player for example ? The answer is you don't. you stick with DC coupling. In my case I don't have such extreme size constraints. I decided to fit in some conventional through hole 63V 100nF polyester caps and forget about using X7R 0805 100nF 50V caps. This is for DC blocking the inputs from the outside world. With a 470kohm load resistance that'll form a 3.1Hz cutoff HPF. The DC offset due to input bias current across a 470k ohm resistor is still well under the intrinsic DC offset voltage of the op-amp. The remainder of the signal chain is DC coupled right through to the outputs. At very worst case, one can use a servo amplifier to correct DC offsets. And probably cheaper too than a bunch of largish capacitors scattered throughout. |
#33
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X7R ceramics for DC blocking ?
Adam S wrote: Eeyore wrote: It doesn't happen with electrolytics, That's exactly my point. Electrolytics exhibit a different 'problem' that's very easily completely avoided. so you don't have to worry unless you change your designs. The Original Poster was asking about cercaps because of their smaller physical dimensions per unit capacitance. It's got to be pretty desperate if there's no room for a 10uF electrolytic. And where will you put a 10uF electro in a portable MP3 player for example ? You get a miniature type. The answer is you don't. you stick with DC coupling I very much doubt that but there's no need to scatter them about like confetti. Graham |
#34
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X7R ceramics for DC blocking ?
"Adam S" not.valid@nosuchaddress wrote in message
u Eeyore wrote: It doesn't happen with electrolytics, That's exactly my point. Electrolytics exhibit a different 'problem' that's very easily completely avoided. so you don't have to worry unless you change your designs. The Original Poster was asking about cercaps because of their smaller physical dimensions per unit capacitance. It's got to be pretty desperate if there's no room for a 10uF electrolytic. And where will you put a 10uF electro in a portable MP3 player for example ? Near the headphone jack, and across the battery, for starts. Since most MP3 players use power with just one polarity, there are generally at least output coupling caps. Typcially they are more like 100 uF, than 10 uF. A standard 10 uF, 10 volt SMT electrolytic is about 3 x 5.5 x 3.3 mm or 4 x 5.5 x 4.3 mm, according to http://panasonic.com/industrial/comp...ABA0000CE2.pdf . That's pretty small! |
#35
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X7R ceramics for DC blocking ?
Arny Krueger wrote:
And where will you put a 10uF electro in a portable MP3 player for example ? Near the headphone jack, and across the battery, for starts. Since most MP3 players use power with just one polarity, there are generally at least output coupling caps. Typcially they are more like 100 uF, than 10 uF. A standard 10 uF, 10 volt SMT electrolytic is about 3 x 5.5 x 3.3 mm or 4 x 5.5 x 4.3 mm, according to http://panasonic.com/industrial/comp...ABA0000CE2.pdf . That's pretty small! For the battery you use a 10uF 6.3V 0805 ceramic which is cheaper, smaller and more reliable. But why bother for the headphones when National, Maxim and others have offered low cost capacitorless headphone driver ICs for a many years now. Adding a 1/2 Vcc virtual ground doesn't take up much more silicon area. http://www.national.com/pf/LM/LM4924.html http://www.national.com/pf/LM/LM4982.html http://www.national.com/pf/LM/LM4985.html .... http://datasheets.maxim-ic.com/en/ds/MAX9725.pdf http://datasheets.maxim-ic.com/en/ds...2-MAX9722B.pdf .... Adam |
#36
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X7R ceramics for DC blocking ?
"Adam S" not.valid@nosuchaddress wrote in message
u Arny Krueger wrote: And where will you put a 10uF electro in a portable MP3 player for example ? Near the headphone jack, and across the battery, for starts. Since most MP3 players use power with just one polarity, there are generally at least output coupling caps. Typcially they are more like 100 uF, than 10 uF. A standard 10 uF, 10 volt SMT electrolytic is about 3 x 5.5 x 3.3 mm or 4 x 5.5 x 4.3 mm, according to http://panasonic.com/industrial/comp...ABA0000CE2.pdf . That's pretty small! For the battery you use a 10uF 6.3V 0805 ceramic which is cheaper, smaller and more reliable. But why bother for the headphones when National, Maxim and others have offered low cost capacitorless headphone driver ICs for a many years now. Adding a 1/2 Vcc virtual ground doesn't take up much more silicon area. http://www.national.com/pf/LM/LM4924.html http://www.national.com/pf/LM/LM4982.html http://www.national.com/pf/LM/LM4985.html ... http://datasheets.maxim-ic.com/en/ds/MAX9725.pdf http://datasheets.maxim-ic.com/en/ds...2-MAX9722B.pdf The MAX 4410 caught my eye. Thanks for the technology update! |
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