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#1
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Microphone Preamp Front-End Architecture
Is there ever a reason NOT to use a differential amplifier as
the first stage of a microphone preamp? -- Randy Yates % "Rollin' and riding and slippin' and Digital Signal Labs % sliding, it's magic." % http://www.digitalsignallabs.com % 'Living' Thing', *A New World Record*, ELO |
#2
Posted to rec.audio.tech
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Microphone Preamp Front-End Architecture
"Randy Yates" wrote in message
Is there ever a reason NOT to use a differential amplifier as the first stage of a microphone preamp? A transformer coupled input makes a differential input at least optional. |
#3
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Microphone Preamp Front-End Architecture
Randy Yates wrote:
Is there ever a reason NOT to use a differential amplifier as the first stage of a microphone preamp? Transformer coupled inputs do not have problems with phantom power. Cheers ian |
#4
Posted to rec.audio.tech
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Microphone Preamp Front-End Architecture
On Tue, 23 Mar 2010 08:40:00 -0700, Randy Yates wrote
(in article ): Is there ever a reason NOT to use a differential amplifier as the first stage of a microphone preamp? I really can't think of any instance where NOT using a differential amplifier as a microphone input stage would, in any way, be considered beneficial. Differential amps have a high common-mode rejection ratio which is the raison d'etre for balanced microphone cables in the first place. Common mode signals are generally characterized as those unwanted signals such as induced hum and RF interference, etc. that appear equally on both signal-carrying conductors on a balanced microphone line. Transformers also can cancel common mode signals, but not as well as a properly designed differential amplifier stage (IOW, they do not as high a common mode rejection ratio). When transformers are employed, it is not necessary to employ a differential stage following the transformer. However, transformers have their own problems in that it is more difficult (and therefore more expensive) to design and build transformers with the very linear frequency response coupled with a minimum of phase-shift that is desirable for coupling microphones to electronics. But, transformers can add voltage gain to a circuit without the added noise of a high-gain active stage, and are therefore probably necessary in tube microphone preamps. They also make applying 48 volts DC of phantom power easier as it is simply applied at the center tap of the primary and shows up equally on both signal-carrying conductors of the balanced input line. The transformer will not couple the DC across the transformer and thus it stays out of the electronics where it can cause a voltage offset which will reduce the signal handling properties of the electronics. It is possible to design an effective phantom powering scheme for a solid-state differential amplifier that is both cheap to build and which does not couple through the electronics by simply making the differential amp work down to DC. This way that +48 volts appears equally on both the inverting and the non-inverting inputs to the differential amp and is "cancelled out causing no DC offset. Capacitor coupling can also be used for this purpose. To see how this works, go to: http://sound.westhost.com/project96.htm for a circuit diagram showing a simple phantom powering setup for a differential amplifier type mic preamp. Go he http://sound.westhost.com/project66.htm to see a decent design for a differential microphone preamplifier stage. From the two schematics. it should be apparent how they work in concert to provide phantom powering to the microphone and keep the +48 volts out of the electronics. |
#5
Posted to rec.audio.tech
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Microphone Preamp Front-End Architecture
Audio Empire wrote:
On Tue, 23 Mar 2010 08:40:00 -0700, Randy Yates wrote (in article ): Is there ever a reason NOT to use a differential amplifier as the first stage of a microphone preamp? I really can't think of any instance where NOT using a differential amplifier as a microphone input stage would, in any way, be considered beneficial. 1. They provide no galvanic isolation (transformers do) 2. If they have phantom power connected to them, user cock ups an destroy the input stage (transformers don't) Cheers ian Differential amps have a high common-mode rejection ratio which is the raison d'etre for balanced microphone cables in the first place. Common mode signals are generally characterized as those unwanted signals such as induced hum and RF interference, etc. that appear equally on both signal-carrying conductors on a balanced microphone line. Transformers also can cancel common mode signals, but not as well as a properly designed differential amplifier stage (IOW, they do not as high a common mode rejection ratio). When transformers are employed, it is not necessary to employ a differential stage following the transformer. However, transformers have their own problems in that it is more difficult (and therefore more expensive) to design and build transformers with the very linear frequency response coupled with a minimum of phase-shift that is desirable for coupling microphones to electronics. But, transformers can add voltage gain to a circuit without the added noise of a high-gain active stage, and are therefore probably necessary in tube microphone preamps. They also make applying 48 volts DC of phantom power easier as it is simply applied at the center tap of the primary and shows up equally on both signal-carrying conductors of the balanced input line. The transformer will not couple the DC across the transformer and thus it stays out of the electronics where it can cause a voltage offset which will reduce the signal handling properties of the electronics. It is possible to design an effective phantom powering scheme for a solid-state differential amplifier that is both cheap to build and which does not couple through the electronics by simply making the differential amp work down to DC. This way that +48 volts appears equally on both the inverting and the non-inverting inputs to the differential amp and is "cancelled out causing no DC offset. Capacitor coupling can also be used for this purpose. To see how this works, go to: http://sound.westhost.com/project96.htm for a circuit diagram showing a simple phantom powering setup for a differential amplifier type mic preamp. Go he http://sound.westhost.com/project66.htm to see a decent design for a differential microphone preamplifier stage. From the two schematics. it should be apparent how they work in concert to provide phantom powering to the microphone and keep the +48 volts out of the electronics. |
#6
Posted to rec.audio.tech
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Microphone Preamp Front-End Architecture
On Tue, 23 Mar 2010 16:44:59 -0700, Ian Bell wrote
(in article ): Audio Empire wrote: On Tue, 23 Mar 2010 08:40:00 -0700, Randy Yates wrote (in article ): Is there ever a reason NOT to use a differential amplifier as the first stage of a microphone preamp? I really can't think of any instance where NOT using a differential amplifier as a microphone input stage would, in any way, be considered beneficial. 1. They provide no galvanic isolation (transformers do) And this is important, when? 2. If they have phantom power connected to them, user cock ups an destroy the input stage (transformers don't) Most modern differential amplifiers are pretty well protected against that. I still wouldn't consider that an overwhelming reason to use transformers and still think that the disadvantages of transformers outweigh the advantages. Cheers ian Differential amps have a high common-mode rejection ratio which is the raison d'etre for balanced microphone cables in the first place. Common mode signals are generally characterized as those unwanted signals such as induced hum and RF interference, etc. that appear equally on both signal-carrying conductors on a balanced microphone line. Transformers also can cancel common mode signals, but not as well as a properly designed differential amplifier stage (IOW, they do not as high a common mode rejection ratio). When transformers are employed, it is not necessary to employ a differential stage following the transformer. However, transformers have their own problems in that it is more difficult (and therefore more expensive) to design and build transformers with the very linear frequency response coupled with a minimum of phase-shift that is desirable for coupling microphones to electronics. But, transformers can add voltage gain to a circuit without the added noise of a high-gain active stage, and are therefore probably necessary in tube microphone preamps. They also make applying 48 volts DC of phantom power easier as it is simply applied at the center tap of the primary and shows up equally on both signal-carrying conductors of the balanced input line. The transformer will not couple the DC across the transformer and thus it stays out of the electronics where it can cause a voltage offset which will reduce the signal handling properties of the electronics. It is possible to design an effective phantom powering scheme for a solid-state differential amplifier that is both cheap to build and which does not couple through the electronics by simply making the differential amp work down to DC. This way that +48 volts appears equally on both the inverting and the non-inverting inputs to the differential amp and is "cancelled out causing no DC offset. Capacitor coupling can also be used for this purpose. To see how this works, go to: http://sound.westhost.com/project96.htm for a circuit diagram showing a simple phantom powering setup for a differential amplifier type mic preamp. Go he http://sound.westhost.com/project66.htm to see a decent design for a differential microphone preamplifier stage. From the two schematics. it should be apparent how they work in concert to provide phantom powering to the microphone and keep the +48 volts out of the electronics. |
#7
Posted to rec.audio.tech
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Microphone Preamp Front-End Architecture
On 3/23/2010 5:40 PM Audio Empire spake thus:
On Tue, 23 Mar 2010 16:44:59 -0700, Ian Bell wrote (in article ): Audio Empire wrote: On Tue, 23 Mar 2010 08:40:00 -0700, Randy Yates wrote (in article ): Is there ever a reason NOT to use a differential amplifier as the first stage of a microphone preamp? I really can't think of any instance where NOT using a differential amplifier as a microphone input stage would, in any way, be considered beneficial. 1. They provide no galvanic isolation (transformers do) And this is important, when? 2. If they have phantom power connected to them, user cock ups an destroy the input stage (transformers don't) Most modern differential amplifiers are pretty well protected against that. I still wouldn't consider that an overwhelming reason to use transformers and still think that the disadvantages of transformers outweigh the advantages. Why are we even discussing microphone transformers here? (I guess this is the Usenet version of "telephone".) I mean, the OP didn't mention them; neither did they mention phantom power, yet everyone seemed to jump to the conclusion that this would be a crucial consideration. I'd say that since a differential amp practically eliminates common-mode noise, there's no reason *not* to use one (which, after all, was the question the OP asked). Sheesh. -- You were wrong, and I'm man enough to admit it. - a Usenet "apology" |
#8
Posted to rec.audio.tech
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Microphone Preamp Front-End Architecture
In article , Randy Yates
wrote: Is there ever a reason NOT to use a differential amplifier as the first stage of a microphone preamp? Differencing doesn't have to be in the first stage. I just has to be performed before common mode noise would be amplified too much. In laboratory circuits, sometimes a high impedance voltage follower buffers the input before the differential amp. The output of that follower also drives a shield on the input wire. The result is a virtually perfect cable with no capacitance and no insulation losses. (It only works as long as the electrical wavelength of the input is much longer than the wire, as with audio.) -- I won't see Google Groups replies because I must filter them as spam |
#9
Posted to rec.audio.tech
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Microphone Preamp Front-End Architecture
On Tue, 23 Mar 2010 20:43:30 -0700, David Nebenzahl wrote
(in article ): On 3/23/2010 5:40 PM Audio Empire spake thus: On Tue, 23 Mar 2010 16:44:59 -0700, Ian Bell wrote (in article ): Audio Empire wrote: On Tue, 23 Mar 2010 08:40:00 -0700, Randy Yates wrote (in article ): Is there ever a reason NOT to use a differential amplifier as the first stage of a microphone preamp? I really can't think of any instance where NOT using a differential amplifier as a microphone input stage would, in any way, be considered beneficial. 1. They provide no galvanic isolation (transformers do) And this is important, when? 2. If they have phantom power connected to them, user cock ups an destroy the input stage (transformers don't) Most modern differential amplifiers are pretty well protected against that. I still wouldn't consider that an overwhelming reason to use transformers and still think that the disadvantages of transformers outweigh the advantages. Why are we even discussing microphone transformers here? (I guess this is the Usenet version of "telephone".) I mean, the OP didn't mention them; neither did they mention phantom power, yet everyone seemed to jump to the conclusion that this would be a crucial consideration. I'd say that since a differential amp practically eliminates common-mode noise, there's no reason *not* to use one (which, after all, was the question the OP asked). Sheesh. That's what I said. I only brought-up transformers as an alternative (after all, they do replace diff amps in some designs) and to show that this older technology was at once more expensive and less effective than are modern differential amplifiers. |
#10
Posted to rec.audio.tech
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Microphone Preamp Front-End Architecture
On Tue, 23 Mar 2010 22:58:47 -0700, Kevin McMurtrie wrote
(in article ): In article , Randy Yates wrote: Is there ever a reason NOT to use a differential amplifier as the first stage of a microphone preamp? Differencing doesn't have to be in the first stage. I just has to be performed before common mode noise would be amplified too much. In laboratory circuits, sometimes a high impedance voltage follower buffers the input before the differential amp. The output of that follower also drives a shield on the input wire. The result is a virtually perfect cable with no capacitance and no insulation losses. (It only works as long as the electrical wavelength of the input is much longer than the wire, as with audio.) Yep. But this technique is rare in most modern microphone preamps because it's more expensive to implement. |
#11
Posted to rec.audio.tech
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Microphone Preamp Front-End Architecture
On 3/23/2010 10:41 PM Audio Empire spake thus:
On Tue, 23 Mar 2010 20:43:30 -0700, David Nebenzahl wrote (in article ): Why are we even discussing microphone transformers here? (I guess this is the Usenet version of "telephone".) I mean, the OP didn't mention them; neither did they mention phantom power, yet everyone seemed to jump to the conclusion that this would be a crucial consideration. I'd say that since a differential amp practically eliminates common-mode noise, there's no reason *not* to use one (which, after all, was the question the OP asked). Sheesh. That's what I said. I only brought-up transformers as an alternative (after all, they do replace diff amps in some designs) and to show that this older technology was at once more expensive and less effective than are modern differential amplifiers. Yes. My exasperation wasn't directed at you, but at some replies up-thread of yours. -- You were wrong, and I'm man enough to admit it. - a Usenet "apology" |
#12
Posted to rec.audio.tech
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Microphone Preamp Front-End Architecture
"David Nebenzahl" wrote in message
.com I'd say that since a differential amp practically eliminates common-mode noise, there's no reason *not* to use one (which, after all, was the question the OP asked). Transformers are an effective means of elimination common-mode noise. They are as a rule more effective than differential amplifiers. If you have an input transformer, you don't need a differential input and you pick up some inherent protection against EMI and casualty losses. If you've ever had a grounding problem that a standard active differential input can't handle, you know what I mean. Trnasformers also provide galvanic isolation, or isolation of signal ground. We would probably be a little less critical of the configuration of safety grounds in audio if we were still in the days of transformers. Transformers would probably rule the inputs of professional grade gear were it not for costs. While it is hard to make transformers have as flat of frequency response and as have ultimately low distortion as differential inputs, it is possible to make them perform well enough to be sonically innocious. Transformers are still the preferred means for obtaining the best possible freedom from grounding problems in areas where the cost issues are not so severe, such as analog and digital communications. |
#13
Posted to rec.audio.tech
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Microphone Preamp Front-End Architecture
Hi Arny et alius,
Thank you all for your comments. They have been very helpful. FYI, the reason for my question is that I am attempting to review the design of the front-end of a heart rate monitoring device. This design is based on the SSM2167 (e.g., from National). My thinking is that this is the wrong part for the job since the sensor is fed to the preamp via a run of cable perhaps 4 feet in length, and there could therefore be lots of common-mode noise. Also, I believe the AGC action is, in this stage, not appropriate. This appears to be an electret type of microphone in that DC power is fed to the device, so the poster who discussed how to feed phantom power and simultaneously receive the input differentially was right on the money. Thanks again for your input and suggestions. --Randy "Arny Krueger" writes: "David Nebenzahl" wrote in message .com I'd say that since a differential amp practically eliminates common-mode noise, there's no reason *not* to use one (which, after all, was the question the OP asked). Transformers are an effective means of elimination common-mode noise. They are as a rule more effective than differential amplifiers. If you have an input transformer, you don't need a differential input and you pick up some inherent protection against EMI and casualty losses. If you've ever had a grounding problem that a standard active differential input can't handle, you know what I mean. Trnasformers also provide galvanic isolation, or isolation of signal ground. We would probably be a little less critical of the configuration of safety grounds in audio if we were still in the days of transformers. Transformers would probably rule the inputs of professional grade gear were it not for costs. While it is hard to make transformers have as flat of frequency response and as have ultimately low distortion as differential inputs, it is possible to make them perform well enough to be sonically innocious. Transformers are still the preferred means for obtaining the best possible freedom from grounding problems in areas where the cost issues are not so severe, such as analog and digital communications. -- Randy Yates % "Maybe one day I'll feel her cold embrace, Digital Signal Labs % and kiss her interface, % til then, I'll leave her alone." http://www.digitalsignallabs.com % 'Yours Truly, 2095', *Time*, ELO |
#14
Posted to rec.audio.tech
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Microphone Preamp Front-End Architecture
Audio Empire wrote:
On Tue, 23 Mar 2010 16:44:59 -0700, Ian Bell wrote (in article ): Audio Empire wrote: On Tue, 23 Mar 2010 08:40:00 -0700, Randy Yates wrote (in article ): Is there ever a reason NOT to use a differential amplifier as the first stage of a microphone preamp? I really can't think of any instance where NOT using a differential amplifier as a microphone input stage would, in any way, be considered beneficial. 1. They provide no galvanic isolation (transformers do) And this is important, when? Not heard of hum loops then? 2. If they have phantom power connected to them, user cock ups an destroy the input stage (transformers don't) Most modern differential amplifiers are pretty well protected against that. So the recent AES lectures on this very point were, er pointless then? I still wouldn't consider that an overwhelming reason to use transformers and still think that the disadvantages of transformers outweigh the advantages. I never said transformers were better, the OP just asked why you would NOT use a diff. amp and I gave two examples. Cheers Ian Cheers ian Differential amps have a high common-mode rejection ratio which is the raison d'etre for balanced microphone cables in the first place. Common mode signals are generally characterized as those unwanted signals such as induced hum and RF interference, etc. that appear equally on both signal-carrying conductors on a balanced microphone line. Transformers also can cancel common mode signals, but not as well as a properly designed differential amplifier stage (IOW, they do not as high a common mode rejection ratio). When transformers are employed, it is not necessary to employ a differential stage following the transformer. However, transformers have their own problems in that it is more difficult (and therefore more expensive) to design and build transformers with the very linear frequency response coupled with a minimum of phase-shift that is desirable for coupling microphones to electronics. But, transformers can add voltage gain to a circuit without the added noise of a high-gain active stage, and are therefore probably necessary in tube microphone preamps. They also make applying 48 volts DC of phantom power easier as it is simply applied at the center tap of the primary and shows up equally on both signal-carrying conductors of the balanced input line. The transformer will not couple the DC across the transformer and thus it stays out of the electronics where it can cause a voltage offset which will reduce the signal handling properties of the electronics. It is possible to design an effective phantom powering scheme for a solid-state differential amplifier that is both cheap to build and which does not couple through the electronics by simply making the differential amp work down to DC. This way that +48 volts appears equally on both the inverting and the non-inverting inputs to the differential amp and is "cancelled out causing no DC offset. Capacitor coupling can also be used for this purpose. To see how this works, go to: http://sound.westhost.com/project96.htm for a circuit diagram showing a simple phantom powering setup for a differential amplifier type mic preamp. Go he http://sound.westhost.com/project66.htm to see a decent design for a differential microphone preamplifier stage. From the two schematics. it should be apparent how they work in concert to provide phantom powering to the microphone and keep the +48 volts out of the electronics. |
#15
Posted to rec.audio.tech
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Microphone Preamp Front-End Architecture
In article , Randy Yates wrote:
Hi Arny et alius, Thank you all for your comments. They have been very helpful. FYI, the reason for my question is that I am attempting to review the design of the front-end of a heart rate monitoring device. This design is based on the SSM2167 (e.g., from National). My thinking is that this is the wrong part for the job since the sensor is fed to the preamp via a run of cable perhaps 4 feet in length, and there could therefore be lots of common-mode noise. Also, I believe the AGC action is, in this stage, not appropriate. This appears to be an electret type of microphone in that DC power is fed to the device, so the poster who discussed how to feed phantom power and simultaneously receive the input differentially was right on the money. Thanks again for your input and suggestions. --Randy Hope your using battery power. Isolated preamps must be used when attached to the line. They usually use switching and transformers. greg "Arny Krueger" writes: "David Nebenzahl" wrote in message .com I'd say that since a differential amp practically eliminates common-mode noise, there's no reason *not* to use one (which, after all, was the question the OP asked). Transformers are an effective means of elimination common-mode noise. They are as a rule more effective than differential amplifiers. If you have an input transformer, you don't need a differential input and you pick up some inherent protection against EMI and casualty losses. If you've ever had a grounding problem that a standard active differential input can't handle, you know what I mean. Trnasformers also provide galvanic isolation, or isolation of signal ground. We would probably be a little less critical of the configuration of safety grounds in audio if we were still in the days of transformers. Transformers would probably rule the inputs of professional grade gear were it not for costs. While it is hard to make transformers have as flat of frequency response and as have ultimately low distortion as differential inputs, it is possible to make them perform well enough to be sonically innocious. Transformers are still the preferred means for obtaining the best possible freedom from grounding problems in areas where the cost issues are not so severe, such as analog and digital communications. |
#16
Posted to rec.audio.tech
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Microphone Preamp Front-End Architecture
Ian Bell writes:
Audio Empire wrote: On Tue, 23 Mar 2010 16:44:59 -0700, Ian Bell wrote (in article ): Audio Empire wrote: On Tue, 23 Mar 2010 08:40:00 -0700, Randy Yates wrote (in article ): Is there ever a reason NOT to use a differential amplifier as the first stage of a microphone preamp? I really can't think of any instance where NOT using a differential amplifier as a microphone input stage would, in any way, be considered beneficial. 1. They provide no galvanic isolation (transformers do) And this is important, when? Not heard of hum loops then? 2. If they have phantom power connected to them, user cock ups an destroy the input stage (transformers don't) Most modern differential amplifiers are pretty well protected against that. So the recent AES lectures on this very point were, er pointless then? I still wouldn't consider that an overwhelming reason to use transformers and still think that the disadvantages of transformers outweigh the advantages. I never said transformers were better, the OP just asked why you would NOT use a diff. amp and I gave two examples. Thanks for the information and examples, Ian. Actually what I should have asked is, "Is there ever a reason NOT to use a differential (balanced) configuration for mic inputs?" whether differential amplifier or transformer. But I get you. --Randy Cheers Ian Cheers ian Differential amps have a high common-mode rejection ratio which is the raison d'etre for balanced microphone cables in the first place. Common mode signals are generally characterized as those unwanted signals such as induced hum and RF interference, etc. that appear equally on both signal-carrying conductors on a balanced microphone line. Transformers also can cancel common mode signals, but not as well as a properly designed differential amplifier stage (IOW, they do not as high a common mode rejection ratio). When transformers are employed, it is not necessary to employ a differential stage following the transformer. However, transformers have their own problems in that it is more difficult (and therefore more expensive) to design and build transformers with the very linear frequency response coupled with a minimum of phase-shift that is desirable for coupling microphones to electronics. But, transformers can add voltage gain to a circuit without the added noise of a high-gain active stage, and are therefore probably necessary in tube microphone preamps. They also make applying 48 volts DC of phantom power easier as it is simply applied at the center tap of the primary and shows up equally on both signal-carrying conductors of the balanced input line. The transformer will not couple the DC across the transformer and thus it stays out of the electronics where it can cause a voltage offset which will reduce the signal handling properties of the electronics. It is possible to design an effective phantom powering scheme for a solid-state differential amplifier that is both cheap to build and which does not couple through the electronics by simply making the differential amp work down to DC. This way that +48 volts appears equally on both the inverting and the non-inverting inputs to the differential amp and is "cancelled out causing no DC offset. Capacitor coupling can also be used for this purpose. To see how this works, go to: http://sound.westhost.com/project96.htm for a circuit diagram showing a simple phantom powering setup for a differential amplifier type mic preamp. Go he http://sound.westhost.com/project66.htm to see a decent design for a differential microphone preamplifier stage. From the two schematics. it should be apparent how they work in concert to provide phantom powering to the microphone and keep the +48 volts out of the electronics. -- Randy Yates % "Watching all the days go by... Digital Signal Labs % Who are you and who am I?" % 'Mission (A World Record)', http://www.digitalsignallabs.com % *A New World Record*, ELO |
#17
Posted to rec.audio.tech
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Microphone Preamp Front-End Architecture
Randy Yates wrote:
Ian Bell writes: Audio Empire wrote: On Tue, 23 Mar 2010 16:44:59 -0700, Ian Bell wrote (in article ): Audio Empire wrote: On Tue, 23 Mar 2010 08:40:00 -0700, Randy Yates wrote (in article ): Is there ever a reason NOT to use a differential amplifier as the first stage of a microphone preamp? I really can't think of any instance where NOT using a differential amplifier as a microphone input stage would, in any way, be considered beneficial. 1. They provide no galvanic isolation (transformers do) And this is important, when? Not heard of hum loops then? 2. If they have phantom power connected to them, user cock ups an destroy the input stage (transformers don't) Most modern differential amplifiers are pretty well protected against that. So the recent AES lectures on this very point were, er pointless then? I still wouldn't consider that an overwhelming reason to use transformers and still think that the disadvantages of transformers outweigh the advantages. I never said transformers were better, the OP just asked why you would NOT use a diff. amp and I gave two examples. Thanks for the information and examples, Ian. Actually what I should have asked is, "Is there ever a reason NOT to use a differential (balanced) configuration for mic inputs?" whether differential amplifier or transformer. But I get you. --Randy No problem. Cheers Ian |
#18
Posted to rec.audio.tech
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Microphone Preamp Front-End Architecture
"Randy Yates" wrote ...
FYI, the reason for my question is that I am attempting to review the design of the front-end of a heart rate monitoring device. This design is based on the SSM2167 (e.g., from National). My thinking is that this is the wrong part for the job since the sensor is fed to the preamp via a run of cable perhaps 4 feet in length, and there could therefore be lots of common-mode noise. It does sound like a less than optimal design. OTOH, if they are only looking for cardiac sounds, they could make the case that everything of interest is *below* the power mains frequency (50/60Hz) and can be simply low-pass filtered. So the kinds of things we do for wideband audio are maybe not necessary for that special case? Also, I believe the AGC action is, in this stage, not appropriate. I agree from an audio perspective. But maybe the full waveform is not what they are looking for? Maybe only the cardiac rate is desired here and preserving the exact waveform is not important? It would presumably be different if they were running a full EKG. This appears to be an electret type of microphone in that DC power is fed to the device, so the poster who discussed how to feed phantom power and simultaneously receive the input differentially was right on the money. There are some clever (and simple/elegant) circuits for using commodity electret elements, feeding power, AND providing a fully balanced cable run from the capsule to the preamp, etc. When I went in for a sleep study, they taped a (disposable) sensor on my finger with a SMD red LED over my fingernail, and what appears to be a small photo-resistive element on the opposite side of my finger. They said that they read both cardiac rate AND blood oxygenation from the sensor. I kept it as a souvenir. It is possible that there is also an IR LED and/or sensor in there, but I haven't put it under the micro- scope yet. |
#19
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Microphone Preamp Front-End Architecture
On Wed, 24 Mar 2010 03:54:59 -0700, Arny Krueger wrote
(in article ) : "David Nebenzahl" wrote in message .com I'd say that since a differential amp practically eliminates common-mode noise, there's no reason *not* to use one (which, after all, was the question the OP asked). Transformers are an effective means of elimination common-mode noise. They are as a rule more effective than differential amplifiers. If you have an input transformer, you don't need a differential input and you pick up some inherent protection against EMI and casualty losses. Now that's interesting. My sources all say that transformers have a lower common-mode rejection ratio than can differential amplifiers. If you've ever had a grounding problem that a standard active differential input can't handle, you know what I mean. Yes, we all have in-line isolation transformers in our kit-bags for just such a contingency. snip |
#20
Posted to rec.audio.tech
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Microphone Preamp Front-End Architecture
Audio Empire wrote:
On Wed, 24 Mar 2010 03:54:59 -0700, Arny Krueger wrote (in article ) : "David Nebenzahl" wrote in message .com I'd say that since a differential amp practically eliminates common-mode noise, there's no reason *not* to use one (which, after all, was the question the OP asked). Transformers are an effective means of elimination common-mode noise. They are as a rule more effective than differential amplifiers. If you have an input transformer, you don't need a differential input and you pick up some inherent protection against EMI and casualty losses. Now that's interesting. My sources all say that transformers have a lower common-mode rejection ratio than can differential amplifiers. 'Have' and 'can (have)' are not identical conditions. Cheers Ian snip |
#21
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Microphone Preamp Front-End Architecture
On Wed, 24 Mar 2010 13:24:45 -0700, Ian Bell wrote
(in article ): Audio Empire wrote: On Wed, 24 Mar 2010 03:54:59 -0700, Arny Krueger wrote (in article ) : "David Nebenzahl" wrote in message .com I'd say that since a differential amp practically eliminates common-mode noise, there's no reason *not* to use one (which, after all, was the question the OP asked). Transformers are an effective means of elimination common-mode noise. They are as a rule more effective than differential amplifiers. If you have an input transformer, you don't need a differential input and you pick up some inherent protection against EMI and casualty losses. Now that's interesting. My sources all say that transformers have a lower common-mode rejection ratio than can differential amplifiers. 'Have' and 'can (have)' are not identical conditions. Cheers Ian snip Well, of course, we're comparing best-case scenarios here. A transformer is a transformer, and while there are a number of makers of microphone transformers, they are all very similar in design and all have transformer characteristics in the same proportion, both good and bad. Differential amplifiers, OTOH, come in a myriad of configurations from cheap 709 style op-amps to very good ones like the latest National Semiconductor LM 497XX series to differential amps made with selected discrete components to potted gain modules made specifically for the pro recording industry. A transformer can be better than a old-school 709 or 741 era op-amp, but newer op-amps are much better than that wrt common mode rejection ratios. |
#22
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Microphone Preamp Front-End Architecture
Audio Empire wrote:
On Wed, 24 Mar 2010 13:24:45 -0700, Ian Bell wrote (in article ): Audio Empire wrote: On Wed, 24 Mar 2010 03:54:59 -0700, Arny Krueger wrote (in article ) : "David Nebenzahl" wrote in message .com I'd say that since a differential amp practically eliminates common-mode noise, there's no reason *not* to use one (which, after all, was the question the OP asked). Transformers are an effective means of elimination common-mode noise. They are as a rule more effective than differential amplifiers. If you have an input transformer, you don't need a differential input and you pick up some inherent protection against EMI and casualty losses. Now that's interesting. My sources all say that transformers have a lower common-mode rejection ratio than can differential amplifiers. 'Have' and 'can (have)' are not identical conditions. Cheers Ian snip Well, of course, we're comparing best-case scenarios here. A transformer is a transformer, and while there are a number of makers of microphone transformers, they are all very similar in design and all have transformer characteristics in the same proportion, both good and bad. Differential amplifiers, OTOH, come in a myriad of configurations from cheap 709 style op-amps to very good ones like the latest National Semiconductor LM 497XX series to differential amps made with selected discrete components to potted gain modules made specifically for the pro recording industry. A transformer can be better than a old-school 709 or 741 era op-amp, but newer op-amps are much better than that wrt common mode rejection ratios. Actually, I just noticed you said 'differential amplifiers' rather than operational amplifiers and to that extent I agree with you when you use selected descrete components. However, when it comes to op amps, they are not the prime determinant of achieved CMRR. It's all the other components around it, and by the time you have included the dc blocking caps and zener diodes, series inductors etc to prevent RFI and damage from phantom power and so on necessary for a mic preamp, on a PCB then actual measured CMMR across the audio band will be little better than a good transformer with phantom power fed through a primary centre tap. Cheers Ian |
#23
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Microphone Preamp Front-End Architecture
"Ian Bell" wrote in message
Actually, I just noticed you said 'differential amplifiers' rather than operational amplifiers and to that extent I agree with you when you use selected descrete components. However, when it comes to op amps, they are not the prime determinant of achieved CMRR. It's all the other components around it, and by the time you have included the dc blocking caps and zener diodes, series inductors etc to prevent RFI and damage from phantom power and so on necessary for a mic preamp, on a PCB then actual measured CMMR across the audio band will be little better than a good transformer with phantom power fed through a primary centre tap. Good points. Another hidden gotcha with specifying CMRR is dynamic range. What is the CMRR of an op amp when the common mode voltage is 5 Kv? 120 volts? Peak common mode voltage VCC which might only be 5 volts. Some or all of these voltages are probable in the real world. Of course they are inceasingly probable as the voltage goes down. The answer can be very close to 0 dB. :-( |
#24
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Microphone Preamp Front-End Architecture
In article , (GregS) wrote:
In article , Randy Yates wrote: Hi Arny et alius, Thank you all for your comments. They have been very helpful. FYI, the reason for my question is that I am attempting to review the design of the front-end of a heart rate monitoring device. This design is based on the SSM2167 (e.g., from National). My thinking is that this is the wrong part for the job since the sensor is fed to the preamp via a run of cable perhaps 4 feet in length, and there could therefore be lots of common-mode noise. Also, I believe the AGC action is, in this stage, not appropriate. This appears to be an electret type of microphone in that DC power is fed to the device, so the poster who discussed how to feed phantom power and simultaneously receive the input differentially was right on the money. Thanks again for your input and suggestions. --Randy Hope your using battery power. Isolated preamps must be used when attached to the line. They usually use switching and transformers. greg Here is what I am familiar with in using chest sensors............. http://www.analog.com/static/importe...eets/AD210.pdf greg "Arny Krueger" writes: "David Nebenzahl" wrote in message .com I'd say that since a differential amp practically eliminates common-mode noise, there's no reason *not* to use one (which, after all, was the question the OP asked). Transformers are an effective means of elimination common-mode noise. They are as a rule more effective than differential amplifiers. If you have an input transformer, you don't need a differential input and you pick up some inherent protection against EMI and casualty losses. If you've ever had a grounding problem that a standard active differential input can't handle, you know what I mean. Trnasformers also provide galvanic isolation, or isolation of signal ground. We would probably be a little less critical of the configuration of safety grounds in audio if we were still in the days of transformers. Transformers would probably rule the inputs of professional grade gear were it not for costs. While it is hard to make transformers have as flat of frequency response and as have ultimately low distortion as differential inputs, it is possible to make them perform well enough to be sonically innocious. Transformers are still the preferred means for obtaining the best possible freedom from grounding problems in areas where the cost issues are not so severe, such as analog and digital communications. |
#25
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Microphone Preamp Front-End Architecture
Kevin McMurtrie writes:
In article , Randy Yates wrote: Is there ever a reason NOT to use a differential amplifier as the first stage of a microphone preamp? Differencing doesn't have to be in the first stage. I just has to be performed before common mode noise would be amplified too much. In laboratory circuits, sometimes a high impedance voltage follower buffers the input before the differential amp. Hi Kevin, Do you mean *two* "high impedance voltage follower buffers", one for the "+" and one for the "-" of the differential input signal? The output of that follower also drives a shield on the input wire. "that" follower? Which one? Are you talking about shielded, twisted pair cable? The result is a virtually perfect cable with no capacitance and no insulation losses. (It only works as long as the electrical wavelength of the input is much longer than the wire, as with audio.) Without the previous clarifications, it's hard for me to grok this last statement, but it sounds like an interesting result. -- Randy Yates % "Though you ride on the wheels of tomorrow, Digital Signal Labs % you still wander the fields of your % sorrow." http://www.digitalsignallabs.com % '21st Century Man', *Time*, ELO |
#26
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Microphone Preamp Front-End Architecture
"Randy Yates" wrote ...
Kevin McMurtrie writes: The output of that follower also drives a shield on the input wire. "that" follower? Which one? Are you talking about shielded, twisted pair cable? The result is a virtually perfect cable with no capacitance and no insulation losses. (It only works as long as the electrical wavelength of the input is much longer than the wire, as with audio.) Without the previous clarifications, it's hard for me to grok this last statement, but it sounds like an interesting result. He appears to be talking about the technique of using a "driven guard" http://ieeexplore.ieee.org/stamp/sta...umber=00706021 I hold a patent for using that technique to probe very low currents on semiconductor wafers. |
#27
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Microphone Preamp Front-End Architecture
In article ,
"Richard Crowley" wrote: "Randy Yates" wrote ... Kevin McMurtrie writes: The output of that follower also drives a shield on the input wire. "that" follower? Which one? Are you talking about shielded, twisted pair cable? The result is a virtually perfect cable with no capacitance and no insulation losses. (It only works as long as the electrical wavelength of the input is much longer than the wire, as with audio.) Without the previous clarifications, it's hard for me to grok this last statement, but it sounds like an interesting result. He appears to be talking about the technique of using a "driven guard" http://ieeexplore.ieee.org/stamp/sta...umber=00706021 I hold a patent for using that technique to probe very low currents on semiconductor wafers. I figured out where my copy of "The Art of Electronics" was hiding. "Driven guard" or "bootstrapped guard" is it. It's funny that a patent can be granted for the application of that to silicon wafers sensors. -- I won't see Google Groups replies because I must filter them as spam |
#28
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Microphone Preamp Front-End Architecture
"Kevin McMurtrie" wrote ...
I figured out where my copy of "The Art of Electronics" was hiding. "Driven guard" or "bootstrapped guard" is it. It's funny that a patent can be granted for the application of that to silicon wafers sensors. Well, of course, *I* think it is a valid application of patent protection. :-) OTOH, there is the claim that someone successfully obtained a patent on a ham sandwich (appropriately disguised in legaleese.) |
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