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#1
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how to resistively load a 57?
can i just put a 1k resistor across pins 2 and 3? to my understanding
this degrades CMRR, since the differential signal is knocked down, but not the common mode. how much would CMRR degrade in dB with a preamp with a ~5k input Z? Du |
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#2
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#3
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#4
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dufus wrote:
can i just put a 1k resistor across pins 2 and 3? Absolutely. This is not that different from what happens when you add many mic attenuators. The mic attenuators I've examined provide load impedances in the 1 to 2.5 Kohm range. Attenuators that are connected across pins 2 & 3 are generally recommended because of their minimal effect on CMRR to my understanding this degrades CMRR, since the differential signal is knocked down, but not the common mode. The real question is not CMRR, its resistance to externally-generated noise. Do you have reason to believe that common mode noise is currently a serious problem with your system? Or, are just worrying about possibilities, as opposed to realities? how much would CMRR degrade in dB with a preamp with a ~5k input Z? Don't know, because this calculation would be based on the source impedance of the SM-57, not the input impedance of the preamp. An interesting related article can be found at http://www.eetasia.com/ARTICLES/2002...MSD_POW_AN.PDF Most interesting is: "Figure 3 shows the effect of preamp input resistance (and capacitance) on frequency response of a Shure SM57 with 100 feet of common cable. The upper curves, 10 kS and 3 kS, are typical of transformer-less mic preamps while the lower curve, 1.5 kS, is typical of a transformer input mic preamp. Note the ultra-sonic peaks in response caused by insufficient damping". This chart gives some insight into the output impedance of a SM57. To summarize, there is negligible change in response at 3 KHz with load impedances varying from 1,500 ohms to 10,000 ohms. IOW, the source impedance of a SM57 in the normal audio range (20 KHz) is quite small. Other sources give it as being 150 ohms or 310 ohms. It may be even less - perhaps 75 ohms or less. The same charts show an approximate 5 dB range of response at 20 KHz but less than 1 dB variation at 10 KHz. None of this is all that audibly significant. aside The high frequency variation relates to a resonance between the source inductance of the SM57 and a cable capacitance on the order of 0.035 uF. The resonance centers at about 45 KHz, from which the equivalent source impedance of the SM57 can be calculated. I would expect less than 1 dB variation due to adding a 1K ohm resistor. This means that even in your pessimistic estimate, there would be a 1 dB loss of CMRR which is almost always negligible, even in a high noise environment. |
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#5
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"Arny Krueger" An interesting related article can be found at http://www.eetasia.com/ARTICLES/2002...MSD_POW_AN.PDF Most interesting is: "Figure 3 shows the effect of preamp input resistance (and capacitance) on frequency response of a Shure SM57 with 100 feet of common cable. The upper curves, 10 kS and 3 kS, are typical of transformer-less mic preamps while the lower curve, 1.5 kS, is typical of a transformer input mic preamp. Note the ultra-sonic peaks in response caused by insufficient damping". This chart gives some insight into the output impedance of a SM57. To summarize, there is negligible change in response at 3 KHz with load impedances varying from 1,500 ohms to 10,000 ohms. IOW, the source impedance of a SM57 in the normal audio range (20 KHz) is quite small. Other sources give it as being 150 ohms or 310 ohms. It may be even less - perhaps 75 ohms or less. The same charts show an approximate 5 dB range of response at 20 KHz but less than 1 dB variation at 10 KHz. None of this is all that audibly significant. ** Fig 3 in the Jensen article shows an overall variation of less than 2dB at 20kHz for the three load impedances at the end of 100 feet of cable driven by an SM57. At 15 kHz, or the highest frequency an SM57 actually reproduces, the variation is less than 1 dB while at 10 kHz that variation is less than 0.5 dB. The HF response variation between different samples of the SM57 is a lot greater than that !! ............. Phil |
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#6
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Phil Allison wrote:
"Arny Krueger" An interesting related article can be found at http://www.eetasia.com/ARTICLES/2002...MSD_POW_AN.PDF Most interesting is: "Figure 3 shows the effect of preamp input resistance (and capacitance) on frequency response of a Shure SM57 with 100 feet of common cable. The upper curves, 10 kS and 3 kS, are typical of transformer-less mic preamps while the lower curve, 1.5 kS, is typical of a transformer input mic preamp. Note the ultra-sonic peaks in response caused by insufficient damping". This chart gives some insight into the output impedance of a SM57. To summarize, there is negligible change in response at 3 KHz with load impedances varying from 1,500 ohms to 10,000 ohms. IOW, the source impedance of a SM57 in the normal audio range (20 KHz) is quite small. Other sources give it as being 150 ohms or 310 ohms. It may be even less - perhaps 75 ohms or less. The same charts show an approximate 5 dB range of response at 20 KHz but less than 1 dB variation at 10 KHz. None of this is all that audibly significant. ** Fig 3 in the Jensen article shows an overall variation of less than 2dB at 20kHz for the three load impedances at the end of 100 feet of cable driven by an SM57. At 15 kHz, or the highest frequency an SM57 actually reproduces, the variation is less than 1 dB while at 10 kHz that variation is less than 0.5 dB. The HF response variation between different samples of the SM57 is a lot greater than that !! This article seems to throw quite a bit of cold water on the many claims of dramatic sonic differences due to real-world variations in the loading of SM57s by various preamps and cables. |
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#7
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Arny Krueger wrote:
Phil Allison wrote: "Arny Krueger" An interesting related article can be found at http://www.eetasia.com/ARTICLES/2002...MSD_POW_AN.PDF Most interesting is: "Figure 3 shows the effect of preamp input resistance (and capacitance) on frequency response of a Shure SM57 with 100 feet of common cable. The upper curves, 10 kS and 3 kS, are typical of transformer-less mic preamps while the lower curve, 1.5 kS, is typical of a transformer input mic preamp. Note the ultra-sonic peaks in response caused by insufficient damping". This chart gives some insight into the output impedance of a SM57. To summarize, there is negligible change in response at 3 KHz with load impedances varying from 1,500 ohms to 10,000 ohms. IOW, the source impedance of a SM57 in the normal audio range (20 KHz) is quite small. Other sources give it as being 150 ohms or 310 ohms. It may be even less - perhaps 75 ohms or less. The same charts show an approximate 5 dB range of response at 20 KHz but less than 1 dB variation at 10 KHz. None of this is all that audibly significant. ** Fig 3 in the Jensen article shows an overall variation of less than 2dB at 20kHz for the three load impedances at the end of 100 feet of cable driven by an SM57. At 15 kHz, or the highest frequency an SM57 actually reproduces, the variation is less than 1 dB while at 10 kHz that variation is less than 0.5 dB. The HF response variation between different samples of the SM57 is a lot greater than that !! This article seems to throw quite a bit of cold water on the many claims of dramatic sonic differences due to real-world variations in the loading of SM57s by various preamps and cables. It looks to me as though it's comparing the impact of different loading resistors across the inputs of a particular preamp on the output frequency response of SM57s. Many of the sonic diffrerence claims (and my own experience) show a marked difference between the sound of an SM-57 feeding a high impedance transformerless preamp when compared with that produced by one feeding a transformer-fronted preamp. |
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#8
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In article ,
Kurt Albershardt wrote: Arny Krueger wrote: This article seems to throw quite a bit of cold water on the many claims of dramatic sonic differences due to real-world variations in the loading of SM57s by various preamps and cables. It looks to me as though it's comparing the impact of different loading resistors across the inputs of a particular preamp on the output frequency response of SM57s. Many of the sonic diffrerence claims (and my own experience) show a marked difference between the sound of an SM-57 feeding a high impedance transformerless preamp when compared with that produced by one feeding a transformer-fronted preamp. Bingo. The low end is one place that a transformer loaded preamp can't be compared to a transformerless preamp. Basically, the input transformer is not ideal, and since it has a finite number of turns in its primary and a core of finite permeability, the input inductance may not be high enough to present the same load at low frequencies as is presented to the mike at 1KHz. To a dynamic mike, this will directly influence the mechanics of the transducer. This load impedance is reflected into the transducer and will change the damping quite readily. Practically, I have found that some transformer input preamps like the Groove Tubes VIPRE that use transformer tap switching (and not high quality resistors) to control the input impedance can offer a fairly complex input impedance that can greatly color a 57. In my experience, this coloration is far from subtle. We're talking 4-8dB of coloration here... hardly the stuff that needs to be argued over with DBT. One odd effect I have also noticed with the GT VIPRE is an added peak around 4-8KHz with low load impedances and a Shure 57/545/547 type of mike. I don't know why this happens, but it can sound pretty nice sometimes, and godawful shrill when you don't want it. That's why the impedance switch is there... right? ;-) One final observation... I have noticed that you (Arny) seem to have a bias that prefers not to hear differences between pieces of gear, even though such differences might be reasonably well known to other engineers and sometimes exploited on a regular basis by those same folks. I'm curious as to how this bias helps you get along with life...? I suppose in the ideal case it more accurately models how things happen in your world, but I find it odd that it doesn't line up with what happens in a lot of other folks' worlds who came to some conclusion without having knowledge that others came to the same conclusion. Regards, Monte McGuire |
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#9
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In article ,
Kurt Albershardt wrote: Arny Krueger wrote: This article seems to throw quite a bit of cold water on the many claims of dramatic sonic differences due to real-world variations in the loading of SM57s by various preamps and cables. It looks to me as though it's comparing the impact of different loading resistors across the inputs of a particular preamp on the output frequency response of SM57s. Many of the sonic diffrerence claims (and my own experience) show a marked difference between the sound of an SM-57 feeding a high impedance transformerless preamp when compared with that produced by one feeding a transformer-fronted preamp. Bingo. The low end is one place that a transformer loaded preamp can't be compared to a transformerless preamp. Basically, the input transformer is not ideal, and since it has a finite number of turns in its primary and a core of finite permeability, the input inductance may not be high enough to present the same load at low frequencies as is presented to the mike at 1KHz. To a dynamic mike, this will directly influence the mechanics of the transducer. This load impedance is reflected into the transducer and will change the damping quite readily. Practically, I have found that some transformer input preamps like the Groove Tubes VIPRE that use transformer tap switching (and not high quality resistors) to control the input impedance can offer a fairly complex input impedance that can greatly color a 57. In my experience, this coloration is far from subtle. We're talking 4-8dB of coloration here... hardly the stuff that needs to be argued over with DBT. One odd effect I have also noticed with the GT VIPRE is an added peak around 4-8KHz with low load impedances and a Shure 57/545/547 type of mike. I don't know why this happens, but it can sound pretty nice sometimes, and godawful shrill when you don't want it. That's why the impedance switch is there... right? ;-) One final observation... I have noticed that you (Arny) seem to have a bias that prefers not to hear differences between pieces of gear, even though such differences might be reasonably well known to other engineers and sometimes exploited on a regular basis by those same folks. I'm curious as to how this bias helps you get along with life...? I suppose in the ideal case it more accurately models how things happen in your world, but I find it odd that it doesn't line up with what happens in a lot of other folks' worlds who came to some conclusion without having knowledge that others came to the same conclusion. Regards, Monte McGuire |
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#10
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Kurt Albershardt wrote:
Arny Krueger wrote: Phil Allison wrote: "Arny Krueger" An interesting related article can be found at http://www.eetasia.com/ARTICLES/2002...MSD_POW_AN.PDF Most interesting is: "Figure 3 shows the effect of preamp input resistance (and capacitance) on frequency response of a Shure SM57 with 100 feet of common cable. The upper curves, 10 kS and 3 kS, are typical of transformer-less mic preamps while the lower curve, 1.5 kS, is typical of a transformer input mic preamp. Note the ultra-sonic peaks in response caused by insufficient damping". This chart gives some insight into the output impedance of a SM57. To summarize, there is negligible change in response at 3 KHz with load impedances varying from 1,500 ohms to 10,000 ohms. IOW, the source impedance of a SM57 in the normal audio range (20 KHz) is quite small. Other sources give it as being 150 ohms or 310 ohms. It may be even less - perhaps 75 ohms or less. The same charts show an approximate 5 dB range of response at 20 KHz but less than 1 dB variation at 10 KHz. None of this is all that audibly significant. ** Fig 3 in the Jensen article shows an overall variation of less than 2dB at 20kHz for the three load impedances at the end of 100 feet of cable driven by an SM57. At 15 kHz, or the highest frequency an SM57 actually reproduces, the variation is less than 1 dB while at 10 kHz that variation is less than 0.5 dB. The HF response variation between different samples of the SM57 is a lot greater than that !! This article seems to throw quite a bit of cold water on the many claims of dramatic sonic differences due to real-world variations in the loading of SM57s by various preamps and cables. It looks to me as though it's comparing the impact of different loading resistors across the inputs of a particular preamp on the output frequency response of SM57s. Right, and that's what I said, isn't it? Many of the sonic difference claims (and my own experience) show a marked difference between the sound of an SM-57 feeding a high impedance transformerless preamp when compared with that produced by one feeding a transformer-fronted preamp. Sure, and the effects of transformer loading are not directly addressed here. But what is addressed here relates to variations in resistive and capacitive loading. We've seen various claims about how preamps with various resistive input impedances can make SM-57s sound dramatically different, right? It appears that those claims have now been soaked with cold water by these careful, real-world measurements. There are substantial measured changes in performance, but they are all outside the audible range. The bottom line is that these tests show that a SM-57 presents a relatively low-impedance source to whatever loads it. They show that SM-57s tend to perform in an audibly similar fashion with various combinations of resistance and reactance. Transformer-input preamps add two dimensions that these tests don't address directly. First there is the response of the transformer itself. It obviously acts like a bandpass filter of some kind. Secondly, transformers can present inductive loads to microphones because the transformers themselves can have inductance. This is shown as Lt in figure 5 for example. The article seems to encourage us to ignore both of these, because it does not mention any effects that are associated with them. It may be that because of their quality, Jensen transformers have reduced the effects of these parameters to the point where they can be safely ignored. |
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#11
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Kurt Albershardt wrote:
Arny Krueger wrote: Phil Allison wrote: "Arny Krueger" An interesting related article can be found at http://www.eetasia.com/ARTICLES/2002...MSD_POW_AN.PDF Most interesting is: "Figure 3 shows the effect of preamp input resistance (and capacitance) on frequency response of a Shure SM57 with 100 feet of common cable. The upper curves, 10 kS and 3 kS, are typical of transformer-less mic preamps while the lower curve, 1.5 kS, is typical of a transformer input mic preamp. Note the ultra-sonic peaks in response caused by insufficient damping". This chart gives some insight into the output impedance of a SM57. To summarize, there is negligible change in response at 3 KHz with load impedances varying from 1,500 ohms to 10,000 ohms. IOW, the source impedance of a SM57 in the normal audio range (20 KHz) is quite small. Other sources give it as being 150 ohms or 310 ohms. It may be even less - perhaps 75 ohms or less. The same charts show an approximate 5 dB range of response at 20 KHz but less than 1 dB variation at 10 KHz. None of this is all that audibly significant. ** Fig 3 in the Jensen article shows an overall variation of less than 2dB at 20kHz for the three load impedances at the end of 100 feet of cable driven by an SM57. At 15 kHz, or the highest frequency an SM57 actually reproduces, the variation is less than 1 dB while at 10 kHz that variation is less than 0.5 dB. The HF response variation between different samples of the SM57 is a lot greater than that !! This article seems to throw quite a bit of cold water on the many claims of dramatic sonic differences due to real-world variations in the loading of SM57s by various preamps and cables. It looks to me as though it's comparing the impact of different loading resistors across the inputs of a particular preamp on the output frequency response of SM57s. Right, and that's what I said, isn't it? Many of the sonic difference claims (and my own experience) show a marked difference between the sound of an SM-57 feeding a high impedance transformerless preamp when compared with that produced by one feeding a transformer-fronted preamp. Sure, and the effects of transformer loading are not directly addressed here. But what is addressed here relates to variations in resistive and capacitive loading. We've seen various claims about how preamps with various resistive input impedances can make SM-57s sound dramatically different, right? It appears that those claims have now been soaked with cold water by these careful, real-world measurements. There are substantial measured changes in performance, but they are all outside the audible range. The bottom line is that these tests show that a SM-57 presents a relatively low-impedance source to whatever loads it. They show that SM-57s tend to perform in an audibly similar fashion with various combinations of resistance and reactance. Transformer-input preamps add two dimensions that these tests don't address directly. First there is the response of the transformer itself. It obviously acts like a bandpass filter of some kind. Secondly, transformers can present inductive loads to microphones because the transformers themselves can have inductance. This is shown as Lt in figure 5 for example. The article seems to encourage us to ignore both of these, because it does not mention any effects that are associated with them. It may be that because of their quality, Jensen transformers have reduced the effects of these parameters to the point where they can be safely ignored. |
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#12
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Arny Krueger wrote:
Phil Allison wrote: "Arny Krueger" An interesting related article can be found at http://www.eetasia.com/ARTICLES/2002...MSD_POW_AN.PDF Most interesting is: "Figure 3 shows the effect of preamp input resistance (and capacitance) on frequency response of a Shure SM57 with 100 feet of common cable. The upper curves, 10 kS and 3 kS, are typical of transformer-less mic preamps while the lower curve, 1.5 kS, is typical of a transformer input mic preamp. Note the ultra-sonic peaks in response caused by insufficient damping". This chart gives some insight into the output impedance of a SM57. To summarize, there is negligible change in response at 3 KHz with load impedances varying from 1,500 ohms to 10,000 ohms. IOW, the source impedance of a SM57 in the normal audio range (20 KHz) is quite small. Other sources give it as being 150 ohms or 310 ohms. It may be even less - perhaps 75 ohms or less. The same charts show an approximate 5 dB range of response at 20 KHz but less than 1 dB variation at 10 KHz. None of this is all that audibly significant. ** Fig 3 in the Jensen article shows an overall variation of less than 2dB at 20kHz for the three load impedances at the end of 100 feet of cable driven by an SM57. At 15 kHz, or the highest frequency an SM57 actually reproduces, the variation is less than 1 dB while at 10 kHz that variation is less than 0.5 dB. The HF response variation between different samples of the SM57 is a lot greater than that !! This article seems to throw quite a bit of cold water on the many claims of dramatic sonic differences due to real-world variations in the loading of SM57s by various preamps and cables. It looks to me as though it's comparing the impact of different loading resistors across the inputs of a particular preamp on the output frequency response of SM57s. Many of the sonic diffrerence claims (and my own experience) show a marked difference between the sound of an SM-57 feeding a high impedance transformerless preamp when compared with that produced by one feeding a transformer-fronted preamp. |
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#13
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#14
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#15
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Arny Krueger wrote:
This article seems to throw quite a bit of cold water on the many claims of dramatic sonic differences due to real-world variations in the loading of SM57s by various preamps and cables. This is a PSA. If you hook a 57 to a Mackie 1202, and then hook it to a GRE MP2 and can't hear a difference, please see your audiologist. -- ha |
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#16
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hank alrich wrote:
Arny Krueger wrote: This article seems to throw quite a bit of cold water on the many claims of dramatic sonic differences due to real-world variations in the loading of SM57s by various preamps and cables. This is a PSA. If you hook a 57 to a Mackie 1202, and then hook it to a GRE MP2 and can't hear a difference, please see your audiologist. Well Hank you charming devil, if you think this discussion suggests that there can't or shouldn't possibly be an audible difference in that situation, then you need to learn the difference between a console and a stand-alone mic preamp. ;-) |
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#17
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hank alrich wrote:
Arny Krueger wrote: This article seems to throw quite a bit of cold water on the many claims of dramatic sonic differences due to real-world variations in the loading of SM57s by various preamps and cables. This is a PSA. If you hook a 57 to a Mackie 1202, and then hook it to a GRE MP2 and can't hear a difference, please see your audiologist. And, it's measurable. I've only done frequency response, not impulse response, though. Anyone who gets a good set of impulse responses with different loads should be able to get a nice JAES paper out of it. Be aware that the GRE MP2 input is reactive, too! --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
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#18
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"Arny Krueger" wrote in message ...
Phil Allison wrote: "Arny Krueger" An interesting related article can be found at http://www.eetasia.com/ARTICLES/2002...MSD_POW_AN.PDF Most interesting is: "Figure 3 shows the effect of preamp input resistance (and capacitance) on frequency response of a Shure SM57 with 100 feet of common cable. The upper curves, 10 kS and 3 kS, are typical of transformer-less mic preamps while the lower curve, 1.5 kS, is typical of a transformer input mic preamp. Note the ultra-sonic peaks in response caused by insufficient damping". This chart gives some insight into the output impedance of a SM57. To summarize, there is negligible change in response at 3 KHz with load impedances varying from 1,500 ohms to 10,000 ohms. IOW, the source impedance of a SM57 in the normal audio range (20 KHz) is quite small. Other sources give it as being 150 ohms or 310 ohms. It may be even less - perhaps 75 ohms or less. The same charts show an approximate 5 dB range of response at 20 KHz but less than 1 dB variation at 10 KHz. None of this is all that audibly significant. ** Fig 3 in the Jensen article shows an overall variation of less than 2dB at 20kHz for the three load impedances at the end of 100 feet of cable driven by an SM57. At 15 kHz, or the highest frequency an SM57 actually reproduces, the variation is less than 1 dB while at 10 kHz that variation is less than 0.5 dB. The HF response variation between different samples of the SM57 is a lot greater than that !! This article seems to throw quite a bit of cold water on the many claims of dramatic sonic differences due to real-world variations in the loading of SM57s by various preamps and cables. I was under the impression that the interesting loads for the SM-57 were reactive rather than resistive. I think Mark McQ added some reactive load (a low pass filter on the input for RF rejection) to his RNP in part for this reason. rossi |
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#19
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Chris Rossi wrote:
"Arny Krueger" wrote in message ... Phil Allison wrote: "Arny Krueger" An interesting related article can be found at http://www.eetasia.com/ARTICLES/2002...MSD_POW_AN.PDF Most interesting is: "Figure 3 shows the effect of preamp input resistance (and capacitance) on frequency response of a Shure SM57 with 100 feet of common cable. The upper curves, 10 kS and 3 kS, are typical of transformer-less mic preamps while the lower curve, 1.5 kS, is typical of a transformer input mic preamp. Note the ultra-sonic peaks in response caused by insufficient damping". This chart gives some insight into the output impedance of a SM57. To summarize, there is negligible change in response at 3 KHz with load impedances varying from 1,500 ohms to 10,000 ohms. IOW, the source impedance of a SM57 in the normal audio range (20 KHz) is quite small. Other sources give it as being 150 ohms or 310 ohms. It may be even less - perhaps 75 ohms or less. The same charts show an approximate 5 dB range of response at 20 KHz but less than 1 dB variation at 10 KHz. None of this is all that audibly significant. ** Fig 3 in the Jensen article shows an overall variation of less than 2dB at 20kHz for the three load impedances at the end of 100 feet of cable driven by an SM57. At 15 kHz, or the highest frequency an SM57 actually reproduces, the variation is less than 1 dB while at 10 kHz that variation is less than 0.5 dB. The HF response variation between different samples of the SM57 is a lot greater than that !! This article seems to throw quite a bit of cold water on the many claims of dramatic sonic differences due to real-world variations in the loading of SM57s by various preamps and cables. I was under the impression that the interesting loads for the SM-57 were reactive rather than resistive. I think Mark McQ added some reactive load (a low pass filter on the input for RF rejection) to his RNP in part for this reason. Check figure 4 in the reference: http://www.eetasia.com/ARTICLES/2002...MSD_POW_AN.PDF for information about the effects of reactive loads on the SM-57. Basically, with a load on the order of a relatively large 25,000pF there a mere 2 dB increase at 15 KHz. A large 2,500 pF load gives just a 1.2 dB increase at 20 KHz. Both peaks are fairly narrow and lightly damped, so their effects in the main part of the audible range (10 KHz) are less than 1.5 dB (25,000 pF) or less than 0.3 dB (2,500 pF). Since the RNP and the GRE have been mentioned, perhaps people who have them can see what's inside the box in this area. Compared to the mic's response variations in the same frequency ranges, this is all chump change. |
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#20
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Chris Rossi wrote:
were reactive rather than resistive. I think Mark McQ added some reactive load (a low pass filter on the input for RF rejection) to his RNP in part for this reason. Sorry to change the subject here, but this may relate. As I understand it, the RNP is a transformerless mic pre. According to a little birdie who told me things, the blocking capacitors, necessary in transformerless pres to protect the input from phantom power, cause a certain amount of signal degradation, particularly in the top end. However, this supposedly is not a factor when it comes to condensors since when the phantom is switched on, the biasing to the capacitors that results, somehow negates this problem. Assuming any of this is true, has anyone ever tested with a dynamic mic, plugged into a transformerless pre, whether or not the sound improves (more clarity in the top end) by switching on the phantom power!? It may not matter with most dynamics, but what about some of the better ones such as the 441 or RE20? And dare I say it...what about ribbon mics, where switching on phantom is generally considered a no-no? Rob R. |
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#21
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Phil Allison wrote:
"Arny Krueger" An interesting related article can be found at http://www.eetasia.com/ARTICLES/2002...MSD_POW_AN.PDF Most interesting is: "Figure 3 shows the effect of preamp input resistance (and capacitance) on frequency response of a Shure SM57 with 100 feet of common cable. The upper curves, 10 kS and 3 kS, are typical of transformer-less mic preamps while the lower curve, 1.5 kS, is typical of a transformer input mic preamp. Note the ultra-sonic peaks in response caused by insufficient damping". This chart gives some insight into the output impedance of a SM57. To summarize, there is negligible change in response at 3 KHz with load impedances varying from 1,500 ohms to 10,000 ohms. IOW, the source impedance of a SM57 in the normal audio range (20 KHz) is quite small. Other sources give it as being 150 ohms or 310 ohms. It may be even less - perhaps 75 ohms or less. The same charts show an approximate 5 dB range of response at 20 KHz but less than 1 dB variation at 10 KHz. None of this is all that audibly significant. ** Fig 3 in the Jensen article shows an overall variation of less than 2dB at 20kHz for the three load impedances at the end of 100 feet of cable driven by an SM57. At 15 kHz, or the highest frequency an SM57 actually reproduces, the variation is less than 1 dB while at 10 kHz that variation is less than 0.5 dB. The HF response variation between different samples of the SM57 is a lot greater than that !! This article seems to throw quite a bit of cold water on the many claims of dramatic sonic differences due to real-world variations in the loading of SM57s by various preamps and cables. |
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#22
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"Arny Krueger" An interesting related article can be found at http://www.eetasia.com/ARTICLES/2002...MSD_POW_AN.PDF Most interesting is: "Figure 3 shows the effect of preamp input resistance (and capacitance) on frequency response of a Shure SM57 with 100 feet of common cable. The upper curves, 10 kS and 3 kS, are typical of transformer-less mic preamps while the lower curve, 1.5 kS, is typical of a transformer input mic preamp. Note the ultra-sonic peaks in response caused by insufficient damping". This chart gives some insight into the output impedance of a SM57. To summarize, there is negligible change in response at 3 KHz with load impedances varying from 1,500 ohms to 10,000 ohms. IOW, the source impedance of a SM57 in the normal audio range (20 KHz) is quite small. Other sources give it as being 150 ohms or 310 ohms. It may be even less - perhaps 75 ohms or less. The same charts show an approximate 5 dB range of response at 20 KHz but less than 1 dB variation at 10 KHz. None of this is all that audibly significant. ** Fig 3 in the Jensen article shows an overall variation of less than 2dB at 20kHz for the three load impedances at the end of 100 feet of cable driven by an SM57. At 15 kHz, or the highest frequency an SM57 actually reproduces, the variation is less than 1 dB while at 10 kHz that variation is less than 0.5 dB. The HF response variation between different samples of the SM57 is a lot greater than that !! ............. Phil |
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dufus wrote:
can i just put a 1k resistor across pins 2 and 3? Absolutely. This is not that different from what happens when you add many mic attenuators. The mic attenuators I've examined provide load impedances in the 1 to 2.5 Kohm range. Attenuators that are connected across pins 2 & 3 are generally recommended because of their minimal effect on CMRR to my understanding this degrades CMRR, since the differential signal is knocked down, but not the common mode. The real question is not CMRR, its resistance to externally-generated noise. Do you have reason to believe that common mode noise is currently a serious problem with your system? Or, are just worrying about possibilities, as opposed to realities? how much would CMRR degrade in dB with a preamp with a ~5k input Z? Don't know, because this calculation would be based on the source impedance of the SM-57, not the input impedance of the preamp. An interesting related article can be found at http://www.eetasia.com/ARTICLES/2002...MSD_POW_AN.PDF Most interesting is: "Figure 3 shows the effect of preamp input resistance (and capacitance) on frequency response of a Shure SM57 with 100 feet of common cable. The upper curves, 10 kS and 3 kS, are typical of transformer-less mic preamps while the lower curve, 1.5 kS, is typical of a transformer input mic preamp. Note the ultra-sonic peaks in response caused by insufficient damping". This chart gives some insight into the output impedance of a SM57. To summarize, there is negligible change in response at 3 KHz with load impedances varying from 1,500 ohms to 10,000 ohms. IOW, the source impedance of a SM57 in the normal audio range (20 KHz) is quite small. Other sources give it as being 150 ohms or 310 ohms. It may be even less - perhaps 75 ohms or less. The same charts show an approximate 5 dB range of response at 20 KHz but less than 1 dB variation at 10 KHz. None of this is all that audibly significant. aside The high frequency variation relates to a resonance between the source inductance of the SM57 and a cable capacitance on the order of 0.035 uF. The resonance centers at about 45 KHz, from which the equivalent source impedance of the SM57 can be calculated. I would expect less than 1 dB variation due to adding a 1K ohm resistor. This means that even in your pessimistic estimate, there would be a 1 dB loss of CMRR which is almost always negligible, even in a high noise environment. |
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