Hi patchwork,
The first thing to know is a driver's resistance changes with frequency. So a single figure doesn't tell you everything. You want to view the driver's impedence plot to know what you're amp will really be driving.
Re is a driver's DC resistance which is measured with a MM with the driver in free air.
The nominal impedence is a kind of average taken from the impedance plot of the driver. Usually means very little.
The first thing to know is a driver's resistance changes with frequency. So a single figure doesn't tell you everything. You want to view the driver's impedence plot to know what you're amp will really be driving.
Re is a driver's DC resistance which is measured with a MM with the driver in free air.
The nominal impedence is a kind of average taken from the impedance plot of the driver. Usually means very little.
Thus the reason to school someone properly to use the correct equipment.
There are too many variables in trying to use a VOM/multimeter. First and formost how accurate is the meter?
I always say the proper tool for the job. This is why we mention the usage of books by Morgan Jones and other people noted for their profound knowledge to school people and provide a proper knowledge base.
kelticwizard
My logic is very simple but some may feel it is flawed. In an ideal world you would teach someone how to use the proper tools.
It would be very simple for patchwork to drive down to his local stereo shop and borrow an accurate impedance meter. This meter would provide him with all the necessary information he would need to evaluate his current driver. Example:
It would provide the impedance at different frequencies. It would also tell him at a glance if there was any voice coil damage to the driver. This is why I push the idea of using this tool which has been designed specifically for this purpose instead of trying to use a VOM meter. I don't care what you factor in for your percentage of impedance it simply will not provide accurate information.
Story time now....
I work for a sound consultant in the midwest. My job consists of fixing what people "in the know" screw up. In other words when a sound system (usually a large one like a stadium system) gets screwed up I fix it. I have had my hands full of people trying to use a VOM meter to analyze a speaker load and making changes based on what their VOM meter tells them. It is simply incapable of providing accurate results and therefore shouldn't be used period.
My logic is very simple but some may feel it is flawed. In an ideal world you would teach someone how to use the proper tools.
It would be very simple for patchwork to drive down to his local stereo shop and borrow an accurate impedance meter. This meter would provide him with all the necessary information he would need to evaluate his current driver. Example:
It would provide the impedance at different frequencies. It would also tell him at a glance if there was any voice coil damage to the driver. This is why I push the idea of using this tool which has been designed specifically for this purpose instead of trying to use a VOM meter. I don't care what you factor in for your percentage of impedance it simply will not provide accurate information.
Story time now....
I work for a sound consultant in the midwest. My job consists of fixing what people "in the know" screw up. In other words when a sound system (usually a large one like a stadium system) gets screwed up I fix it. I have had my hands full of people trying to use a VOM meter to analyze a speaker load and making changes based on what their VOM meter tells them. It is simply incapable of providing accurate results and therefore shouldn't be used period.
Andrew T
I have been told there are good impedance meter circuits that employ signal generators that one can make from scratch. I haven't surfed the web for circuits but this certainly would make a good thread. An ecconomical impedance meter certainly has a place on any technicians or diy's bench. Calibration wouldn't/shouldn't be that bad.
patchwork
I certainly don't mean to sound like I am picking on you by any means. I would just like you to know and learn the proper way and why. I am certain a local stereo shop will let you take a reading or do it for you. A look at the reading will tell you in an instant what your driver is as well as the condition of the drivers voice coil.
I have been told there are good impedance meter circuits that employ signal generators that one can make from scratch. I haven't surfed the web for circuits but this certainly would make a good thread. An ecconomical impedance meter certainly has a place on any technicians or diy's bench. Calibration wouldn't/shouldn't be that bad.
patchwork
I certainly don't mean to sound like I am picking on you by any means. I would just like you to know and learn the proper way and why. I am certain a local stereo shop will let you take a reading or do it for you. A look at the reading will tell you in an instant what your driver is as well as the condition of the drivers voice coil.
hi guys, thanks for the replies.
BurnedFingers - don't worry, I didn't think you were picking on me at all!
I was already aware that drivers change impedances at different frequencies. I know a decent amount about audio from having been an interested audiophile for a couple of years. I just don't like all the myths in the hi-fi audio world, hence my interest in DIY
I was really just curious to know if that reading from the multimeter would be giving me any information whatsoever that was useful or interesting, however it seems that this is not the case.
Thanks again guys!
BurnedFingers - don't worry, I didn't think you were picking on me at all!
I was already aware that drivers change impedances at different frequencies. I know a decent amount about audio from having been an interested audiophile for a couple of years. I just don't like all the myths in the hi-fi audio world, hence my interest in DIY
I was really just curious to know if that reading from the multimeter would be giving me any information whatsoever that was useful or interesting, however it seems that this is not the case.
Thanks again guys!
A voltmeter can be used to measure the magnitude of the speaker impedance at its +/- termainals IF:
- the speaker is being driven by a steady state signal
- you know the current through the speaker at the time the voltage measurement is made
once you have the voltage and current....
Plotting the impedance curve is one of the basic steps in obtaining the speaker paramerters,... am I missing something here?
- the speaker is being driven by a steady state signal
- you know the current through the speaker at the time the voltage measurement is made
once you have the voltage and current....
Plotting the impedance curve is one of the basic steps in obtaining the speaker paramerters,... am I missing something here?
I guess it is too apparent that some do not do this for a living like I do. Having been in the hobby in excess of 30 years myself I too had formed some ill opinions and these changed as a result of being in the trade working on large systems for the last 15 years.
The only thing a VOM is going to tell you is an approximate resistance and if its burned open. Granted there are other more accurate methods of determining speaker impedance but then again I don't hall around numerous pieces of equipment to do what I can accomplish with a hand held piece.
The only thing a VOM is going to tell you is an approximate resistance and if its burned open. Granted there are other more accurate methods of determining speaker impedance but then again I don't hall around numerous pieces of equipment to do what I can accomplish with a hand held piece.
roddyama
You are correct with what you have stated. If I haven't missed something I am under the assumption that patchwork asked if he could get a impedance reading using a Multimeter/Vom. Speaker parameters /Theil small parameters weren't mentioned and even if they were one would want to use a RMS meter and not a VOM meter.
As I have tried in vain to point out here....you cannot get an accurate reading or a condition of a driver using a vom and this is why they are not tolerated or used in the trade.
Part of diy audio if I understand it correctly is to lay a proper foundation and guide people in their understanding of electronic principals. I have tried to do this with proper information as a result of my training and years in the sound field.
You are correct with what you have stated. If I haven't missed something I am under the assumption that patchwork asked if he could get a impedance reading using a Multimeter/Vom. Speaker parameters /Theil small parameters weren't mentioned and even if they were one would want to use a RMS meter and not a VOM meter.
As I have tried in vain to point out here....you cannot get an accurate reading or a condition of a driver using a vom and this is why they are not tolerated or used in the trade.
Part of diy audio if I understand it correctly is to lay a proper foundation and guide people in their understanding of electronic principals. I have tried to do this with proper information as a result of my training and years in the sound field.
burnedfingers said:
I see where you're coming from, but you're NOT answering the original question, but have wandered off on your own little crusade.
For a crude check of a speaker an ohm reading is all you need 99% of the time, certainly from a servicing point of view - just to make sure you're not putting a 4 ohm load on your 8 ohm amp.
I see very little reason for an impedance meter in a service setting, and none at all in a PA setting.
We're all aware that a speaker 'impedance' is merely a nominal value, usually measured at 1KHz, and the actual impedance will vary massively over it's full operating range. But you don't really need to know that! - unless you're designing and building speakers.
Re: Why is this so...?
If you have an amplifier that's rated at 100W into 8 ohms, and you connect a 4 ohm speaker, then the amplifier will try and supply 200W to it - this generally kills the amp!.
As long as the volume is kept down it won't damage it, as it won't try and supply more power than it can - but run the amp hard and you're asking for trouble.
As you mentioned 'taps' presumably you're either talking valve amps or 100V line - the same applies, but they are not as easily damaged.
JandG said:
If you have an amplifier that's rated at 100W into 8 ohms, and you connect a 4 ohm speaker, then the amplifier will try and supply 200W to it - this generally kills the amp!.
As long as the volume is kept down it won't damage it, as it won't try and supply more power than it can - but run the amp hard and you're asking for trouble.
As you mentioned 'taps' presumably you're either talking valve amps or 100V line - the same applies, but they are not as easily damaged.
Dear Mr. Nigel Goodwin
I believe I have answered the question 100% to the point. To obtain an accurate measurement of the actual impedance of the driver a impedance meter is needed. Anything else is hit and miss.
Therefore an accurate measurement will provide patchwork with the information he will need in order to come to a decision that his amplifier is or is not capable of driving the load. In addition to this quick measurement a more detailed measurement can be obtained by checking the impedance at various frequencies. In addition to this one can easily make a judgement of the drivers condition based on the impedance measurement.
Well sir, once again we differ. In the world I live in I have to document all loads on all amplifiers. This is usually typical of professional sound equipment service here in the states. At
least in the region where I reside. What this provides is an accurate record of the speaker load status for the given service date. My customers rely on this information.
To say that impedance meters are not used in the PA setting is well lets just say that someone is very lacking in common sense and experience.
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