User Manual - LinearX Systems Inc
Contents
1. of power levels two ranges are provided for the voltage and current sensitivities Essentially the ranges provide OdB or 40dB output levels from the XLRs Accurate measurements can be made at almost any power level from 10mW to 2500W by proper range selection The V Box is built entirely with 1 precision low TC components and features 4 wire Kelvin shunts a precision voltage divider and balanced output line impedance The user selects the range by connecting the amplifier to either the LO RANGE or HI RANGE amplifier binding posts The V I signals are then taken from the matching pair of XLR connectors Typical Connection and Setup The figure below shows the typical V Box application circuit If a single channel analyzer or volt meter is used then two sweeps or measurements will be required while alternating the measurement connection from the current to voltage XLR outputs For most purposes the relative polarity of the speaker connections to the Red Blk terminals of the V Box is not important for the electrical data but will be important if absolute SPL phase is also to be measured The power amplifier should NOT be a bridged type output but rather a normal single ended ground referenced output Almost any ana lyzer or meter can be used however the analyzer input s should be balanced or floating for maximum accuracy The connections to the power amplifier must be connected BIk BIk and Red Red This is very important if highly accurate m2. required Electrical Specifications E Maximum Load Voltage 100 Volts E Maximum Load Current 25 Amps E Shunt Resistors 1 0Q and 10mMQ 1 7W E Load Bridging 10kQ in 40dB range E Signal Output Impedance 100Q x 2 E Selectable V I Ranges 0dB 40dB Mechanical Specifications l i i E Weight 2 Ibs w E Width 3 5 Inches g gt E Height 2 55 Inches E Length 5 625 Inches E Output Signal Connectors XLR Male 3 Pin E Amp Spkr Connectors 5 Way Binding Posts O O LinearX Systems Inc A 9500 SW Tualatin Sherwood Rd L N E R x Y S T E M S Tualatin OR 97062 8586 USA 7 TEL 503 612 9565 FAX 503 612 9344 Web www linearx com 2003 LinearX Systems Inc All rights reserved Page 4 VI Box User Manual
3. E EHP ACEN E E A ee ee eee EE EEEE U corral 188 135 i Frequency 1686 568 x Mag ce gt dBm Voltage Measurement a Phase Deg Hames ee Hi p Se SIRE PIRE l menen Impedance Curve Z V I Inpedance Measurement lt Phase Deg i i BH ital Impedance measured by CVS and CCS methods ing normal operation It is not at all uncommon to see a significant difference between CCS and CVS measurements In this example the impedance data taken using the simple CCS method would be grossly inaccurate at low frequencies relative to the normal operation of the speaker The measurement of small speakers often has similar results Due to their small size the speakers are operating well into their nonlinear region even at low drive levels Using the V Box CVS data is easily obtained and is the most accurate representation of the speaker s electrical characteristics under realistic operating conditions Page 2 VI Box User Manual Multi Level Impedance Measurement E e aes In this more typical example a high power 15 speaker was mounted in a 3 8 EEE E EEE ee he ele EF Ft enclosure with a 4 diameter port V I data was taken at four power lev al iis els 0 1W 1W 10W and 100W For the 100W level both of the V Box high range selections for V amp were used attenuation OdB or 40dB then dividing the curves cancels the attenua sit eee ne tion and no scaling is ever nece
4. LINEARX User M anual Precision measurement of loudspeaker LO RANGE AMPLIFIER INPUT SPEAKER OUTPUT HI RANGE AMPLIFIER INPUT MAXIMUM CURRENT 2 5A RMS MAXIMUM VOLTAGE 250V RMS voltage current impedance and power CO KY e OD BOX The VI Box is a specialized device designed to facilitate the easy and accu rate measurement of electrical parameters for loudspeakers and passive crossover components under actual constant voltage source CVS drive eee Sa oe Ta urren oltage oltage urren conditions pik LINEARX While the commonly used constant current source CCS method provides easy and quick impedance measurement it sacrifices the quality and accu SA racy of the data Loudspeakers are normally operated in a constant voltage inal a en aE 1VAO0A 404b source CVS mode by direct connection to a power amplifier a Device Description The V Box provides all of the components circuitry and interface connectors necessary to produce CVS measurements The only other required elements are a power amplifier oii A ea cs and an analyzer or suitable volt meter The VI Box is placed in series between the ampli fier and the load to be measured Three dual binding posts are provided for connection to the amplifier and speaker load as well as two pair of XLR signal line outputs for the load current 1 and load voltage V To allow measurement over a very wide range
5. easurements of current are to be produced by minimizing the common mode voltage at the internal current shunt The cable impedance to the amplifier is not important but the cable impedance to the speaker will appear as part of the load For this reason it may be desirable to locate the V Box near the speaker to reduce that cable length Power Range Selection The maximum current allowed in the low range is 2 5 Amps In the high range the maximum current is 25 Amps As a general rule of thumb use the low range for any measurements at 10 Watts or below For measurements at higher power levels above 10 Watts the high 25A range must be used Connect the amplifier and V I XLRs as appropriate for the range in use Warning Exceeding the current limit of 2 5A in the low range will cause the current shunt to burn out Please make sure to use the high power 25A range for higher ower measurements PEAKER OUTPU CS GS In the low range the output signal voltage from the V VI and l current sense XLRs will be 1 1 proportional to the CR OF ge oes speaker s voltage and current 0dB gain This means 4 Cy that if 1 Amp flows through the load 1 Volt is produced at KJ E amp Current m EE Voltage amp Curre Measurement the I XLR output In the high range a 1 100 ratio
6. evel It should be noted that a drive level of 10mA through an 8 Ohm load corresponds to a power level of 0 0008 Watts P I x R This is less than 1mW and is 1000 times lower than the previous drive level used with the V Box at a mere 1W level The graph below on the right shows the two impedance curves as mea sured under CVS conditions using the V Box solid line and using the CCS method dashed line At mid and high frequencies the two impedance curves are nearly identical showing very little difference between them However below 60Hz there is a dramatic difference between the two curves The CCS measurement shows another resonance hump at 14Hz while the CVS measurement shows basically a flat line Why the difference The loudspeaker used in this example had an extremely small port relative to the piston size of the speaker This causes a dramatic velocity increase for the air moving in the port relative to the velocity of the cone The acoustic resistance of the port is very large and is also very nonlinear Changing the drive level from 1mW to 1W causes the port to completely saturate becoming almost entirely resistive and damps out the resonance This is an extreme example but clearly illustrates the need to measure a loudspeaker under the same drive conditions and levels as will occur dur Ohms lt Magnitude 2 18 Frequency a8 Magnitude gt dBm Voltage Measurement lt Phase Deg A BHT MGT i E EA
7. is produced 40dB gain meaning that 100 Amps flowing Ld Ld through the load will yield 1 Volt at the XLR output and ANALYZER 100Volts across the speaker will produce 1 Volt from the ANALYZER V XLR output It should be noted that a 10k resistance will appear in parallel with the load This is insignificant PO RANGE CONNEC TIONS for typical low impedance speaker load applications H RANGE CONNECTIONS VI Box User Manual Page 1 Loudspeaker Impedance Measurement In this example a 15 speaker was used mounted in a very small box with a very small port This example was chosen to illustrate some of the more dramatic differences between CVS and CCS impedance measurements In the following examples an LMS analyzer is used This analyzer is single channel so two sweeps are required to obtain both the voltage and current data each time changing the XLR line input from the V output to the I output of the V Box We first begin by connecting the V Box between the power amp and speaker These tests were conducted at a nominal 1W level meaning 2 83V into an 8 Ohm load The OSC level was adjusted to produce that ae from the V output Both of the low ranges in the V Box were used for voltage and current The LMS analyzer was set to measure Volts from the line input and the first voltage V sweep was taken This is shown here on the right The curve is a somewhat bumpy or wavy flat line This is very typical and ref
8. lects the voltage division ratio produced from the speaker load and the 1 Ohm current shunt in the V Box This curve represents the actual volt age across the speaker terminals with respect to frequency Next we change the LMS line input to the I XLR output and take another sweep Of course this time placing the data curve into a different entry The data type is kept on Volts The I curve is shown on the right below The I curve represents the actual current flowing through the speaker ter minals with respect to frequency It is very much like the shape of an im pedance curve but inverted Now we have the two curves necessary to produce the impedance curve Using the processing features in LMS the V curve entry is divided by the I curve entry However the units of the resulting curve are now Ratio or dimensionless We can change these to Ohms using the Left Vert data drop down list box in the Curve Library dialog The resulting impedance curve is shown here on the right below The impedance curve has a typical electrodynamic shape with inductive rise being shown at high frequencies and two resonance peaks in the 300Hz region However note that below 50Hz the Z curve tends to flatten out to approximately 10 Ohms without reaching another peak As a comparison the direct LMS impedance measurement mode was used to measure the speaker likewise This method is essentially constant cur rent source CCS using a 10mA drive l
9. mpedance should be at least 20k Ohms Additional Applications The VI Box is an extremely useful device which facilitates a wide variety of different kinds of measurements for many different applications The ability to measure the current through the load and the voltage across the load produces many different measurements including com plex impedance complex power current voltage distortion temperature rise temperature coefficients and loaded network transfer functions The following is a brief description of some of the additional applications E Simultaneous Measurement of SPL and Z Since the speaker is driven under normal voltage source conditions it is possible to measure the acoustic SPL output at the same time as acquiring the V I data for impedance without the need of setup changes This is an attractive choice for QC testing where time is a key factor E Measurement of Crossover Components The behavior and impedance characteristics of passive components can also be determined Both components and entire networks can be measured while under actual load and drive conditions for impedance distortion and transfer function E Determination of Speaker Parameters at any Power Level The impedance curves produced at any drive level can be used to determine the speaker parameters at that drive level CVS data is far less sensitive to noise allows for faster sweep times and more accurately represents the nonlinear operation of the speaker f
10. or characterization E Measuring Electrical THD and or Rub amp Buzz Noise The current sense output can be used to measure the reflected acoustical artifacts such as distortion or rub amp buzz noise The measurement of low frequency distortion and cone noise can often be made electrically using the current waveform without the need of a microphone E Determination of Temperature Coefficient Temperature rise and Power Handling Limits Knowing the actual current and voltage allows for accurate calculation of the power dissipation in the load Moreover the change in resis tance of the load can also be used to determine the temperature rise This data can then be used to fully characterize the temperature related parameters of the loudspeaker VI Box User Manual Page 3 VI Box Technical Information Constant Voltage vs Constant Current Measurements The measurement of loudspeaker impedance has traditionally been accomplished through the use of constant current source CCS tech niques This method supplies a constant drive current typically 10mA to the speaker and produces a variable voltage across the speaker which is proportional to the speaker s impedance While this method is convenient to use it has the following well known problems a Extremely nonlinear drive level across frequency b No resonance damping provided by the current source c Setup and connections must be changed to measure SPL d Measurements taken at e
11. quirements As previously mentioned the amplifier should not be a bridged type output but rather a normal single ended output Blk terminal is GND For impedance measurement purposes the actual frequency response of the amplifier is relatively unimportant Since the same amplifier transfer function exists in both the voltage and current measurements the end result of dividing these curves to obtain impedance also can cels out the transfer function of the amplifier This is another benefit of the CVS measurement method Analyzer or Voltmeter Requirements For manual measurements a simple oscillator and voltmeter can be used to obtain useful data However do not assume that the frequency response of your voltmeter or DVM DMM is infinite Many of the low cost DMM s have very poor frequency response and are actually only designed to measure AC voltage at power line frequencies 50 60Hz If you are using a DMM it is highly advisable to check and know the frequency response of the meter prior to use Most DMM s with RMS detectors have adequate flat response over the audio frequency range As for more sophisticated analyzers one or two balanced differential inputs are required Unbalanced inputs cannot be used to take reliable measurements from the V Box and are not recommended Either sinewave FFT or noise type analyzers can be used depending on your final data requirements Any analyzer input with CMR of 40dB or higher is adequate and the input i
12. ssary of the resulting Z data oe N A e EA Multi Level es teow rT impedence curves The graph on the right shows the resulting impedance curves at four differ _ 4 135 ent power levels Several points of interest can be noted CE ont PL x 189 Note Since both of the Lo Hi ranges for voltage and current use the same a mmea 1 At the lower resonance hump near 12Hz there are significant changes that occur at each power level Even between the 0 1W and 1W levels changes still occur in the impedance At 100W the resonance hump is dramatically reduced due to saturation of the air in the port 2 In the lower impedance areas 30Hz and 200Hz where the impedance is dominated by the resistance of the voice coil a significant rise in the valleys is observed for the 100W curve This is due to heating of the voice coil The 100W sweep causes the copper in the voice coil to heat resulting in an increase in the resistance of the coil This results in a substantial increase in the Revc value If you wish to obtain high power data without such a dramatic increase in the Revc value a low duty cycle sweep can be made using a gated sweep mode 3 It is also interesting to note that at high frequency the speaker s impedance varies with drive level as well This is not due to temperature rise but due to increased iron loss in the magnetic circuit Here the impedance above 1kHz increases about 25 from 0 1W to 100W Power Amplifier Re
13. ts in substantially greater precision and consistency for dynamic measurements b The nonlinear characteristics of electroacoustic devices imply that all measurements be taken under identical conditions as to which the device will operate For CCS methods measuring the device at a drive level of 1mW is 1000 times lower than the power level employed when commonly measuring SPL response at 1W Moreover the voltage produced from the CCS method follows the impedance curve itself and changes radically near the resonance frequency This can exaggerate and intensify nonlinear effects in the magnetic and compliance systems of the device Using the CVS method produces measurements based on device operation identical to normal operation The mea surement level can be 1W or at any other desired power The nonlinear effects of the device are measured accurately under identical drive conditions to and are representative of normal operation Determination of electrical and mechanical loudspeaker parameters is commonly performed from measured impedance data The quality and accuracy of the parameters produced is therefore a direct function of the quality and accuracy of the original impedance data In order for the parameters to accurately represent the device under normal operating conditions the impedance data must also accurately represent the same characteristics The CVS method should always be used where accurate repeatable and high precision parameters are
14. xtremely low fixed power level typically 1mW i i i a The operating conditions imposed by the CCS method are in reality the exact opposite of normal operation In normal operation a loud speaker is driven directly from a power amplifier at virtually zero source impedance The drive method is by constant voltage source CVS In CVS operation the speaker is driven with a linear voltage vs frequency characteristic with virtually zero source impedance and at a much higher power level In order to determine impedance using the CVS method a current shunt is employed to measure the current flow through the device Know ing the current flow through the device and the voltage across the device the true impedance can then be calculated using basic Ohm s law as Z V I and the true power determined as P V l All loudspeakers to varying degrees are nonlinear resonant devices Measuring the impedance for these devices presents the following difficulties a When a resonant electroacoustic device is driven from a current source of high impedance all damping of the resonance behavior is determined solely by the mechanical Qms losses in the device Under these conditions dynamically measured impedance can be greatly effected by the type of stimulus and or the rate at which a sine wave is swept Under CVS conditions the damping is determined largely by the electrical system Qes and offers dramatically higher damping than the CCS method This resul
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