grounding audio equipment

8/6/2019 Grounding Audio Equipment

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Jensen AN-004

HUM & BUZZ IN UNBALANCED INTERCONNECT SYSTEMSby BillWhitlock

ORIGINS OF HUMOften sound systems exhibit strange and perplexing behavior suchas hum that appears and disappears when power to otherequipment, not even part ofthe audio system, isswitched on or off!Traditional methods to eliminate hum often seem more likevoo-doothan engineering and, more often than not, are trial and errorexercises that end only when someone says "Ican live with that".This author has previously written about balanced lines in audiosystems, so this paper will be strictly confined to unbalancedsystems. [2]

In contrast to a balanced system, an unbalanced system uses onlytwo wires, one for signal and one for ground. Its use stillprevails inconsumer audio, probably because it is cheap to make and itperforms acceptably wellin very small systems such as typical homestereo setups. However, any unbalanced scheme has an inherentproblem called common impedance coupling. From Ohm's lawwe know that when current flows in a resistance, a voltage dropappears across that resistance. With the exception ofsuperconductors, any conductor (wire)has resistance. If two differentcircuitsshare the same conductor or wire, a current flowing in eithercircuit will produce a voltage drop across the wire. As shown inFigure 1, a partial schematic of the simplest possible system, theshield conductor of the interconnecting cable becomes theoffending common impedance.

Since the cable shield is effectively connecting the grounds of thedevices together, itcarries a current derived from the power line aswell as the audio signal current. Although this fact is oftenoverlooked or ignored, itisfundamental to this discussion. Whatevervoltage ispresent between itsinputs, points A and C in Figure 1, willbe amplified by Device B.It cannot tellthe difference between signaland hum, and will amplify both if they are present. To determinewhat the input "sees",we must trace the circuit loop from point A toB to C. Since the voltage A to B (shield voltage drop) isinseries withthe voltage B to C (the signal), the voltages willdirectlyadd. Clearly,it would be very desirable for the shield voltage drop to be zero toavoid contaminating the signal. Inthe real world, regardless ofshieldconstruction, material or gauge, we cannot make the shield

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conductor resistancezero. Our only remaining choice isto somehowreduce the interchassis current, I in Figure 1,to an acceptable level.

INTERCHASSIS CURRENT: THEORYThis current is caused by the charge and discharge of capacitancesbetween power line and chassis. An undesired but unavoidableprimary to secondary capacitance exists inthe power transformer ofevery piece of AC operated equipment. Sometimes intentionalcapacitors and resistors are added from power line to chassis tosuppress RFI and/or meet safety regulations. To predict the severityof the hum problem these capacitances create, we can analyze thecircuit using the steps of simplification shown in Figure 2.

THEVEN IN EQUIVALENT:

SINGLE C & V EQU IVALENT:

CAPACITIVE REACTANCE at 60 Hz: Xc1

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~= REFERENCE POINT AT RECEIVER GROUND

~= HUM VOLTAGE E AT DRIVER GROUND = I x R

© = HUM VOLTAGE ~ to ~ + SIGNAL ~ to ©

CHASSIS

Figure 1 - THE HUM GENERATINGMECHANISMThe basic problem is that the shield is a common path for both inter-chassis "ground" and signal currents.

Jensen AN-004



Note the Vx ± Vyterm in the single capacitor equivalent circuit. The± accounts for the fact that most consumer equipment has a two-prong AC plug which can be inserted into an outlet either of twoways. A special example illustrates how extreme the effects of plugreversal can be. Consider the case where C PSl is V z the value of C PS2

and C pS3 is V z the value of C pS4 (this condition would be highlyunlikely in the real world). If the two plugs are connected to theAC line as shown in the diagram, each pair of capacitors forms avoltage divider with a 3:1 division ratio, making chassis voltages Vxand Vy each 40 volts AC with respect to ground. Since no current

willflow in a wire connecting two points of equal voltage, current Iwillbe zero. However, ifone ofthe ACplugs isreversed, the chassisvoltages willno longer be equal and current I willflow. Except forthis special case, plug reversals simply cause a change in theinterchassis current, rather than the total cancellation seen in thisexample. For this reason, reversing AC plugs will almost alwayschange the hum level in a system..

It is also very unlikely that the two capacitances, C PSl and C PS2 orCpS3 and C pS4 , would be exactly matched in any piece of equipment.Mismatch ratios oftwo to one are common. Since allutility120VACpower in this country is distributed asymmetrically with respect toearth ground, one side called "neutral", is grounded. The other,called "hot" or "line", is at 120 volts with respect to ground.[4]

Recently, proponents of a scheme called "Balanced ACPower" haveclaimed that "[hum reduction] results are often quite dramatic".[3]Balanced power uses a center-tapped transformer to make each sideof the line 60 volts with respect to ground. Although intuitivelyattractive, this approach can completely cancel interchassis currentsin a system of three or more devices only in the case where each ofthe devices had such matched capacitances. This would be anextremely rare occurrence. Although 10 to 15 dB hum reductions,which would be more routinely achieved, might be considered"dramatic" in a video system, this author cannot recommend this orany other "lineconditioning" method as a cost effective solution foraudio system hum problems.

INTERCHASSIS CURRENT: MEASUREMENTWhen designingor troubleshooting a system, a highlyrecommendedfirststep isto measure actual ground currents ofthe system devices.This can be done quite simply using an AC voltmeter adapted, asshown inFigure 3, to measure ACcurrent. Thissame setup can alsomeasure chassis current between devices.The 1 kQ resistor converts current to voltage at 1 millivolt permicroamp while the capacitor limitsthe measurement to frequenciesunder about 1 kHz. One lead of this current meter is connected tothe shield of an input or output jack on the device under test. AnIHF/RCA plug ishandy for this and it generally won't matter whichjack you choose, since allshield grounds are usually tied togetherinside the device. The other lead of the current meter isconnected

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Figure 3 - MEASURING THE CHASSIS CURRENT

to a ground equivalent to power line neutral. The safety ground ofany modern outlet is convenient for this. It is required by safetycodes to be tied to earth ground, as is the neutral. The currentmeasurement should be taken under four conditions: device "off",device "on", then repeated with the device's AC plug reversed.Taking the highest reading willgiveus a "worstcase" number whichcan then be used, along with Table 1, to estimate the system humlevels produced when this device isconnected to others via cables.

The author has tested a variety of consumer devices, including CDplayers, cassette decks, tuners, receivers, and power amplifiers, forchassis current to ground. The broad categories of typical groundcurrents developed from the testing were used in Table 1. Groundcurrent isgenerally related to ACpower consumption of the device,since this dictates the size of its power transformer and, to someextent, its interwinding capacitances. Ground current L, 5 p.ARMS,is typical of "lowpower" consumer gear drawing under 20 watts.This includes most CD players, cassette decks, and turntables.Current M, 100 p.A RMS, is typical of "medium power" consumergear drawing 20 to 100 watts. This includes most tuners, low tomedium power receivers or power amplifiers, and some small TVreceivers. Current H, 1 rnA RMS, is typical of "high power"consumer gear drawing, or capable of drawing, wellover 100 watts.This includes most high powered amplifiers or powered subwoofersand large screen or projection TV receivers. Table 1 shows thecalculated effect of these currents when they flow in interconnectcable shields in an unbalanced audio system. A contact resistance of50 mQper connection was used and the 0 dB reference level is 300mV RMS or about -10 dBV. Allresults have been rounded to thenearest dB.

Please note that this characterization of chassis current applies onlyto devices wi th two-prong AC plugs. Three-prong plugs effectivelyconnect the device chassis to safety ground, making the chassis avoltage source. System effects of this willbe discussed later.

11 , - . .T-i orh 3ft~lm 10 ft ~ 3 m 20 ft ~ 6 m 50 ft ~ IS m 100 ft ~ 30 m

IIent L M H L M H L M H L M H L M H

#26 GA (41 mn per ft) -109 -83 -63 -101 -75 -55 -96 -70 -50 -89 -63 -43 -83 -57 -37

Shield Conductor

#24 GA (25 mn per ft) -III -85 -65 -lOS -79 -59 -100 -74 -54 -93 -67 -47 -87 -61 -41

Shield Conductor

#22 GA (16 mn per ft) -112 -86 -66 -107 -81 -61 -103 -77 -57 -96 -70 -50 -91 -65 -45

Shield Conductor

2

Table I - CALCULATED HUM LEVEL, dB re 300 mV, vs GROUND CURRENT, CABLE LENGTH, and SHIELD GAUGE

Jensen AN-004



AUDIBILITY OF HUM & BUZZJust what levelof hum or buzz is audible depends on many factors.A recent AES paper indicates that noise artifacts should be under-120 dB to be inaudible for serious listening in residentialenvironments. [5] The experience of this author indicates that levelshigher than about - 80 dB are annoying to most listeners. The noisesoriginating with the power line are generally described as either"hum", which ispredominantly 60 Hz, or "buzz",which consists of amixture of high-order harmonics of 60 Hz. These harmonics are theresult of power line waveform distortion, which commonly reaches

5% THD and is caused by many types of non-linear power lineloads. Because the human ear ismuch more sensitive to frequenciesin the 2 kHz to 5 kHz range at these very low levels, buzz is usuallymore audible than hum, even though the hum level may beelectrically larger.

BREAKING THE INTERCHASSIS CURRENT PATHTo eliminate hum, we must effectivelyeliminate interchassis groundcurrent. We could eliminate it by simply breaking the chassis tochassis shield connection. Of course, this alone would not solve ourproblem. We must break the signal line as well and insert a devicewhich willsense the voltage at the output of deviceA and regenerateit into the input of device B, while ignoring the voltage that existsbetween the now disconnected device grounds. These properties

generally describe a differential responding device with highcommon-mode rejection, usually called a ground isolator. Seereference [2] for more information on this subject.

Two basic types of differential responding devices, active differentialamplifiers and audio transformers are available at reasonable cost.We won't consider active optical or carrier modulated isolationamplifiers here because such devices which also have acceptableaudio performance are still quite expensive.

Active differential amplifier circuits are used in a number ofcommercially available devices. To a greater or lesser extent, they allshare several disadvantages: they can further complicate the groundsystem by contributing interchassis currents of their own, since theyrequire AC power; they cannot handle ground voltage differencesover about 10 volts RMS; they use semiconductors or integratedcircuitswhich are prone to degradation or failure caused by powerline or lightning induction voltage transients; and, worst of all, theyare exquisitely sensitive to source impedance. This sensitivity limitshum rejection, even in a balanced system (for which they areintended), but it makes them nearly useless in an unbalancedsystem. [2]A typical example ofsuch devices isthe popular SonanceAGI-l (which uses the Analog Devices SSM2141). Labmeasurements on this unit, shown in Figure 4, reveal that over the200 Q to 1 kQ range of source (output) impedances typical inconsumer equipment, its hum rejection is only 15 to 30 dB.

High quality audio transformers are, by their nature, relativelyinsensitive to source impedance and exhibit excellent hum rejectionperformance in either balanced or unbalanced systems. Under thesame conditions and same ran~e ofsource impedances, the passivetransformer based ISO-MAX model CI-2RR measures 90 to110 dB. Asshown inthe Figure 4 graph, itsmeasured hum rejectionis over 70 dB better than the active device. It requires no powerof any kind and can handle ground voltage differences up to 250volts RMSwithout malfunction, degradation, or damage.There is a widespread belief that all audio transformers haveinherent limitations such as high distortion, mediocre transient

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10

20

30

40

50

60 . - ..70 -..-80

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10 0

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Figure 4 - HUM REJECTION vs SOURCE IMPEDANCE

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response, and large phase errors. Unfortunately, many suchtransformers do exist and not all of them are cheap. The vast

majority of available audio transformers, even when used asdirected, do not achieve professional performance levels. As CalPerkins wrote "Withtransformers, you get what you pay for. Cheaptransformers create a host of interface problems, most of which areclearly audible."[6] If well designed and properly used, however,audio transformers qualify as true high fidelity devices. They arepassive, stable, reliable, and require neither trimming, tweaking, norexcuses.

DIAGNOSIS OF A LARGER SYSTEMMost systems consist of more than two devices and often consist ofa mixture of floating (2-prong AC plug) and safety grounded (3-prong AC plug). In addition, devices may be connected to externalsources of ground currents, such as cable TV. Our previous analysisof a generalized two device system allows us to apply the sameprinciples to analyze and treat hum problems in larger systems. Ourexample system, shown in Figure 5, consists of a large screen TVreceiver with audio outputs, a stereo preamp control center, a sub-woofer with internal power amplifiers, and a stereo power amplifierforthe satellite speakers. Alldevices have 2-prong ACplugs, exceptthe sub-woofer, which has a 3-prong plug. Initially,we willnot makethe cable TVconnection shown by the dotted line. The interconnectcable is a foil shielded type with a #24 gauge drain wire having aresistance of 25 mQper foot. Now, let's go through the process step-by-step.

Step 1 is to measure or estimate the worst case ground current ofeach device having a 2-prong ACplug (as described under heading3). To keep our ana lysis process as easy as possible, we will be using

some simplifying assumptions and approximations throughout.Therefore measurements need not be made with laboratoryprecision. Our calculated hum levels will generally be pessimistic byseveral dB.

Step 2 isto measure or calculate the interchassis resistance foreachcable run. Vendor data usually provides either resistance per unitlength or equivalent wire gauge information for the cable's shield.Remember to include some shield contact resistance at eachconnector (normally one at each end) as part of the total. If the

Jensen AN-004 3



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Figure 5 - TH E EXAMPLE SYSTEM "BEFORE"

cable run is a stereo pair, remember to divide by two because thetwo shields are in parallel. The 50 foot run from TV to preamp inFigure 5 was calculated as follows:

50 ft cable shield @ 25 mQ/ft = 1.25 Q

2 connector contacts @ 50 mQ each = 0.1 QTotal resistance per cable = 1.25 Q + 0.1 Q = 1.35 QResistance of 2 paralleled cables = 1.35 Q -i- 2 = 0.68 Q

Step 3 isto estimate current flowineach cable (or cable pair, inthiscase). At this point we willmake some simplifying assumptions, butaccording to the following rules:

A - If two devices with different ground currents areconnected, the current flowbetween them islimited to thelower of the two currents. If the ground currents haveequal values, flowbetween them isthat value.

B - Current flowbetween two high ground current devices canflowthrough a lower ground current device chassis fromconnector to connector.

C - A device connected to safety ground or any external pathto earth ground or AC neutral can support unlimitedcurrent flow.

D - If current into a device may flow out in multiple paths,assume that value willflow in all paths that can support it(according to rule A).

Make a simple diagram of the system, similar to Figure 5, and enterthe measured or estimated worst case ground current for eachfloating (2-prong plug) device near itsAC plug symbol. Applying therules, determine the current flow in each cable (or cable pair) andenter the value on the diagram. In our example, although thepreamp can support only 30 pA through its power connection, notethat the 1 rnA ground current from the TV will flow through it toeither the power amplifier (which can support 1 rnA through itspower connection) or the sub-woofer (which can support unlimitedcurrent through its safety ground).

Step 4 isto calculate the hum voltage drop for each cable. Knowingthe values of interchassis resistance (from step 2) and current flow(from step 3) for each cable, allows us to find the hum voltage.According to Ohm's law, E = I x R where E is the hum voltage, I isthe interchassis current, and R is the interchassis resistance.

4

Therefore, in Figure 5, the hum voltages are:

1 rnA x 0.68 Q = 0.68 mV for TV to preamp cable,1 rnA x 0.55 Q = 0.55 mV for preamp to sub-woofer, and1 rnA x 0.11 Q = 0.11 mV for preamp to power amplifier.

Step 5 expresses the ratio in dB of each of these hum voltages tothe signal voltage, since the hum voltage directly adds to the signalat the receive end of each cable. Because each cable carries anominal 300 mV (sometimes expressed as -10 dBV) maximumsignal level, this will be our reference level. For each voltagecalculated in step 4, the hum level, in dB relative to the referencesignal, is calculated as dB = 20 x log (E -i- 300 mV). Expressing ourexample voltages in dB gives us:

-53 dB at preamp input from TV,-55 dB at sub-woofer input from preamp, and-69 dB at power amplifier input from preamp.

These numbers do not include the additional hum caused by adevice which may amplify the hum appearing at its input. In thisexample, even ifthe preamp volume control is "off",unacceptablyhigh hum levelsexist at the inputs ofboth the sub-woofer and poweramplifiers. If we advance the volume to unity preamp gain (preampinput and output levels the same), the hum level to both poweramplifierswillfurther increase. Calculations should assume thatall hum voltages are in-phase and additive. While it is possibleforthe hum at the preamp input to be anti-phase to the hum at thepower amplifiers (as volume is advanced, hum would decrease to a"nul/" and then increase beyond the null), we won't rely on thispossibility.

TREATMENTS TO REDUCE HUMAs a general rule, problem areas involve the longer cables and

higher interchassis currents. Inour example system of Figure 5, theseare the 40 and 50 foot cable runs. Figure 6 shows the same systemwith transformer isolators added to the long runs. Note how theisolators reduce interchassis currents (the 1 /LAflow through eachisolator is due to interchassis voltage now present - more on thatlater). Since the isolators block the high chassis currents from the TVand sub-woofer, the preamp to power amplifier current isnow only30 /LA and flows in the shortest, lowest shield resistance

Jensen AN-004



cable. Following steps 3, 4, and 5 as before, the new hum estimatescalculate as:


Inreality, we can't achieve the -113 dB and -115 dB levels. Recallthat Figure 2 showed a Thevenin equivalent circuit for each device,consisting of voltage source Vxand capacitance Cx. If an isolatoreffectively disconnects a device from ground, its chassis will "float"above safety ground at its Thevenin voltage. This voltage, whichappears as common-mode (onboth input lines)to the isolator, couldrange from 0 to 120 V.Areasonable typical is 60 Vwhich is +46 dBrelative to a 300 mV reference signal. Even the best real isolator,with a CMRRof 120 dB, willhave an output hum level of +46 dB- 120 dB or - 74 dB.

If, as shown inFigure 6, we ground each device through a separatewire (not through the audio cable shields), we can essentially removethe common-mode voltage from the isolator. More and moreequipment, TV receivers especially, have plastic cabinets and theonly exposed metal may be screwheads and connectors, making itunclear how to ground the "chassis".DONOTmake the ground wireconnection to anything INSIDE the cabinet - you may severely

damage the equipment and/or create a lethal shock hazard. You canconfirm that a screwhead, for example, is an effective "chassis"ground by: 1) disconnecting the equipment from everything exceptACpower, 2)with an ACvoltmeter, monitor the voltage between anaudio output jack's outer (shield) contact and the AC outlet's safetyground pin, and 3) verifythat the voltmeter reading drops to undera volt when the grounding wire connected to the AC outlet's safetyground pin istouched to the screwhead. When grounded this way,the output hum levelof our example system willdrop from - 74 dBto about -110 dB (the thermal noise f loor of a Jensen ISO-MAX®CI-2RR isolator). With isolators and added grounds in place, ourestimate becomes:


The - 99 dB figure can be improved further by lowering the shieldresistance of the 5 foot cable which uses a foilshielded cable with#24 gauge drain wire (25 mQ per foot). Cable using a #18 gaugeequivalent braided copper shield (6.5 mQ per foot) willlower humlevel by 5 dB from -99 dB to -104 dB.

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The cable TV connection isshown to illustrate the problems it cancause. Most cable systems supply ACpower to their trunk mountedrepeater amplifiers through the trunk cable itself. This 60 Hz ACcurrent flows through the shield of the coaxial trunk cable and(surprise) causes AC voltage drops. For safety reasons (lightningstrikes to the trunk line, for example) the residential "drop" cable isusually "grounded" near its point of entry to the building. Thisground may be to a water pipe, a separate earth ground rod, thesame ground point used bythe main ACpower panel, or the groundconnection may not even exist. In any case the cable TVshield willtypically carry several volts of hum with respect to the building'ssafety ground wiring. When the dotted cable connection ismade inFigure 5, this ground voltage difference can cause very high currentsOustunder 200 rnA in our example) to flowfrom cable shield to TV,through audio cable to the preamp through more audio cable to thesub-woofer system and itssafety ground. The resulting voltage dropsin the audio cables produce truly horrible hum levels of about - 6dB. One might be sorely tempted to "lift"the safety ground of thesub-woofer to reduce the hum (which it would).

DO NOT DEFEAT SAFETY GROUNDS!A "ground adapter" is intended to provide a safety groundfor 3-conductor power cords when used with 2-prongoutlets, NOT defeat the safety ground provided by a 3-prong

outlet. Defeating a safety ground could allow lethal voltagesto appear on all equipment in an interconnected system.

In most systems, the best way to deal with the cable TVproblem isto stop the current flowwith a 75 QRFisolator. Mostcommercial RFisolators are simply high-pass filterswhich use capacitive coupling,resulting in input to output capacitances up to 4 nF. Although theseisolators certainly reduce the current injected into the ground system,RFtransformer type isolators, with capacitances under 50 pF, workabout 40 dB better, to effectively eliminate the current. The JensenISO-MAX®VR-1FF issuch a transformer type isolator. Because ourmodified system of Figure 6 prevents the TV chassis currents frompassing through any audio cables, it does not need an RF isolator.Generally, neither RF isolator type can be used with DSS receiversbecause they do not pass DC current.

An unbalanced audio system with two or more safety groundeddevices (3-prong AC plugs) will virtually always require audioisolators to prevent hum. Without isolation, the voltage dropsoccurring in the safety ground wiring ofthe building are forced ontothe audio cable shield system through the two or more safety groundconnections. Thisproblem willgenerally be made more severe as the

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5ensen AN-004



distance (and length of intervening building safety ground wiring)increases. The worst case istwo safety grounded devices operatingfrom two different branch circuits of the building's AC power. Thebest case is two safety grounded devices located very near eachother and operated from the same AC receptacle.

Magnetic pickup loops can easilybe created by improper routing ofcables. Any closed loop of wire in the proximity of an ACmagneticfieldwillhave a current induced inthe loop. When this current flowsin the shields, it produces voltage drops which contaminate theaudio just as interchassis currents do. Common sources ofstrong ACmagnetic fields are power transformers, motors, fluorescent lights,AC power wiring (even inside walls and conduit), TV sets, andcomputer CRT displays. A simple stereo pair of interconnect cablesbetween two devices forms a loop because the shields are tiedtogether at both ends. Since loop pickup is directly proportional tothe area inside the loop, the cables should be dressed as physicallyclose together as possible. In fact, all cables that connect between thesame two devices should be bundled together in order to minimizeloop area. The bundles should, of course, be kept as far as possiblefrom magnetic field sources.

CONCLUSIONS AND TIPSHum and buzz in unbalanced audio systems is caused by commonimpedance coupling in the shield resistance of the interconnecting

cables. This coupling can be minimized by reducing shieldresistance, reducing or eliminating interchassis currents, or both.

Choose shielded cable for low shield resistance. A cablewith 6.5 mQ/ft (6.5 Q per 1000 ft) shield resistance and90% shield coverage is much preferred over one with25 mQ/ft shield resistance and 100% shield coverage.

Use high quality, transformer based audio ground isolatorsto eliminate high interchassis currents.

Bundle together all audio cables connecting the same twodevices and keep the bundle away from power cabling orother AC magnetic fields.

NEVER, NEVER DEFEATTHE SAFETYGROUNDINGof any device having a 3-prong power cord. The results ofdoing so can be deadly to you and/or your customer.

If the system contains more than one safety groundeddevice, use audio ground isolators to eliminate current flowthrough audio cables connecting them.

Add external grounding, ifpossible, to devices without anyother system ground path in order to reduce common-mode voltage at the isolators.

Use a transformer type RFisolation device, ifnecessary, toprevent ground currents that may result from the cable TVconnection.

References

[ 1 ] B. Hofer, "Transformers inAudio Design", Sound & VideoContractor, March 15, 1986, p. 24.

[2] B. Whitlock, "Balanced Lines in Audio Systems: Fact,Fiction, and Transformers", J. Audio Eng. Soc., vol. 43,

6

[3]

[4]

[5]

[6]

pp. 454-464 (1995 June).

M. Glasband, "Lifting the Grounding Enigma", Mix,November 1994, pp. 136-146.

D. Engstrom, "The AC Connection: A Tutorial, Part 1",Sound & Communications, February 25, 1995, pp. 28-38.

L. Fielder, "Dynamic Range Issues in the Modern DigitalAudio Environment", J. Audio Eng. Soc., vol. 43, pp. 322-339 (1995 May).

C. Perkins, "To Hum or Not to Hum", Sound & VideoContractor, March 15, 1986, p. 41.

ISO-MAX®is a registered trademark of Jensen Transformers, Inc.Copyright 1996, Jensen Transformers, Inc.

Jensen Transformers, Inc.7135 Hayvenhurst AvenueVan Nuys, California 91406

Tel (818) 374-5857 Fax (818) 374-5856http://www.jensen-transforrners.com

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