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Frequently Asked

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  1. Q.  Can you answer a specific technical question for me?
    A.  First check the list of tech FAQs found later on this page.
          (Or continue in this list for more general FAQs, covered by other pages on this site.)
  2. Q.  Where in the UK can I buy PA equipment and spare parts?
    A.  See Suppliers.
  3. Q.  Which are the most commonly used brands?
    A.  See Links to Manufacturers' Sites.
  4. Q.  How do I put together a PA system?
    A.  See Getting Started − Assembling a System.
  5. Q.  What safety issues do I need to think about?
    A.  See Safety.
  6. Q.  What kinds of things do I need to consider when operating my system?
    A.  See Getting Started − for Mixing Engineers.
  7. Q.  What do all the controls do on my mixer?
    A.  See Mixing Facilities.
  8. Q.  I know the technical basics, but where can I find some useful practical advice?
    A.  Try PA Proverbs.
  9. Q.  I am a performer new to using a PA, can you help?
    A.  See Getting Started − for Performers.
  10. Q.  My controls / level meters have markings in dB − what exactly does it mean?
    A.  See Decibels.
  11. Q.  What is amplifier bridging and when should I use it?
    A.  See Bridging on the Amplifiers and Speakers page.
  12. Q.  Can you help me with all the terminology used in PA?
    A.  See the Glossary.
  13. Q.  I want to become more knowledgeable about PA, what kinds of things do I need to know?
    A.  For a structured list of topics, see Training.
          Or for experience-based advice and tips, see PA Proverbs.
  14. Q.  I am having difficulties with the PA at my local church (or other place of worship) − how can I get better results?
    A.  See PA for Places of Worship.
  15. Q.  What other sources of information can you suggest?
    A.  See PA-Related Links.

See also the list of technical FAQs below. You might also find some of the guidance on the PA Proverbs page helpful.

"The only thing more expensive than education is ignorance." − Benjamin Franklin.

The FAQs below are provided to suggest solutions to some specific technical issues commonly encountered with PA systems. If you have a technical problem that is not covered by the questions below then please feel free to contact PAforMusic.

  1. Which speakers / microphones / mixer / amplifiers / etc should I buy?  View
  2. How can I avoid feedback?  View
  3. How can I solve a hum (or buzz) problem?  View
  4. How can I avoid background hiss?  View
  5. How powerful does my system need to be?  View
  6. How should I set up my channel Gain controls?  View
  7. How should I set up my channel EQ controls?  View
  8. Should I use mono or stereo?  View
  9. How should I mike up a ...?  View
  10. How do I wire up a microphone cable?  View
  11. My amplifier has a quoted output power into 4 ohms, but my speakers are 8 ohms. Is this a problem, and will it affect how much output power I will get?  View
  12. Why do my speakers have different quoted maximum power values for 'RMS' and 'music power'?  View
  13. I changed my speakers − why do they sound louder (or quieter) than my old ones, even though they have the same impedance value?  View
  14. How do I connect the balanced output from my mixer to the unbalanced input of my power amp, without getting hum problems?  View
  15. Can I use a different model of external power unit to replace a lost or faulty one?  View
  16. I'm bi-amping my speakers − what crossover frequency should I set?  View
  17. Why do UK mains plugs contain a fuse, and what fuse value should I use in my IEC power cable plugs?  View
  18. What's the difference between earthing and grounding?  View
  19. What's the difference between level and volume?  View
  20. How do I calculate ...?  The formulae and calculators have now moved to this new page.

Q 1.
Which speakers / microphones / mixer / amplifiers / etc should I buy?

A 1.
PAforMusic cannot provide answers to questions such as "Which speakers should I buy?" because choosing the most suitable equipment (of any type) depends on so many factors. Often compromises between the different factors may be necessary, depending on their relative importance in your particular case. Some of the most important factors will usually include:

So, after considering the above questions (and perhaps making some brief notes), your best course of action would probably be to contact a reputable PA equipment supplier for guidance on which makes and models would be most appropriate to your particular circumstances and requirements − though for microphones and mixers you might find that the PAforMusic microphone selector and 'choosing a mixer' notes give you a useful starting point.

Information on specific items of equipment can be downloaded from many of the manufacturers' websites − see the Equipment Manufacturers page. Some UK suppliers are listed on the Suppliers page.

Above all, be sure to select equipment that fully meets your most important requirements − regardless of what attractive 'extra' features are offered by models that do not. Likewise, beware of being swayed towards unsuitable equipment due to it being available as a "cut-price bargain".

Back to the Tech FAQ Contents.

Q 2.
How can I avoid feedback?

A 2.
Acoustic feedback, the most common variety, results from too high an overall gain around a complete loop through the PA system, from microphone, through mixer and amplifier, to speaker and back (through the air) to microphone again. To avoid the feedback, the overall gain around this loop at the feedback frequency must be reduced. This may be achieved through one or more of the following actions (not all of which will be relevant or practicable in every situation):

If your system has an induction loop, and if the feedback is more of a "screech" than a whistle or a low note, and starts and stops suddenly, then it is possible that the feedback is magnetic rather than acoustic. In such a case, you can check for magnetic feedback by seeing if the problem goes away when you switch off the induction loop. This kind of feedback occurs when the magnetic field of the induction loop is picked up too much by other parts of the system. For example, if an electric guitar is connected to the PA system (or its backline is miked-up or DI'd) and its signal is fed to the induction loop, then pick-up of the induction loop by the guitar pick-up will complete a magnetic feedback loop. Even if the guitar is not amplified by the PA system but only by its backline, pick-up of the induction loop signal by the guitar may still occur and be emitted as sound from the backline speaker(s). If this backline sound is picked up by mics that are fed into the induction loop then the resulting feedback path will involve both magnetic and acoustic sections.

Magnetic feedback problems can usually be eliminated by changing the relative positions or orientations of the magnetic field source and/or the equipment that is picking it up, for example by changing the location or angle of a problematic guitar. It may also be necessary to reduce the level of guitar signal fed into an induction loop. To minimise the likelihood of these problems in advance, use guitars with humbucking pick-ups, and route the loop cable so as to avoid including the stage area within the loop. In addition, it is always preferable to aim to minimise the extent to which backline sound is picked up by mics that are provided for other purposes.

Back to the Tech FAQ Contents.

Q 3.
How can I solve a hum (or buzz) problem?

A 3.
This depends on where the hum is being introduced into the system, and on the way that it is being introduced.

Considering the "where" aspect of this first, you should determine if the hum is coming into the mixer on one (or more) of the channels, and if so which, by the following procedure:

Now considering the "way" aspect, there are five main ways that hum can be introduced:

  1. By inductive coupling from a source of a mains-frequency magnetic field − usually from a mains transformer within an item of equipment, or occasionally from mains cables. Possible solutions in this case are:
    • Increase the distance between the equipment or cable which is producing the offending field and the equipment or cable which is picking it up.
    • Use a system with balanced inputs to the mixer, and ensure that all connections to balanced circuits are made using only balanced cables.
  2. By capacitive coupling from mains cables (not necessarily ones that are powering equipment associated with the PA system!). Often a hum caused this way has a relatively high harmonic content and so its sound is often quite 'thin' or 'edgy', more usually described as a 'buzz' than a 'hum'. Possible solutions in this case are:
    • Check that screened cable is used for all connections except for speaker and mains connections.
    • Check that the cables or their connectors are not faulty (e.g. a screen not properly connected).
  3. By means of an earth loop. This is a condition whereby a "circular" arrangement of earth connections exists because of the interconnection of two or more items of earthed equipment (typically backline amplifiers and mixers). Earth leakage currents flowing in the earth connections develop a hum voltage between the chassis of the interconnected equipment, which effectively adds to the signal voltage passed between them. To avoid this problem:
    • Make use of balanced feeds wherever possible. Hum voltages caused by earth loops will then be added equally to the 'hot' and 'cold' connections, enabling the destination equipment to largely cancel out the hum that it receives.
    • For the connection of backline amplifiers and other instruments with an unbalanced output, use a DI box, located as close to the signal source as is practical, to balance the signal as soon as possible. These often also have the facility to provide electrical isolation of signal earth connections, and so reduce hum even further. (Note that if phantom power is present on a balanced connection then a DI box must always be used when connecting an unbalanced source to it.)
    • Never attempt to avoid earth loop problems by the disconnection of safety earths, as this would create a lethal hazard.
  4. Because faulty or poor-quality equipment is directly adding the hum to the signal passing though it. Or, because one or more items of equipment have a 'pin 1 problem'.
    • Get it fixed or buy better equipment!
  5. Because of a problem with the mains power arrangements.
    • Get it investigated and fixed by a qualified expert − and urgently as there may be an associated safety hazard.

Back to the Tech FAQ Contents.

Q 4.
How can I avoid background hiss?

A 4.
The background "hissing" noise introduced by equipment can never be totally eliminated, but it can usually be rendered insignificant in relation to the level of the signal by ensuring that the equipment is carrying a level of signal that is no lower than the minimum that the equipment is intended to handle. This is especially important in the case of any equipment that has a relatively poor dynamic range. (It is noteworthy that some lower-quality radio-link equipment and effects processing equipment suffers from this problem.)

In practice, this usually means ensuring that the original source signal (e.g. from a microphone or instrument pick-up) is initially adequately amplified by a sufficiently high-quality pre-amp, and is not subsequently unduly attenuated (e.g. by a level control being turned down too much). For guidance on the correct setting of pre-amp gain controls see the Setting gain controls item on this page.

Another source of hiss that is sometimes encountered occurs when balanced outputs of equipment are connected to unbalanced inputs, or are connected to using unbalanced cable. If a balanced output is of the quasi-floating type (also known as 'cross-coupled'), then if it is connected to an unbalanced input, or is connected to using unbalanced cable, a significant amount of noise (hiss) can be added by the line driver unless the unused leg (typically the cold leg, i.e. pin 3 of an XLR or the ring of a 3-pole jack) of the output is linked with signal earth (i.e. pin 1 of an XLR, or the sleeve of a 3-pole jack), ideally at the destination end of the interconnection (in the case of balanced cable being used).

For further information on minimising hiss levels, see Gain structure and Signal-to-noise ratio.

Back to the Tech FAQ Contents.

Q 5.
How powerful does my system need to be?

A 5.
You first need to know how loud you want your system to be. This will depend upon the type of programme material, the size of your audience, the level of other sound sources (e.g. audience sound and background noise), etc.

As the sound level will decrease with increasing distance from the speakers, you need to consider the average sound level (SPL) that is needed at the front and at the back of the audience area. Then you can work out the sound level required at a distance of 1 metre in front of the speakers (see Inverse square law).

The amount of average electrical power (watts) that is needed to create a particular average sound level 1 metre in front of the speakers depends on the sensitivity of your speakers. More sensitive speakers give the same sound level output for less electrical power input. For example, a speaker with a sensitivity figure of 90 dB/W will need a power input of 1 kW to provide 120 dB SPL at a distance of 1 metre, but a 96 dB/W speaker only needs an input of 250 W to give the same sound level at that distance.

Your speakers will need to have a continuous average power rating that is at least as high as the average power input level that they require to produce the average sound level that you want. For example, if the sensitivity value of your speakers means that they need 500 W in order to give the average sound level that you want, then they will need to have a continuous average power rating of at least 500 W if they are to produce that sound level without being damaged. (The maximum continuous average power input level that a speaker can handle is often referred to as its RMS power rating, though this is strictly incorrect terminology.) For more information see Speaker Sensitivity on the Amplifiers and Speakers page.

Next you need to consider how much headroom you require − this will depend on how 'peaky' your programme material is, and on whether you are using a limiter. The important thing to ensure here is that the amplifier is able to deliver the peak power that is required without clipping (preferably with some margin to spare). For example, if the average power level is 500 W per channel and the amplifier has a power rating of 2 kW per channel then this will provide a headroom of 6 dB to accommodate the programme peaks. Note that this will often mean that the amplifier power rating is greater than the speaker power rating (unless your speakers are rated considerably higher than they need to be).

As a final check, be sure that the peak power rating of your speakers is high enough to cope with the peaks of the programme, and be sure that you do not abuse the headroom of the amplifier by driving the speakers at too high an average level. For more information see Power Ratings on the Amplifiers and Speakers page.

If the power requirement is high, the total power needed to achieve the needed sound level can be split between several speakers, provided that they are positioned and angled so as to cover the same audience area. If, however, multiple speakers are positioned and/or angled so that they each cover just a part of the audience area, then the required power input to the speaker (or group of speakers) covering each area will need to be considered separately if the audience areas are of different size or are different distances from their respective speakers. In large systems incorporating multiple speakers, several amplifiers are often used − or powered speakers are utilised.

For more detail on the selection of amplifiers and speakers see Amplifer and speaker selection on the Amplifers and Speakers page.

Back to the Tech FAQ Contents.

Q 6.
How should I set up my channel Gain controls?

A 6.
As each model of mixer is different, the only safe answer is "Set them according to the advice given in the mixer's operating instructions". If you don't have the instructions, they can often be downloaded from the manufacturer's website − see the Manufacturers page for links.

If for some reason you need alternative/additional guidance, here goes:

Remember to keep an eye on the channel levels (meters and/or LEDs) during the event. In particular, you may need to re-adjust Gain controls in the following circumstances:

Also see Overload or level indication on the Mixing Facilities page.

Back to the Tech FAQ Contents.

Q 7.
How should I set up my channel EQ controls?

A 7.
From an artistic viewpoint, the simple answer is "To get the best out of the sound from the source connected to each channel", or "To get as close as possible to the kind of sound that you want on each channel", or "To get the best fit of the sound of each channel in the overall mix". For an engineering perspective, see the Equalisation section of the Mixing Facilities page. For further advice on setting EQ, see the Mixing section on the PA Proverbs page.

Back to the Tech FAQ Contents.

Q 8.
Should I use mono or stereo?

A 8.
This depends entirely upon whether the performers using the PA system, or the recordings that are to be played, rely on stereo sound to achieve their desired effect. If not, it is better to use mono because it is simpler and cheaper.

When using stereo, remember that a true stereo effect is only heard by listeners who are situated an equal distance from the left and the right speakers. The more unequal the distance, the less impressive the effect. This means that stereo works best in long, narrow rooms, and less well in short, wide rooms. Listeners whose location results in them effectively hearing only the sound from the speaker nearest to them (Haas effect) will probably get a less satisfactory sound than if mono were being used.

Back to the Tech FAQ Contents.

Q 9.
How should I mike up a ...?

A 9.
There are two aspects to this:

For the first of these, you may find the PAforMusic microphone selector useful. But, in general, this is a 'technical' site and does not prescribe specific ways to do 'artistic' things such as miking up instruments, backline cabs, etc. For some technical guidance see Use of Microphones.

The best rule-of-thumb is 'experiment and find out what sounds best' with your available microphones, instruments and circumstances. And remember that there is no definitive 'right' and 'wrong' way of doing things in the artistic world. What sounds good to you, or is an ideal microphone technique to use in one situation, may be quite different to someone else's opinion, or to what would be best in different circumstances.

However, if you need a suggested starting point, or inspiration, or you just don't have the time to experiment, good sources of reference are provided by the top microphone manufacturers e.g. Shure.

Back to the Tech FAQ Contents.

Q 10.
How do I wire up a microphone cable?

A 10.
If, as is usually the case, the cable is to have XLR connectors on both ends, then proceed as follows:

See also the diagram and information on the System Assemblers page.

Back to the Tech FAQ Contents.

Q 11.
My amplifier has a quoted output power into 4 ohms, but my speakers are 8 ohms. Is this a problem, and will it affect how much output power I will get?

A 11.
The figure quoted in ohms for your speaker is the impedance value of the speaker. This is a measure of how much it opposes the flow of current supplied by the amplifier. So, connecting a speaker with a higher impedance figure than is quoted for the amp will do no harm to the speaker nor to transistor-based amplifiers, because your speaker will draw a smaller amount of current from the amp than what the amp is capable of supplying.

However, the connected overall speaker impedance must never be less than the minimum value specified for the amplifier, as this would cause too much current to be drawn from the amp. (We say 'overall impedance' to cover the case where several speakers are connected to a single output of the amplifier. In this case you must work out the combined impedance of all the speakers that you are connecting, taking into account the way in which they are interconnected − usually in parallel.)

In contrast, amplifiers having valve-based outputs usually required a specific load impedance to be connected (no less and no more), to ensure proper operation and avoid damage. Some types have a switch that enables the required load impedance to be selected from a range of values.

Now we will consider how much power we will get. Since power is voltage times current, we have to consider both of these quantities. The voltage provided at the output of an amplifier, at a given point in time, is essentially unaffected by the load impedance connected (though the maximum output voltage attainable will be affected to some degree). However, as we have already said, the current is very much affected by the load impedance. From Ohm's Law, if the load impedance is doubled then the current will be halved.

So, with an 8 ohm speaker, half as much current will flow as with a 4 ohm one, causing half as much power to be obtained from the amplifier (given that its output voltage remains constant).

We can perhaps see this more clearly from an alternative formula for power: P = V2 / R i.e. the amount of power is the voltage squared, divided by the impedance (or, more strictly, the resistive part of the impedance). In this formula, when R is 8 instead of 4 then we are dividing V2 by twice as much, so the resulting value for the power is only half as much.

Now in practical terms we are often concerned with the maximum power that we can get from an amplifier. In this case, we have to consider the maximum amount of voltage that it can supply, as well as what load impedance we are connecting. Connecting a higher load impedance will put less of a burden on the internal power supply of the amplifier, so in this case the maximum output voltage that the amplifier can produce will be a little higher. This means that it will be able to supply a little more power than you might expect from the simple theory above. For example, if an amplifier is rated at 200 W into 4 ohms, the theory would indicate that the maximum output power than can be obtained into 8 ohms would be half as much, i.e. 100 W. But, in practice, the reduced burden on the power supply means that the maximum output power into 8 ohms might be around 120 W.

So why is it so common for amplifiers to be rated into a 4 ohm load, when most speaker cabs are 8 ohms? This is because many users will want to connect more than one speaker to each channel of the amplifier. When plugging several speakers direct into the same channel of the amplifier, or plugging one speaker into the amplifier and then 'looping on' from another connector on that same speaker to a further speaker, then in practice you are connecting those speakers in parallel. In such cases, the maximum number of 8 ohm speakers that you can supply from each channel of a 'minimum 4 ohm rated' amplifier is two − because the overall impedance of two 8 ohm speakers connected this way will be 4 ohms. In this example, you would then have available the full '4 ohm rated' power output of the channel − half of that value into each speaker. This arrangement needs two speakers per channel, but that can be helpful because, for FOH, it gives more flexibility in how they are positioned and pointed, in order to provide the best coverage of your audience area. (Or in the case of monitors, it is often useful to have more than one speaker per monitor mix, whether for different performers or perhaps two speakers for a lead vocalist, for example.)

For more information see Impedance on the Amplifiers and Speakers page.

Back to the Tech FAQ Contents.

Q 12.
Why do my speakers have different quoted maximum power values for RMS power and music power?

A 12.
'RMS power' is (in practice) short-hand for 'continuous average sine-wave power', and the rating is usually given on the basis of some standard test signal such as band-limited pink noise. The important word here is continuous − the test sound just goes on and on at the same level, so it puts a lot of stress (especially heating) on the speaker. That's why the RMS rating figure has to be lower than other kinds of rating.

But real programme material (whether music or speech) isn't like that. It has variations in level: peaks and quieter passages. So, when considering the rating of the speaker for music programme signals, the speaker can be rated at a higher value than for the continuous test signal, because it only has to stand that maximum amount of power for short intervals − on the peaks of the music. For more information see Power Ratings on the Amplifiers and Speakers page.

Back to the Tech FAQ Contents.

Q 13.
I changed my speakers − why do they sound louder (or quieter) than my old ones, even though they have the same impedance value?

A 13.
Since they have the same impedance as your old ones, they will be drawing the same amount of audio power from your amplifier (see Q 11) − assuming that you haven't changed any settings of your equipment. But, a given power level into the speakers does not directly indicate the sound level they will produce − that depends on how efficient your speakers are at converting electrical power into sound. We call this 'efficiency' value the sensitivity of the speaker, and it can vary a great deal between different makes and models of speaker. So, 100 W of power into one type of speaker can easily sound like 200 W or even 400 W of power into another type! For more information see Speaker Sensitivity on the Amplifiers and Speakers page.

Back to the Tech FAQ Contents.

Q 14.
How do I connect the balanced output from my mixer to the unbalanced input of my power amp, without getting hum problems?

A 14.
This is a common situation when using semi-professional amplifiers, and may well need extra care to avoid problems with hum. Firstly we have to say that the ideal solution would be to use an amplifier having balanced inputs (and, preferably, a signal-earth lift switch). But if that isn't practicable...

When using good quality screened cable (such as a standard multicore), an unbalanced interconnection between the mixer and amplifier can often be satisfactory − depending on the distance involved and the amount of magnetic and radio-frequency interference present in the vicinity.

However, there are two main ways that hum can be introduced into an unbalanced interconnection, even when it is well-screened:

Now provided that your amp is of the 'double insulated' (also called Class II) type, designed to be safely operated without a safety earth connection (i.e. is fitted with a 2-core mains cable), you should not be troubled by the first of these hum sources. (However, note that other equipment connected at the amplifier end of the interconnection − such as an active crossover or a graphic equaliser − may have a safety earth, or the chassis of the amp or other equipment may be in contact with earthed racking.) And, provided you have laid out your cables and equipment so as to keep mains and audio as separate as possible, the second hum source is often also not a problem. This is because the signal level out of the mixer is relatively high (much greater than the signal level from microphones or guitar pick-ups, for example), so any hum induced into the mixer-to-amplifier interconnection is much less significant than if the same amount of hum were induced into a microphone-to-mixer interconnection, say. (This is why mic-level interconnections are pretty much always balanced.)

Hence the statement above, that unbalanced interconnections can often be satisfactory. To make such an unbalanced interconnection, the cable screen should interconnect pin 1 of the multicore XLR with the sleeve terminal of the jack input to the amplifier, and the inner screened core of the cable should interconnect pin 2 of the XLR with the tip terminal of the jack − leave pin 3 of the XLR (or any core connected to it) unconnected. (This will work in 99% of cases, but some small older mixers of US origin have a semi-balanced output with pin 3 'hot' instead of pin 2 − when using these you will need to connect XLR pin 3 to the tip terminal and leave pin 2 unconnected.)

However, if your amplifier is of the Class I type, which require a safety earth (3-core mains cable), then there will be an earth loop through the audio interconnection from the mixer, and this loop might cause some noticable hum − especially if the amplifier is fed from a different mains outlet from the one used for the mixer.

N.B. Never disconnect safety earths to resolve earth loop problems, as this will create a potentially lethal electric shock hazard.

So to be sure of avoiding hum problems in the Class I amplifier situation, you need to effectively adapt the input of the amplifier to accept a balanced feed, and have the facility to break the earth loop by interrupting the signal earth path (NOT the mains earth path!). When doing this, it is best to have a balanced cable run to the amplifier location, and to make the adaptation to unbalanced at that end.

The best way of doing this is to use the tool made for the job − a line-level balanced-to-unbalanced convertor unit (sometimes called an 'earth isolator' or a 'ground isolator'), with a 1:1 impedance ratio. These are available ready-made, but professional quality units are fairly expensive. (I wouldn't recommend the very cheap ones.) You can also make them yourself quite easily, if you're into that sort of thing, using a suitable high quality audio transformer that is designed to carry the required level. Make sure that the unit has an earth-lift switch, which you should set to the 'Lift' position to break the connection between the signal earth of the audio interconnection and the safety earth of the amplifier. (Note, however, that if you are using an earth isolator unit with a Class II amplifier then the earth-lift switch may need to be set to the 'Earth', or 'Ground', position.)

Alternatively, it can be done using a DI box 'in reverse' (i.e. balanced in, unbalanced out). But it must be a passive DI box − i.e. one which uses no power source (battery, mains, or phantom power). The passive types achieve the balanced/unbalanced conversion by using an internal audio transformer (rather than electronically), and such a transformer operates happily 'in reverse'. The DI box needs to have an 'earth lift' facility (as most of them do), which, as for the earth isolator unit, should be set to the 'Lift' position. Set its pad switch (if it has one) to the '0 dB' position.

However, there are two possible problems with the DI-box method. The first problem is that DI boxes are generally designed with low-level signals in mind, so (to keep the cost down) they often use transformers which are not intended to carry the high level that you get from a mixer output. The result of putting this high level through such a transformer would be some distortion of the signal − possibly just on the signal peaks. (This will not harm the transformer, though.) So be sure to use a good quality DI box, as these are more likely to have good quality transformers. Also, try setting the amplifier input level control at or near maximum − this means that, for a given output from the amp, it needs less drive signal from the mixer than if you had set its input level control to a lower setting, hence less chance of exceeding the level at which the transformer would start to introduce distortion. (Be sure, though, that the power rating of your speakers is appropriate for the amp's maximum audio output power, just in case you accidentally drive the amp with a full-level signal.)

The second problem is that as DI boxes are intended as impedance matching devices, they intentionally do not have a 1:1 impedance ratio. So, when used in reverse, their output impedance will be relatively high and there will be an increase in signal level. However, unbalanced inputs of amplifiers are likely to be fairly high impedance (compared with balanced inputs), and so may not unduly load the output of the DI box. And the increase in level can be compensated for by simply reducing the output level from the mixer. (Don't use the pad switch of the DI box for this purpose, as the pad is not designed to be used 'in reverse' and would probably further increase the output impedance of the DI box.)

In the few instances where the impedance change and/or level change from a DI box is a problem, you could try using two identical passive DI-boxes 'back-to-back', i.e. link their jack connectors with a short jack-to-jack cable. Connect the multicore feed from the mixer to the XLR of one of the boxes, and connect the XLR of the other box to the amplifier input jack as follows: pin 1 and pin 3 of the XLR to the sleeve of the jack, and pin 2 of the XLR to the tip of the jack. Set the earth lift switch to 'Lift' on only one of the boxes − it shouldn't matter which. Ensure that both of the pad switches are set to '0 dB'. This arrangement should eliminate the problems of using a single DI box, because the two boxes will cancel each other's impedance and level changes.

Finally, for more general information on the causes and avoidance of hum, see the FAQ entry How can I solve a hum problem?.

Back to the Tech FAQ Contents.

Q 15.
Can I use a different model of external power unit to replace a lost or faulty one?

A 15.
Although this might seem to be a useful thing to do for convenience and to reduce costs, it is strongly recommended that only the advised genuine manufacturer's power unit is used for each item of equipment. This is because of the many vital factors involved, including:

If you must use a power unit other than that recommended by the manufacturer, be sure to take at least all the above factors into consideration.

Back to the Tech FAQ Contents.

Q 16.
I'm bi-amping my speakers − what crossover frequency should I set?

A 16.
When bi-amping, in order to get the best possible performance from the speakers, and to avoid causing serious damage to them, it is very important to adjust the settings of the crossover unit correctly before you start putting any sound through the bi-amped system. The crossover frequency, and any other adjustable parameters such as the slope, must be set to the values advised by the speaker manufacturer. Refer to the operating manual for your particular model of speakers. (Manuals can often be downloaded from the manufacturer's website − see the Equipment Manufacturers page.)

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Q 17.
Why do UK mains plugs contain a fuse, and what fuse value should I use in my IEC power cable plugs?

A 17.
A fuse (type BS 1362) is needed in UK mains plugs because of the arrangements for fixed wiring that are typically used in the UK between the distribution board (DB) and the socket outlets. In this arrangement, called a ring main, many socket outlets are supplied from each of a relatively small number of circuit breakers (MCBs) or fuses on the DB. To cater for the likely current demand from so many sockets, relatively thick (2.5 mm2) wiring conductors are needed, and the MCB or fuse requires a relatively high rating of 32 or 30 amps. This rating is too high to safely protect the relatively thin power cables of the individual items of equipment plugged into the sockets from the harmful effects of excessive current flow (e.g. in the event of an overload or short circuit), so plug fuses are needed in order to provide that protection. Furthermore, the current rating of each BS 1363 socket outlet is 13 amps, so the maximum plug fuse rating permissible is 13 amps. (In contrast, in other countries each MCB or fuse on the DB typically supplies only one or two sockets. The MCBs or fuses at the DB can therefore have a lower current rating, making the use of plug fuses unnecessary in those countries.)

Now for the second part of the question. If an IEC equipment power cable is dedicated for use with one particular item of equipment and is the cable originally supplied with that equipment by its manufacturer, then the plug fuse value should be as recommended by the equipment manufacturer − refer to the equipment's user manual. However, if an IEC cable will (or might) be used with two or more different items of equipment then refer to the current rating marked on the cable's IEC connector. If the connector is rated at 6 amps then fit a 5 amp plug fuse. If the connector is rated at 10 amps then fit a 13 amp plug fuse.

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Q 18.
What's the difference between earthing and grounding?

A 18.
Nothing. Grounding is simply the American term for earthing. So, both terms can refer to either a safety earth connection or to a signal earth connection − assuming that both of the terms are being used correctly. (Use of the term 'ground' is becoming more and more common in the UK because of the large quantity of PA equipment in use whose panel markings and documentation use that term to suit the US market. Nevertheless, the official UK term for electrical safety-related purposes remains 'earth'.)

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Q 19.
What's the difference between level and volume?

A 19.
The most important difference is that 'volume' can only describe a sound, but 'level' is a more general term that can either describe a signal (usually measured in dBu, but sometimes in volts or millivolts) or describe a sound (usually measured in dB SPL). A high signal level at a particular point in a system doesn't necessarily mean that it will produce a high sound level (or volume), because that high signal level may, at a point further on in the system, be reduced to a small one before it gets converted into sound. Similarly, a low signal level might be later increased to a large one.

There's also a couple of more subtle (but important) differences between sound level and volume. Firstly, sound level is a definite measurable quantity, whereas volume is subjective; what might be considered 'too loud' by one person might be 'just right' for another. (In fact, volume is more technically referred to as 'loudness', and many professional sound engineers dislike the term 'volume'.) Secondly, sound levels can be quoted either as 'unweighted' or 'weighted' values (numbers). In simple terms, an unweighted value refers to the actual sound power, while a weighted value incorporates an adjustment to enable the value to more closely represent the subjective sound level as experienced by an 'average listener' − i.e. something more similar to loudness. This adjustment is necessary because of the ear's different response to different frequencies. Unweighted sound level values are usually expressed in dB SPL and weighted ones in dB (A) SPL (denoting the particular type of weighting known as 'A weighting'). In contrast, because 'volume' is already a subjective matter, there's no such thing as 'weighted' or 'unweighted' volume.

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This page last updated 07-Jun-2019.