How to measure insulation resistance using a multimeter?
In order to measure the insulation resistance of a cable or wire we need:
- disconnect the cable in the switchboard and remove the load, i.e....
- connect the measuring device to two wires, between which we will measure the resistance;
- apply increased voltage and measure the presence of leakage current from one core to another through the insulation;
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Ohm. Resistance is futile!
October 11, 2011 | Maxim Ivanov, (c) KOMBIK.COM
Hello! Long time no see Today we'll talk about impedance matching and why they write such strange numbers on cabinets and amplifiers: 8, 16, 4 Ohms. What does this mean for us and what should we do about it? If you don't want to burn your amplifier unknowingly, read on.
We all know what a speaker, loudspeaker, speaker is. Well, at least in general terms there is an idea. We know that this thing takes an electrical charge and converts it into sound. Street magic lies in how an amplifier and speaker work together to create the sound we hear. Do you know how the signal passes from the amplifier to the speaker and why it is important to connect them correctly? Terms such as Ohm, voltage, resistance are not fully understood by everyone. We, as guitarists, want to just pick up the guitar, crank up the amp and hit the strings and make everyone fly away. like in the movie "Back to the Future". But this is only one side of the coin. There are certain requirements and rules for properly connecting amplifiers to cabinets and speakers. So today we will have to figure out the matching of resistances, which cabinet should be connected and how, what wires to use for this, and yes, there’s no way without physics. We ’ll have to figure out ohms!
Start over. The term Ohm (Ohm, symbol Ω). This unit of measurement was named after Georg Simon Ohm, who studied electrical voltage. Essentially, an ohm is a unit of measurement for resistance. If something interferes with the flow of electrical charge, it can be measured in Ohms. The greater the Ohm, the greater the resistance, the less current flows. The smaller the Ohm, the lower the resistance, the greater the power. This is a key point in understanding how and what works in music equipment. Speakers are specially made in such a way that their impedance matches the impedance of the amplifier to which they will be connected. If the resistance is too high, the amplifier will sound quieter. If the resistance is too low, the amplifier will send too much current to the speakers. Manufacturers indicate the resistance of the speakers on stickers and labels, often indicating it in the model name itself, for example Eminence does this. If you come across an unmarked speaker, a digital multimeter will help.
But this is only if you are a radio amateur or are simply addicted to the smell of rosin.
The important point here is that if you measure the speaker's resistance manually, you will only see 2/3 to 3/4 of the value for which it is designed. This is because the resistance on the speakers is specified based on the supply of a certain current to them. So you can see 8 ohms provided that a current of 30 amperes is constantly supplied to the speaker. Why? Read on Wikipedia! The most common speaker impedance ratings are 4, 8, and 16 ohms. The speaker resistance is a constant value and does not change. At the same time, amplifiers often have the ability to switch resistance depending on which cabinet you are going to connect. The amplifier's voltage must be limited on its way to the speakers. Thus, the speakers are supplied with exactly the current that is required for the speakers to operate in optimal mode for as long as possible. This is resistance matching. Read carefully what is written on the cabinets and amplifiers on the back side! Multimeter
Resistance matching is great and let's assume you understand and remember that these 2 numbers on the cabinet and on the head should be the same. Now it’s worth mentioning something that almost everyone forgets - the cable that connects the cabinet and the head. The amplifier is connected to the cabinet using a speaker cable . Not a guitar cable, not an iron cable, not a piece of God knows what was lying under the table. This cable must withstand high voltage. How to distinguish a speaker cable from a regular guitar cord? It is usually much thicker, weightier and less flexible. Speaker cables do not have a shield, and the wire gauge is larger than guitar wires.
If you use guitar cords to connect heads to cabinets, you are taking a risk. The copper conductor can overheat, melt the cable sheath and short out or shock someone.
Never use guitar cords to connect cabinets to amplifiers. Love your equipment and it will love you back. A dead amplifier is not fun at all. The amplifier should always be connected to the cabinet and always only the speaker cable. We also do not advise you to turn on the amplifier without a cabinet connected to it.
A speaker with a lower impedance than recommended for your amplifier will overheat the tubes and output transformer because too much current is flowing through them. The amplifier will be very loud, but not for long. You will know that it is ready by the pleasant smell of burnt transformer windings. Introduced? This is how your money, earned with blood and sweat, burns.
Too much resistance on the speaker, not matched with the amplifier, is also fraught with consequences. Firstly, the volume will not be enough, and the sound will be unpleasant, which in itself already sucks, because you paid a lot of money for your amplifier! Secondly, you risk burning the lamps and transformer, since the voltage does not pass through them, remaining inside. The speaker won't let him in! As a result - the same smell of an amplifier with a crispy crust. Everything we love!
The math is simple. If we have one speaker in the cabinet, there is nothing easier to match it with the amplifier. 4 to 4 ohms, 8 to 8, 16 to 16. Easier than steamed turnips. But difficulties arise when we have a cabinet with 2 or even 4 speakers, or even 2 4*12 cabinets. How to connect correctly in this case?
There are three ways to connect speakers: serial, parallel and mixed (series-parallel).
Let's assume that the tip of the jack connector on the speaker cable is respectively connected to the + or red ear on the speaker, then plus to plus, red to red from speaker to speaker. After this, black is connected to black or minus to minus. This is a serial connection . In this case, the speaker resistance adds up. If we have two of them, we get 8+8 = 16.
The second method is parallel . This can best be understood by looking at the diagram in the figure below. This is reflected in numbers as follows: take the resistance of one speaker (8 Ohms) and divide it by the number of speakers connected in parallel. In our case there are 2 of them, respectively 8/2 = 4 Ohms.
The mixed type of connection involves maintaining the original resistance value. That is, if there were 16 ohms on all speakers, it will remain so.
You can read more here and here.
Well, that's all for today! Now you know the basics of switching and can easily figure out what to do if an unfamiliar device catches your eye. Take care of your equipment because you bought it to work. Good luck!
conclusions
What conclusions can be drawn?
In most cases there is no point in chasing zero resistance. For headphones with strong resonance in the low frequencies, either a voltage amplifier or a current amplifier may be suitable, and this will be a compromise between control of low frequencies and transparency of the sound in the rest of the range. For a number of headphones where the manufacturer has tried to reduce the dependence of resistance on temperature, there may be no difference at all with what output impedance the amplifier is used.
High-impedance headphones (such as Sennheiser HD 650, HD 800, Beyerdynamic DT 880 Pro) have the advantage that their resistance fluctuations have little effect on the signal amplitude, and perhaps that is why high-impedance headphones are associated with “high-quality sound”.
And in the end, the “amplifier + headphones” combination is selected based on subjective sound, where technical characteristics provide primary information and what features should be paid attention to first. For example, when evaluating a current amplifier, you need to pay attention to the quality of the low frequencies, while when using a voltage amplifier, you need to pay attention to whether there is excessive harshness or a feeling of “mudiness” in the sound. When using armature or hybrid headphones, is the final frequency balance appropriate?
Author Kuznetsov Roman romanrex
How are the power ratings of JBL Professional speaker systems calculated?
As noted, speaker damage can occur due to overheating of the voice coil or excessive low-frequency vibration. Ideally, we need a standard test signal that takes into account both of these types of damage and is practical at the same time. It is described in standard No. 268-5 of the International Electrotechnical Commission (IEC). This is a 6 dB load factor pink noise signal, filtered at 12 dB per octave below 40 Hz and above 5 kHz.
To determine the power rating, test loudspeaker samples are fed a test signal with a gradual increase in power to determine the level they can withstand for eight hours. The details of the testing process are shown in Fig. 3.
We mentioned above that the load crest factor is 6 dB. Let's explain this in detail. Crest factor refers to the crest factor of a signal and is a precise measure of the peak value of noise associated with the average "heating capacity" of the signal for the moving coil. A value of 6 dB means that the signal is sent to a particular speaker or transducer with four times the average power. For example, when testing a speaker rated at 150 watts, this method will drive the speaker at 600 watts of instantaneous power for eight hours. This method is so suitable for real-world speaker systems that JBL is now applying it to all speakers, abandoning all previous evaluations.
What is a speaker
A speaker (often called a loudspeaker) is an electrical device that converts an electrical signal into sound. This transformation occurs due to the oscillatory motion of an electric coil in a constant magnetic field (this field is provided by a permanent magnet or, in more rare cases, an electromagnet). This coil moves the diffuser, which in turn creates air vibrations, and this allows you to hear the reproduced sounds.
Despite the simplicity of the design, malfunctions that arise during the operation of the speakers can still occur periodically. If the speaker begins to reproduce sound with interference, hissing or crackling, or stops working altogether, the first step is to localize the problem area.
First, you need to conduct a visual inspection of the device, namely, check for damage:
- housings;
- power cord;
- signal cord;
- corrugations;
- diffuser
If there is damage, this part should be repaired or replaced (if the corrugation or diffuser is damaged, the speaker will have to be replaced). But if no external defects are found, and the sound is reproduced with interference or is absent altogether, then the problem could arise in the contacts or coil of the speaker, and in this case, checking the speaker with a multimeter will help.
Amplifiers with characteristic output impedance graphs
Amplifiers with flat output impedance
The graph shows typical impedance lines of 20, 50, 100, and 300 ohms.
When the resistance is less than 3 ohms, the resistance is called “zero”. Amplifiers with “zero” resistance include Violectric amplifiers.
Current amplifiers include Erzetich amplifiers, where the output impedance is above 60 Ohms.
Close to zero with an increase in the low frequencies
This impedance curve can be seen in single-supply amplifiers, where the DC offset voltage is eliminated by a capacitor at the output. When low-impedance headphones are connected to such an amplifier, the frequency response usually shows a decrease in low frequencies in level. Such amplifiers belong to the category of amplifiers with “zero” resistance. Most often, such amplifiers are found in players and other mobile devices.
Differences in car and home subwoofers
The most obvious difference between car subwoofers and those used for home theaters is size. Not only is the speaker cabinet sized to accommodate a more powerful subwoofer, it is also necessary if the subwoofer will be active.
An active subwoofer is a subwoofer that has an internal amplifier built into it. It doesn't have to rely on a stereo amplifier for power, so it has a lot of flexibility and can be much more powerful.
Active subwoofers, because they have their own amplifier, also need their own power supply. The easiest way to tell if your home theater subwoofer is active is to determine whether it needs to be plugged in. Passive subwoofers only need to be connected to an amplifier. Powered subwoofers require an outlet.
As you can imagine, the subwoofer in your car will be passive - unless you really put in the effort to do something active. When you hear bass coming from a passing car 3 blocks away, you can be sure they installed a powered subwoofer.
The difference in size also results in a difference in the efficiency of the two subwoofers. The subwoofer in your home is designed to move large amounts of air and produce sound over a large area. The subwoofer in your car is designed to fill the car's interior with sound, taking up much less space.
How are car amplifiers different from home amplifiers?
In fact, there is nothing special about the amplification part itself. Except that the power supply here does not come from household 220 volts, but from the on-board 12-volt network. In my opinion, there are two fundamental differences.
Switching power supply takes up most of the internal space of the amplifier
Firstly, most car amplifiers are universal in terms of connection: they provide both a regular load connection and a bridge for a pair of channels. This function allows you to use conventional wideband amplifiers not only to connect speakers, but also to connect subwoofers. The latter are even more “gluttonous” in terms of power. The simplest option, which can rightfully be called classic, is a 4-channel amplifier, two of which work for the front speakers, and the other two are bridged to the subwoofer.
A circuit with a 4-channel amplifier for front speakers and a subwoofer is rightfully considered a classic car audio system
The second key difference between car amplifiers and home amplifiers is their equipment. Most household ones are pure “advice”. At the very least, you should use filters in your car.
For example, the same simple four-channel circuit to which the front speakers and subwoofer are connected. In most cases, the acoustic design of the front low-frequency / mid-frequency speakers is implemented in ports. Let's face it, it's not the best, which means it's advisable to attenuate the lower bass and need a high-pass filter in these channels. On the other hand, the subwoofer should not have a singing voice, which means that a low-pass filter is needed on these channels. The optimal crossover frequency between speakers and subwoofer is usually between 50 Hz and 100 Hz, which is slightly lower than what is typically used in home systems. Passive separation won't work here, winding coils the size of a spare tire - no thanks. But active separation is a real thing. Thus, historically it has been established that minimal amplification equipment consists of active low-pass and high-pass filters. Mostly adjustable, but there are also fixed frequency settings.
Adjustable low-pass and high-pass filters are used in the vast majority of car amplifiers
And then more. For example, if channel-by-channel switching of acoustics with active separation is expected, the amplifier should have filters with a wider adjustment range. Some old-school models even have filter tilt switches. Let's say JL Audio traded this way. And gradually increasing until the amplifier is equipped with an integrated processor and turns it into a real “sound processor”.
