Active speaker system circuit on tda2822. Miniature amplifier based on TDA2822L. Let's look at the microcircuit housing
Resistance electrical circuit- this is its ability to resist passing current. Units of measurement this parameter are called ohms. We remembered this definition back in school, studying a physics textbook. That is, the greater the resistance value, the less electric current can pass through this device, and vice versa. This also applies to, only their resistance is called loudly and beautifully - impedance. It is this that largely determines such qualities as the level of interference and sound volume.
To hear rich and clear sound in your headphones, select their resistance value as close as possible to the parameters of the amplifier. Otherwise, even the most expensive headphone model will not be able to show itself in all its glory, and you will be very disappointed. In addition, a mismatch between the input and output impedance can even lead to equipment damage. For example, if you take headphones with a low impedance and connect them to a high-impedance output device, you will end up with a lot of distortion, and often just an unpleasant grinding sound.
If you do everything exactly the opposite, you will have to constantly turn up the volume of the sound, but it will still be too quiet. The very high impedance of headphones, of course, fights interference well, but at the same time it also noticeably reduces the signal level. Therefore, such equipment is only suitable for professional recording studios, where the cost is quite high. powerful amplifiers. In this case, the high impedance also protects the delicate thin membranes of the headphones - after all, they can simply tear from a very loud sound. When you turn off the music, you will hear only silence without any interference - this is a big plus of high resistance.
But in order to listen to a player or radio, high-impedance headphones are not required at all - as mentioned above, they will make the sound too quiet. The most acceptable impedance value for everyday use is 32 ohms. There are also 16 ohm models. They make it possible to listen to music louder, so these “ears” can be recommended to lovers of incendiary compositions. If you need headphones for listening to audiobooks, for example, then a resistance of 32 ohms or a little more will be quite suitable.
So, when choosing headphones, carefully read their characteristics. An impedance of 250 ohms and higher is needed only for powerful studio equipment. If you are looking for headphones for a computer or player, choose models with an impedance of 32 to 80 ohms. Those who like very loud sound should pay attention to the impedance value of 16 ohms.
Prepared by the online store "130 dB"
For amplifiers, you can often see such a characteristic as “recommended headphone impedance” in the form of a range of impedances.
Manufacturers usually determine this characteristic subjectively by testing their amplifier with different models headphones. Others focus on the characteristics of competitors and adapt to them if other characteristics are similar. With this approach, products inevitably appear, for some of which this characteristic becomes overestimated, and for others underestimated. There is no single formula for this characteristic.
But, as often happens, if many manufacturers indicate a similar characteristic (similar to “ frequency range"for headphones), then there is a false impression that there is some kind of standard for this.
This characteristic can be derived and calculated if one approaches such a problem from the original meaning of the characteristic and generalizes data from manufacturers. If these data are taken subjectively from manufacturers, then they can be considered empirical and made a certain starting point. Understanding the calculation algorithm, you can always make the necessary correction.
Characteristic Definition
The main essence of the characteristics is to determine with which headphones the amplifier can produce an acceptable volume level and at the same time work efficiently (have good sound).We know that for high-impedance headphones the main limitation is voltage, and for low-impedance headphones it is current. Since the main difference is mainly indicated for the boundary of high-impedance headphones, we need to determine what sound pressure level, from the point of view of most manufacturers, is acceptable specifically for high-impedance models.
The selection based on the RAA database included products whose manufacturers recommend for headphones with an impedance of up to 600 Ohms.
Model/maximum voltage level:
ASUS Essence One MK2 16.2 dBV
ASUS Essence 3 Balanced 16.5 dBV
ASUS Essence 3 Unbalance 15.7 dBV
ASUS Xonar Essence STX Hi 16.3 dBV
ASUS STRIX RAID DLX Hi 15.4 dBV
ASUS STRIX RAID PRO Hi 15.4 dBV
ASUS ROG Xonar Phoebus 15.4 dBV
Cary Audio HH-1 18.9 dBV
Creative Sound Blaster X7 LE 15.7 dBV
Creative Sound BlasterX AE-5 14.5 dBV
Creative Sound Blaster E5 14.8 dBV
Erzetich Perfidus 17.0 dBV
Erzetich Bacillus Tilia 18.1 dBV
Musatoff HA-4 14.8 dBV
MyST DAC 1866OCU V.2 17.6 dBV
OPPO HA-1 16.7 dBV
OPPO HA-1 Balance 24.5 dBV
From the list you can see that the minimum voltage value for 600 Ohms is 14.5 dBV (the voltage is already expressed in decibels).
For headphones, the sensitivity parameter is expressed in terms of both voltage and power. The first option allows us to draw a conclusion about the “loudness” of the headphones without taking into account their resistance, which is very convenient when we compare headphones with different impedances. The second option shows their efficiency. Moreover, if you make the same headphones, but with different inductor resistance, then their efficiency will be the same (same sensitivity to power), but sensitivity to voltage will differ depending on their ratios of their resistances as 10*Log10(R1/R2).
When calculating voltage ratios in decibels, the formula 20*Log10(V1/V2) is used, with a difference in power sensitivity of 10*Log10(R1/R2).
The 16 ohm/600 ohm impedance ratio is -15.7 dB and 600 ohm headphones will be 15.7 dB quieter than 16 ohm headphones, all other things being the same design.
Thus, we need to determine the average level of power sensitivity among high-impedance headphones, eliminating headphones with a non-standard design.
Because sampling is needed primarily based on the design of headphones for high-impedance models, and not strictly only high-impedance models, this allows you to sample across a wide range of models in a larger range of resistances and obtain a more adequate average value for sensitivity.
AKG K 340 ED 91.7 dB/mW SPL 398.6 Ohm
Beyerdynamic DT 770 Pro 250 Ohm 94.5 dB/mW SPL 254.4 Ohm
Beyerdynamic DT 880 Pro 95.6 dB/mW SPL 252.9 Ohm
Beyerdynamic DT 990 Pro 250 Ohm 93.4 dB/mW SPL 267.4 Ohm
Beyerdynamic DT 150 250 ohm 99.3 dB/mW SPL 280.1 ohm
Beyerdynamic DT 250 250 ohm 101 dB/mW SPL 268.7 Ohm
Beyerdynamic DT 48 E 200 ohm 99.2 dB/mW SPL 200.1 ohm
Beyerdynamic DT 660 mk 2 85.6 dB/mW SPL 568.6 Ohm
Beyerdynamic DT 770 600 ohm 96.6 dB/mW SPL 570.8 ohm
Beyerdynamic DT 797 99.5 dB/mW SPL 259.4 Ohm
Beyerdynamic DT 831 106.2 dB/mW SPL 348.2 Ohm
Beyerdynamic DT 911 106.5 dB/mW SPL 298.6 Ohm
Beyerdynamic DT 931 107.3 dB/mW SPL 328.2 Ohm
Beyerdynamic DT 990 600 ohm 95.4 dB/mW SPL 612 ohm
Beyerdynamic T1 99.3 dB/mW SPL 819.3 Ohm
Beyerdynamic T70 99.6 dB/mW SPL 394.2 Ohm
Beyerdynamic T90 97.6 dB/mW SPL 406.2 Ohm
Beyerdynamic DT 250 250 Ohm Mod 103.1 dB/mW SPL 238.9 Ohm
German Maestro GMP 400 98.2 dB/mW SPL 292.8 Ohm
German Maestro GMP 435 S 111.9 dB/mW SPL 347.4 Ohm
German Maestro GMP 450 PRO 105.8 dB/mW SPL 301 Ohm
German Maestro GMP 8,300 D 99.9 dB/mW SPL 303.2 Ohm
MB Quart QP 400 98.8 dB/mW SPL 292.5 Ohm
Sennheiser HD 280-13 103.9 dB/mW SPL 405.9 Ohm
Sennheiser HD 700 94.4 dB/mW SPL 235.1 Ohm
Sennheiser PC 350 Xense 101 dB/mW SPL 253.9 Ohm
Sennheiser HD 600 100.6 dB/mW SPL 352.2 Ohm
Sennheiser HD 650 100.5 dB/mW SPL 344.9 Ohm
Sennheiser HD 800 100.1 dB/mW SPL 429.4 Ohm
Sennheiser HD 800s 101.6 dB/mW SPL 442.8 Ohm
The average value turned out to be 99.6 dB/mW. For 600 Ohm this is 101.82 dB/V (Conversion can be done using an on-line calculator)
Let's round all values to whole numbers for the upper limit of 600 Ohms:
- Minimum voltage level: 14 dBV
- Headphone sensitivity to 600 Ohm: 102 dB/V
Thus, to determine the upper limit, we only need to determine, when connecting what load, the amplifier will not be able to provide 116 dB SPL with a headphone sensitivity of 100 dB/mW SPL.
At first glance, the statement also looks correct - in what range of connected resistances will the amplifier provide a power of 16 dBm for headphones with a sensitivity of 100 dB/mW. (16 dBm is 40 mW on-line calculator).
But the second option is incorrect for calculating the lower limit, because The default power is calculated for the sine wave, not for the music signal. For voltage limitation this is not significant, but for current it is necessary to introduce an additional correction of +12 dB and operate with “musical” power. These subtleties will be discussed in a separate article.
116 dB SPL - is it a lot or a little? In fact, if you turn on music and take measurements with a sound level meter, then in reality, instead of the calculated 116 dB SPL, the average value will be about 20 dB lower. This is because the music track is not continuous noise on maximum volume, but consists of quiet and loud sections with different energy densities. Thus, these 20 dB are a kind of “crest factor” and when calculating for the sound from music, an additional correction is made. If you conduct larger studies than those carried out in the RAA laboratory, then this value will be adjusted up or down, but within small limits.
Those. the calculated value is 116 dB SPL, but in reality this will be music with an average sound pressure level of 96 dB SPL.
Examples of determining the range of headphone impedances
Visually on the maximum voltage graph using the example of a new sound Creative cards Sound BlasterX AE-5 it will look like this:The dotted line shows the required voltage level for headphones with a sensitivity of 100 dB/mW to produce a sound pressure level of 116 dB SPL.
The X axis shows the range of loads, here for clarity of the example the scale is from 0.001 Ohm to 2000 Ohm. In general reports, the scale is shown in a narrower range from 10 to 1000 ohms.
The Y axis is the voltage at the amplifier output in dBV.
Because The amplifier has an internal resistance of 1.1 Ohm, then in the region of low-impedance loads the graph goes down gently.
The blue area indicates the operation of the amplifier in equivalent class A ( best mode, in which the amplifier can operate sound card AE-5). Green mode shows the operating area in equivalent class AB mode. The transition limit to mode B and maximum before clipping is less than 1 dB and is not visible. The upper part of the gray area shows the possible voltage level without current limitation. About equivalent classes on the chart.
Lower limit
On the left side of the low-resistance loads we observe three intersection points.Gray area
Crossover at 0.0007 ohms / -63 dB(ohms). Whatever signal is being played, if a lower impedance is connected, it is physically impossible to apply a voltage higher than necessary to obtain 116 dB with a headphone sensitivity of 100 dB/mW SPL. The gray zone is built without current limitation and in most cases this means that in this area the musical signal will be accompanied by strong distortion from clipping. Those. This is the level where it is theoretically possible to reproduce some pulse signals or fabricate high numbers for power in the PMPO.Thus, it is impossible to navigate in the gray zone.
Red/yellow/green zone
For the AE-5, these zones coincide.Crossover: 0.17 ohms / -15dB(ohms). This is the load value, below which wheezing will be clearly audible due to lack of current. In fact, it is this boundary that should indicate the minimum allowed when connecting headphones.
But there is one caveat. Since the intersection occurs in the current limiting zone (oblique slope of the graph), this already implies that the volume knob should not be set to maximum, but in this case to a level of -19 dB (the difference between the maximums of the gray and green zones).
Those. In order for us to hear a musical signal without wheezing and obvious distortion, the voltage at the output of the amplifier without load should be set to 14.5 dBV -19 dB = -4.5 dBV.
If the volume knob is set to maximum, then at a load of 0.17 Ohm we will have a voltage of -3 dBV, and up to 0.7 W of power, and a voltage of 13.25 dBV will be established at the internal resistance of the amplifier and will dissipate up to 4.8 W (When calculating power, the current is calculated with a -12 dB correction due to the density of the music signal). Yes, it is possible that the actual numbers will be lower due to the current limitation on the power supply, but it only gives the chance that the power supply section may fail.
The question here is whether the resistor at the output of the amplifier is powerful enough and whether it will fatal problem Random setting of the maximum volume level? What if the output is not music, but a technical signal in the form of a sine or meander with high density and, accordingly, greater power?
Because of this, the lower limit would be desirable, but it cannot be determined only by the volume at the headphone output.
Blue Zone
Crossover: 0.35 ohms / - 9 dB(ohms). This is the load value at which high-quality sound will be maintained for the amplifier to operate in an equivalent class A. Here the volume level should be set to -24 dB from the maximum.Total at the lower limit
When determining the lower limit, two factors must be taken into account:- Required volume level without distortion
- The capabilities of an amplifier that will not burn out if you accidentally set the volume higher than necessary.
A reasonable option today would be to check for a conditionally low typical impedance for headphones. Today it will be 8 Ohms for rare headphones and 16 Ohms for mass ones.
For the AE-5 there will be an output, then the amplifier supports a load of 8 ohms.
Taking into account that in RAA the minimum load for testing is 16 ohms, then when corrected by 12 dB for a music signal, the assessment can be made at the lower limit of up to 4 ohms.
Upper limit
The upper limit is crossed for all zones at 707 Ohms / 57 dB(Ohms). There are no pitfalls here and everything is quite simple, the AE-5 amplifier supports 600 Ohms. Above 700 Ohms the volume will not be enough for some.In-ear headphones
Returning to the beginning, 100 dB/mW and 116 dB were taken for over-ear headphones.In-ear headphones, due to their design with a lighter membrane, have a higher sensitivity and, accordingly, the range for them should be considered not for a sensitivity of 100 dB/mW, but for approximately 109 dB/mW.
Otherwise, we will get the result that the vast majority of portable players, smartphones and tablets are too quiet by default and in-ear headphones with an impedance of 16 Ohms are insufficient. But in practice, in-ear headphones are usually quite sufficient in terms of volume.
Let's take a smartphone Xiaomi Redmi Note 4X, when playing sound from the PowerAmp software player with output in Hi-Rez (the only player currently capable of outputting data bypassing Android 6/7 and thereby providing best quality sound in these OS).
If we determine by the limit for full-size headphones, we find that the lower limit of 8 ohms fits. But it remains the upper limit.
In this case, it will be absurd in the sense that the lower limit of modern full-size headphones starts from the rare 16 Ohms and the more common 24 and 32 Ohms. 8 Ohm is usually found only among vintage headphones, such as TDS-7 and many older ones with paper full-range speakers.
If we draw a line for a sensitivity of 109 dB/mW, we see a more adequate picture. The lower limit confidently fits into 8 ohms, and the upper limit is approximately 60 ohms.
Subjectively, headphones with an impedance of 16 Ohms (Sennheiser CX300-II, IE-8) and 32 Ohms (Sennheiser CX 980, HiFiMan Re400) with Xiaomi Redmi Note 4X really play with adequate volume and there is a small margin for adjusting the volume up.
Below are examples of smartphones in which the volume was not enough with the Sennheiser IE-8, but was only at the “not loud” level.
Thus, two ranges should be determined for the headphones used: full-size and in-ear. For some sources, this range will become “too quiet for all standard headphones.” This characteristic included in the “simple” amplifier reports in the RAA database.
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