Hi-Fi output cattle amateur measurement knowledge
Let's talk about how to measure the output cattle in amateur conditions. The tools used are: digital voltmeter, ammeter, common inductance meter, and 0 ~ 100KHz signal generator (signal generator). In order to observe the waveform distortion, it is best to have an oscilloscope and distortion meter Etc. (For example, when measuring the frequency response width, the output value sometimes measured in the high frequency band is not small, but the waveform has been distorted). However, we can know whether it is good in the important 20Hz ~ 20KHz range through other measurement methods.
The signal generator recommended by the author is the typical XD-1, the second-hand price is only about 300 yuan; the oscilloscope is also the most commonly used dual-track oscilloscope SR-8, and the second-hand price is only a thousand yuan. The price of these two must-use instruments is low, amateur enthusiasts are easy to own, and can fully meet the test requirements, very practical (of course, if you have other high-end instruments, it is better). The output cows are generally measured with a dedicated comprehensive tester at regular manufacturers. Some data is of little significance to audiophiles. With the simple test method here, you can measure several practical parameters of the output cow, and the relative Almost, very useful.
1: Inductor
Single-ended (single-ended) output cattle have serious DC magnetization, so a DC current close to the actual working circuit must be added during measurement. As shown in Figure 1, the power transformer (preferably with high power) provides AC and DC voltages, and passes the AC and DC components at the same time (simulates the actual working state of the single-ended cattle) at the primary of the measured output cattle. R series non-inductive resistance (noniducTIve), (AC ammeter is difficult to find, the current is replaced by the resistance buck method) to display the size of the loop DC current. Measure the AC voltage value UR on R and the output primary AC component UL respectively, then calculate the primary inductance as follows
L = U2 · R / 2πf · U1
Note that none of the above three meters can use ordinary pointer multimeters to avoid the mutual interference of AC and DC components, making the test inaccurate (the same below).
Push-pull output cattle, because there is no DC magnetization (or only a little magnetization). You can use the test circuit of the figure to add an adjustable DC current of 0 ~ 15mA to an arm of the output cow (if the current of the two tubes of your circuit is completely equal, you can not add DC). Where: R≤0.05 (2πfL), and then get it as follows:
L = R√ (U / UR)-(1 + r / R) / 2πf, where U is the AC input voltage, UR is the voltage on R, and r is the primary (pri) internal resistance of the output cattle.
2. Leakage inductance (leakage inductance), the general output cattle leakage inductance is only a few millihenries ~ tens of millihenries, its value can be measured with a common inductance meter or multimeter (general test frequency is 1KHz, 100Hz), which is ominously described.
3. Self-resonance (resonance) frequency: Whether it is series or parallel resonance, it is best not to fall within the audio range, so as not to damage the sense of hearing. Generally, it appears in dozens of KHz, and some may be within 10KHz. You can measure according to Figure 3 in your spare time. Among them, R series non-inductive resistance (noniducTIve), the smaller the value, the better, but it must be able to ensure that the voltmeter has a suitable voltage display. The signal source outputs a fixed voltage, and then adjust the output frequency of the signal source. When the minimum reading of the voltmeter appears, check the frequency of the signal source. This frequency is the parallel self-resonant frequency; the frequency when the maximum reading of the voltmeter appears is Series self-resonant frequency.
4. Frequency response (freguency response): The test circuit of push-pull output cattle is shown in the figure, where the R value is equal to half of the primary impedance and RL is equal to the output impedance. First adjust the output voltage of the signal source to the maximum (it is best to increase the voltage to the voltage required for the output cattle to work) and record it. Then adjust the output frequency, and record the voltage UR low, UR medium, and UR high at low frequency, medium frequency (1KHz) and high frequency respectively, and then calculate according to the following formula:
Low frequency response value (dB) = 20lg UR low / UR medium;
High frequency response value (dB) = 20lg UR high / UR medium.
R1 matching resistance. The resistance value is the difference between the transformer primary impedance and the output impedance of the signal source (use a non-inductive resistor)
RL is the impedance value of the secondary measured terminal of the transformer (with non-inductive resistance)
C1 is a DC blocking capacitor, use a non-polar capacitor of more than 30uf
C2 is the transformer output matching capacitor, generally 0.5uf
test:
The test voltage refers to the excitation voltage DC of the primary-side signal voltage 4V (RMS), which is 1/10 of the maximum primary current value of the transformer
Single-ended (single-ended) output cattle, because of the DC magnetization in the single-ended cattle, the DC current component needs to be added to the test circuit. However, the high frequency band is only related to leakage inductance and distributed capacitance. To facilitate testing, the DC component may not be added when measuring the frequency response of the high frequency band. In Figure 5, R is equal to the input impedance of the measured output cattle. Adjust W so that the ammeter A indicates the actual working current value of the output cattle, and C1 is the DC blocking capacitor. The test method and calculation formula are the same as the push-pull cattle.
5. Input impedance: The input impedance is affected by the primary inductance, leakage inductance and distributed capacitance, etc., showing irregular impedance changes in high and low frequency bands, sometimes with an error of 30- 50%. The push-pull test of the bull is shown in Figure 6. The output voltage of the bull primary is measured in different frequency bands, and then calculated as follows:
Input impedance (input impedance) Z = Ra UL / Ua Ua: voltage on Ra; single-ended (single-ended) cattle test also need to add DC current, according to Figure 7 test. Adjust W to indicate A to normal working current. The remaining test methods and calculations are the same as push-pull cattle.
6. Measurement of distributed capacitance: Distributed capacitance is formed by factors such as the structure of the transformer itself, winding process and materials. The measurement circuit of the primary distributed capacitance is shown in Figure 8. Adjust the output voltage of the signal source to the maximum (preferably to the required voltage for operation), and then change the output frequency. When the output voltage indicates maximum, it indicates that the circuit is resonant, and the frequency at this time is f (KHz). Then calculate as follows:
Pri distributed capacitance
C (pF) = 25.3 × 10 / 2πfLs where Ls is the primary leakage inductance (mH).
The measurement of the secondary distributed capacitance (sec distributed capacitance) is shown in Figure 9. Adjust the frequency of the signal source. When the voltmeter indicates the maximum, it means that series resonance occurs, and the frequency at this time is f (KHz).
The secondary distributed capacitance C (pF) = 25.3 × 10 / 2πfLs Ls is the secondary leakage inductance (mH).
The measurement of pri-sec distributed capacitance between the primary and secondary can be tested according to Figure 10, the frequency of the signal source is adjusted to 15KHz, and the output voltage is 17.9v. Then directly read the reading of the voltmeter and calculate it according to 10mV / pF.
For the measurement of the withstand voltage and other parameters of the output cattle, the reader can refer to other newspapers and periodicals, which will not be repeated here.
The above test is only measured under the condition that the output cow does not work or simulates the ideal working state, and uses pure resistance as the load. The speaker in the actual circuit is a reactance component, and the influence of the frequency divider often makes the test results unrecognizable. But this general test method is still convincing. After the above items are tested, the output cow can be tested in the working state of the machine and half output power or full output power. At this time, several parameters such as frequency response width and waveform distortion can be tested. The data measured in the low-power state is not inconsistent with the previous test results, and the larger the output power, the worse the results, sometimes more different, but the general manufacturers do not rely on the test data at full power.
Through the above self-test of the output cattle, amateur enthusiasts can have a "base" and have a physical measure of the quality of the amplifier they are listening to. The sound quality and timbre are of course related to other factors, but it is also a matter of benevolence and wisdom. Some people like warm colors, and some prefer delicate colors. But even if the output of the above test results is not good, no matter how complicated the circuit is, there will definitely be no good sound performance (unless your ears have problems). After the above tests, you can reasonably match peripheral equipment (such as preamps, speakers) and cables according to the shortcomings or advantages of the indicators, so that your amplifier can achieve the best performance, or when you are evaluating a tube amplifier ". In addition, some small and medium-sized bile machine manufacturers can understand and improve the output cattle they use, and make the bile machine market enter a healthy development track.
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