Fans of flea markets and music? You got an old tube or transistor amp for three under last weekend and would like to use it to play your good old electric guitar, but it does not work anymore? Here are some tips for refurbishing it. In this lesson, we are going to start using a different evaluation method than usual. We are going to deliver WB files with our amplifier circuit but with damaged components. You have to run it on your computer, verify the failure and repair it using all the WB virtual instruments and the knowledge acquired in this course.
The only information we are going to give in addition to the file is what the user could say when bringing his amplifier to repair. This is what is said to be an actual repair practice, and as far as we know, it is the first time it is used in the whole world. I hope you copy the idea so that everything related to the virtual laboratories and especially the use of the WB is disseminated.
Measuring the output power of an amplifier
To measure the output power of an amplifier you need three things that usually have no repairer. However, the three can be replaced with a little ingenuity and a desire to work well. The ingenuity runs on my own. The desire to work has to put you. We are going to suppose that you want to work repairing electronic equipment or dedicate yourself to electronics as a hobbyist but of the good ones; spending little but performing all the steps leading to assemble and test good power amplifiers. You will need:
- a stereophonic charge
- a signal generator
- an oscilloscope or something that replaces it
Stereo charging 8 Ohms 200 W
And the first thing is to realize a load resistance of 8 ohms 200W. The fastest way to do it is using 25W wire resistors. As you will probably not get resistors of 1 Ohm 25W to put them in series, the only thing left to do is use eight resistors in parallel of 68 Ohms 25W that give a value of 8.5 Ohms that is within an acceptable tolerance although it is not ideal. As surely these resistors are not going to be easy to find we recommend another very simple method that is to use enameled copper wire with a diameter of approximately 0.30 mm. In general, the source of this wire is a motor winding shop. Ask about your neighborhood to see if you can get a roll with traces of wire.
- Take 10 meters and measure them with the digital tester well adjusted and without using the tips of the tester.
- Simply tin the tip of the 10-meter wire and place them on the terminals of the tester and tighten them with a wedge of rounded wood.
- Then you must make a rule of three simple of the type: if 10 meters of wire has a resistance of x Ohms a resistance of 8 Ohms should have z meters of wire.
- The calculation formula would be z = 10. (8 / x)
- Then take a wooden board about 20 cm wide and wind the wire to separate turns (approximately 1mm) so that it dissipates better.
- Then place two sheets of black (or natural) anodized aluminum, somewhat larger than the wood by tightening the winding and using silicone grease between the winding and the aluminum (no screws were drawn for clarity).
Note: As a reference value we indicate that 10 meters of wire of 0.20 mm diameter have a resistance of 6.6 Ohms. At the moment all the equipment is stereophonic. This means that you have to face the load resistance in duplicate since an exact measurement involves measuring the two channels at the same time to take into account the voltage drop in the power supply that is not regulated. In general, the current equipment is 8 or 4 Ohms; This means that if you want to have a well-equipped laboratory, you need four equal resistors to put them in parallel of two and form 4 Ohms. You will be thinking that what I’m asking is very complex, but you do not have a simpler or more economical solution. You will also be thinking that you never saw such an artifact in an audio “Lab” and it is true. Generally, the watts of the amplifiers of artisan manufacture, are more imaginary than real and the manufacturers do not know well what power has what they manufacture; in that way he can lie more happily without having charges of conscience. Do not even think about using the speakers of the equipment to make the measurement, first because if you can support 400W real inside your laboratory you must be deaf, or you will be in a short time and secondly because it is likely that the speakers do not support a 200W continuous power and return the burned speakers to the user will not be very easy. This is not a free comment; the author knows that in Argentina stores are sold speakers of unknown origin with fantasy names, which have a magnet so deficient that its audio output power is very low. And if the power goes in but does not come out, the voice coil gets hot and burns in a few minutes. With music, they hold a bit more, but the real test with an audio tone cannot stand it.
Audio signal generator
The second instrument that you need is an audio generator. In the previous installment, we indicated a solution that involved the use of the computer and a free program downloaded from the Internet but also has another solution that is a CD recorded with audio tones and played in a music center or on a DVD. It is not the same to use one team or another. The musical center is the most appropriate because it already has a low output impedance because it has its audio amplifiers. The problem is that the output voltage for the speaker is very high and can burn to the input circuit of the amplifier under measurement.
Note that we represent the musical center with a function generator to which we assign an output voltage of 30V peak that is a very common value as maximum voltage. By adding a splitter for 34 times, we take that output to approximately 1V which is twice what an amplifier needs as an input signal for its auxiliary input. The addition of diode D1 and capacitor C1 allows us to measure the output with an analog or digital tester. Remember that the input signal of the amplifiers is usually given in effective voltage so you should divide by 1.41 first and then by 34 to obtain the effective value of the input.
In a word you must take what the tester indicates and divide it by 50 to know what the output voltage of our home generator is. It is obvious that in order to obtain minor signals, the volume control of the music center must be operated.
- The problem when using a DVD is different since it has no internal amplifier and only generates 660 mV with a load of 75 Ohms applied to its left or right output. This voltage is usually sufficient because it goes to 1.32 V if charged at high impedance (usually the inputs of the amplifiers are 1KOhms).
- Another problem is that the DVD does not have output level control. Therefore, a potentiometer of 1 KOhms must be placed between the output and ground and take the signal from the cursor.
- Another problem is to determine the output level easily. The only resource that we can advise is to use the RF Probe that transforms the digital tester into a millivoltmeter and connect it between the cursor and the mass.
An oscilloscope or alternative method with a 1N4148 diode and a capacitor
Finally, we must measure the output of the amplifier under test and detect the moment when the output signal is trimmed. The ideal instrument is an oscilloscope but let’s assume that the reader does not have it.
The idea is to lower the input signal and begin to increase it gradually. It will be noted that the output voltage increases proportionally to the input voltage until it reaches a certain value and then does not increase further. This value is the positive trimming voltage that in this case is approximately 5.3V. Then the diode D1 must be inverted to be able to measure the negative peak of the output. If the tester is a needle, its tips should be inverted. Now we must start a procedure identical to the previous one until the indication of the tester does not increase more, that is the negative cut voltage that in our case is also of 5.3V indicating that the amplifier has the cut very even. Now we just have to calculate the output power that we already know how to make of previous deliveries: the peak voltage is the smaller of the two measured voltages if they were different; in our case, it is 5.3V. The effective value of this voltage is Vef = 5.3V / 1.41 = 3.76V and that voltage applied to a resistor of 8 Ohms develops a power Pmax = (3.76) 2/8 = 1.76W For those who are not very skilled in mathematics calculate the effective value of the output divides the value indicated by the tester by 1.41. Then take the effective value, multiply it by itself and divide it by the resistance of the speaker to obtain the true power output. It seems a very small value but it is not possible to increase the output power if the source voltage is not increased, or the value of the load resistance is reduced or the circuit is changed, because this circuit is optimized and its maximum power if the transistors ideal would be: (6 / 1,41) 2/8 = 2,26W which is very close to the previous one.
Sensitivity measurement at trimming level
An amplifier is an intermediate block between the signal source (in our case the output signal of the radio) and the speaker. Sensitivity is the input voltage that is required to bring the amplifier to the trim level. The same measurement above gives us the answer. The amplifier has a sensitivity of 640 mV peak or 640 / 1.41 = 453 mV effective. For our case, it reaches perfectly but could not reach or be excessively large. How can the sensitivity level of our amplifier be adjusted? The ideal mode is by variation of the negative feedback resistor for the AC which in our case is R6. We ask the reader to modify the value of R6 to 4.7 Kohms and test the new cut sensitivity value. You will see that it will now be 430 mV peak or 305 mV effective. But is nothing lost by reducing negative feedback? Several very important things are lost. In principle, we can assure that the distortion increases since the negative feedback were added to reduce it and the response is lost at low and high frequencies. Later we will see that feedback also influences other factors.
The student must understand the concept of generalized performance for any machine such as: the energy delivered by the machine divides the energy it consumes.
This is not an electronic concept but is a mechanical concept that electronic devices must fulfill at all costs because it corresponds to what we might call a higher law. And the performance must always be less than unity because otherwise, we would have invented continuous movement. Many times the author observes the musical centers coming from the East that clearly indicates the consumption over the network as 200W and magically has an audio output power of 250W + 250W (stereo). This means that they deliver 500W and consume 200W. If with normal performance less than 1, the equipment heats up with that performance of 500/200 = 2.5, the equipment should cool down. Yes, look good because you will surely find an ice bucket somewhere. Let’s go back to reality; Our equipment belongs to the physical world and therefore will heat up indicating that it has a performance lower than one. How much less? It is what we are going to measure. Since we adjusted the vacuum current to reduce cross-over distortion, our amplifier was left with an ammeter in series with the source. We leave it placed because it will indicate what the consumption is and therefore the performance of our amplifier. In our simulated circuit, we use a regulated 12V source. Therefore we do not need to measure the source voltage to perform the calculation of the maximum output power we know that it will always be 12V. In the real case the energy source is not regulated and when we consume a lot, it falls. In this case, source voltage and source current must be measured. And as surely the amplifier will be stereophonic and will use a single common source, not only should the amplifier under measurement be functioning, but also that of the other channel, and both should have the nominal load that in our case is 8 Ohms. The power delivered by the source will be 12Vx0,25A = 3W. The performance is then quite low since it is only 1.76 / 3 = 0.6 or 60%. That is to say that 40% of the source power is transformed into heat in the two power transistors because of 3W x 0.2 = 0.6W in each transistor. Is this power output the minimum of the amplifier in any condition? Do not; when the amplifier works at the lower output has lower performance. So you must perform the performance measurement at different output powers to graph or tabulate it. The WB avoids the problem because it has a wattmeter that can do the work for us if we connect one on the output and another on the source.
Note that the wattmeter is a four-terminal, two current and two voltage instrument. The current section is connected in series with the load and the voltage section in parallel. In the following table, you can see the performance of different input and output signals.
|Pot. Output [W]||Pot. of font [W]]||Ent. Tension [mV]||Redimiento [%]|
Performance table of a bipolar push-pull amplifier Note that the collector resistors of the R1 and R4 driver are not connected to the wattmeter, that is, the actual performance considering the excitation power is even lower than that considered because the driver manages a considerable power to be able to excite the output transistors that have a relatively low beta. The performance of a real amplifier considering the output and the driver is of the order of 50%. How much fossil fuel disappears per hour around the world due to such waste?
If in a home laboratory of manufacture of amplifiers, not even the most elementary measurements are made, the reader can imagine that the distortion is an impossible parameter to measure because it requires specific instruments. The measurement requires an instrument called a total harmonic distortion meter that is difficult to find in a repair shop. We are only going to analyze the influence of the feedback in the distortion and we will leave the problem of building a distortion meter for later because it is not impossible to manufacture it in a homemade way, with operational amplifiers and helped by the filter calculations section of the WB
The distortion meter is already set by default to perform an audio measurement at a frequency of 1 KHz with a resolution of 100 Hz and with a display that directly indicates distortion percentage. As we can see our amplifier has a DTH (Total Harmonic Distortion) of 0.33% which indicates an adequate behavior of the amplifier. The distortion is a function of the negative feedback of the amplifier. Remember that it was added to avoid an appreciable apartment in the form of a sinusoidal input signal. The best way to verify the operation of the negative feedback is to reduce it by half, bringing the value of R6 to 6.8Kohms, readjusting the input signal to the nominal value of the output signal and measuring the distortion again.
Frequency response measurement
It is common for students to refer to distortion when the output signal is not proportional to the input signal (distortion of shape). But if the amplifier has an inadequate frequency response, it has another form of distortion but does not doubt that it is a distortion that in this case is called distortion by cutting the frequency response. The human ear has a response range of approximately 10 Hz to 20 KHz depending on age and ear education. Below 10 Hz, the ear perceives the sine-wave cycles as separate strokes of the speaker cone and above 20 KHz it has no auditory sensation. As the ear is unable to distinguish two signals whose amplitude differs by less than 33% (3 dB), the frequency response is measured considering a drop in that value at high and low frequencies.
- You only have to measure the output voltage with our diode and capacitor at 1 KHz, increase the frequency and observe the voltage drop in the tester by 33%.
- That is the higher cutoff frequency.
- Then do the same towards the low frequency and determine the lower frequency cut.
The WB facilitates the measurement with the Bode meter that we already use in this course to measure the response of the coils to the RF. As the meter reaches audio frequencies, it allows us to automatically measure the frequency response curve and modify it if necessary.
As you can see, the tracer tells us that the response in high frequency is excessively high since it reaches more than 1 MHz. Of course, this can not harm the hearing of the highs, but an abnormally high response can generate spurious oscillations when placing a real speaker, with inductance, in addition to resistance. At low frequencies, the cutoff is close to 80 Hz, which is considered appropriate in principle since the amplifier must have little response to the resonance frequency of the speaker and the acoustic cabinet to avoid generating a drum sound at low frequencies. The best way to cut the treble is through capacitive negative feedback added to the existing network.
As you can see now, the cut occurs around 50 KHz so that there is no loss of treble but avoiding the same time the possibility of oscillations.
To close this release, we will present some WB files with our amplifier circuit that has a damaged material. The reader should run the files and find the damaged material by making measurements with the virtual instruments. When you are sure of the damaged material, you should change it and check if the fault was repaired.
Note: take into account some characteristics of the WB. Each time a simulation is started the presets are automatically set to half value. In our case, before running the simulation, take the preset to 80%. The function generator starts at 1 Hz with 10V of amplitude. Use the correct parameters. All repairs must begin with the measurement of source voltage; then you must measure the continuous voltages of the circuit that can be obtained from running the file without failures that is BUD1923.ms9 (with the addition of the treble core capacitor) if you have any doubts, use this circuit as if it was the service manual of the real equipment (do not forget to bring the generator to 1mV or less when measuring continuous voltages).
A feedback circuit is very difficult to repair measuring continuous voltages because one voltage modifies the others. To measure continuous voltages, you must open the negative feedback loop. Disconnect resistor R5 from the output and connect it to a source of approximately 6V. Measure the output and vary the voltage of the source by a few mV up and down until the output is just half the source voltage of the amplifier. If the output does not change it is because the equipment has a fault, leave the source at 6V and try to follow the voltages from the input to the output until finding the damaged component.
The digital tester in its ohmmeter function can be used if it is necessary to desolder the components, practically in all cases. The reason is that the ohmmeter works with a 0.5V virtual battery. With this voltage value, the diodes and the bipolar transistors do not operate because that voltage is below the barrier of the silicon junctions. In modern equipment with SMD mounting components (surface mounting or without wire terminals), the use of non-invasive techniques is essential. It is impossible to solder and desolder a resistor or a capacitor without altering it in any way. The only cases that require disconnection of the component are when it is in parallel with a component winding or very low resistance. The WB can perform a perfect simulation of an analog tester, but it must take into account that for it to work, the simulation must be operating. Disconnect the 12V source from the amplifier, activate the simulation and measure with the tester as you would in the real world. Remember that for a change in the circuit to be recognized by the WB (for example, changing the voltage value of the open loop source) the simulation must be turned off and on again.
“Divide to repair” is a maxim that you must remember at all times.
Always look for the central point of a rough team and make a measurement that allows determining if the fault is before or after that point. In our case, for example, After opening the loop, the bases of the output transistors can be disconnected and measured in the driver’s manifold. The tension does not change too much with or without the bases connected. If the problem is solved, the damage is in one of the output transistors that pull down or up the collector voltage.
In this way, we carry out an analysis of the main parameters of an amplifier and explain how they are measured in a virtual and real way. If you apply the techniques learned here, you can be sure that you have a deeper knowledge of the subject than many who are professionally dedicated to the audio and who will repair changing components at random. In the next lesson we will continue making measurements on the amplifier and above all, we will explain how you can increase the output power by making a connection of two bridge amplifiers.