Making of a small amplifier
It is a good idea to transform the scrapped tube radio into a small amplifier. The audio of the radio, or the use of CD as the signal source, the sound of puff is endless. It is easy to make sound from the amplifier, but difficult to make good sound. Although everyone has different tastes for the so-called "good voice". But there is an indicator that must be achieved. That is static! When the music stops, if you want to enter the realm of "no sound wins at this time", the speakers should be quiet. The low-frequency AC hum of the amplifier is the result of one or more interference sources that interfere with the machine. The source of the interference comes from the machine itself. A friend of mine made the amplifier in one day. But it took three months to get rid of the exchange hum. How can I not rework at one time, so that the amp will refuse to hum, the following text may give the first bird who just entered the Tao, a little inspiration.
There are several sources of interference for AC hum: 1: magnetic field leakage of transformer, 2; poor filter capacitor, 3; interference of filament to negative level. 4: Disturbance of the input signal of the previous stage, 5: The phase of the negative feedback is wrong. If your machine is powered on after it is done once and finds an exchange hum, it is difficult to check if it is caused by the interference. You should gradually discover and gradually eliminate.
One: Elimination of transformer magnetic field leakage interference: Before making the rack, first place your fire cow in a certain position you like by default, or on the left, right, or in the middle. , And then suspend your Huoxiu secondary, the primary power is 220V, and then suspend the primary of one of your output calves, and the secondary is connected to the horn. In a quieter environment, if the position of the two transformers is not appropriate . There will be electromagnetic disturbance squeak sound, at this time you only need to move the calf until the squeak sound is eliminated, and then locate another one as usual. Now the position of your three transformers can be determined. For the rest of the time, you should consider the placement of the electronic tube. According to experience, the relative position of the calf from the fire cow should not be less than 3 cm. If your pair of Mavericks are disassembled parts, it is best to do the same phase test, unless it is the same factory, the same type of product.
The method is as follows: use one battery to touch the primary of two calves, use the minimum gear of the micro-ammeter, connect the secondary, instead of the horn. If the winding direction of the two calves is the same, the direction of the hands will be the same. Touch a few more times until you see clearly. Because the finger width of the hands is not large. And fleeting.
Two: Poor filter capacitance and elimination of filament interference to the negative level: At this time, you have fixed the transformer and the electronic tube seat. The welding machine is started, the filament wire should be twisted, and the loop shelter must be grounded a little. You don't want to weld after finishing, stop. Connect the speaker, plug in the power amplifier tube, and power on. If the filter capacitor is bad, or the filament interferes with the negative level, there will be exchange hum. You first connect the end of the Huoxiu 6.3V filament to the ground. If the AC hum is eliminated, there is no problem with the filter capacitor. If the AC hum is not eliminated, the filter capacitor will have a problem. Changing the filter capacitor should be a simple matter. Then, intermodulate one end of the filament, and select the end with the lowest AC hum to ground. In the quiet night, your ears are no more than 10 centimeters away from the speaker, and you cannot hear the exchange hum. The capacity of the filter capacitor is enough. If you are using a stone rectifier, the capacity of the filter capacitor is no big deal. If you supplement with inductive filtering, the effect will be better. Now you can connect the signal to the rear stage and listen to the sound. Don't worry it will be very loud because your front stage has not been welded yet.
Three: Elimination of pre-stage input signal interference: Weld the pre-stage. At this time, if there is further disturbance, the input signal part of the front stage is the only original culprit. Be sure to use a good volume potentiometer, don't be afraid to spend money, one point of silver, one point of goods! The input signal line should be dual-core, and the outer layer should be grounded at one end. The two lines of the left and right channels should be the same length to achieve balance. As close as possible to the bottom plate, away from the AC electric field.
All right! You can now plug in all the tubes to power up and put your ears 5-8 cm into the speaker. "The dawn here is quiet."
At this moment, you can connect the signal source CD, and make sure that the power of the CD is turned on. Turn the volume potentiometer of your new tube amplifier to the maximum, the dawn here is still quiet. If there is hum, and the hum changes with the volume potentiometer. That is your CD is not good. If there is no hum, just put the disc.
remember! There are no good or bad amps, only different tastes! ! Otherwise you will fall into the trap of burning money.
Four; adjust the phase of the negative feedback: if you use the post-stage large-loop negative feedback circuit, you can connect it at this time. If the sound is not correct and there is howling, it means positive feedback. Adjustment welding output Mavericks primary or secondary is OK. When the phase of negative feedback is correct, the sound should be reduced.
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