Previously, in this thread, I discussed my modifications to a class AB amplifier in a stereo console unit from the 1970s. Now I have another similar unit that I got at a rummage sale that I would like to do something very similar to, and now that I've already done it once, for the most part it's been easier. But, with the other stereo I had the original manual containing the schematic, and with this one I have no documentation. So, just by examining the PCB, I created this sketch of the heart of one channel of the amplifier. There are of course many more components in the circuit, these are just the ones that I particularly care about (and most of the other ones I wouldn't quite know what they're for).
My plan is to replace every component displayed here in order to allow the amplifier to sound better and be much more powerful. The DC blocking capacitor just before the connection to the speaker will be replaced by one that is 2200 μF instead of 470 μF to allow for much better bass, the 2.2 Ω resistors will be 5W instead of 1W, and the transistors will be TIP41AG for the NPN and TIP42AG for the PNP. Since the A738 and C1368 transistors were in a TO-126 package, and the new ones will be in a TO-220, the pin configuration will be backwards of what it was, but I have taken this into consideration.
My main questions, then, are these:
1. Does anyone know why a resistor and a diode in series may have been used instead of two diodes in order to bias the transistors? Would it be acceptable to replace the diode and resistor with two new diodes? There is a ~1.3V drop over the resistor and diode, so I would imagine two diodes would be pretty much the same, and since the goal is to match the characteristics of the diodes with the transistors, I would imagine it might be better to replace this configuration, especially since I am replacing the transistors. Last time, I used these same transistors and 1N4003 diodes, and it seemed to work great.
2. The use of 2.2Ω resistors seems to be quite inefficient. The previous stereo I modified used 0.5Ω resistors, which seemed much better to me. If I were to play a sine wave just at where it clipped (and I realize this isn't what music is like, but it makes the math easier), and I had a 4Ω load (which I plan to do, since I will have two sets of 8Ω speakers configured in parallel), if my math is correct, each resistor would dissipate 2.51W. And, even worse, the maximum power to the speakers would be at 21.9W with no resistors, and only 9.1W with these resistors. Would it be possible to reduce the value of these resistors, say to the 0.5Ω used in the other stereo (or to any other value)? Would this require additional modifications of the circuit?
If it would be helpful to know any additional details about the circuit, just let me know, and I will try to provide them.
Thanks in advance!
GeorgeTR
My plan is to replace every component displayed here in order to allow the amplifier to sound better and be much more powerful. The DC blocking capacitor just before the connection to the speaker will be replaced by one that is 2200 μF instead of 470 μF to allow for much better bass, the 2.2 Ω resistors will be 5W instead of 1W, and the transistors will be TIP41AG for the NPN and TIP42AG for the PNP. Since the A738 and C1368 transistors were in a TO-126 package, and the new ones will be in a TO-220, the pin configuration will be backwards of what it was, but I have taken this into consideration.
My main questions, then, are these:
1. Does anyone know why a resistor and a diode in series may have been used instead of two diodes in order to bias the transistors? Would it be acceptable to replace the diode and resistor with two new diodes? There is a ~1.3V drop over the resistor and diode, so I would imagine two diodes would be pretty much the same, and since the goal is to match the characteristics of the diodes with the transistors, I would imagine it might be better to replace this configuration, especially since I am replacing the transistors. Last time, I used these same transistors and 1N4003 diodes, and it seemed to work great.
2. The use of 2.2Ω resistors seems to be quite inefficient. The previous stereo I modified used 0.5Ω resistors, which seemed much better to me. If I were to play a sine wave just at where it clipped (and I realize this isn't what music is like, but it makes the math easier), and I had a 4Ω load (which I plan to do, since I will have two sets of 8Ω speakers configured in parallel), if my math is correct, each resistor would dissipate 2.51W. And, even worse, the maximum power to the speakers would be at 21.9W with no resistors, and only 9.1W with these resistors. Would it be possible to reduce the value of these resistors, say to the 0.5Ω used in the other stereo (or to any other value)? Would this require additional modifications of the circuit?
If it would be helpful to know any additional details about the circuit, just let me know, and I will try to provide them.
Thanks in advance!
GeorgeTR