I am replacing some faulty output transistors and while the new transistors are spec’d equal or better than the originals. They are smaller in physical size. It was recommended that I add some output transistors as there is plenty of room on the heat sink just to give the output transistors an easier time I haven’t had to do this before, but I like the idea of it, as it gives me a new method to learn. The amplifier is a class H design, and here is a portion of the schematic for the outputs.
I have the collector wired directly in parallel with the other collectors. The emitter going to an additional emitter resistor (.22 ohm or .1)which then ties to where the other emitter resistors go. The base connects to a resistor (2.2 ohm) and then the other side of that resistor connects to where the other base resistors are.
Is the appropriate way in adding transistors? There are two output sections for each channel and each section uses two pairs of outputs. I would be increasing it to three pairs per section, a total of six pairs per channel. Thank you
Dan
I have the collector wired directly in parallel with the other collectors. The emitter going to an additional emitter resistor (.22 ohm or .1)which then ties to where the other emitter resistors go. The base connects to a resistor (2.2 ohm) and then the other side of that resistor connects to where the other base resistors are.
Is the appropriate way in adding transistors? There are two output sections for each channel and each section uses two pairs of outputs. I would be increasing it to three pairs per section, a total of six pairs per channel. Thank you
Dan
Why WOULDN’T the driver be able to drive it? The 3 outputs are still driving the same load the original two were. If anything, total base current will come DOWN reducing the load on the driver. Sure, input capacitance increases, but that will only cause short lived transient current to go up, and that’s NOT what dominates the heating in the driver.
If NJW02xx devices are used, the total capacitance of 3 of them will probably be about the same as two of the original Sankens anyway.
If NJW02xx devices are used, the total capacitance of 3 of them will probably be about the same as two of the original Sankens anyway.
Also, obviously the correct way of adding extra outputs. The only values to ever change would be if you were revisiting the protection circuits when doing this. One could increase all the emitter resistors for the master outputs from 0.22 to 0.33 ohms, causing any current sensing that is employed to scale. Not strictly necessary. The ones on the upper/outer transistors would stay the same.
Thank you, so leave the emitter resistors the same then at .22? Here we go, I’ll post a photo as I go. Ignore the MT-200s.
I’m assuming that 3 pair of NJW0302/0281 would be a better option than 2 pair of 2SC5200 and 2SA1943 considering their all rated at 150w.
Dan
I would be inclined to use all original mounting holes, rather than do a bunch of machine shop work. Threading holes is a &!+(#, and the in between position might not have enough space between fins to put a 4-40 nut. The spacing between the 4 holes looks good, and existing threads can be used. Put the driver in the 4th position.
In any case, way better than 2 pair C5200/A1943. If you were going to use only 2 pair it would be 3281/1302 or 21193/4.
In any case, way better than 2 pair C5200/A1943. If you were going to use only 2 pair it would be 3281/1302 or 21193/4.
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Damn, I really wish I would have seen this before I got going. I wanted to be able to use the original solder tabs on the boards so I was putting them in the original spot.
So here it is, the additional transistors are the ones that are in the middle of each pair and are inset towards the center of the heat sink a little bit. I was considering going to the very edge for the extra four transistors, but I figured that it would be better to have the heat towards the center of the heat sink versus the very edge.
Dan
I would have thought about re-spinning power boards. Double sided with 2 ounce copper. But hey, you’re done with the hard part already, and if you are re-using boards it’s best to keep things in original positions.