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Transistor amplifier wiring diagram9/6/2023 ![]() I have had a few constructors comment on the quiescent current, which is somewhat higher than they expected. The oscilloscope output indicates that most of what I see is noise - even integrating the output (my scope can do that) to eliminate the noise reveals very little at all. The noise and distortion figures are somewhat pessimistic - there is so little distortion at 1V (or 20V for that matter) that my distortion set has great difficulty in getting a readable measurement. In its lifetime, many of the mods mentioned above were made anyway, since I could never find the circuit diagram when I needed it, so often made it up as I went along! It is a fair testament to the amp that all sorts of resistor and capacitor substitutions can be made, and it still works fine. Best to wait until the temperature has stabilised first, but it won't change very much anyway.Īpart from these changes, the amp is pretty much original, and with a nominal ☓5V supply as shown, will provide around 70W into 8 Ohms quite happily. If you really wanted to, you could even use a trimpot (2k), and adjust this for minimum DC offset. If this is found, you might need to 'tweak' this resistor to obtain minimum DC offset. ** It is conceivable that with some transistors, the value of 560 Ohms may not be correct. Added the output inductor and damping resistor (see UPDATES).Reduced stabilisation caps to 100pF (they used to be 220pF).Increased the value of the bootstrap capacitor to 100♟.Reduced the value of the Class-A base resistor to 560 Ohm **.Noise - DC Offset - Less than 100mV (Open Loop Gain - 125dB (no load), 80dB (8 Ohm load).Harmonic distortion 1kHz - 0.05% (maximum typical).Frequency response (-3dB) - 10Hz to 23kHz 1W.Input sensitivity for 60 W output - just under 1V (1V gives 66W).Because of the cost of the devices used (minimal), it is a cheap amp to fix even if you do manage to blow it up.įigure 1 - 60W Power Amplifier Original Circuit Diagram (Don't Use This Circuit!)īasic specs on the amp are as follows. Since this amp was built commercially, the savings were worth the risk - most of these amps were installed in the speaker box, so shorting was not likely (unless the loudspeaker voice coil shorted as happened a few times). Note that there is no output short circuit protection, so if speaker leads are shorted while the amp is working (with signal), there is a very real risk of the transistors being destroyed. As with anything on any auction site, beware of fakes. I have seen similar devices offered on an auction website, but there were no details. The package is TO127 which is no longer used by most manufacturers. At the time they were 'premium' Motorola devices (note the gold plated leads), and they were remarkably rugged. These devices were considerably better than the modern TO220 versions, but there is no data on the Net because they are so old. Photo Of Original (Long Obsolete) MJE2955/3055 Transistorsįor a bit of history, the photo above shows the transistors that were used when hundreds of these amps were built. The transistors recommended today are the TIP35C and TIP36C (schematics have been updated), which are better than any TO220 devices. The modern ones come in a TO220 package, and are severely limited in nearly all respects compared to those used in the 1970s. Note that although MJE2955 and MJE3055 transistors were shown, the original amps used a version of these transistors that has long since been discontinued. Project 3A is the version intended for construction, and it is a far better amplifier overall. ![]() Please note that I do not recommend that you build this version of the amplifier. The amp is capable of driving 4 Ohms, but it is starting to push the limits of the transistors, however, even when used at 4 Ohms, very few failures were encountered. Most were operated as small PA or instrument amps, but many also found their way into home hi-fi systems. ![]() The design featured is a slight modification of an amp I originally designed many years ago, of which hundreds were built. It is a simple amp to build, uses commonly available parts and is stable and reliable. This amplifier does not claim to be 'state of the art', and in fact the base design dates back to the earl y 1970s. Please Note: PCBs are available for the updated version of this project (P3A) ![]()
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