Schematic | PCB | Component layout on PCB| Upgrades

Presented is a design of my push-pull bipolar amplifier. First was build in July 1988. After 11 years I am totally sure in this baby, because numerous copies have been built by friends and myself, ( more then 50 ), with use of slightly different components. Presented is most recent, but generic version. This amplifier works always, even if you use different transistors, ( but use data books when choose replacements - hFe is most important, and other things-of course ). you can try more things then you can imagine, but always follow original schematic, and do only one by one modification until you are sure in reliability. Few guidelines:
Resistors: Max. tolerance is +-10%, but you MUST use same value for every specific value in BOTH channels. Tip: you decided to change 47 Ohms resistor in 47.5 (standard value in metal-film resistors) you must use 47.5 Ohm for EVERY ex-47 Ohm. Following this rule, you can safely use carbon, metal-film or oxide, wire-wound, HOLCO, etc.
Transistors: I use this generic, easy to get 3-leg creatures, but if you are familiar with transistor data books, hey-playground is bigger ! Tip:BC transistors are generic input small signal 50Volts/100mA/300mW types, but MUST have hFe greater then 350. BD transistors are generic driver medium signal 80Volts/1A/8W types, but with hFe greater then 60, and -16 units MUST have hFe greater then 120. Prefer also types with greater bandwidth ( 100 MHz) on this positions. Output transistors are TO-3 160Volts/25A/250W types, but I use what I can get. If you have speakers with 8 Ohm impedance freely use BD249B/C and BD250B/C or some other TOP-3 80Volts/12A/117W min. types.
Always try to check hFe on your multimeter-specially BC (input) and BD (driver)  transistors. If you don't have one, borrow it from friend. Russian no-name transistors, even those from dead VCR work fine if you apply this rules.
Capacitors: For picofarad (pF) values use ceramic or foil, but NEVER 50, 60 or 100 Volt types. They works fine in digital 5 Volt circuits, but in audio they are source of troubles! Capacitor on output (0.1uF) must be a foil, min. 250V, and impulse-resistive. Electrolytic must be new, buy new ones, double the number of them, but always keep in mind: electrolitics are usually dead after seven years, used or not. If your friend or yourself cant get sound from some Hi-Fi component, first thing to do is to replace all (ALL) electrolytic anywhere if the component is older then hmm...... 5 years.
Tip: Use separate power supply for each channel. This means -transformer, diode bridge, and 8 electrolytics- two times. More electrolitics is better, increase number of them, not value. Big electrolytics are often a way to make sound heavy and less vivid. It is better to spend money in increased number of low-cost electrolytics and then bypass each one with 0.1uF foil capacitor, then spend big money on fancy, gold lettered audio-grade electrolytic capacitors. They are better, but 16 cheapo bypassed is better then two fancy for 10-time more money. Transformer can be smaller if you use one for each channel, but never  below 500 VA. (However, 100 VA for both channels works fine, but the sound   is gone.)

Heatsinks and case: Heathsinking is very important, because all transistors, specially in input and voltage gain stages run with very high static current-way beyond normal class A. But, this is a secret of this, relatively simple design. So, put biggest heat sinks you can fit and find on BD139-16 and BD140-16 (narrow and long one is best-and with min. 8C/W). Output stage heatsinks are not very important, until you decide to switch from class AB (as in original) to class A operation. For normal, class AB, you can use aluminium sheet min. 3mm thick, and dim. 400x200 and bigger-maybe front or bottom panel. I prefer regular heatsinks in range 0.55C/W for both channels and all four output transistors, min. 1.5C/W if use one per channel (two output transistors on one heathsink-two heath sinks for stereo amplifier), or 2.2C/W for each output transistor (four heatsinks for every output transistor). In any case, let some air to them, never seal them in case. Case. you have no idea where and in what my friends and myself put all this stuff, wired them, and they still work, wiping all commercial, expensive amplifiers, making them silly and make his owners desperate.
Always take great care when working with lethal voltages, specially AC mains.

Back to Welcome| 50W schematic| 50W PCB| 50W components layout| Upgrades