A simple germanium power amplifier. Simple germanium power amplifier Proven amplifier circuits using germanium transistors
The main feature of the UMZCH published below is its use of broadband OOS, the frequency response of which, unlike the OOS of conventional multi-stage UMZCHs, does not have a deep cutoff at higher audio frequencies. To implement the linearizing capabilities of broadband OOS, it was decided to abandon the multi-stage UMZCH and limit the number of its stages to only the absolutely necessary ones. In addition, it was necessary to abandon the use of elements that create a delay in the amplified signal, which made it possible to use negative feedback in the frequency spectrum of switching distortions. As a result, with the help of OOS operating in the range of 40..60 kHz, it was possible to reduce the coefficient of nonlinear distortion at a frequency of 20 kHz to 0.05...0.01% when using the output stage operating mode with zero quiescent current.
The pre-terminal voltage amplifier is built on two transistors UT1 and VT 2. Through capacitor C1 to the base of the transistor VT 1 the input signal arrives, and through resistors R 3, R 4 – balancing voltage of the power supply. To guarantee stable operation of the amplifier, the capacitances of capacitors C1, C6 and C8 should not differ from those indicated on the circuit diagram by more than 50%. In order to protect against accidental current overloads, a resistor is included in the collector circuit of the UP transistor R 7. Cascade on a transistor VT 2 provides the main signal amplification. Resistor chain Rl 1 R 12 with traditional voltage boost through capacitor C8 gives an increase in the amplitude of the amplified signal by 10..12%. The synchronization of functional processes in the amplifier arms is ensured by capacitor C5.
The final current amplifier is built on a complementary pair of transistors VT 5- VT 8 , connected according to a circuit with a common collector. Transistors interconnected by emitters VT 3, VT 4 connected by bases to bases of transistors VT 7, VT 8, and the collectors to the bases of the transistors VT 5, VT 6. Using a variable resistor included in the current feedback circuit R 13 adjusts the voltage at the transistor bases VT 3, VT 4 and thus ensures the voltage setting at the bases of the transistors VT 7, VT 8 is 0.1..0.2 V lower than usual and the terminal transistors operate in amplification mode with zero quiescent current. The UMZCH is powered by an autonomous rectifier without galvanic connection with a common wire. Thanks to this, it was possible to reliably protect the speaker from the direct current component of the terminal transistors, without introducing complex relay-transistor protection devices into the amplifier.
The UMZCH is made in a single unit with a rectifier. Its dimensions (135X90X60 mm) are determined by the dimensions of the heat sinks and filter capacitors. The mass of the block is 560 g. The block is mounted on two plates measuring 130X58, between which heat sinks and filter capacitors are sandwiched. One of the plates houses the rectifier diodes and output circuits, and the other contains all the transistors, capacitors and resistors.Most connections are made by the components' own terminals. Resistor R 6, capacitors C11 and C12, input circuits and load circuits are connected to a common wire at one point. If the recommendation for a monoblock construction of the UMZCH is not used, then blocking the power circuits with capacitors with a capacity of 0.1 μF will be required.
To check the parameters of the assembled amplifier and the efficiency of the technical solutions used in it, it is recommended to assemble a defect signal selector. Its diagram is shown in the figure. Variable resistors – R 1 and R 8 provide balancing and compensation for the delay of the controlled signal.
I would like to express special gratitude for the printed circuit board and preparation in the description to my friend and just a good person under the nickname Chetlanin.
Power unit:
The quality can be improved by using better transistors for the outputs, for example KT814-815 on 2SC4793-2SA1837, and instead of KT818-819 put KTB688-KTD718 or 2SD718-2SB688. True, these outputs are in the TO247 package, the board will need to be adjusted.
In the program at maximum power, the amplifier consumes (not exceeding): 1.6-1.7 A.
A wirewound resistor is needed when you turn it on for the first time, so as not to kill the output transistors if there is any mistake in the installation.
When you turn it on for the first time with a resistor, if everything is fine, then we remove it and set the settings, set it, set the fuse, turn it on and listen.
A fuse (or a jumper instead, it doesn’t matter) is required specifically for my board layout, since to configure it you need to break the + power bus.
The printed circuit boards (.lay) and amplifier circuit (.spl) are located.
From some of my friends I heard good reviews about the sound of ULF on germanium transistors. And I decided to assemble the usual classical circuit using complementary germanium transistors GT703/705. For the build-up - the SRPP cascade on 6N30P to obtain the lowest possible output impedance.
The scheme is as follows:
Resistor VR2 sets zero at the output, resistor VR1 sets the quiescent current of the output transistors. Zener diodes are needed to prevent the appearance of dangerous voltage for transistors between the floors of the SRPP in the event of failure of one of the halves of the lamps. Preliminary listening to the prototype showed very good sound, maximum sinusoidal power - 8 Watts, bandwidth at minus 1 dB from 20 Hz to 80 kHz. Sensitivity – 0.6 volts. The prototype played for about 10 minutes at maximum volume (as long as the ears could hold it) and the radiators of the output transistors did not even heat up to 50 degrees, only the quiescent current increased from the initial 40 mA to 100. Power supply: 
For further experimenters, a mock-up was assembled in stereo version. The first tests were done without a surge protector. The addition of this element brought back the clarity of sound inherent in tube amplifiers. In general, of course, this is not a 2A3, but given the simply captivating simplicity of the design, the sound is very, very decent. The general impression is that it is typically triode, that is, clean, detailed, accurate, but therefore somewhat unemotional and rustic. It is difficult to say whether the reason for this is the tube or transistor part of the circuit, or the circuit itself - this will be shown by further experiments - they will certainly be continued.
And finally, a couple of pictures of what it looks like:
Updated February 21, 2013. Apparently, it is possible to power the output stage using LM7812 and LM7912 installed on a radiator.
Nikolay Troshin
A simple germanium power amplifier.
Recently, there has been a noticeable increase in interest in power amplifiers based on germanium transistors. There is an opinion that the sound of such amplifiers is softer, reminiscent of “tube sound”.
I bring to your attention two simple circuits of low-frequency power amplifiers using germanium transistors, which I tested some time ago.
More modern circuit solutions are used here than those used in the 70s, when “germanium” was in use. This made it possible to obtain decent power with good sound quality.
The circuit in the figure below is a reworked version of the low-frequency amplifier for “germanium” from my article in Radio magazine No. 8, 1989 (pp. 51-55).
The output power of this amplifier is 30 W with a speaker load impedance of 4 ohms, and approximately 18 W with a load impedance of 8 ohms.
The amplifier supply voltage (U supply) is bipolar ±25 V;
A few words about the details:
When assembling an amplifier, it is advisable to use mica capacitors as constant capacitors (in addition to electrolytic ones). For example, the CSR type, such as below in the figure.
MP40A transistors can be replaced with MP21, MP25, MP26 transistors. Transistors GT402G - on GT402V; GT404G - to GT404V;
The GT806 output transistors can be assigned any letter indices. I do not recommend using lower-frequency transistors such as P210, P216, P217 in this circuit, since at frequencies above 10 kHz they work rather poorly here (distortion is noticeable), apparently due to a lack of current amplification at high frequencies.
The area of radiators for output transistors must be at least 200 cm2, for pre-terminal transistors - at least 10 cm2.
For transistors of the GT402 type, it is convenient to make radiators from a copper (brass) or aluminum plate, 0.5 mm thick, 44x26.5 mm in size.
The plate is cut along the lines, then this workpiece is shaped into a tube, using for this purpose any suitable cylindrical mandrel (for example, a drill).
After this, the workpiece (1) is tightly placed on the transistor body (2) and pressed with a spring ring (3), having previously bent the side mounting ears.
The ring is made of steel wire with a diameter of 0.5-1.0 mm. Instead of a ring, you can use a copper wire bandage.
Now all that remains is to bend the side ears from below to attach the radiator to the transistor body and bend the cut feathers to the desired angle.
A similar radiator can also be made from a copper tube with a diameter of 8 mm. Cut a piece of 6...7 cm, cut the tube along the entire length on one side. Next, we cut the tube into 4 parts half the length and bend these parts in the form of petals and place them tightly on the transistor.
Since the diameter of the transistor body is about 8.2 mm, due to the slot along the entire length of the tube, it will fit tightly onto the transistor and will be held on its body due to its springy properties.
Resistors in the emitters of the output stage are either wirewound with a power of 5 W, or type MLT-2 3 Ohm, 3 pieces in parallel. I do not recommend using imported films - they burn out instantly and imperceptibly, which leads to the failure of several transistors at once.
Setting:
Setting up an amplifier correctly assembled from serviceable elements comes down to setting the quiescent current of the output stage to 100 mA using a trimming resistor (it is convenient to control the 1 Ohm emitter resistor - voltage 100 mV).
It is advisable to glue or press the VD1 diode to the heatsink of the output transistor, which promotes better thermal stabilization. However, if this is not done, the quiescent current of the output stage from cold 100mA to hot 300mA changes, in general, not catastrophically.
Important: Before turning on for the first time, you must set the trimming resistor to zero resistance.
After tuning, it is advisable to remove the trimming resistor from the circuit, measure its real resistance and replace it with a constant one.
The most scarce part for assembling an amplifier according to the above diagram is the GT806 output germanium transistors. Even in the bright Soviet times it was not so easy to acquire them, and now it is probably even more difficult. It is much easier to find germanium transistors of types P213-P217, P210.
If for some reason you cannot purchase GT806 transistors, then we offer you another amplifier circuit, where you can use the aforementioned P213-P217, P210 as output transistors.
This scheme is a modernization of the first scheme. The output power of this amplifier is 50W into a 4-ohm load and 30W into an 8-ohm load.
The supply voltage of this amplifier (U supply) is also bipolar and is ±27 V;
Operating frequency range 20Hz…20kHz:
What changes have been made to this scheme;
Added two current sources to the “voltage amplifier” and another stage to the “current amplifier”.
The use of another amplification stage on fairly high-frequency P605 transistors made it possible to somewhat unload the GT402-GT404 transistors and boost the very slow P210.
It turned out pretty good. With an input signal of 20 kHz and an output power of 50 W, distortion is practically not noticeable under the load (on the oscilloscope screen).
Minimal, barely noticeable distortions of the output signal shape with P210 type transistors occur only at frequencies of about 20 kHz at a power of 50 watts. At frequencies below 20 kHz and powers below 50 W, distortion is not noticeable.
In a real music signal, such powers at such high frequencies usually do not exist, so I did not notice any differences in the sound (by ear) of an amplifier with GT806 transistors and P210 transistors.
However, with transistors like GT806, if you look at it with an oscilloscope, the amplifier still works better.
With an 8 Ohm load in this amplifier, it is also possible to use output transistors P216...P217, and even P213...P215. In the latter case, the amplifier supply voltage will need to be reduced to ±23V. The output power will, of course, also drop.
Increasing the power supply leads to an increase in output power, and I think that the amplifier circuit in the second option has such potential (reserve), however, I did not tempt fate with experiments.
The following radiators are required for this amplifier - for output transistors with a dissipation area of at least 300 cm2, for pre-output P605 - at least 30 cm2, and even for GT402, GT404 (with a load resistance of 4 Ohms) are also needed.
For transistors GT402-404, you can do it easier;
Take copper wire (without insulation) with a diameter of 0.5-0.8, wind the wire turn to turn on a round mandrel (4-6 mm in diameter), bend the resulting winding into a ring (with an internal diameter less than the diameter of the transistor body), connect the ends by soldering and put the resulting “donut” on the transistor body.
It will be more efficient to wind the wire not on a round, but on a rectangular mandrel, since this increases the area of contact of the wire with the transistor body and, accordingly, increases the efficiency of heat removal.
Also, to increase the efficiency of heat removal for the entire amplifier, you can reduce the area of the radiators and use a 12V cooler from the computer for cooling, powering it with a voltage of 7...8V.
Transistors P605 can be replaced with P601...P609.
The setup of the second amplifier is similar to that described for the first circuit.
A few words about acoustic systems. It is clear that to obtain good sound they must have the appropriate power. It is also advisable, using a sound generator, to go through the entire frequency range at different powers. The sound should be clear, without wheezing or rattling. Especially, as my experience has shown, this is especially true for the high-frequency speakers of speakers like S-90.
If anyone has any questions about the design and assembly of amplifiers, ask, I will try to answer if possible.
Good luck to all of you in your creativity and all the best!
