DIY car amplifier conversion
Typically, in such amplifiers, power field-effect transistors in the power supply (converter) or powerful transistors in the final stages of the power amplifier burn out, which are not difficult to replace.These transistors can be checked for functionality using a conventional tester, measuring the resistance between the “legs”-terminals - for “broken” elements these resistances will be almost zero for any combination and polarity of the measuring probes. If replacing powerful transistors does not restore the functionality of the amplifier, then the reason is deeper, and determining the serviceability of microcircuits and other small elements, especially in SMD cases without desoldering, is quite problematic. In this case, it is much easier and faster to assemble a new unit to replace the faulty one. For example, if the amplifier does not work, but its power supply converter produces normal operating voltages, then you can remove from the board all the parts related to the amplification stages and install a new circuit, assembled yourself, in the free space on the board. In principle, it will be enough to remove the powerful transistors of the output and pre-final stages, and the entire low-current part (pre-amplifiers) can be left on the board. They do not take up much space and will not interfere in any way. All power paths going to this part from the converter should, of course, be found and carefully cut to avoid possible short circuits when installing additional boards.
This way we will only have a working power supply converter and enough free space to accommodate the new circuit.
Here is an example of such a repair-conversion of a two-channel amplifier into a single-channel one, for a subwoofer:
The picture shows the remaining “native” part - a voltage converter and an added homemade circuit - an adder and filter block and a final power amplifier. Below are schematic diagrams of the new, added “part”.
Power amplifier circuit
It is assembled according to a fairly simple scheme, which provides quite decent characteristics. Depending on the terminal transistors used and the supply voltage, such an UMZCH can produce up to 200 watts into a 4 Ohm load:
If the supply voltage of the converter of your amplifier is not +/-32 volts, but less (for example +/-24 volts), then the output power of the amplifier will be less. This situation can only be corrected by replacing the pulse power transformer in the converter (or rewinding its secondary winding to a greater number of turns) and replacing the filter electrolyte capacitors also with a higher voltage. At a voltage of 32 volts, the output power is about 150 watts. At lower voltages, for example 24 volts, the values of resistors R10R11 should be reduced to 910 Ohms; no other circuit changes will be required. Operational amplifier OP1 can be used like LM2904, LM324N, BA4558N, TL062 (072, 082) or any other similar, single or dual (the diagram in parentheses shows the numbering of the second channel in the case of a dual amplifier). For all the microcircuits listed above, the pinout is the same; when using other analogues, you should pay attention to the pinout (!).
Zener diodes VD1VD2 - any, with a stabilization voltage of 15 volts (typical power supply value for most op-amp chips). Transistors T1T2 of the pre-terminal stage of type KT815G (817G) and KT814G (816G), respectively, or any of their foreign analogues. These transistors need to be installed on small heat sinks. The circuit is not critical to the parts used and the transistors do not require special selection according to parameters. It is better to install more powerful output transistors T3T4, for example, types 2SA1943 and 2SC5200.They are attached to the body (which acts as a heat sink) through electrical insulating gaskets made of mica or a special heat-conducting material. All resistors have a power of 0.25 watts or more, except for R9 - it will get very hot at high powers and it is better to set it with a power of at least 2 watts. Capacitors - any type, with an operating voltage no less than the supply voltage, and preferably 50-63 volts. When setting up, you should select the resistance values of R6R7 so that in the “rest” mode and with the speaker turned off, there is a constant voltage of about 0.4-0.6 volts at the bases of transistors T1 and T2. Capacitors C4, C5C6 and C7 are responsible for the stability of the circuit to self-excitation at HF and are selected in the event of such excitations. With the correct layout of the printed circuit board tracks, as a rule, no excitations are observed. Resistor R1 sets the feedback depth and determines the overall gain of the amplifier.
It is undesirable to greatly overestimate its rating, as this can also lead to instability of the amplifier. The diagram shows its optimal value.
Channel combiner and adjustable filter block
This block is also assembled according to a rather simple “classical scheme”:
The circuit has a regular linear input (Line In) and a high level input (Hi In). The high-level input is designed to connect the entire amplifier directly to speakers operating, for example, from another amplifier and are used if there are no linear outputs in the car radio. If such an input is not intended to be used, then elements C3C4R3R4R5R6 can be excluded from the circuit. A 100 kOhm variable resistor regulates the gain of the cascade and is displayed on the front panel of the case as a “Level” control.It can be replaced with a nominal value from 50 to 200 kOhm and connected to the board with a shielded wire (!). A dual 33 kOhm resistor regulates the filter cutoff frequency (from 50 to 500 Hz) and can be replaced with a value from 22 to 56 kOhm. It is also displayed on the front panel of the case and is connected to the board by a wire in the screen. The operational amplifiers here can be the same as in the power amplifier and the zener diodes VD1VD2 too. With proper assembly and serviceable parts, this circuit does not require any adjustment.
As a result, we get a functional amplifier for a subwoofer with quite decent parameters and power. All the circuits used here were repeated more than once and showed high reliability, much higher than the “native” ones for this car amplifier - Chinese...
