I have started electronics because of a book I have received from my uncle as a birthday present many years ago. At first I was interested in audio amplifiers the most, as I liked to listen to ‘noise’ – even back then.
This amplifier came to me recently – after I have acquired a fairly good amount of knowledge in amps – with the hope that I would be able to turn it into a working one.
1. 230V socket
2. power switch
3. fuses (T2A)
4a. soft-start resistors
4b. soft-start circuit with relay (a couple resistors and a capacitor – very easy)
5. ~ 300W toroid power transformer in a steel housing to shield the audio circuit
6. diode bridge
7. secondary capacitors and fuses (T3,15A)
The main problem with this amplifier was – apart from the obvious lack of space – that it’s left side only produced a strange noise regardless of the input signal. After hours of search and replacing all suspected components I have found that one of the small signal transistors in the class A output stage was the culprit.
The circuit is pretty interesting. It starts with an opamp (IC1) input stage, then a class A output stage (T1-T4 + T7) that amplifies the signal for good quality low power use (it also removes the crossover distortion of the following stage) and also feeds the class B output stage (T8 + T9-T10) for higher power output.
The modifications include:
– exchanging the general TL071 opamp for a precision device ti OPA 604
– exchanging the small signal transistors for MPSA42/92 types
– exchanging the driver transistors (T7-T8) for MJE15030/31 types
– exchanging the output transistors for MJL21193/94 types
– exchanging the R7-R8-D1 R9-R10-D2 15V stabilizer networks for one 2.2 kOhm resistor and a 2 W zener diode each
– exchanging the R30-R31 class A output resistors for 5W types, because they tend to get very hot
– exchanging old capacitors
– exchanging old cables
– insulating components from the chassis
Apart from the above there was a wish to include speaker protection for both channels and a soft start circuit.
I have also raised the current on safe operating circuits (around T5-T6) so that the output transistors may deliver up to 5A instead of the previous 3A.
All this means that the peak power is raised from 100-150 to 250 W – something that the transformer can’t handle for both amp panels simultaneously.
Unfortunately I don’t have an adequate testing equipment right now to test it thoroughly, but it sounds great with my bass speakers. 🙂