My uncle’s quad amplifier redone

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.

Top view of the amplifier (The pictures show the ‘refurbished’ state.)

Functional blocks of the amplifier.
Functional blocks of the amplifier.

Power path:
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)

Audio path:
a. input RCA connectors
b. volume control
c. amplifier boards
d. speaker protection (two panels under the relay panel)
e. 6,3 Jack output connectors

Front view.
Front view.
Back view with connectors.
Back view with connectors.





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 schematic diagram
The schematic diagram

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. 🙂

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