Anyway, I've used this little bench amp on and off for over 30 years, most recently with circuits connected to an Arduino. I used it to listen to the output of a SID chip that I wired to an Arduino, for instance. But recently, I've begun to notice problems with it. For one, it's in a wooden box and picks up DECT and GSM phone interference rather badly. Another snag is its use of PP9 batteries, a now-obsolete 9V type (it needs two of them to run on 18V). The most recent circuit I wired it up to, an MCP4822 dual 12-bit DAC chip, revealed another limitation: it has very poor low-frequency response.
Time to make some measurements! There's no point acquiring all this fine HP test gear from eBay unless I'm going to wire it all up to a circuit now and then. So, we have the HP 3310A Function Generator to supply an input signal and the HP 427A Voltmeter to measure the AC voltage at the loudspeaker. The 427A runs on another obsolete battery, a 22.5V type. The previous owner has installed two 4mm sockets on the back of the instrument for an external power supply, so I hooked that up to a DC bench PSU that I made in the mid-1970s. The bench amp that I'm testing is running off my more recent (1990s) Farnell TOPS 4D power supply. There's an HP 3456A, a 3466A and a 3476B nearby, as well as a 1980B scope and a 1630G logic analyser. If I'm in any doubt about frequencies, there's an HP 5326A Timer-Counter with Nixie tubes, no less.
Looking at the circuit, I was suspicious of the 10nF coupling capacitor between the 741 op-amp input stage and the 100kOhm volume control. Seems way too small. So, I worked out the RC time constant, and that was 1ms, which led me to expect a fall-off in response below about 1kHz. Wait a minute -- below 1kHz? Surely not! Middle 'C' is 262Hz, and the 'A' above that is 440Hz; an amplifier should have a level frequency response well below that. Something's wrong with that 10nF capacitor! And that was confirmed by the testing: frequency response falls off dramatically below about 1kHz.
To fix the problem, I increased all the coupling capacitors in the amplifier. The main problem was with the 10nF (C3), which I increased to 4.7uF. The first capacitor that came to hand in the little drawer of recycled caps was a 2200uF, so that went in place of the LM380 output capacitor (previously 470uF). A 47uF axail electrolytic went in place of the original 4.7uF input coupling capacitor. With the HP 3310A as an input again, the output signal retained a constant ampltude down to about 25Hz. The small speaker had no chance of reproducing such a low frequency, but at least the signal was there.
To fix the other problems, I glued aluminium foil to the inside of the wooden case, and earthed it. I used a small spring to make contact with the foil on the inside of the lid of the box. As for the PP9 batteries, I simply wired up an 18V mains adaptor "brick" -- another item that would have been hard to find in 1977. There's room in the box for a stack of 12 AA cells if I really need to use the amplifier on batteries one day.