17-01-2021, 03:49 PM
Yes, I wondered about that supply. I don't think the 303 or 405 has anything connected to pin 4, but I'm no Quad expert. An obvious use would be to remotely turn on a power amplifier - that's quite common today using 12V triggers. Much nicer than switching the mains with the pre-amp's power switch, but wasn't common practice back then. It's strange that the manual makes no mention of it outside of the schematic.
Have you measured the voltage yet? It would be lovely if it was enough for the NE5532, but I'm not optimistic as the caps are only rated at 12V. Really need 10V, but there are plenty of good op-amps about that will work at lower voltages, but would need to check how far they can swing relative to what you have and what you need. As you suggest, coming from C502 might be better, but what a shame as you're so close to a no-mod solution.
Of course, going fully-balanced gives you 6dB extra swing for a given power supply. But the current draw of the extra op-amp section will drop the rail further. The NE5532 is quite thirsty (8mA), but the TL072 draws less than 4mA. The TL062 only draws 0.4mA, but it has higher distortion as a result. Though probably less than the 33 does, and if it allows you to make better use of the supply coming out on pin 4 as standard, I'd definitely consider it. They are extensively used in DI boxes because of their low consumption (DI boxes are usually powered by phantom). You occasionally find them in some quite expensive microphones (e.g. AKG C460). Oh, and despite the datasheet recommending they be powered from a minimum of 10V total, I've happily ran them from 9V batteries - they work decently at 6 or 7V in my experience. All commercial DI boxes I've seen that are based around TL06x op-amps use a PP3 for when phantom isn't available.
It might be safest to just have a captive lead on the box with the 4 pin DIN on the end.
Have you measured the voltage yet? It would be lovely if it was enough for the NE5532, but I'm not optimistic as the caps are only rated at 12V. Really need 10V, but there are plenty of good op-amps about that will work at lower voltages, but would need to check how far they can swing relative to what you have and what you need. As you suggest, coming from C502 might be better, but what a shame as you're so close to a no-mod solution.
Of course, going fully-balanced gives you 6dB extra swing for a given power supply. But the current draw of the extra op-amp section will drop the rail further. The NE5532 is quite thirsty (8mA), but the TL072 draws less than 4mA. The TL062 only draws 0.4mA, but it has higher distortion as a result. Though probably less than the 33 does, and if it allows you to make better use of the supply coming out on pin 4 as standard, I'd definitely consider it. They are extensively used in DI boxes because of their low consumption (DI boxes are usually powered by phantom). You occasionally find them in some quite expensive microphones (e.g. AKG C460). Oh, and despite the datasheet recommending they be powered from a minimum of 10V total, I've happily ran them from 9V batteries - they work decently at 6 or 7V in my experience. All commercial DI boxes I've seen that are based around TL06x op-amps use a PP3 for when phantom isn't available.
It might be safest to just have a captive lead on the box with the 4 pin DIN on the end.
