I'm beginning to agree. Doug Self points this out after his tests using bog standard electros. Indeed, the original design (and subsequent kits sold by the thousands here in Australia from 1981), all used standard caps (including green caps). The test results Doug shared in his signal design book, as well as results published by David Tilbrook in 1981, all used low cost off the shelf components.
Complete schematic... less PSU. This is the Australian magazine ETI Series 5000 Control Preamplifier by David Tilbrook.
Hi Mark.
Yes, my bad, coupling. Last night I read about a test jig by Douglas Self where he tested values up to 1,000uF - the higher the value, the lower the distortion figures. He recommends 470uF (1,000uF improving, but the curve improvement is negligible at LF).
Quoted from the chapter 'Components' in 'Small signal audio design' by Douglas Self:
Replacing (coupling capacitor) with a 100uF 25V capacitor drops the distortion at 20Hz from 0.0080% to 0.0017%, an improvement of 4.7 times; the voltage across the capacitor at 20Hz has been reduced from 1.66V rms to 790mV rms. A 220uF 25V capacitor reduces the voltage across itself to 360mV and gives another very welcome reduction to 0.0005% at 20Hz, but it is necessary to go to 1,000uF 25 V to obtain (virtually flat) trace, which is indistinguishable from the noise floor of the test system. The voltage across the capacitor at 20Hz is now only 80mV. From this data it appears that the AC voltage across an electrolytic capacitor should be limited to below 80mV rms if you want to avoid distortion. I would emphasise that these are ordinary 85°C rated electrolytic capacitors and in no sense special or premium types.
Note that Doug was testing at several volts RMS into the capacitor. The residual 80mV he cites is much higher than most input signals at everyday listening levels.
