Op-amp differences: NE5532P and NE5532AP.
The part specified by Piet in his BOM
http://www.echotapper.nl/wp/?p=151 is the AP version but in my kit from Newtone I was supplied with the P versions. The P type is a little cheaper but, other than that, the difference, according the Texas Instruments data sheet, is that the 'A' variants have guaranteed Maximum Input noise voltage limits.
The typical value for both types is 5uV at 1kHz but the A type is guaranteed not to exceed 6uV. There is no max figure quoted for the other variant.
Could I have been unlucky and got a noisy pair of NE5532P's ...?
I also found this recent link on the TI forum where someone is complaining about noise issues even with the NE5532AP.
http://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/t/344.aspxI will try swapping my NE5532's with a pair from my mates kit and see if it makes any difference though there could be a problem with this batch.
This is still an itch I can't quite reach!