Just want to double check my ill/ tired brain has these right:
Used a wheatstone bridge this evening to match up the 2.2k resistors that need to be within 0.4%. Am I reading the schematic right that they're the ground reference/ virtual centre tap for the secondary side of the transformer? Presumably it's easier to get an accurate centre tap this way than by putting a centre tap inside the secondary itself coming out with a 5th wire from the transformer?
So... four 2.2k 1% resistors go in the Wheatstone bridge "diamond". I had 20, so that means 17 to substitute. I record the mV difference across the two parallel pairs as I go. The two that give the closest value difference to each other are best matched.
The thing I don't understand is, the instructions I've found then say to make those matched resistors side A, and start subbing two different resistors into side B to find the value closest to 0mV, and then THOSE, on side B, are the match. But haven't you already found the match to populate side A? Why bother with the last step if you only need to find 2 matched resistors for the circuit? By the end of the instructions you've got two matched pairs - side A and side B, even if they differ from each other?
As it stands, I'm making two mics so I did want to find two pairs;
I put 4 random resistors in, measured the mV difference, cycled the other 16 resistors through that position recording the mV difference for each, popped the results into a spreadsheet to find the two nearest values (7.31 & 7.53 mV), those two resistors became my first matched pair, used the spreadsheet to find the *next* two nearest values (1.57 & 1.8mV), and those became the second pair. Double checked by putting the two matched pairs into the Wheatstone bridge, and the Multimeter read 0.00mV. So... Am I done? Have I made a terrible mistake?
I did a sanity check of just measuring the resistance of my pairs & got 2.204k for the first pair, and 2.199k for the second, so it seems like they're matched closer than 0.4%...