I want to try out this preamp from Aliexpress but am not sure what they mean by testing the voltage at the points indicated. If the board already has a transformer built in and is delivering 18v--what is there to test for? And why would they create a break in the circuit with pads that have to be manually connected? images of the board and the schematic are here:
https://www.aliexpress.us/item/3256...gvDM28hl&utparam-url=scene:search|query_from:
https://www.aliexpress.us/item/3256...gvDM28hl&utparam-url=scene:search|query_from:
It is always a good idea to verify the power supply voltage(s) before connecting to an amplifier, so that is why the jumper is there. Note that there must be some sort of adjustment for the voltage as they note. I wouldn't bet on those 2SK170 JFETs being genuine either.
The power supply on the board is regulated with an LM723. You need to power up the supply, adjust the potentiometer for 18 volt output as measured at the test point. Then you can complete the power to the rest of the circuit by shorting the two pads.
Why would it be made in such a way that the user has to adjust it to 18v? Why not just make it so that it cannot be other than 18v?
Probably, the onboard voltage regulator design is used with different circuits with various voltage requirements, so they added an ability to adjust the voltage.
I think it's a bonus to be able to adjust the voltage. A potentiometer is certainly more expensive than a couple of resistors.
I think it's a bonus to be able to adjust the voltage. A potentiometer is certainly more expensive than a couple of resistors.