the LTA preamp and AGD monoblocks both connected to a Puritan 156, so they are both on the same AC line ... I disconnected the RCA cables between the LTA and AGD units, the hum went away through the speakers. According to ChatGPT, it’s probably a ground loop issue between those components.
If the amp and preamp are on the same line, then the origin of the ground loop is elsewhere - such as a source component connected to another AC line or ground. The preamp then sees the source ground potential through the interconnect. To troubleshoot, you can disconnect your sources one at a time to test if the hum goes away.

