I recently got a used RB985 on the cheap and trying to make it perform. I have an issue that I have not come across before and am hoping for some ideas or pointers what might be going on.
The odd thing is that the noise changes with what signal cable I use and with the cable disconnected it's "silent".
Adding a pic of how it looks on REW and a Focusrite G3. Red is the signal cable disconnected and green with it connected to the preamp muted and I have obviously tested with other amps, in this case the Rotel RB 980 and with that there is no difference with the cable disconnected or not.
The odd thing is that the noise changes with what signal cable I use and with the cable disconnected it's "silent".
Adding a pic of how it looks on REW and a Focusrite G3. Red is the signal cable disconnected and green with it connected to the preamp muted and I have obviously tested with other amps, in this case the Rotel RB 980 and with that there is no difference with the cable disconnected or not.
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Would you describe noise? Harmonic-rich buzz? Which channel/all channels? Grounding problem?
Good luck!
Good luck!
All channels and a buzz, as the the FFT shows it seems to be rich mains harmonics.
Thanks for the link but I have that schematic already and its not fully accurate so if anyone has another version it would also be very helpful.
Thanks for the link but I have that schematic already and its not fully accurate so if anyone has another version it would also be very helpful.
If you have cable connected at amp but floating at the source end, is hum still present? What if you short shield to center conductor, but unconnected to source? If you grasp cable, does hum increase?
i.e. what madis64 just said! 😀
i.e. what madis64 just said! 😀
I've used the same preamp for many amps without an issue but since you asked I tried a Topping D70 DAC and its the same
If I understand correctly, seems like problem is within the PA. If you install a shorting plug at amp input (i.e. no cable), then problem still present? That is, no other equipment other than speakers?
The power amp now has a very solid (6mm2) star ground with a 2.2ohm and 0.1 uF in parallel connected to the casing. My goto has been how it was grounded and I have reworked it getting the noise much lower but it seems I cant get it lower with any additional ground changes I can come up with.
The preamp has performed flawlessly in this respect, so far supporting maybe 20 amps and as I said it's the same connected directly to a Topping D70 DAC.
No and I just now tested a direct connection between the chassis but it did not change anything.
But with the questions I decided to disconnected the left input while measuring the "still" connected right input and the right output was clean which indeed indicates a ground loop type of issue?
Happy to provide photos but the signal input layout is very straight forward now, shielded cable from the connector directly to the pcb and the pcb layout is "std" rotel as compared to the RB 980 and many others. There is one power ground point for each the two left and right channels with a solid connection to the star ground and center tap on the trafo.
That said there is obviously a problem and maybe it will help adding a small resistance (2 ohm'ish) between signal and power ground?