Buzz / Hum with Adcom 555 and SDA 2 speakers (fixed)
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Posts: 1
I'm posting this so that anyone with the same problem can find this solution via google or the forum search to prevent one of these: https://xkcd.com/979/
I recently purchased a used Adcom 555 and used Polk SDA 2 (version 1), and upon hooking them up found they had moderately annoying 60/120hz hum. BTW if you happen to have a piano, the B one octave and one note below middle C is right around 120 Hz and the B below that is right around 60 Hz if you need reference. This wasn't LOUD, but was annoying at proper listening distances, and the higher harmonics made a nasty "crt tv is on in the room" kind of hiss. This buzz was only present when an input device was plugged in. I already had an older somewhat decrepit Sony receiver that was not rated to 4 ohms that also buzzed with these speakers (this was a red herring that made me suspect my house wiring).
I checked polarity of my outlet with multimeter, all good. No dimmers, flourescent lights, or other weird electronics on this circuit.
I did not yet have a preamp, so I was temporarily plugging in an ipad to do initial tests. One of my first suspicions was that I had a bad cable that was causing a ground loop, but I tried several cables to the ipad with the same results. Then I suspected the ipad not being grounded to the circuit the amp was on. Also plugging in directly to a mixer plugged into the same outlet via RCA caused the same buzz.
Early on I noticed that unplugging the SDA interconnect cable dramatically lessened the hum, but it was still present. In hindsight this was a big clue.
Looking at the SDA handbook, I ran through the checklist for polarity of speakers and measured all resistance ratings to find my speakers were within specs for all measurements (page 32 ofthe SDA handbook) : http://vr3mods.com/LCSDAUpgrade.php This wasn't helpful to solving the problem, but was good to do for my general edification.
Checking inputs, I made shorted RCA connectors. When no inputs were plugged into the amp there was no buzz. With both shorted RCA connectors connected to inputs, there was no buzz. When I tied those shorted input cables together there was a buzz (!).
In the end I found that someone had reinstalled one of the RCA connectors incorrectly, forgetting one of the little nonconductive spacers that isolates the connector from the chassis. My left channel RCA connector was grounded to the chassis directly and via the input circuit board, but my right channel RCA connector was tied to ground via the input circuit board. Isolating the left input RCA from the chassis solved my problem.
I recently purchased a used Adcom 555 and used Polk SDA 2 (version 1), and upon hooking them up found they had moderately annoying 60/120hz hum. BTW if you happen to have a piano, the B one octave and one note below middle C is right around 120 Hz and the B below that is right around 60 Hz if you need reference. This wasn't LOUD, but was annoying at proper listening distances, and the higher harmonics made a nasty "crt tv is on in the room" kind of hiss. This buzz was only present when an input device was plugged in. I already had an older somewhat decrepit Sony receiver that was not rated to 4 ohms that also buzzed with these speakers (this was a red herring that made me suspect my house wiring).
I checked polarity of my outlet with multimeter, all good. No dimmers, flourescent lights, or other weird electronics on this circuit.
I did not yet have a preamp, so I was temporarily plugging in an ipad to do initial tests. One of my first suspicions was that I had a bad cable that was causing a ground loop, but I tried several cables to the ipad with the same results. Then I suspected the ipad not being grounded to the circuit the amp was on. Also plugging in directly to a mixer plugged into the same outlet via RCA caused the same buzz.
Early on I noticed that unplugging the SDA interconnect cable dramatically lessened the hum, but it was still present. In hindsight this was a big clue.
Looking at the SDA handbook, I ran through the checklist for polarity of speakers and measured all resistance ratings to find my speakers were within specs for all measurements (page 32 ofthe SDA handbook) : http://vr3mods.com/LCSDAUpgrade.php This wasn't helpful to solving the problem, but was good to do for my general edification.
Checking inputs, I made shorted RCA connectors. When no inputs were plugged into the amp there was no buzz. With both shorted RCA connectors connected to inputs, there was no buzz. When I tied those shorted input cables together there was a buzz (!).
In the end I found that someone had reinstalled one of the RCA connectors incorrectly, forgetting one of the little nonconductive spacers that isolates the connector from the chassis. My left channel RCA connector was grounded to the chassis directly and via the input circuit board, but my right channel RCA connector was tied to ground via the input circuit board. Isolating the left input RCA from the chassis solved my problem.