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Chip Amps Amplifiers based on integrated circuits

hum with RCA
hum with RCA
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Old 26th October 2017, 11:14 AM   #1
arifs is offline arifs  Pakistan
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Default hum with RCA (SOLVED)

Hi everyone,
UPDATE: Solution found see this post (post 17)
hum with RCA






I need some expert help as I am stuck and cannot find my answers on google. My setup is a 6 channel TDA7294 amps (2 per board = 3 boards),

Problem is that whenever I connect RCA to any device the hum starts coming from speakers. (not very loud but can be heard from 1-2 ft )

1. RCA is isolated from chasis,
2. we dont have earth connections here (yup)
3. even when I connect RCA to a portable mp3, hum starts coming (makes me wonder, how can loop be happening here)
4. I have tried connecting all input grounds to star ground , individual to their respective boards, but no use, as any external device is connected , hum is present)







Any advise please. Thankyou
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File Type: jpg amplifier ground loop.jpg (225.5 KB, 157 views)

Last edited by arifs; 18th July 2018 at 09:43 AM. Reason: solved
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Old 26th October 2017, 12:34 PM   #2
ctrlx is offline ctrlx  United Kingdom
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try wiring with 1 "star" only
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Old 26th October 2017, 12:59 PM   #3
AndrewT is offline AndrewT  Scotland
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simplify first.

Work with one PSU and one amplifier and find if it behaves first with shorted input and then with an interconnect that is shorted at the far end and finally with an interconnect that is fitted to a source that is powered OFF and then powered ON.

Once you know how the simple mono behaves, then progress to adding on one stage at a time to find when the new problem occurs. Measure the Hum+Noise and output offset at each stage of testing so that you (and we) can compare results.
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Old 26th October 2017, 01:22 PM   #4
arifs is offline arifs  Pakistan
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tried one star too. no difference.
single amp = less noise (audible 4-5 inch from speaker).

trying again though to further confirm.
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Old 26th October 2017, 01:34 PM   #5
arifs is offline arifs  Pakistan
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just to add, the amps are dead silent when I disconnect input cables. Problems start when inputs are connected, as more channels are connected, more noise appears
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Old 26th October 2017, 02:56 PM   #6
arifs is offline arifs  Pakistan
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Thanks you for responses, I have tried, as suggested, here are the results:

Amplifier board 1 / PSU 1 : Almost no sound only hearable from 2 inch from speaker, dc voltage on output = 5mv

Amplifier board 2 / PSU 1 : much more noise than single amp

Amplifier board 2 / PSU 2 : same as above


I am not sure about the red circles, kindly advise.
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File Type: jpg amplifier ground loop-01.jpg (268.4 KB, 137 views)
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Old 26th October 2017, 03:25 PM   #7
AndrewT is offline AndrewT  Scotland
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the speaker return must not go back to the PSU charging node. Doing this will bring in avoidable hum and buzz.

What is the Hum+Noise measurement using the 199.9mVac scale of your DMM?
Do post6 tests a, b, or c, measure 0.1mVac, or 0.0mVac?

Each input to the 2channel amplifier should be two pairs of signal flow and signal return.
Adopting commoned Signal Returns will complicate your arrangement.
That's why I need to see your measurements for a single channel, then for two channels, so that we can identify the source of the hum due to an error in the wiring.
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Last edited by AndrewT; 26th October 2017 at 03:28 PM.
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Old 26th October 2017, 04:44 PM   #8
arifs is offline arifs  Pakistan
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Thanks Andrew,

Its late night here, I will double check and post results in morning.
I have this dmm (attached) I should be measuring with ac setting right?, this dmm has only option to measure 750v and 200 Vac
Thanks again.
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File Type: jpg dt830b1_1_.jpg (144.1 KB, 131 views)
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Old 26th October 2017, 04:51 PM   #9
scottjoplin is offline scottjoplin  Wales
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Can you show a photo or schematic of the boards? Speaker return should go back to the amplifier. Your DMM is of no use for measuring low ac voltage I'm afraid
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Old 26th October 2017, 10:19 PM   #10
Travis Clarke is offline Travis Clarke  United States
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"1. RCA is isolated from chasis,
3. even when I connect RCA to a portable mp3, hum starts coming (makes me wonder, how can loop be happening here)
"

Hi, I ran across your thread. From personal experience with a somewhat similar setup, including an isolated AC-DC suppy, I have seen this same behavior.

I've seen the same behavior with my Android device connected to the RCA cable to my amplifier. In my case, the cable seems to pick up radiated noise and the cable acts as an antenna, just as you're seeing.
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