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Old 19th October 2014, 04:37 AM   #1
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Default radio through the amp

I just put together one channel of audiosector 3875 chip amp, but when I turned it on, I could hear the amp picking up a radio station. Does anyone know how I can fix this? Many thanks in advance.
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Old 19th October 2014, 05:33 AM   #2
Vasquo is offline Vasquo  United States
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Do you have schematic?
Is it in a metal case? or plastic case?
Is the metal case grounded?
Is the PCB design good? <known to be noise free>?
Is your soldering job clean?
Cleaned the board? from flux and junk like that?
Cut the leads on the underside of the PCB close to the board? And not sticking out too high like little antennas?
Is your cable good? shielded?
Is your internal wiring clean? Critical areas like input wires twisted?

You can try putting a 47pf, or a 100pf capacitor across the input line to ground for starters.
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Old 19th October 2014, 06:41 AM   #3
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Thank you, Vasquo. It is in a metal box, which as of yet is not grounded. I should add that there is also a humming sound that accompanies the radio and both sounds disappear completely when I disconnect the RCA at the input.
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Old 19th October 2014, 04:22 PM   #4
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Here is some additional information that might be relevant to the problem.

1. There is hum and radio pickup whenever the input is connected to a source, regardless of whether the source is turned on or off. But when I disconnect the RCA cable from the source, the amp is completely quite.

2. The power supply is in another chassis, connected with a two feet umbilical cord that carries AC signal to the rectifier board (which is connected to the amplifier board). The rectifier board does not have the optional 10uf caps and the amplifier board is not grounded to the chassis.
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Old 19th October 2014, 10:44 PM   #5
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most probably a grounding issue/loose connection issue.
post schematics and actual build.
otherwise there is no way on eearth anyone could help.
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Old 20th October 2014, 12:27 AM   #6
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If you can hear the information, (Music, Voice) it's an AM radio station. Frequency is between .5 and 1.7 mHz. unless it's some shortwave station somewhere close..

Find some ferrite ring chokes that will attenuate the above frequencies and wrap each of your audio cables through them 4-5 times near the amp. That should help with the R.F. issue..

The hum could be a ground loop issue. Install a ground loop isolator between the source and the amp..
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Old 20th October 2014, 01:00 AM   #7
rayma is offline rayma  United States
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Quote:
Originally Posted by mozarti View Post
1. There is hum and radio pickup whenever the input is connected to a source, regardless of whether the source is turned on or off. But when I disconnect the RCA cable from the source, the amp is completely quite.
If the input jack is isolated from the metal box (it should be), connect a 0.01 uF ceramic capacitor from the input jack common to the box,
using very short leads to a ground lug. Scrape off any paint or other finish to mount the lug.
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Old 20th October 2014, 02:27 PM   #8
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Quote:
Originally Posted by rayma View Post
If the input jack is isolated from the metal box (it should be), connect a 0.01 uF ceramic capacitor from the input jack common to the box,
using very short leads to a ground lug. Scrape off any paint or other finish to mount the lug.
Well NO, the input jack must be connected to the chassis!

Jim Brown, who was the past AES committee chair on EMI/RFI and a Ham radio operator, has many papers on EMI/RFI and audio equipment.

Audio Systems Group, Inc. Publications

In a new PowerPoint, this problem is covered, starting at slide #28.

"Power, Grounding, Bonding, and Audio For Ham Radio"
http://www.audiosystemsgroup.com/GroundingAndAudio.pdf
__________________
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Old 20th October 2014, 02:58 PM   #9
rayma is offline rayma  United States
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Quote:
Originally Posted by Speedskater View Post
Well NO, the input jack must be connected to the chassis!
In audio, the input RCA sockets are universally isolated from the chassis. Otherwise will cause many problems, depending on the installation.
The shell of XLR conectors is grounded at the input, but this is only a shield and does not carry signal current.
There are conflicting requirements in audio and RF equipment, but grounding the input RCA sockets directly to the chassis in audio equipment
is not good practice.

Last edited by rayma; 20th October 2014 at 03:08 PM.
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Old 20th October 2014, 07:45 PM   #10
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Default thank you

all for your help. The problem is solved. I put a lundahl 1 to 1 transformer between the preamp and amp. Both the hum (which was actually more like an annoying buzz) and the radio interference disappeared. Offgridkindofguy you were right. Incidentally, before I did this I changed the loudspeakers, moving from 4 ohm speaker to 8 ohm one and this led to a substantial reduction of both the buzz and the radio.
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