Hello,
I can't seem to solve this problem:
Technics amplifier SA 150.
Using Aux in via a cable (RCA/3.5mm stereo) from my laptop
Laptop has mains power supply, no earth.
I get a severe computer like random noise.
Quiet when disconnected on the amp side.
Noise goes away when I run a short wire from the amplifier antenna shield ground to one of the RCA input grounds. - Solved and almost usable.
However, I now have some mains hum which goes up and down with the volume so it is from the input.
It stops when I disconnect the cable on the laptop side.
It does not stop when I disconnect the laptop power supply (run on battery).
I previously used my Marantz SR340 amp for this purpose with identical setup and there was NO computer noise and NO mains hum.
All this has been introduced by swapping amplifiers.
How can I troubleshoot and fix this this?
Best regards
Bernt
I can't seem to solve this problem:
Technics amplifier SA 150.
Using Aux in via a cable (RCA/3.5mm stereo) from my laptop
Laptop has mains power supply, no earth.
I get a severe computer like random noise.
Quiet when disconnected on the amp side.
Noise goes away when I run a short wire from the amplifier antenna shield ground to one of the RCA input grounds. - Solved and almost usable.
However, I now have some mains hum which goes up and down with the volume so it is from the input.
It stops when I disconnect the cable on the laptop side.
It does not stop when I disconnect the laptop power supply (run on battery).
I previously used my Marantz SR340 amp for this purpose with identical setup and there was NO computer noise and NO mains hum.
All this has been introduced by swapping amplifiers.
How can I troubleshoot and fix this this?
Best regards
Bernt
OK, but for what reasoning?
Do you suspect a common mode voltage problem? I did too but because the hum remains when the laptop runs of batteries, I assume that is moot.
If all else is identical, then the problem has to be caused by a difference in the amplifier and input circuit design somehow.
Input impedance perhaps?
Do you suspect a common mode voltage problem? I did too but because the hum remains when the laptop runs of batteries, I assume that is moot.
If all else is identical, then the problem has to be caused by a difference in the amplifier and input circuit design somehow.
Input impedance perhaps?
I just noticed the following:
As me mentioned previously, unplugging the laptop side stops the hum.
However, then shorting the unplugged cable 3.5mm connector ground ring to one of the channel rings, starts the same hum again.
As me mentioned previously, unplugging the laptop side stops the hum.
However, then shorting the unplugged cable 3.5mm connector ground ring to one of the channel rings, starts the same hum again.
The "computer like random noise" may be RFI blasting through the tuner and input switching. Does shorting the antenna terminals eliminate the noise?
Don't connect the antenna ground to the RCA ground. That creates a ground loop within the receiver.
The Technics is either too cheaply designed or just plain broken. I think the best option is to junk it.
Ed
Don't connect the antenna ground to the RCA ground. That creates a ground loop within the receiver.
The Technics is either too cheaply designed or just plain broken. I think the best option is to junk it.
Ed
There is definitely something different with the Technics amp.
No, the computer noise was coming through the aux input. It also stopped when unplugging the laptop power supply.
The antenna earth I mentioned before is a little screw on the back and seems to be the just chassis of the Technics amp. It has no mains earth connection.
I do not actually use the AM/FM receiver for anything. Only the ampifier part.
I am still no wiser. Cheaply designed? I do not know as I have no schematics to compare. I thought Technics was a good brand in its day.
Regards
Bernt
No, the computer noise was coming through the aux input. It also stopped when unplugging the laptop power supply.
The antenna earth I mentioned before is a little screw on the back and seems to be the just chassis of the Technics amp. It has no mains earth connection.
I do not actually use the AM/FM receiver for anything. Only the ampifier part.
I am still no wiser. Cheaply designed? I do not know as I have no schematics to compare. I thought Technics was a good brand in its day.
Regards
Bernt
Would you confirm my interpretation that you are connecting a short wire from rear chassis bonding screw to shell of RCA jack at rear panel of the amp? And this mostly solves digital noise, but leaves mains hum?
I'll offer some guess-work suggestions.
I suspect the digital noise is introduced via the AC adapter which is likely a switcher, and the noise currents probably flow via the AC mains. A line filter might help localize the switcher currents and prevent them from conducting along the AC mains. You might see improvement if you arrange an ordinary non-switcher power supply for the laptop.
Another experiment you might try is to replace the wire between chassis screw and RCA shell with a capacitor. There's some chance you may find a value that's sufficient to attenuate digital noise but with large enough reactance at 50Hz to avoid hum. Experiment with value. Try 0.1uF, larger and smaller, to see if you can find a value that is acceptable.
Yet another experiment is to incorporate a low pass filter in series with the RCA cable. Try a 1k in series, 4.7nF load cap at jack input (~ 33.kHz), and 100 ohm in the ground lead, all located near the aux input. The 100 ohm in the ground is to limit ground currents, and its value should should be experimental.
Experimentation is key. Applying both of the latter techniques may help, or may fail.
Good luck!!!!
I'll offer some guess-work suggestions.
I suspect the digital noise is introduced via the AC adapter which is likely a switcher, and the noise currents probably flow via the AC mains. A line filter might help localize the switcher currents and prevent them from conducting along the AC mains. You might see improvement if you arrange an ordinary non-switcher power supply for the laptop.
Another experiment you might try is to replace the wire between chassis screw and RCA shell with a capacitor. There's some chance you may find a value that's sufficient to attenuate digital noise but with large enough reactance at 50Hz to avoid hum. Experiment with value. Try 0.1uF, larger and smaller, to see if you can find a value that is acceptable.
Yet another experiment is to incorporate a low pass filter in series with the RCA cable. Try a 1k in series, 4.7nF load cap at jack input (~ 33.kHz), and 100 ohm in the ground lead, all located near the aux input. The 100 ohm in the ground is to limit ground currents, and its value should should be experimental.
Experimentation is key. Applying both of the latter techniques may help, or may fail.
Good luck!!!!
You might minimize the mains hum by using a 3-wire twisted cable with differential signals. Ground noise is the same in both wires and is cancelled.
Rod Elliot has much to say on this topic. https://sound-au.com/articles/balanced-io.htm
Rod Elliot has much to say on this topic. https://sound-au.com/articles/balanced-io.htm
I think that once the ebay DC speaker protection PCB arrives, I will switch back to using the Marantz amplifier. (see other thread: https://www.diyaudio.com/community/...-speaker-possible-causes.413921/#post-7729798 ).
@BSST : Yes, a short wire from amp chassis to the RC outer shell on Left side input only.
That removes the noise from the laptop power supply but then seem creates the hum problem. - which is a lot less than the noise.
@fubar3 : That is an idea but in theory, the amplifier does not have differential inputs so noise induced in the ground wire will not cancel the same noise in the input wire. I think that is why they used shielded cable to start with.
@BSST : Yes, a short wire from amp chassis to the RC outer shell on Left side input only.
That removes the noise from the laptop power supply but then seem creates the hum problem. - which is a lot less than the noise.
@fubar3 : That is an idea but in theory, the amplifier does not have differential inputs so noise induced in the ground wire will not cancel the same noise in the input wire. I think that is why they used shielded cable to start with.