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FM radio interference on DAC + headphone amp stack

bsdinis

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I am using a Topping D10 Balanced with a Sabaj A20h and I just found something really weird, I am picking up FM radio interference but only on 3 separate volume spectrums, between -99dB and -85dB, between -3dB and -1dB, and at 0dB.

Between -99dB and -85dB is the strongest, I can no longer hear it when I increase the volume to -84dB.
Between -3dB and -1dB it's still there but not as noticeable due to the higher noise floor, I can no longer hear it when I decrease the volume to -4dB.
At 0dB it's a bit lower, but still there.

I am using the single-ended output and the FM radio interference can only be heard on the Left channel.
It doesn't matter which input is selected on the amp.
It doesn't matter which gain setting is used, although it is the only thing that seems to have any effect on the volume of the FM radio interference.

I am using very short 30cm XLR>6.35mm cables without any adapters, the interference only goes away when I unplug both cables even though it can only be heard on the Left channel.
There is nothing connected to the amp's single-ended input.

Touching the Topping's chassis or its USB cable or moving the stack seem to alleviate the problem, but what is the explanation for this phenomenon and what can be done to mitigate it? And why exactly only between -99dB~-85dB, between -3dB~-1dB and at 0dB? This amp uses a stepped resistor ladder controlled by relays.

The radio station that I'm hearing is transmiting on 103.40MHz or 105.00MHz, not sure which one, or maybe on both.
 

Dunring

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I have the exact same setup and found the power brick has a polarity to the plug, though it's not mentioned in the manual or anywhere else. I have two, and the first I plugged in wrong and it sounded weird, not defective, but something was wrong. So I just by chance plugged the second one in the right way and it sounded great. I then checked both and saw the plug that goes into the brick was opposite, and flipping it around fixed it. Here's a photo of how it should be. Worth checking for a quick fix if that's all it is.

PXL_20230222_203602774.jpg
 
OP
bsdinis

bsdinis

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I have the exact same setup and found the power brick has a polarity to the plug, though it's not mentioned in the manual or anywhere else. I have two, and the first I plugged in wrong and it sounded weird, not defective, but something was wrong. So I just by chance plugged the second one in the right way and it sounded great. I then checked both and saw the plug that goes into the brick was opposite, and flipping it around fixed it. Here's a photo of how it should be. Worth checking for a quick fix if that's all it is.

View attachment 266685
If that's the case, flipping the plug around on the wall would have the same effect as flipping it on the PSU side. I can give it a try in a moment.
 
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Blumlein 88

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The designer at Topping says not to use the D10 balanced single ended. You can do it with most balanced output devices, but not the D10B. My guess is this has something to do with your noise pickup. Swapping the PSU might help, but the quality of the output signal is significantly degraded by single ended use. Someone did some basic tests into an RCA jack and distortion and noise were 40 db worse.

index.php
 
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OP
bsdinis

bsdinis

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The designer at Topping says not to use the D10 balanced single ended. You can do it with most balanced output devices, but not the D10B. My guess is this has something to do with your noise pickup. Swapping the PSU might help, but the quality of the output signal is significantly degraded by single ended use. Someone did some basic tests into an RCA jack and distortion and noise were 40 db worse.

index.php
The DAC is connected to the amp's differential input..
 
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OP
bsdinis

bsdinis

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I have the exact same setup and found the power brick has a polarity to the plug, though it's not mentioned in the manual or anywhere else. I have two, and the first I plugged in wrong and it sounded weird, not defective, but something was wrong. So I just by chance plugged the second one in the right way and it sounded great. I then checked both and saw the plug that goes into the brick was opposite, and flipping it around fixed it. Here's a photo of how it should be. Worth checking for a quick fix if that's all it is.

View attachment 266685
And as I suspected, flipping the (AC) cable has absolutely no effect.

If (both of) you read the original post carefully, the interference goes away as soon as I unplug both cables from the amp's input, so the amp is not the problem, it is just amplifying the incoming radio signal.
 
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fpitas

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Clip-on ferrite beads might help. Not sure where they are readily available though. Amazon might have something.
 

fpitas

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antcollinet

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Have you tried swapping the dac to amp cables to see if the interference follows the cable - is it possible one of the cables has a broken connection leaving one of the inputs floating?
 
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bsdinis

bsdinis

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Have you tried swapping the dac to amp cables to see if the interference follows the cable - is it possible one of the cables has a broken connection leaving one of the inputs floating?
I can check them with a multimeter, but everything is brand new.
 

Blumlein 88

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A shot in the dark, since you say touching the Topping or USB cable stop this. Maybe connect the coax jack on the Topping and one RCA jack on the input of the Sabaj with an RCA cable that has the center pin disconnected. In essence just an extra ground connection between the two units. Also try a different USB cable if you have not already.
 

HarmonicTHD

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Can you return it?

(Personally I never had luck with ferrit beads and I always ended up swapping the equipment, but admittedly the context was not hifi).
 

antcollinet

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A shot in the dark, since you say touching the Topping or USB cable stop this. Maybe connect the coax jack on the Topping and one RCA jack on the input of the Sabaj with an RCA cable that has the center pin disconnected. In essence just an extra ground connection between the two units. Also try a different USB cable if you have not already.
Also worth trying grounding the case of either the dac or the amp. (not both)
 
OP
bsdinis

bsdinis

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Have you tried swapping the dac to amp cables to see if the interference follows the cable - is it possible one of the cables has a broken connection leaving one of the inputs floating?
The cables are fine, I also confirmed they are truly balanced with each pin going to a separate pin on the other side of the cable, none of the contacts are paralleled.
 
OP
bsdinis

bsdinis

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Also, just having any of the cables plugged into the amp's input makes the interference stronger, doesn't matter if I plug a cable into the right or into the left input.
I can still hear the radio station without the cables, but it's much stronger when I plug them in.
It's much much easier to hear it with my IEMs than with my over-the-ear headphones.
image.png
 
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pma

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Also, just having any of the cables plugged into the amp's input makes the interference stronger, doesn't matter if I plug a cable into the right or into the left input.
I can still hear the radio station without the cables, but it's much stronger when I plug them in.
You know, EMI immunity is almost last thing the producers care about and they do not make their products bullet-proof in terms of connection into complete audio chain. And it is not tested here.

A small hint - what happens if you place Topping DAC and the amplifier separately, side by side? (not the DAC on top of the amp)
What happens if you change the placement or wall outlet?
 
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bsdinis

bsdinis

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You know, EMI immunity is almost last thing the producers care about and they do not make their products bullet-proof in terms of connection into complete audio chain. And it is not tested here.

A small hint - what happens if you place Topping DAC and the amplifier separately, side by side? (not the DAC on top of the amp)
What happens if you change the placement or wall outlet?
The interference was even stronger when I moved the Topping away from the amp with the cables still dangling on the amp's input, especially when I held my hand close to them. The placement of the power supply has no effect.
 
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