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I measured voltages of 3 different headphone amps using test tones. Topping L30 is acting strange with 60hz tone. OK or not?

Pdxwayne

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I have the following on hand to compare:
Topping L30
Sabaj A10h
SMSL HO200

Before I did any measurements, I did listening comparisons first. With my AKG371, my impression was that, often L30 sounded slightly "thin" in bass (like low bass guitar tones in music) as compared to the other amps. But, sometimes it sounded strong and not thin. It just confused me. What happened to my hearing?

The song I used for comparison is this:
Bass guitar starts at 46 seconds of the song.

So I decided to do voltage match and do listening test again. Initially I found channel imbalance issue when using "mid" volume setting. I then decided to switched to "low" volume setting and that helped with channel imbalance issue.

With all amps in LOW volume setting, I first played 250hz tones at a good volume level (as determined by listening to music via k371) to matched all left channel to same voltage. Then I check right channel.

Left and right channel voltage measured for 250hz tone:
L30 0.092 0.093
A10H 0.092 0.092
HO200 0.092 0.093
All three voltages are stable playing the 250hz tone for a minute.

I listened again and it seems L30 again sometime sounded a little thin in bass, sometimes sounded just fine.

I decided to check with 60hz tone next, using the same volume knob level as 250hz.

Left channel voltage measured when playing 60hz tone:
L30 keep moving around between 0.088 to 0.096 V
A10H move a bit between 0.092 to 0.093, mostly stable to 0.092
HO200 move a bit between 0.092 to 0.093, mostly stable to 0.092.

So, A10H and HO200 voltages are stable for both 250hz and 60hz tones.

L30 is not stable at 60hz tone.
Every few seconds it would move by 0.001 V, stop at one extreme for 10+ seconds, then move 0.001 V every few second to another extreme. It would repeat that up and down voltage swing.

Using the first calculator in http://www.sengpielaudio.com/calculatorVoltagePower.htm,
I estimate that the dB difference between 0.088 to 0.096 is about 0.75db. So it would seem for every half a minute or so, I would hear 0.75db louder or softer in 60hz tone, when using my AKG371.

Is this voltage swing of 0.75db at 60hz acceptable behavior?

Thanks!
 
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Pdxwayne

Pdxwayne

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Since my other two amps are stable at 60hz tone, should I assume L30 is not working as expected? @JohnYang1997, can you comment?
 

solderdude

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When your mains is 60Hz you can be almost certain it is caused by a ground loop.
A: do you hear a 60Hz tone (when no music is playing)
B: You can use Pauls software and see how high the 60Hz tone is.
 
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Pdxwayne

Pdxwayne

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When your mains is 60Hz you can be almost certain it is caused by a ground loop.
A: do you hear a 60Hz tone (when no music is playing)
B: You can use Pauls software and see how high the 60Hz tone is.
It is not about ground loop.

It is about voltage output instability of L30 when the amp is playing real 60hz tone.

I created 60hz tone wav file using Audacity app. I then used foobar2000 to play the 60hz tone wav file. Laptop usb to Topping E30, then E30 RCA to the amps.
; )


Here is the cable I use. One end plugs into the headphones jack of the amp. Then measure the voltage using my multi meter at the other end.

If you have L30 on hand, you can also check if your L30 behaves the same when playing 60hz tone. Play the 60hz file for a minute and observe your multi meter reading over that 1 minute. Volume mode is low. Volume knob is at about 1 o'clock.

Thanks!
20211215_224243.jpg
 
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solderdude

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When you have a groundloop and mains frequency is 60Hz and your test tone is 60Hz (poorly chosen, should use 40 or 80Hz for instance) then a small 60Hz from mains will be in phase and out of phase with a slow drift. Exactly as you reported. Use a different frequency.

I don't have an L30 and live in 50Hz country so even when I had a ground loop it would not be obvious using 60Hz (the warble would be 10Hz, to fast for a multimeter)
 
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Pdxwayne

Pdxwayne

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What happens for a nearby tone, say 55 or 70 Hz?
When you have a groundloop and mains frequency is 60Hz and your test tone is 60Hz (poorly chosen, should use 40 or 50Hz for instance) then a small 60Hz from mains will be in phase and out of phase with a slow drift. Exactly as you reported. Use a different frequency.
I will check with other tones tomorrow. Maybe 50hz next.

Interesting that the other two amps worked just fine with 60hz tone....
 

solderdude

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No groundloop issues (could be layout, cables, power supply or whatever other cause)
 
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Pdxwayne

Pdxwayne

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@solderdude, regarding your post #3's question A, the answer is that there is nothing heard when no music is playing. My multi meter also shows no voltage when nothing is playing. So, no typical goundloop noise that I could hear when nothing is playing.

Here are the measurements results for today. It would appear the swing of voltages are at 60hz (the most swing), 120hz (second most), and 240hz. All other tones so far a lot more stable.

L30 voltage swing, left channel and right channel
30hz 0.001 0.00
50hz 0.001 0.00
60hz 0.007 0.006
70hz 0.001 0.00
90hz 0.001 0.00
120hz 0.003 0.004
240hz 0.002 0.001


60hz left channel swing is between 0.088 to 0.095. This translates to about 0.66db difference.

120hz right channel swing is between 0.091 to 0.095. This translates to about 0.37db difference.

Is this really ground loop? I can't hear noise when nothing is playing. Multi meter also detect no voltage when nothing is playing.

Thanks!
 
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solderdude

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Listen while measuring.

Something, somewhere in the measurement (maybe the leads of the voltmeter) are picking up hum.
At 60Hz you seem to be picking up 2.5mV hum (32dB below your signal so 52dB below 1V)
At 120Hz (2nd harm) 1.4mV hum
At 240Hz (4th harm) 0.7mV hum

So... when you want to measure your amp the way you are doing... do not measure at 60Hz or multiples.
 
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Pdxwayne

Pdxwayne

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Pdxwayne

Pdxwayne

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Listen while measuring.

Something, somewhere in the measurement (maybe the leads of the voltmeter) are picking up hum.
At 60Hz you seem to be picking up 2.5mV hum (32dB below your signal so 52dB below 1V)
At 120Hz (2nd harm) 1.4mV hum
At 240Hz (4th harm) 0.7mV hum

So... when you want to measure your amp the way you are doing... do not measure at 60Hz or multiples.
I saw you added more info to your post.

If my leads are picking up hum, like @MeZoX said, I would pick up same swing of voltages when measuring my other amps. This is not showing up when measuring a10h and ho200. So that part is strange.


Edit 2:
The multi meter actually was showing 0.003 without connecting to L30. When connected to L30, without anything playing, it still shows 0.003. This part I will further investigate.
 
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Pdxwayne

Pdxwayne

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@solderdude, I found out that a10h was doing fine because the preouts were connected to my stereo system. My stereo system's amps are grounded. Maybe that was why it did fine with measurements for 60hz. Once I unplugged the preout RCA cables, a10h also not doing well with 60hz measurements.

Once I plugged the preout cables to L30, 60hz measurements showed no movement.

So, like you said, ground related issue.

H0200 is stable without needed preout connections. I think this is because it is already grounded since it uses 3 prong plug.

Anyway, I also found out something interesting:
If I simply hold the multimeter leads and touch the leads together, without it touching anything else, and my body not touching anything else, it is reading 0 V.

Once I plugged in headphones to a10h or L30 (when both without preout connections), simply touching the headphone wire on my body, the multi meter would give a reading of 0.002V.

In this case, would that 0.002V flow from headphones to my body cause any audible effect to music?

Thanks!
 
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solderdude

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I suspect it only influenced the meter.
 
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