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Old 17th July 2015, 02:04 PM   #1
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Default Noise, too much noise.

Just made a headphone amp, but there is a 50Hz(the frequncy of the power supply in here) noisy when I turn on the power. Here's the PCB layout, please check and tell me what the problem you think maybe.

Click the image to open in full size.

Last edited by ShadowMike; 17th July 2015 at 02:09 PM.
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Old 17th July 2015, 02:27 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by ShadowMike View Post
Just made a headphone amp, but there is a 50Hz(the frequncy of the power supply in here) noisy when I turn on the power.
Here's the PCB layout, please check and tell me what the problem you think maybe.
How about a schematic instead?
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Old 17th July 2015, 03:25 PM   #3
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Here is the Sch. Change R6 to 22k, R14 and R15 to 100 Ohm.

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Old 17th July 2015, 05:41 PM   #4
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Assuming PCB is correct and PSU has good enough after-rectifier filtering then you probably have oscillation. You can check it with oscilloscope that is fast enough to up to 20Mhz ringing (so audocard-based software scopes bad for that).
Simulation doesn't show huge instability, however there'is small ringing on pulse edges that can be fixed by adding 51..100pF in parallel to R11
Also capacity like that can be useful on input (parallel to R6).
However your correction to 100 Ohm on R14/R15 looks rather strange
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Old 18th July 2015, 07:59 AM   #5
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When using 10 ohm on R14/15 the drain current is too much that causing the mosfets be too hot, so I changed them to 100 ohm.
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Old 18th July 2015, 09:35 AM   #6
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This current must be adjusted with R1 - decrease it by 1..2K to get noticable quiscence current decreasing.
From your PCB image it seems you're using bc550 for Q6 and 12K for R1. Simulation tells that drain current will be 87mA with such values. Changing R1 to 10K as on sch decreases current to 29mA.
But such R14/R15 values are not a reason of noise. They only limit output amplitude.
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Old 18th July 2015, 09:43 AM   #7
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Originally Posted by ShadowMike View Post
When using 10 ohm on R14/15 the drain current is too much that causing the mosfets be too hot, so I changed them to 100 ohm.
Did it ever occur to you that this is supposed to be a Class A output stage, which tend to require adequate heatsinking? 15 V times 75-150 mA is 1-2 W per output transistor, no way around that. At least I assume that was projected target level, simulation is not of too much help with the IRF530/9530 models that I have, predicting zero output stage quiescent current, which does not make an awful lot of sense.

Besides, R1/R2 would have been the correct spot if you want to adjust bias.

I hope you either have some LSK170s or some truly genuine 2SK170s there, there's any amount of fakes of the originals around that aren't even close in performance.

Do you have a schematic for the power supply? I think I spot some 3-terminal regulators there (7815/7915?), is input voltage to these high enough? With a lowish-loop-gain circuit like this, I would assume dropping out of regulation would give fairly audible hum.

Last edited by sgrossklass; 18th July 2015 at 09:48 AM.
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Old 18th July 2015, 10:47 AM   #8
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How did you construct the power supply (including the transformer)?
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