Hello,
I'm a total beginner and working on a fairly high gain audio preamp circuit with two stages. I'm using a fairly dirty 9v wall wart supply in order to test how well the circuit can handle bad supplies. The only two caps coupling the Opamp currently are a 0.1uf (ceramic) and a 10uf (electrolytic). They are mounted very close to the power pin of the opamp. I notice there is still some light background high pitched whine. So, when I attempt to add a 47uf, 100uf or 220uf to ground to hopefully filter the bad supply a bit more they actually boost this high pitched whine further. Can anyone tell me what the normal cause of this would be?
The 9v supply is being used to drive an old LA3161 chip. When I use a 150ohm feed resistor to VCC the whine seems louder than when I use a 10ohm feed resistor.
Thank you
I'm a total beginner and working on a fairly high gain audio preamp circuit with two stages. I'm using a fairly dirty 9v wall wart supply in order to test how well the circuit can handle bad supplies. The only two caps coupling the Opamp currently are a 0.1uf (ceramic) and a 10uf (electrolytic). They are mounted very close to the power pin of the opamp. I notice there is still some light background high pitched whine. So, when I attempt to add a 47uf, 100uf or 220uf to ground to hopefully filter the bad supply a bit more they actually boost this high pitched whine further. Can anyone tell me what the normal cause of this would be?
The 9v supply is being used to drive an old LA3161 chip. When I use a 150ohm feed resistor to VCC the whine seems louder than when I use a 10ohm feed resistor.
Thank you
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The noise is only judged by hearing it through speakers. The 0.1uf and 10uf caps are very close to the +V pin of the opamp. When I try to add the other filter caps but closer to the power supply input (DC Jack) I just get this increased noise. Thanks
LA3161 Stereo Preamplifier for car stereo, 12V operation,
The circuit is similar to this circuit. The issue happens regardless weather I use a single stage as seen here or use the stereo chip as two stages.
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Yea similar on both. Input and output grounds are closer to the DC Jack.
The noise is a combo of white noise (from the high gain) with a high pitched noise behind it. I'm more worried about the high pitched noise as I think the white noise may just be unavoidable?
Edit: the high pitched noise is what is being amplified by extra coupling caps.
ok, why run an amp in starvation and expect normal operation? and while your at it you say it's a 9v wall wart your running i hope it's DC and not AC!
the ic referenced will run at 9vdc but only into a 10 k load for preamp applications certainly not a 8 ohm loudspeaker as a load! or are you indeed sending the output to a power amp?
the ic referenced will run at 9vdc but only into a 10 k load for preamp applications certainly not a 8 ohm loudspeaker as a load! or are you indeed sending the output to a power amp?
The PSU is a regulated DC supply.
I'm not sure what you mean "try again short circuiting the input of the amp"? What should I experience/learn from doing that?
Thanks
Yes why? I've tried running it with a 12v supply as well and gotten the same results but yea I agree. Yes I'm using a regulated DC supply.
How does one figure a 10k load btw, I'm just not familiar as a beginner. I'm running it into a line level mixer not an 8ohm loudspeaker. Hopefully that info is helpful.
Thanks
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I'm replacing a preamp unit in an old cassette player, just running the head straight to this circuit to learn a bit.
Would an RF signal explain why adding the larger caps amplify the high pitched noise? I'm just not sure why that would strengthen the RF. Thanks
It would help if the OP distinguished between coupling caps and decoupling caps. He seems to be talking about the latter but calling them the former.
'Bypassing' caps can make things worse, by raising impedance over a certain frequency range. Maybe this is what is happening? Also, simple bolting extra caps onto an existing circuit can result in noise being injected into a ground.
'Bypassing' caps can make things worse, by raising impedance over a certain frequency range. Maybe this is what is happening? Also, simple bolting extra caps onto an existing circuit can result in noise being injected into a ground.
The high pitched noise may be related to switching in the wall wart.
A quick check would be to run the circuit from a couple of batteries. If the whine is gone, you have your answer. If not, that's also an answer. Win-win ;-)
And yes I assume you really mean decoupling caps.
Jan
A quick check would be to run the circuit from a couple of batteries. If the whine is gone, you have your answer. If not, that's also an answer. Win-win ;-)
And yes I assume you really mean decoupling caps.
Jan
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Sounds like poor grounding.
The power supply and audio ground should be star grounded.
Many years ago I designed a audio mixer with power supply on the pcb.
I just mixed in the audio and power supply grounds as they came.
The result was 1 volt of hum on the output even with inputs shorted !
The charging impulses into the smoothing capacitors caused a voltage in the ground line which modulated the audio signal.
I reworked the pcb keeping audio and power supply grounds separate buit only connected at star point on pcb power connector and it worked great.
The power supply and audio ground should be star grounded.
Many years ago I designed a audio mixer with power supply on the pcb.
I just mixed in the audio and power supply grounds as they came.
The result was 1 volt of hum on the output even with inputs shorted !
The charging impulses into the smoothing capacitors caused a voltage in the ground line which modulated the audio signal.
I reworked the pcb keeping audio and power supply grounds separate buit only connected at star point on pcb power connector and it worked great.
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