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Audio Mixer Whooshing Sound....

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mech3d

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I have a mixer circuit that I bread boarded and it worked but when I built a PWB and assembled it, it doesn't anymore.

I was wondering if anyone experienced this symptom in an audio circuit....

1) I power up the circuit (9VDC).
2) It works for about 3 to 5 seconds.
3) You hear a swooshing sound for about 1 to 2 seconds.
4) It stops working.
5) If I turn it off and them back on, it repeats the symptoms.

I am puzzled....
 

mneary

New Member
I would suspect oscillations, at high frequencies. You might see a change if you add or change the power bypass or filter capacitance.
 

mech3d

New Member
ok will do with the pics, but let me ask this, could this be caused by a bad solder joint also?
 

bychon

New Member
Your description of the problem is not sufficient to diagnose it. You can ask all day if a bad diode, solder joint, light bulb, filter capacitor, wall outlet, will cause this problem, but we need more information.
 

mneary

New Member
I'm assuming the problem appears whether the filter U1 is switched in or out.
U2 section 3 (C) is open loop with VCC/2 nominal on both pins, with P4 open circuit on the + pin acting like an antenna - very likely to oscillate.
A lot of parts are not populated on the board. Can these be identified on the schematic?
What does the bottom etch look like? Is there a ground fill?
Q1 collector, Q2 collector, and 3OUT of U2 are connected together. The low impedance of U2 should prevent Q1 and Q2 from introducing any useful signals.
U2 could be oscillating if care isn't taken in routing of high impedance I/O.

[edit]I should also mention that the "stereo" input is not symmetrical and if P3 is supposed to be a balance pot, it isn't.
And the "stereo" output isn't.
 
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It looks really impressive, the "swooshing sound" (that's a great electronics term) is probably white noise in the circuit, and will always be there. The reason that it is shutting off as best i can think of (I don't know how your circuit works because I couldn't find most of the data sheets). is that the darlington pair on your output is charging the 100uF cap. Try using a bigger cap. Have you tried to start it up with the inputs active?
 

mneary

New Member
Which data sheets do you need? I'm only missing the audio filter which appears to be something like HT8958 which google says not. If you want TL074, BC547, BC557, BC558 I'll post the ones you need.

[edit] U1 is Holtek HT8950 Voice Modulator. Since Q1 is shorted anyway it probably isn't contributing to the noise. [/edit]
 
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mech3d

New Member
Here is the bottom side picture: 2circuit_backside.jpg picture by mech3d - Photobucket

Sorry for the non-enectrical terminology, but that is exactly what it sonds like - a swoosh.

I placed all the resistors on the back side to make it easier for me to route and to save space.

The entire bottom layer is a filled ground plane.

I don't remember if I started it up with the inputs active.

P3 is not for balance but to control how much voice is removed to simulate a karaoke and P4 is suppose to behave like a volume control.

I am not looking for stereo output, just to mix the 2 mics and audio in. I did use a stereo jack though since I am plugging this in a stereo unit.

I really appreciate your help, you have all been great - Thanks!
 
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mech3d

New Member
I don't know, I can give it a try though.

Ok, I removed U2 and the mono inputs worked, so some how U2 is causing the circuit to hickup.
 
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mneary

New Member
The problem remains that the circuit still has almost no gain. U2c and Q4 are fighting for control of the audio output and nobody wins. You need a resistor in series with C9 and C10. (One resistor each, 1k would be good.) You should simulate the circuits of Q1 and Q2 and determine how you want them to work.

Can you tell us the source of the original design? As drawn, I have serious doubts. Perhaps some important parts were omitted, or there are more errors. Particularly urgent is that R1 and R4 should NOT be 2k2.

As an added bonus, a legible schematic would be nice. It took me literally half an hour to guess all the part numbers (assuming I actually do have them right).
 
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