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Old 20th June 2006, 01:28 AM   #11
clem_o is offline clem_o  Philippines
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Hi,

If I understand this correctly, the LM386 circuit is used to drive your car amplifier's INPUTS, and thus you are using your car amp to drive an external (PA) speaker?

Cheers

Clem
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Old 20th June 2006, 02:44 AM   #12
anatech is offline anatech  Canada
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Hi Clem,
That's what it sounds like.

-Chris
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Old 20th June 2006, 04:48 AM   #13
CBS240 is offline CBS240  United States
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Hi


I think a single side supply OPamp may be in order here since the output impeadence required is around a K Ohm or so. Electrolytics in signal path.................
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Old 20th June 2006, 06:07 AM   #14
clem_o is offline clem_o  Philippines
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Ok, since it's driving such a high impedance, and for voice:

- reduce output capacitor to 1uF
- put a NC relay contact where the speaker normally is connected. Wire the relay such that it energizes along with the amplifier (so when the amp is turned on, relay contacts become "open")...

Not sure if the relay will be fast enough to catch all of the noise - perhaps people reading this will have a better idea - but the change from 470uF to 1uF output coupling should reduce the transients significantly!!

Cheers!!
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Old 20th June 2006, 02:23 PM   #15
anatech is offline anatech  Canada
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I think the problem is more fundamental. Your DC point at the output is biased around B+ / 2. That means you must charge the cap and discharge the thing every time you use this. I can see two solutions.

Either add a bleed resistor across the output and reduce the cap size, leave the unit powered on before and after you use it. Current drain is low, so let it run all the time the car is running or in ACC mode.

Or, generate a negative voltage and use a standard op amp circuit. Your output DC would then be close to 0 VDC. I'd still recommend letting the unit run before and after use to prevent switching transients.

-Chris
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Old 20th June 2006, 02:56 PM   #16
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Quote:
Originally posted by anatech
I think the problem is more fundamental. Your DC point at the output is biased around B+ / 2. That means you must charge the cap and discharge the thing every time you use this. I can see two solutions.

Either add a bleed resistor across the output and reduce the cap size, leave the unit powered on before and after you use it. Current drain is low, so let it run all the time the car is running or in ACC mode.

Or, generate a negative voltage and use a standard op amp circuit. Your output DC would then be close to 0 VDC. I'd still recommend letting the unit run before and after use to prevent switching transients.

-Chris
I'd be more inclined to worry about the DC potential at the mike. When the switch is opened, the mike discharges the input capacitor....with gain..

Cheers, John
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Old 20th June 2006, 03:03 PM   #17
anatech is offline anatech  Canada
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Hi John,
Yup. Very good point.

Allowing the circuit to run will avoid the noise, but not the issues.

-Chris
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Old 21st June 2006, 01:21 AM   #18
clem_o is offline clem_o  Philippines
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Agree with the comments - though it will take a bit of effort to really get the 1/2Vsupply to zero (additional circuits etc). Why not just shrink the capacitors - perhaps a little 'tick' on turn-on/off is tolerable to the user? In that case, maybe simply changing the output coupling capacitor to a 0.1uF will do...

Cheers!
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