Do i need a diode?

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Below is the schematic of my amp that i made for my microphone. long story short i made a PA system. when the amp powers off, it makes that loud crack/click sound. im guessing thats a voltage spikewhen it turns off, correct? i remember reading that placing a didode between the + and - would correct that problem, since the voltage would go through that instead of the whole amp making that sound.

Also if I do need a didode, does it matter what kind i get? i have a 15/30 watt soldering iron that my brother ran over today. the reason y its 15/30 is that it has a switch. i saw how it works, the 15 watt setting is conencted through the didode so it only gets one half of the 60hz electrical AC cycle; the 30 watt setting gets all of the cycle. COuld I use that one thats in there? its broken so im going to throw it away anyways.


by the way thanks for all the help everyoneh as given me allready
 
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Hi Clem,
You are correct. That circuit is a natural thumper.

Even with the switch, you may get a turn on click or thump. One problem is the charge / discharge current through the cap (C3). Depending on grounding and lead dress, you could be picking up a surge current through the wiring.

-Chris
 
the way that i have this hooked up is the S1 is hooked up through a SPST relay in my car. the "speaker" output is converted into a pair of RCAs that go into my cars cd player(it has aux inputs on the back).

i have all my speakers hooked up to spdt relays . i have all 4 speakers insde hooked to "30" and "87a" so when the 12v switch i have powers on, it turns all the relays inside to 87(which turns the speakers off) and turns a SPST relay "on" that connects a PA speaker that i have under my hood of the car. also it feeds the 12v to my microphone amp. So when i turn on that switch, it turns off all the inside speakers, turns on the mic amp and the speaker outside so i can have a PA system.

when i turn off/on the 'network', i get half of hte crack on the inside speakers nad half on the outside PA speaker since it switches the cd players output fro the inside speakers to the outside.


so a didode would not work at all?
 
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Hi silentblackhat,
I think you are talking about a diode across the relay coil. You should always put one across DC coils. Orient so the diode is reversed biased under energized coil conditions.

I don't think it will help with this problem, but it should be there.

-Chris
 
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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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
 
anatech said:
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
 
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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