Hmmm...
Yes and no.
The circuit - as presented - gives a gain of 1, which the 3886 does not support. It will oscillate!
This is NOT an issue of DC offset, too long feedback path or anything else.
However, you are correct in observing that there's no DC path from the non-inverting input to ground, so if the output of the pre-amplifier has a capacitor as its' last component, the DC operational point will be indeterminate. So a resistor from non-inverting input to ground with the same value as the feedback resistor is a good idea!
Rune
Yes and no.
The circuit - as presented - gives a gain of 1, which the 3886 does not support. It will oscillate!
This is NOT an issue of DC offset, too long feedback path or anything else.
However, you are correct in observing that there's no DC path from the non-inverting input to ground, so if the output of the pre-amplifier has a capacitor as its' last component, the DC operational point will be indeterminate. So a resistor from non-inverting input to ground with the same value as the feedback resistor is a good idea!
Rune
Well guys I switched that 1k resistor so it came before the 33k resistor and I plugged it in with just a Distortion circuit before the LM3886 stage and my Guitar and turned it on and BAM Really LOUD Guitar, But it cut out after a couple minutes probably because I still had the 30v,0v,30v Transformer hooked up on that small Heatsink....
So after this I put in my 18v,0v,18 transformer and I tried something net to try to get my preamp working properly and put in a Bigger (But Lighter) heatsink and turned it on a Everything works Great and sounds Great through my adjustable JBL Crossover and the 12 in sub and 6 in Midrange speakers...
I went and checked my other Power amp PCB from a different PCB design to see if it had the Same Switched resistor problem and It did...I can believe I made the same mistake on 2 totally seperate layouts?? Must be my Dislexia...LOL
Well here"s a Big "Thanx" To those who helped me figure this out, Finally after a Month I have it all working ....
Thanx again....
So after this I put in my 18v,0v,18 transformer and I tried something net to try to get my preamp working properly and put in a Bigger (But Lighter) heatsink and turned it on a Everything works Great and sounds Great through my adjustable JBL Crossover and the 12 in sub and 6 in Midrange speakers...
I went and checked my other Power amp PCB from a different PCB design to see if it had the Same Switched resistor problem and It did...I can believe I made the same mistake on 2 totally seperate layouts?? Must be my Dislexia...LOL
Well here"s a Big "Thanx" To those who helped me figure this out, Finally after a Month I have it all working ....
Thanx again....
OK... I now have a More serious problem,...
I don"t know if I should post a new question in the forum about this or Not but I"ll post it here for now and see if I get any responces....
Well here"s the new deal, I tried playing the amp for about 10 minutes at low volume and suddenly I see smoke comeing from the Cone on my 12in Speaker (But not in the 6 in speaker)....
What would cause a Speaker to start smokeing??? (Becides a stressfull day)
Could it be a Bad speaker?? Or a bad Crossover?? Or DC on the output?? (I have the Input capacitor)
I also noticed when playing that certain frequencies cut out really quick while others cary on...Could this be because of the 2.4uF input capacitor??
Thanx
I don"t know if I should post a new question in the forum about this or Not but I"ll post it here for now and see if I get any responces....
Well here"s the new deal, I tried playing the amp for about 10 minutes at low volume and suddenly I see smoke comeing from the Cone on my 12in Speaker (But not in the 6 in speaker)....
What would cause a Speaker to start smokeing??? (Becides a stressfull day)
Could it be a Bad speaker?? Or a bad Crossover?? Or DC on the output?? (I have the Input capacitor)
I also noticed when playing that certain frequencies cut out really quick while others cary on...Could this be because of the 2.4uF input capacitor??
Thanx
Well, it sounds like DC on the output. Simplest way to determine that is of course to measure the output with a multimeter.
However, if that's the case two things should happen:
1. The 3886 should be very hot, since it's providing the current to the speaker.
2. There should be a substantial "thump" when you switch the amp on.
You haven't said if there's a resistor between the input pin and ground. If not, your DC voltage is undefined since there must be a DC path of equal resistance from both inverting and non-inverting inputs to ground.
Another question is how your speaker is connected. One end is obviously to pin 3 of the chip, but where does the other end connect? It should be to ground, but it's easy to make the mistake of connecting it to V- (or V+) instead.
Rune
However, if that's the case two things should happen:
1. The 3886 should be very hot, since it's providing the current to the speaker.
2. There should be a substantial "thump" when you switch the amp on.
You haven't said if there's a resistor between the input pin and ground. If not, your DC voltage is undefined since there must be a DC path of equal resistance from both inverting and non-inverting inputs to ground.
Another question is how your speaker is connected. One end is obviously to pin 3 of the chip, but where does the other end connect? It should be to ground, but it's easy to make the mistake of connecting it to V- (or V+) instead.
Rune
Hi, No I don"t have a resistor from the Input to ground...
What Value should I use??
I do have the 2.4uF Input cap but I think it is Limiting my frequency responce as some Notes just don"t seem to come through...Is that Possible?? if so what would be a better Value to use??
The speakers are connected from the output and to ground so they are connected correctly and they are connected through a Crossover network which should also block DC from going to the Speakers......
Would a 16 Ohm load be easier for the LM3886 to drive?? cuz I can disconnect the crossover and just wire the 2 speakers in series....
Thanx
What Value should I use??
I do have the 2.4uF Input cap but I think it is Limiting my frequency responce as some Notes just don"t seem to come through...Is that Possible?? if so what would be a better Value to use??
The speakers are connected from the output and to ground so they are connected correctly and they are connected through a Crossover network which should also block DC from going to the Speakers......
Would a 16 Ohm load be easier for the LM3886 to drive?? cuz I can disconnect the crossover and just wire the 2 speakers in series....
Thanx
Yes, you should definitely have a resistor there. The inputs must have a path for DC to ground. Without it the output can be at pretty much any voltage.
Ideally the inputs should see equal resistance at DC. This means the resistance from non-inverting input to ground should be the same as the feedback resistor between output and inverting input. In your case I think that's 33kOhm or 47kOhm.
Just get it there, and MEASURE THE VOLTAGE AT THE OUTPUT! If you don't have a multimeter, get right up and get one. DIY electronics without one is like wood working without a yard stick. It's just luck if it works.
Yes, 16 ohms is easier to drive than 8 ohms, but wiring two dissimilar speakers in series is probably not a good idea, particularly when one is a bass driver and the other a midrange driver. The midrange will most likely not handle the power without a crossover.
And speaking about the crossover: I doubt it blocks DC to the woofer. There's typically an inductor in series with the woofer, and sometimes a capacitor across the woofer. Either way, DC will go right through.
Rune
Ideally the inputs should see equal resistance at DC. This means the resistance from non-inverting input to ground should be the same as the feedback resistor between output and inverting input. In your case I think that's 33kOhm or 47kOhm.
Just get it there, and MEASURE THE VOLTAGE AT THE OUTPUT! If you don't have a multimeter, get right up and get one. DIY electronics without one is like wood working without a yard stick. It's just luck if it works.
Yes, 16 ohms is easier to drive than 8 ohms, but wiring two dissimilar speakers in series is probably not a good idea, particularly when one is a bass driver and the other a midrange driver. The midrange will most likely not handle the power without a crossover.
And speaking about the crossover: I doubt it blocks DC to the woofer. There's typically an inductor in series with the woofer, and sometimes a capacitor across the woofer. Either way, DC will go right through.
Rune
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