LM3886 very sensitive {makes noise}

[Carel]

Ok, at first I had problems with the LM3886 chip, but now it's sorted. I've modified a PC PSU to provide me with enough power for this chip. The PSU's output voltage is adjustable, I've put in a 20k variable resistor to control the output voltages the TL494 PWM chip generates. I've also modified the -12v rail, usually it can only output 0.5A, but now it can give up to 14A of power because I've changed the 2x 1A diodes they used in the unit to 2x more powerfull rectifiers that can handle 10A each. The +12v can output more than 20A. I've modified the primary side too, so that there are 4 BJT Transistors working together instead of 2x {I've put some extra diodes in parallel for the bridge rectifier just to ensure they don't fail}. So this unit can now give anything between +12v and +32v at the output respective to ground, as well as -12v to -32v respective to ground - it's adjustable now because of the 20k variable resistor I've put between Pin 1 & GND, I can now get a total of 64V from the PSU :-) {BTW - I've found a way to disable the Overvoltage and Overcurrent protection by connecting Pin 4 (DTC Pin) to GND, however it wasn't necessary}. I also had to change the capacitors at the secondary side, because the are rated 16v each, so I changed them to 35v, 2200uF. I've tested my LM3886 with this modified PSU and it works excelent {partially}.
There is just one little problem I do experience. The chip is VERY sensitive {I do know the tab is internally connected to V-}. If I put the volume between Minimum and Max the speaker makes a loud grrrrssssshhhhh!!! sound {constantly}, I use my cellphone as input {just temporary for testing}. The chip needs a larger input volume than I expected. Also I found that other items, such as an electric cables nearby do interfere easily with this chip. The chip doesn't want to work so good when I attach it to a very large heatsink, it makes noise {loud scrambled sound}. I don't know why? If I touch the tab, the sound goes away or become less. Does anyone maybe know where I can found a schematic for this basic amp, which is better than National's basic schematic? I really don't know why I'm experiencing such sensitivity with this chip, if I move it to other places, or turn it around the noises goes away... weird... If I can just see other schematics of how I can connect this chip, then it really could help. Have anyone experience this with the LM3886?
Pictures {I don't know how to insert images properly :-( }:
PSU_mod.jpg picture by utersCarel - Photobucket
-12v_Modified.jpg picture by utersCarel - Photobucket
20k-VR.jpg picture by utersCarel - Photobucket
Bridge_Rectifier.jpg picture by utersCarel - Photobucket
Chip_Front.jpg picture by utersCarel - Photobucket
Chip_Front2.jpg picture by utersCarel - Photobucket
Chip_Under.jpg picture by utersCarel - Photobucket

1.) Is there a way to make the Gain higher / better. If I use my PC, then I need to set the input volume to full before it's loud enough {and even then it's not VERY loud}. If I use my laptop however, then it does go VERY loud {but the Laptop has got a mini-earphone amp inside}. Any idea how to make the Gain bigger or the Input vs. Output Ratio?
2.) Any idea on how to fix the sensitivity problem would help too. I've tried to mount it inside a BoomBox AMP, but the large heatsink gave me problems with noise errupting out of my speakers like volcanos. The audio is only clear when either the volume knob of the amp it set to Min or Max. In the midddle it makes noise and prrrrrrrr... A little help or any tips would be great and gladly appreciated.

[tangmonster]

HI

If you look at that drawing.

http://diyaudioprojects.com/Chip/LM3...86-chipamp.png

What size did you use for Rf and R3 ?

Did you change wav or main volume? programs normally just change the wav volume and then the main volume is still very low. I only ask because this is a problem i always have.

how does your circuit diagram look?

Do you have R2 in your circuit?

[Carel]

Originally Posted by tangmonster

HI

If you look at that drawing.

http://diyaudioprojects.com/Chip/LM3...86-chipamp.png

What size did you use for Rf and R3 ?

Rf = 20K, R3 = 1K, Ci = 22uF
Oops, I don't have R2 in my circuit and it's function is to terminate the input signal to ground {I didn't think of that before}... Ok, I'll put R2 in my circuit and then I'll check again.

Originally Posted by tangmonster

Did you change wav or main volume? programs normally just change the wav volume and then the main volume is still very low. I only ask because this is a problem i always have.

how does your circuit diagram look?

Do you have R2 in your circuit?

I built the amp exactly like this one, except Rin isn't a 10k veriable resistor, I only had a 20k variable resistor {it can't be this, can it?}: http://i578.photobucket.com/albums/s...pplication.jpg
You mean the wav volume of my PC?
No, I only have 2x volume controls {I use Windows 7}. 1st Volume is for my Digital Signal Processor, the 2nd is for the Master Volume. Both of them is on 100% output. Media Player itself is also set to 100%, but I normall use my cellphone as Input {before I make nice casing for this amp, so my phone is just temporary}. I could maybe use a preamp for this chip, looks like I'm gonna need one, but preamps suck, because they add noise to the output {unless you know a better one, please tell me where to find a diagram}. Even if my phone's volume is set to 100%, the amp is loud {at about 25Watt}, but with my laptop I can adjust it to full power {because it has a very high output}. I know with Toshiba's TA8254BHQ you can adjust the Gan with a resistor to a scale of 1 to 300. But it looks like there are no way to do this with National's LM3886? Is there a way I can amplify a small or low input signal a bit more? Maybe by putting in a preamp?

[Carel]

Ok, I've checked with connecting R2, but no luck. However I've noticed that if I drop the input voltage from 24v to 12v, then for some reason it stops doing that...

[gmphadte]

One doubt, is the SMPS radiating noise into the nearby chipamp.

Gajanan Phadte

[AndrewT]

Hi,
that 3886 schematic has no input filters.
It also shows all the grounds with the same ground symbol.
Did you build it with the Zobel Ground connected to the Signal Ground?

What exactly is the Signal Ground connected to?

Have you checked for oscillation?

[Carel]

But if that was the case, then it should rather be hearable at high volumes. At the moment I'm only experiencing problems when the volume is not tuned to full volume. I doubt it could be that, because I would've heard noise when the audio VR is tuned to max. I'll build a linear psu tnnight, but I doubt it's that. I've tested the psu with my other DIY amp, which has a higher amplification, and it doesn't do this.

[AndrewT]

the output impedance of an unbuffered volume control is highest when set to ~-6dB.
Maybe your amp goes into oscillation when it is connected to a high source impedance.
This may be made worse by omitting the input filters that are not shown in the schematic.

[Carel]

Ok, sorry, I'm not sure what is the Zobel Ground? All my grounds are connected together, like in the schematic. All the grounds come together where they are connected to the PSU, like the one in my schematic.

The minute I connect my amp to an external component, like my phone, it immediately becomes silent and clear, even if the source isn't playing any audio there isn't any noise. I'ts only working perfectly when the impedance is high {when the volume of the amp itself, the 20K variable resistor is set to max}. But when I turn the variable resistor towards the ground {reducing volume}, then it begins to make a loud and constant noise. But once it's completely tuned to min, the noise is gone, as well as when it's completely turned to max. It's only inbetween I experience problems.

[AndrewT]

Originally Posted by Carel

Ok, sorry, I'm not sure what is the Zobel Ground? All my grounds are connected together, like in the schematic. All the grounds come together where they are connected to the PSU, like the one in my schematic.

The minute I connect my amp to an external component, like my phone, it immediately becomes silent and clear, even if the source isn't playing any audio there isn't any noise. I'ts only working perfectly when the impedance is high {when the volume of the amp itself, the 20K variable resistor is set to max}. But when I turn the variable resistor towards the ground {reducing volume}, then it begins to make a loud and constant noise. But once it's completely tuned to min, the noise is gone, as well as when it's completely turned to max. It's only inbetween I experience problems.

No, no, no.
When the volume pot is set to max the input of the amplifier sees the output impedance of the connected source.
If you set the volume pot to -6dB then the 50k volume pot presents it's own output impedance of ~0.25 * {50k + Rs} ~ 12k5
That 12k5 is in parallel with the 22k you have added.
The source impedance seen by the chipamp is [12k5 + 1k] // 22k ~8k3
When you disconnect the source the amplifier sees a source impedance of 22k.
Most designs are stable with this source impedance. but your's appears not to be. That I put down to bad choices in schematic and wiring and omitting components.

The Zobel is Cz + Rz. It must not connect directly to the Signal Ground.

You need to do a lot of learning to get up to speed.
At present I guess your best efforts are to copy exactly a working amplifier without any changes, until you understand what each component and each connections does.