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Old 14th May 2008, 10:18 AM   #1
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Join Date: May 2008
Default LM3886 background noise

Greetings

I've recently build a 50w guitar amplifier based on ne5532 opamps and a lm3886 poweramp. It works ok, it has 3 band eq similar to a marshall eq, a clean channel using a single ne5532 opamp and an overdrive channel similar to a tubescreamer using 2 ne5532 opamps. It is working fine however i encountered 2 different noises and i couldnt seem to find a way to get rid of them. One of the noises is the background noise ("hssssss") of the distortion channel when i crank the drive and volume pots to max (tubescreamer). The preamp and thepoweramp gain stages are close to 115dB total when the background is clearly audible even at a room which is not silent at all. All guitar amplifiers have this kind of noise more or less so the 2nd noise is the one im really concerned about.

This other noise is a low noise (silent buzzing coming at random periods) which is not due to preamp or the powersupply. I used a 2nd regulated (and isolated in a sheilded box) powersupply at 15V dc, buzzing continues. I isolated the preamp and even shorted the input of the poweramp (used a 10k resistor as well) and the buzzing continues. The buzzing levels are audible if I place my ears close to the speaker or when it is silent at night. When I don't connect an input or decrease the volume of preamp to 0 the buzzing increases and is clearly audible in a normal room during the day. I used a 15cm coaxial cable (as a shielded cable) between the preamp and the poweramp and got the shield ground from the preamp side. Speaker cables were passing around 1 cm below the coax (and 1 cm above the bottom plate of the metal box). I noticed that when i touched the coax, buzzing level increased. So i took the shield ground from the poweramp side and seperated the two cables 1 cm more (I'm out of cabels so i couldnt seperate them more ) Now the buzzing does not increase when I touch the speaker cables or the coax. However it's still there clearly audible when you are in the room at night or get close to the speaker during the day.

I couldnt take a picture of the poweramp schematic but it is almost identical to the typical application chart of lm 3886 in the datasheets. I used 220uF and 100nF (yellow 63v caps) for suplly bypasses. (I already have 4700uF caps at powersuplly circuit) A 22k resistor at feedback between inverting input and output. A 22uF cap and a 470 ohm resistor at negative feedback to ground. 1uF cap at input with 1k in series and a 10k to ground at input. (This was normally 22k, however our instructor told me that higher the input resistor is, more noise the amp would pick, so i dropped it down to 10k). Lastly a 22k between the mute and negative suplly pins as well as a 22uF between the mute pin to ground. I didnt use the resistors or the inductor at output (i dont have an inductor or resistor capable of carrying 2-3 amps ) . Is the inductor at output important?

Is there a way I can lower these 2 noises (essipecially the one caused by the poweramp). I searched the forum as background noise but couldnt find a post with a similar title from search. I'm an engineering student and this is my first try for building an amplifier. I have very little technical or practical information about amplifier designs so please excuse me if i talked about irrelevant stuff. I'll try to put some photos soon and be more precise about the poweramplifier in case you are suspecting a layout problem or some sort. Thank you for your help.
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