My First Gainclone have background noise

[Felix]

This is my first DIY project . I am happy with the sound . Except, I have a constant si--si--si sound if I put my ear near to the speaker unit . The background noise do not go up with the volume ,but I still think it could be a problem when listening . Any suggestion to kill the background noise ? Please .

[Nuuk]

Felix, to help you we will need to know what chip/circuit you have used. (Diagram if possible please).

Also details of your power supply, grounding arrangements and casing. And what type of volume control are you using?

[Felix]

I used LM3875TF and followed Greg's NIGC design . The 50K pot is omitted as I use pre-amp to control music volume . I built it with cheap componets and hardwire them together . all carbon film resistors, NCC 1000uF and philips MKT box capacitors . I am using single Airlink 300VA dual 25V AC toroidal for both channels .

[Felix]

I am using alumium enclosure for heatsink as well . The surface is warm to touch.

[Felix]

I hardwired everythings together with the shortest length as possible . The red/black wire is +/- power . I used copper wires to connect groundings .

[GregGC]

Originally posted by Felix
This is my first DIY project . I am happy with the sound . Except, I have a constant si--si--si sound if I put my ear near to the speaker unit . The background noise do not go up with the volume ,but I still think it could be a problem when listening . Any suggestion to kill the background noise ? Please .

Felix,

Looks very nice. I hope when you tryed the amp you used a heatsink .

First check if the noise is there if you disconect the amp from the preamp, but connect a resistor of 47k or so paralel to the input (you can connect the input to gnd too). It could be that the preamp is noisy and this capable amp amplifies everithing that shows at its input.

I didn't get that kind of noise level.

After you are sure it's not the preamp, make sure it's not the amp oscilating. Do you have the RC network paralel to the speaker output?

Last:
Try 10k/220k or 4.7k/100k FB resistors. It may decrease the noise.

Let's see if it's the preamp first that's making most of the noise.
My guess is for the preamp.

Greg

[Nuuk]

Yes, I agree with Greg, it's a nice neat job so you should be able to get it working OK. I don't think that there is a problem with the circuit per se.

As Greg says, let's iliminate the preamp first.

[Felix]

Originally posted by GregGC

Looks very nice. I hope when you tryed the amp you used a heatsink .

I used the enclosure as heat sink . The heat generated is not that much . It is so nice to touch it in winter .

Originally posted by GregGC

First check if the noise is there if you disconect the amp from the preamp, but connect a resistor of 47k or so paralel to the input (you can connect the input to gnd too). It could be that the preamp is noisy and this capable amp amplifies everithing that shows at its input.

I didn't get that kind of noise level.

I disconnected the pre-amp and short the input to ground with the resristor . The noise is still here , but level is slightly lower .

Originally posted by GregGC

After you are sure it's not the preamp, make sure it's not the amp oscilating. Do you have the RC network paralel to the speaker output?

Yes, I have a resistor of 1 ohm in series with a .22uF box cap . I have tried not having the RC network . The chip become unstable . It sometimes have humm sound when power up. When I measure the DC . It has 1.2V at output !

Originally posted by GregGC

Last:
Try 10k/220k or 4.7k/100k FB resistors. It may decrease the noise.

I think that lower the gain would be benefit to my problem . Many thanks for your hints .

[macboy]

Two tricks for reducing noise:

1. Smaller resistors. Resistors have thermal noise which is proportional to resistance. Noise created in the gain-setting resistors does not change with the volume control. Reduce the two gain-setting resitors by the same proportion so that the gain remains the same. Also be aware of possible effects on the input impedance.

2. Add a small cap (around 0.1 uF) across the two power supply pins (V+ and V-) very close to the IC. I have found this to reduce hiss. Make sure that the cap is rated for the total voltage!

Also, if you disconnect the preamp and short the input to ground, does the hiss disappear or remain? If it disappears, then the pre-amp may be the source of the noise. If it remains, then the gainclone is the source.

[peranders]

Originally posted by Felix
This is my first DIY project . I am happy with the sound . Except, I have a constant si--si--si sound if I put my ear near to the speaker unit . The background noise do not go up with the volume ,but I still think it could be a problem when listening . Any suggestion to kill the background noise ? Please .

Could you take a picture of the solderside too?

This IC is a high current device and care must be taken how you design the current paths. I suspect that you have a ground/decoupling problem or problems with the output filter (oscillations). Have you carefully read the datasheet and the AN-1192 ?
Don't forget to check these too.
http://www.national.com/appinfo/audi...gn_Guide13.xls
http://www.national.com/appinfo/audi...sign_Guide.pdf