gainclone buzzing

[philb]

I just built my first gainclone. I powered it up and it was very cool and everything looked fine.

Then i put a 10ohm resistor (.5W) on the output and it burned. I thought it was only because the amp was putting out more than .5W .

I attached a 40W speaker to the ouputs and turned it on and the speaker buzzed at about 60Hz. Connected my MP3 player as an audio source and still only humming, not even distorted music.

The chip, the wires and the PS stayed cool the whole time.
I used a resistor instead of pot, and I am sure this is wired correctly.

Trafo is 24vct 240VA EI type, maybe this is to low?

thank you for any help you can offer.

[philb]

I searched many threads and found many buzzing gainclones, and tried many solutions. The buzzing remained! the chip is a LM3886TF and i changed the pin wiring to accomadate. It is wired P2P and everything is protected.

[carlosfm]

Where did you get that schematic?
That thing doesn't work!
What are you trying to do?
Inverting?
Non-inverting?

[Svante]

Seems like you swapped the + and - inputs. The feedback is positive. This "amp" will act like a schmitt-trigger, and the output will be either v+ or v- (which contains a 60 Hz component).

[philb]

I found this schematic off the diyaudio threads. I figured it was the simplest one, and hence the easiest.

Thank you for the replies, i will attempt to change the inputs and see if this solves my problem. But I think that they are correct for the other schematic which i have from decibel dungeon looks similar. I will attach it so you can see what i mean.

[philb]

I found this schematic off the diyaudio threads. I figured it was the simplest one, and hence the easiest.

Thank you for the replies, i will attempt to change the inputs and see if this solves my problem. But I think that they are correct for the other schematic which i have from decibel dungeon looks similar. I will attach it so you can see what i mean.

[carlmart]

Originally posted by philb
I found this schematic off the diyaudio threads. I figured it was the simplest one, and hence the easiest.

It is the simplest one, and very much like the amp I assembled with no problems. Except for some important questions:

1) There should be some resistance from the non-inverting input to ground. Something like 22K should do. Offset will be lower.

2) There should be a series resistance at the output, from 0.1 to 0.47, 5W type.

3) The speaker ground should be connected to the star ground, not between the caps.

Originally posted by philb
Thank you for the replies, i will attempt to change the inputs and see if this solves my problem. But I think that they are correct for the other schematic which i have from decibel dungeon looks similar. I will attach it so you can see what i mean.

If your chip is the LM3875, and you are connecting the right pins to the external parts, that should not be the problem. Are you using a pcb or p2p soldering?

Carlos

[philb]

thanks for the reply carlmart

my chip is an lm3886, but i changed the pinout accordingly, i will try to implement your advice, and thank you for taking the time to help.

i will try to post my results on sunday.

[cjd]

While the original schematic doesn't seem to be very well thought out (I could see it easily leading to improper grounding) it does seem to be correct, though it took me a couple looks till I realized it.

Do you have both V+ pins on the 3886 supplied? What value mute resistor are you using?

That aside, look to your grounding scheme carefully. Power ground and signal ground should be separate (with the resistor across the input going to signal ground star) and then connect the two stars with a thin wire. Output ground should come from power ground, and the caps from V+ and V- to ground should also go to power ground. I think I have this right.

It may be worth triple-checking your pins and making sure there are no shorts, solder bridges, etc.

C