not good! not good!

[nerd of nerds]

ummm...soooooo...

i just got done soldering together my first gainclone...i used a radio shack transformer, the best one they had...

i checked my lm3875 gainclone several times and when i plugged it into my speakers all i got was a LOUD humming noise...sounded kind of like a really painful fart to but it bluntly...

any ideas what the hell is causing this?

[ocool_15]

The first thig i would check is the input. make sure that its connected properly. If there is no input your amp may oscillate and make a humming sound. If you have a wrong connection across the input it may sound similar. Does the audio signal go through at all above the humming?

[carlmart]

Originally posted by nerd of nerds

i just got done soldering together my first gainclone...i used a radio shack transformer, the best one they had...

i checked my lm3875 gainclone several times and when i plugged it into my speakers all i got was a LOUD humming noise...sounded kind of like a really painful fart to but it bluntly...

any ideas what the hell is causing this?

Ground loop is most likely, as that is the most common mistake.

To start with: what did you check several times? was there any DC at the output?

If you don't have an oscilloscope it's quite difficult to trace problems.

What type of wiring are you using: p2p or pcb? If pcb, did you do it or bought it somewhere?

Radio Shack is not a very good source for transformers. What voltages are you getting after rectifying, between ground and V+, ground and V-?

Can you draw how your wiring is?

Carlos

[nerd of nerds]

i wired it P2P...

but i think i found my problem: my potentiometer isn't grounded...


i'm getting 35V across one of em...haven't measured the other...

[nerd of nerds]

i measured the voltage coming from the rectifier and i got 17.2 volts...


is that good? bad?

[nerd of nerds]

SUCCESS!!!!

well.. sort of...

now i can get audio coming out of it but there still is a hum...quite audible too...and when i have the volume low it tunes a radio station





so, would putting the power supply in a different enclosure fix the humming? or do i need some more stuff???

[carlmart]

Just do your ground wiring correctly, with a star separating signal and supply grounds, as you may find on many threads here, and your hum will vanish.

Your RFI probably too, except if you have an FM transmitter nearby, where you will have to parallel a cap at the input as low-pass filter.

17.2 volts are a bit on the low-side, but should do fine. Are you getting the same voltage on + and - terminals?


Carlos

[nerd of nerds]

Originally posted by carlmart
Just do your ground wiring correctly, with a star separating signal and supply grounds, as you may find on many threads here, and your hum will vanish.

Your RFI probably too, except if you have an FM transmitter nearby, where you will have to parallel a cap at the input as low-pass filter.

17.2 volts are a bit on the low-side, but should do fine. Are you getting the same voltage on + and - terminals?


Carlos

so do i keep ALL grounds seperate until they meet at the star? and how do i make a star when i'm using part to part wiring?

[demogorgon]

Originally posted by nerd of nerds



so do i keep ALL grounds seperate until they meet at the star? and how do i make a star when i'm using part to part wiring?

I dont think it's nesecary to have a physical stargrounding, but rather a logical one, if you know how the two differ?

[soundNERD]

I've had the same exact problem.

With the radio station on the volume control, means you have it wired wrong:

INPUT
|
|
|
\
/
\<--------- OUTPUT
/
\
/
|
|
|
GROUND

The 35V on the output is probably due to not having a 10k resistor from input to ground and the input not being connected properly (result of incorrectly wired potentiometer)