| 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? |
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| 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? |
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| carlmart |
| quote: | 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 |
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| 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... |
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| nerd of nerds |
i measured the voltage coming from the rectifier and i got 17.2 volts...
is that good? bad? |
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| 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:whazzat:
so, would putting the power supply in a different enclosure fix the humming? or do i need some more stuff??? |
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| 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 |
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| nerd of nerds |
| quote: | 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? |
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| demogorgon |
| quote: | 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? |
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| soundNERD |
I've had the same exact problem.
With the radio station on the volume control, means you have it wired wrong:
INPUT
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\
/
\<--------- OUTPUT
/
\
/
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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) |
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| nerd of nerds |
i've fixed the problem for the most part...
now i'm getting radio stations and i decided to do away with the potentiometer and build a nice preamp with volume control:rolleyes:
but i still get that damned humming...i've managed to transform it into a square wave tho:xeye: it amplifies the input fine now...but i'm still getting radio interference and a small hum...
how the hell do i ground this thing?! i'll be damned if i get it to ground properly...
i was using a 3 connector plug for power but now i've switched to the 2 connector style and it hasn't changed much...
do i need the 3 conductor connector for ground? all it did when i connected it last time was amplify the humming:xeye: |
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| Nuuk |
Yes you need a three pin plug so that you can earth your amp for safety! :att'n:
In the picture below, you can see the signal ground star which is n the rectangular piece of copper. The power ground star is the connection between the two 1000 uF caps.
You may benefit from studying all the information here . ;) |
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| Morse |
Hi nerd of nerds;
I strongly suggest that you:
1. Go back to a 3 prong plug for safety's sake.
2. Use a separate "signal ground" and "power supply ground".
3. Connect the 2 common's together at a final "star point" near the attachment point to the IEC inlet or mains cord.
4. Make CERTAIN that the chassis isn't forming a "ground loop" somewhere (i.e. if you have a "ground lead" going to a component, is it also in electrical contact with the chassis?)
5. Make CERTAIN that components that DO use the chassis as a "ground plane" ARE in electrical contact with it (i.e. you may need to scrape some paint off - is the trafo's outer shell earthed?).
6. Did you use separate bypass caps for each channel's V+ and V-?
If all that fails,
1. Are the signal grounds at the RCA's connected to the chassis? Try using separate leads to earth for your signal in, and use washers to make sure the RCA's are not forming a ground loop with the chassis. That business with radio reception coming in makes me VERY suspicious of your signal lead arrangement.
2. Post some pics - that will help the rest of us see what's going wrong.
3. Persevere - proper earthing ("grounding") technique is one of the trickiest things for a first-timer. But once you get it right, you'll be able to knock out amps that work every time.
Finally, don't worry about using a Radio Shack trafo - I've used their 25.2VCT at 2A units in quite a few amps and haven't had any troubles off them. As far as trafo/signal leads connection, if the outer shell of the trafo is properly earthed, I've never had any bother. On the other hand, if you were using an UNSHIELDED trafo (one without that metal outer shell that encloses the windings) that's a different story....
Good luck and all the best,
Morse |
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| nerd of nerds |
the only reason that i ditched the 3 prong connector was that i cracked it while i was screwing it to part of the chassis...
i'll find another one eventually or just use the 3 prong wire for quickness...
while i was messing with the grounding i noticed that the hum went away when i put my finger in between 2 of the wires, that is if i had a certain grounding scheme set up...which i forgot:rolleyes:
i'm going to try and get into the shop at my school after all my classes and finish making the chassis...
is aluminum good for it? most of the case is made out of MDF and the front is MDf "plated" with aluminum to look pretty...
the top and sides of the case wil be one big piece of bent aluminum sheet metal...is that what i should use for my ground point?
thanks sooooo much for all of your help, you people are wonderful!
P.S. this is my first amp:) |
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