| b<ben>n |
i was bored today so i went to the shops grabed the stuff needed for this preamp http://www.diyguitarist.com/PDF_Fil...uitarPreamp.pdf and put it togeather.
it seems to work fine but there is a strange humming noise that only goes down when i touch a metal part of the guitar or the ground of the 6.35mm plug. i was using this preamp to plug my guitar into my computers line in. any ideas what this hum is from and how i can get rid of it. |
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| Nigel Goodwin |
| That PDF isn't very clear, do you have the input and output grounds connected together?. C4 is also shown the wrong way round!. |
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| b<ben>n |
the input and output grounds are on the same veroboard track.
and do you think reversing c4 will help?
and i didnt follow there layout i just worked it out from the schematic |
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| Nigel Goodwin |
| quote: | Originally posted by b<ben>n
the input and output grounds are on the same veroboard track.
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Yes, but do you have the actual socket grounds connected to it?. And does touching both the input or output plugs stop the hum?.
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and do you think reversing c4 will help?
and i didnt follow there layout i just worked it out from the schematic |
I wouldn't have thought it would affect the hum, but C4 is likely to fail fairly quickly. |
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| b<ben>n |
i fixed c4.
and the hum dose not go away when i touch the output it just gets quiter. i cant see what happens if i touch the output becuase its a 3.5mm thats pluged into the computer. |
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| psychosteve |
even though its battery powered, id put in a supply bypass cap, 100 or 200nf or 1uf is fine, because the earth is batt -ve any pickup through the cabbles/ box/ ext wiring will get to the chip, find something with longish leads and solder pin to pin strait ove the chip. i use the ne5534 for first stage mic preamps (ne5534 10times gain then tl07xxto make up rest of gain) and they can get a bit funny at low gain.
try and get R A Penfolds books, preamp and filter circuits, audioamplifier projects, or there is one for musicians, but i cant remember the name. there is some great stuff in there.
hope this helps, steve.. .. |
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| psychosteve |
ps, id use the tl 071 op amp anyway its much more forgiving and unless you need very very low noise isnt worth the extra hassle of the instabiliity.
plus id be inclined to use a fixed gain of 10x and a pot at the output to controle the level, but every one has their own preferences as to topology.
see you soon, steve.. .. |
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| b<ben>n |
| thanks il give that a try later when i have some time again. ahhh exams suck. |
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