Hi,
Trying to build a simple guitar single stage preamp.
Im using a OPA4134 surface mount.
I dont actually play guitar got a cheap as chineese bass just wanted to muck around with building circuits really, possbily make a amp for my friend in the future.
Anyway just wanted to know what issues are with my circuit. I'm running it to Auxilary in on my Audigy2 zs soundcard. It has a little background noise when plugged in which im certain there should be close to none?
When I plug in the guitar it hums like crazy. Its cheap chineese crap it has a volume pot and a tone i guess it is(a pot in series with a cap to earth). Pretty much no shielding in the guts of it(i mean like foil or something the internal wiring is sheilded).
Anyway help with my circuit would be appreciated. Thanks.
The unused opamps have positive inputs shorted to ground and outputs to negative inputs. Power supply pins have 0.1uf caps on them. Im guessing this circuit gain is pretty high? so would a 2 stage play nicer?
Trying to build a simple guitar single stage preamp.
Im using a OPA4134 surface mount.
I dont actually play guitar got a cheap as chineese bass just wanted to muck around with building circuits really, possbily make a amp for my friend in the future.
Anyway just wanted to know what issues are with my circuit. I'm running it to Auxilary in on my Audigy2 zs soundcard. It has a little background noise when plugged in which im certain there should be close to none?
When I plug in the guitar it hums like crazy. Its cheap chineese crap it has a volume pot and a tone i guess it is(a pot in series with a cap to earth). Pretty much no shielding in the guts of it(i mean like foil or something the internal wiring is sheilded).
Anyway help with my circuit would be appreciated. Thanks.
The unused opamps have positive inputs shorted to ground and outputs to negative inputs. Power supply pins have 0.1uf caps on them. Im guessing this circuit gain is pretty high? so would a 2 stage play nicer?
I can't for the life of me find the edit button.
The attachment I posted didnt work so here it is.
The attachment I posted didnt work so here it is.
The gain is (44k + 1k)/1k = 45.
What kind of cap did you use at the input? I think you would probably want a little larger cap and a smaller resistor, there, maybe a factor of 22 higher for the cap (2.2uF) and a factor of 22 smaller for the resistor (or anything close to that, or even half that). I would use a nice polypropylene film cap there. At least don't use a ceramic cap there. Polyester would be OK too, but not as good as polypropylene.
Resistors generate their own noise. And they generate more noise for higher resistances. Ideally, I would want to use nothing over about 10K, although it might not be noticable unless everything else is perfect. So maybe you could put two 2.2uF film caps in parallel, at the input, and use a 10K R to ground. But that would raise your high-pass filter's cutoff frequency from 1.6 Hz to 3.6 Hz. If you think that's too high, you could increase the C or the R. Maybe 22K would be a better value, since it would also be easier for the source to drive.
I would definitely add a 10uF electrolytic in parallel with each of the 0.1 uF on the power supply pins. I might even use something larger than 10 uF.
I would add a 1K resistor in series with the input (between the R to ground and the + input), to somewhat balance the effects of the input bias currents. Then I would add a few hundred pF to ground from the + input, to form an RF low-pass filter. You could vary the 1K while measuring the output voltage with the input shorted to ground, and use whatever R is needed to make the output's DC offset about zero.
And I would probably add a 10 Ohms to 100 Ohms or so resistor in series with the output. I would probably also add a 10 pF or so ceramic cap in parallel with the 44k feedback resistor, to roll off the gain at RF frequencies.
How have you routed the ground returns?
Tom
What kind of cap did you use at the input? I think you would probably want a little larger cap and a smaller resistor, there, maybe a factor of 22 higher for the cap (2.2uF) and a factor of 22 smaller for the resistor (or anything close to that, or even half that). I would use a nice polypropylene film cap there. At least don't use a ceramic cap there. Polyester would be OK too, but not as good as polypropylene.
Resistors generate their own noise. And they generate more noise for higher resistances. Ideally, I would want to use nothing over about 10K, although it might not be noticable unless everything else is perfect. So maybe you could put two 2.2uF film caps in parallel, at the input, and use a 10K R to ground. But that would raise your high-pass filter's cutoff frequency from 1.6 Hz to 3.6 Hz. If you think that's too high, you could increase the C or the R. Maybe 22K would be a better value, since it would also be easier for the source to drive.
I would definitely add a 10uF electrolytic in parallel with each of the 0.1 uF on the power supply pins. I might even use something larger than 10 uF.
I would add a 1K resistor in series with the input (between the R to ground and the + input), to somewhat balance the effects of the input bias currents. Then I would add a few hundred pF to ground from the + input, to form an RF low-pass filter. You could vary the 1K while measuring the output voltage with the input shorted to ground, and use whatever R is needed to make the output's DC offset about zero.
And I would probably add a 10 Ohms to 100 Ohms or so resistor in series with the output. I would probably also add a 10 pF or so ceramic cap in parallel with the 44k feedback resistor, to roll off the gain at RF frequencies.
How have you routed the ground returns?
Tom
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Despit the many other suggestions I gave you, which should still be used, I think that your primary problem is probably being caused by grounding issues. If you can post a diagram or photo of the actual connections and circuit, it would probably be helpful. In any case, check out the thread linked below, which should help you to fix the grounding problem:
http://www.diyaudio.com/forums/chip-amps/115698-understanding-star-grounding.html
http://www.diyaudio.com/forums/chip-amps/115698-understanding-star-grounding.html
wow that is a huge response. Thank you very much!
The input cap is polyester. I forgot to mention I'm running off 2 rechargeable 12v battery's.
Looked everywhere for my camera, Dad has taken his good camera on holiday.
The 0.1 is all i could find. I might be off for a trip to the shops. Will have a look around see if i can find anything else and hopefully the other camera in the process.
As I understand it with the grounding you only ever want one ground path for anything to go through. Is this right?
Will get a pic up soon.
Thanks again for the nice reply.
The input cap is polyester. I forgot to mention I'm running off 2 rechargeable 12v battery's.
Looked everywhere for my camera, Dad has taken his good camera on holiday.
The 0.1 is all i could find. I might be off for a trip to the shops. Will have a look around see if i can find anything else and hopefully the other camera in the process.
As I understand it with the grounding you only ever want one ground path for anything to go through. Is this right?
Will get a pic up soon.
Thanks again for the nice reply.
You also don't want the wrong grounds sharing the same path. Have a look at the link I posted. It's pretty good.
Sound cards can have some weird inputs and your problem might not be the circuit at all. I know my Emu needs a good DC path to ground and hates to be overloaded. As an experiment, you might try a resistive pad between the amp and the sound card, say 1k in series with the amp output, and 1k from the sound card input to ground. Maybe a large coupling cap too, if the amp has any dc offset. If that improves things by more than 2X, then it's an input problem.
CH
CH
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