Op amp guitar buffer noisey when volume down - Page 1
Forum section: electronics / projects

author: dfox416
author_rank: Contributor
timestamp: February 12, 2020, 12:42:37 pm
content: I built a battery powered op-amp buffer for an electric guitar, and I am having a problem that as I turn the volume knob down on the guitar, I hear the noise/hum of the buffer increase. Basically, the guitar pickup is connected to a 500k volume potentiometer, which is connected to the input of the buffer. The noise is worst when the volume is about half way, presumably because this is where the input source impedance is largest. However, I cannot wrap my head around exactly what causes this, and why the same does not happen when I plug my guitar into a normal amplifier or effects pedal.

author: dzseki
author_rank: Frequent Contributor
timestamp: February 12, 2020, 01:22:56 pm
content: Your input impedance is high. When the volume pot is half way your output impedance also rather high, therefore your guitar cable and even the guitar picks up noise easier. Try to lower your input impedance on the amplifier.

author: Yansi
author_rank: Super Contributor
timestamp: February 12, 2020, 01:33:41 pm
content: What opamp have you used and what was the circuit supplied from?

author: Yansi
author_rank: Super Contributor
timestamp: February 12, 2020, 01:35:10 pm
content: Quote from: dzseki on February 12, 2020, 01:22:56 pmA guitar requires high input impedance. In this case the 1M is good enough.

author: dmills
author_rank: Super Contributor
timestamp: February 12, 2020, 01:45:39 pm
content: Resistors generate noise in proportion to their value.Opamps have current noise which (if you pick the wrong part for your impedance value) can get converted into noise voltage by the source impedance.You probably want a jfet input opamp (There is a reason TL071s are everywhere in guitar circuits), and if noise is a real concern you might do better to buffer the guitar and then use a much lower value pot (Think 10k) at the output of the buffer.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 12, 2020, 02:26:17 pm
content: 500k is a pretty high impedance. Usually guitar volume pots are not wired in usual volume pot arrangement, where the "output" would be shorted to ground when down. In guitars, it's usually the opposite: the pickup(s) are shorted to ground when the volume pot is fully down, and the ouput impedance of the guitar is at its maximum.So here, with a 500k pot, it's getting close to what you'd get with the input of your buffer just floating. A lot of noise.

author: Renate
author_rank: Super Contributor
timestamp: February 12, 2020, 07:14:31 pm
content: Electronically speaking, put the preamp in the guitar.Ok, this solution may not appeal to you.Electric guitar sound is based on magnetic pickups into high impedance inputs.Putting low level, high impedance, unbalanced signals over a long cord wasalwaysa bad idea.My (acoustic) guitar has an internal preamp and puts out a high level, low impedance, balanced signal.

author: sam[PS]
author_rank: Contributor
timestamp: February 28, 2020, 03:35:47 am
content: Have you measured the noise higher ? Or just hear the noise higher ? For this kind of things only a proper measure will tell you if there is a problem or not.As mentioned before if problem there is it's probably an impedance related issue.Another consideration is that an electric guitar pickup is basically a coil, so with the volume pot it forms a LR low pass filter, the higher the parallel resistor (lower part of the pot) the lower the cut frequency and the more the high freq is attenuated. So it might just be that your pickup is picking up some high freq parrasitic signal that is just attenuated enough with the pot up but less and less when the pot goes lower. Some ballpark/back of the envelope calculation using your pickup's typical inductance (good pickup manufacturer do spec that) and the pot value might confirm if that can be part of the issue or not, this cut frequency might well be beyond audio spectrum, on you to check for that.If the noise are high frequencies consider low passing instead of a wide band follower, guitar signal don't have much high freq even in audio sens of the term so that can't harm. Somewhere around 5kHz is probably a good place to start.In a more general scope, lots of noise problem in audio electronics comes from the layout, not from the schematic, so you might check that as well if nothing else seams to work.Just my 2cts...