Post by Jerry on Feb 13, 2024 18:04:24 GMT
Have you ever wondered why your guitar makes more hiss (white noise) when you roll off the volume pot, compared to either all the way up or all the way down? This is especially noticeable with high gain amps or pedals, and -- surprise! -- comes mostly from your guitar, not your pedals or amp.
Any electrical conductor (such as a resistor, pot, wire, or the resistance of a guitar pickup's windings) produces thermal noise, aka Johnson noise. It's caused by the thermal agitation of the electrons in the material, and it appears as a small random noise voltage across the resistor. The higher the resistance, the higher the noise.
A typical guitar pickup has a DC resistance of somewhere between about 5kΩ and 20kΩ. Let's just use 10k as an average. A common value for a passive guitar volume pot is 500k. This value is chosen to be much higher than the pickup to avoid heavily loading it, which would cause it to sound dark and lifeless. Higher pot resistance causes increased thermal noise though, except for these special cases:
- When the pot is 100% up, it's basically a 500k resistor directly in parallel with a 10k pickup. Their combined resistance is lower than 10k (about 9.8k), so the pot's high resistance doesn't actually degrade the unavoidable, but fairly small amount of hiss produced by the pickup's 10k.
- When the pot is all the way down, its wiper is shorted to ground (approx. 0 Ohms). A short circuit makes no thermal noise, so this is your guitar at its quietest. Any hiss you still have in your signal chain at this point is not coming from the guitar, but exclusively from what comes after it.
But what happens when you roll off the volume? You can think of it as two resistors wired in series. The top resistor comes from the pickup(s) and goes to the wiper, and the bottom one goes from wiper to ground. Let's say the volume is at 50%. We have the pickup's own resistance, say 10kOhm, in series with the top half of the pot (250k), for a total of 260k. On the lower side, there's 250k from wiper to ground. The top 260k appears in parallel with the bottom 250k, for an equivalent 127 kOhm between the pot's wiper and ground. This is what the guitar amp sees at this point: a 127k source resistance. This will make quite a bit more hiss than the 10k of the pickup alone (with the pot at max). This is also the cause of "tone suck" (loss of treble) with a long guitar cable. Our 127k source resistance forms a low pass RC filter with the cable's capacitance, which can easily be on the order of 1nF (0.001uF) or more. 127k with a 1nF capacitor to ground creates a nice low pass filter with a corner frequency of 1.25kHz. Dull, eh?
So, how can we fix this? Easy. We get rid of the 500k pot and use an active buffer directly after the pickups, followed by a low value volume pot that will not cause significant hiss (or tone suck due to a long cable) when part of the way down.
This is what active guitar electronics do, in a nutshell. Less noise/hiss when backing off the post-buffer, low value volume pot, and consistent tone at all volume settings, thanks to the low output impedance that's not significantly affected by the cable capacitance.
Of course, the active buffer itself should have low self noise, so it doesn't contribute significant hiss to the unavoidable pickup noise. Ideally, you'll want the buffer to produce no more noise than a single 5k resistor, which is a typical resistance of a single coil pickup. A 5k resistor has a voltage noise density of about 9nV/rtHz. I won't explain this unit of measure here, but it's the same unit used to specify op amp voltage noise density, so we can go through an op amp's datasheet to see how if it's quiet enough for the job.
To give an example, a commonly used TL072 op amp has a voltage noise density of 18nV/rtHz. THis is the same as a 20k resistor, so it's not the best
choice for a guitar input buffer if you want the lowest noise. LM4250, which I believe is what EMG uses as a buffer inside their active pickups, has it much worse, at about 40nV/rtHz (similar to a 100k resistor).
Check out the noise simulation below, it's quite revealing.
Any electrical conductor (such as a resistor, pot, wire, or the resistance of a guitar pickup's windings) produces thermal noise, aka Johnson noise. It's caused by the thermal agitation of the electrons in the material, and it appears as a small random noise voltage across the resistor. The higher the resistance, the higher the noise.
A typical guitar pickup has a DC resistance of somewhere between about 5kΩ and 20kΩ. Let's just use 10k as an average. A common value for a passive guitar volume pot is 500k. This value is chosen to be much higher than the pickup to avoid heavily loading it, which would cause it to sound dark and lifeless. Higher pot resistance causes increased thermal noise though, except for these special cases:
- When the pot is 100% up, it's basically a 500k resistor directly in parallel with a 10k pickup. Their combined resistance is lower than 10k (about 9.8k), so the pot's high resistance doesn't actually degrade the unavoidable, but fairly small amount of hiss produced by the pickup's 10k.
- When the pot is all the way down, its wiper is shorted to ground (approx. 0 Ohms). A short circuit makes no thermal noise, so this is your guitar at its quietest. Any hiss you still have in your signal chain at this point is not coming from the guitar, but exclusively from what comes after it.
But what happens when you roll off the volume? You can think of it as two resistors wired in series. The top resistor comes from the pickup(s) and goes to the wiper, and the bottom one goes from wiper to ground. Let's say the volume is at 50%. We have the pickup's own resistance, say 10kOhm, in series with the top half of the pot (250k), for a total of 260k. On the lower side, there's 250k from wiper to ground. The top 260k appears in parallel with the bottom 250k, for an equivalent 127 kOhm between the pot's wiper and ground. This is what the guitar amp sees at this point: a 127k source resistance. This will make quite a bit more hiss than the 10k of the pickup alone (with the pot at max). This is also the cause of "tone suck" (loss of treble) with a long guitar cable. Our 127k source resistance forms a low pass RC filter with the cable's capacitance, which can easily be on the order of 1nF (0.001uF) or more. 127k with a 1nF capacitor to ground creates a nice low pass filter with a corner frequency of 1.25kHz. Dull, eh?
So, how can we fix this? Easy. We get rid of the 500k pot and use an active buffer directly after the pickups, followed by a low value volume pot that will not cause significant hiss (or tone suck due to a long cable) when part of the way down.
This is what active guitar electronics do, in a nutshell. Less noise/hiss when backing off the post-buffer, low value volume pot, and consistent tone at all volume settings, thanks to the low output impedance that's not significantly affected by the cable capacitance.
Of course, the active buffer itself should have low self noise, so it doesn't contribute significant hiss to the unavoidable pickup noise. Ideally, you'll want the buffer to produce no more noise than a single 5k resistor, which is a typical resistance of a single coil pickup. A 5k resistor has a voltage noise density of about 9nV/rtHz. I won't explain this unit of measure here, but it's the same unit used to specify op amp voltage noise density, so we can go through an op amp's datasheet to see how if it's quiet enough for the job.
To give an example, a commonly used TL072 op amp has a voltage noise density of 18nV/rtHz. THis is the same as a 20k resistor, so it's not the best
choice for a guitar input buffer if you want the lowest noise. LM4250, which I believe is what EMG uses as a buffer inside their active pickups, has it much worse, at about 40nV/rtHz (similar to a 100k resistor).
Check out the noise simulation below, it's quite revealing.
