midwayfair
Well-known member
I made some changes to the Aurycle FET mic circuit during a recent build:
http://aurycle.com/manual/a460-diy-schematic.pdf
I wanted to make a PCB with my changes (and some other mods) pre-made on it, but I'd like to understand what's happening before I do.
I made most of these one at a time, and reduced the noise a little bit each time. Some of them I didn't expect to make a difference, but they seem to have. Others may have had no effect.
My testing method isn't that great, but it does work a little: I set the gain to a certain level on my interface (45 dB for this test), record a clip, run an analyzer, and stack the comparison clips one behind the other on a loop. So I can tell if the noise difference is a decibel or two but I can't make fine measurements, and I don't replace the capsule with a capacitor. I also can't measure the raw self-noise figure because my analyzer doesn't go below -100 and because the pre adds its own noise.
Stuff in bold is things I KNOW reduced the noise.
0. Changed the 2SK170 for a 2SK30. The 2SK30 is lower gain. I was also under the impression that it's lower capacitance than the 170, though. This one might be a wash or might have been a mistake. I have no way of testing that will produce identical gains between the two transistors and I mostly made this change because I didn't really trust the transistor that came with the kit. Should either transistor be expected to contribute less noise?
1. Removed R1, R2, and C1. I came across a post (I think it was here) from a while ago that explained that these add noise and serve no other purpose (they were for something in the U87's testing circuit). This was a huge reduction in noise. Why does a connection to ground on a part of the capsule that is only connected to the polarization circuit (and not the FET amplifier) add noise, though?
2. Changed out C4 with a 100uF low-ESR electrolytic ... and ALSO bypassed R6 with a 100uF low-ESR. I got less noise this way than (a) bypassing the source completely with only one capacitor or (b) bypassing only one resistor. And by this I mean "less noise in the abstract" not just "better signal:noise." My guess is that the single capacitor can't fully filter this node. I'm also going to guess that a 2SK170 would have been able to make more use of the gain, possibly with ill effects on the headroom.
I also replaced R7 with a trimpot; I'm guessing that the trimpot wasn't great for noise performance, but when I pulled it out and replaced it with a 1K temporarily before adding the second capacitor, I was unable to measure a noise difference.
Another thought: Since the source is now fully bypassed, I'm not sure if there's much of a reason to bias the FET via the gate instead of just putting the gate to ground and trimming the source resistance. Gain is at max either way. Current consumption would go up but probably not by much. Would referencing the gate to ground be likely to have any effect on the noise performance compared to referencing it to a slightly positive voltage that's completely filtered? Not really something I know how to answer.
3. Removing the NFB cap (C5, the 470nF) had the opposite effect from what I expected: I got far more white noise when it was removed but didn't get a corresponding increase in gain.
4. I flipped the capsule connections and I'm taking the output from the backplate. I did this to add patterns (see the other thread). I can't remove C2 or R4 without a negative polarization voltage, but I've seen conflicting information about the leakage of certain capacitor types. I used a box cap for C2. Some mics use polystyrene there. Is one or the other a better choice if I must use an input cap?
5. The Aurycle has a pad created by connecting 150pF across the capsule. I didn't have a 150pF film cap, so I used silver mica for the pad. Is this a poor choice? I figured I'm looking at hundreds of megs of leakage resistance in a silver mica. I'm not sure if I should be worried about that dividing with the 1M or with the 1G. If it's dividing with the 1M, then I would say it's fine because the noise contribution from the leakage can't be a big deal. If it's with the 1G, though, then any noise contribution would matter. I'm okay with opening the mic back up and replacing it with a different cap if needed; the closest I have is a 220pF Panasonic ECQ-V.
6. I increased C12 (4u7), C13 (4u7), and C14 (10uF) and used low-ESR alumimum. I didn't have tantalum caps to try just increasing the capacitance first, so I changed both the cap type and value at the same time. I'm guessing this was more the type of capacitor than the capacitance, though, because 150K > 2u2 is still below 1Hz, and 4k7 > 10uF isn't that much different than 4k7 > 6u8. The C13 & C14 change seemed to have a bigger effect than the C12 change. Any thoughts here? Would going bigger with the filter caps help things, or is there a detrimental effect to increasing their value?
7. Why does this use a PNP follower instead of NPN? I thought "upside down" transistor arrangements (meaning, PNP running off a + supply) added noise, but is that only when flipping them in voltage amplification?
This circuit almost identical to the Mxl V67G, 2001, and several other mics, even some of the numbering is the same, so knowledge of those mics applies here.
Complete modified schematic -- all the numbering is the same.
https://dl.dropboxusercontent.com/u/9878279/Jon%20Patton%27s%20layouts/Circuit%20ideas/FET%20mic%20EF%20plus%20xfo.png
http://aurycle.com/manual/a460-diy-schematic.pdf
I wanted to make a PCB with my changes (and some other mods) pre-made on it, but I'd like to understand what's happening before I do.
I made most of these one at a time, and reduced the noise a little bit each time. Some of them I didn't expect to make a difference, but they seem to have. Others may have had no effect.
My testing method isn't that great, but it does work a little: I set the gain to a certain level on my interface (45 dB for this test), record a clip, run an analyzer, and stack the comparison clips one behind the other on a loop. So I can tell if the noise difference is a decibel or two but I can't make fine measurements, and I don't replace the capsule with a capacitor. I also can't measure the raw self-noise figure because my analyzer doesn't go below -100 and because the pre adds its own noise.
Stuff in bold is things I KNOW reduced the noise.
0. Changed the 2SK170 for a 2SK30. The 2SK30 is lower gain. I was also under the impression that it's lower capacitance than the 170, though. This one might be a wash or might have been a mistake. I have no way of testing that will produce identical gains between the two transistors and I mostly made this change because I didn't really trust the transistor that came with the kit. Should either transistor be expected to contribute less noise?
1. Removed R1, R2, and C1. I came across a post (I think it was here) from a while ago that explained that these add noise and serve no other purpose (they were for something in the U87's testing circuit). This was a huge reduction in noise. Why does a connection to ground on a part of the capsule that is only connected to the polarization circuit (and not the FET amplifier) add noise, though?
2. Changed out C4 with a 100uF low-ESR electrolytic ... and ALSO bypassed R6 with a 100uF low-ESR. I got less noise this way than (a) bypassing the source completely with only one capacitor or (b) bypassing only one resistor. And by this I mean "less noise in the abstract" not just "better signal:noise." My guess is that the single capacitor can't fully filter this node. I'm also going to guess that a 2SK170 would have been able to make more use of the gain, possibly with ill effects on the headroom.
I also replaced R7 with a trimpot; I'm guessing that the trimpot wasn't great for noise performance, but when I pulled it out and replaced it with a 1K temporarily before adding the second capacitor, I was unable to measure a noise difference.
Another thought: Since the source is now fully bypassed, I'm not sure if there's much of a reason to bias the FET via the gate instead of just putting the gate to ground and trimming the source resistance. Gain is at max either way. Current consumption would go up but probably not by much. Would referencing the gate to ground be likely to have any effect on the noise performance compared to referencing it to a slightly positive voltage that's completely filtered? Not really something I know how to answer.
3. Removing the NFB cap (C5, the 470nF) had the opposite effect from what I expected: I got far more white noise when it was removed but didn't get a corresponding increase in gain.
4. I flipped the capsule connections and I'm taking the output from the backplate. I did this to add patterns (see the other thread). I can't remove C2 or R4 without a negative polarization voltage, but I've seen conflicting information about the leakage of certain capacitor types. I used a box cap for C2. Some mics use polystyrene there. Is one or the other a better choice if I must use an input cap?
5. The Aurycle has a pad created by connecting 150pF across the capsule. I didn't have a 150pF film cap, so I used silver mica for the pad. Is this a poor choice? I figured I'm looking at hundreds of megs of leakage resistance in a silver mica. I'm not sure if I should be worried about that dividing with the 1M or with the 1G. If it's dividing with the 1M, then I would say it's fine because the noise contribution from the leakage can't be a big deal. If it's with the 1G, though, then any noise contribution would matter. I'm okay with opening the mic back up and replacing it with a different cap if needed; the closest I have is a 220pF Panasonic ECQ-V.
6. I increased C12 (4u7), C13 (4u7), and C14 (10uF) and used low-ESR alumimum. I didn't have tantalum caps to try just increasing the capacitance first, so I changed both the cap type and value at the same time. I'm guessing this was more the type of capacitor than the capacitance, though, because 150K > 2u2 is still below 1Hz, and 4k7 > 10uF isn't that much different than 4k7 > 6u8. The C13 & C14 change seemed to have a bigger effect than the C12 change. Any thoughts here? Would going bigger with the filter caps help things, or is there a detrimental effect to increasing their value?
7. Why does this use a PNP follower instead of NPN? I thought "upside down" transistor arrangements (meaning, PNP running off a + supply) added noise, but is that only when flipping them in voltage amplification?
This circuit almost identical to the Mxl V67G, 2001, and several other mics, even some of the numbering is the same, so knowledge of those mics applies here.
Complete modified schematic -- all the numbering is the same.
https://dl.dropboxusercontent.com/u/9878279/Jon%20Patton%27s%20layouts/Circuit%20ideas/FET%20mic%20EF%20plus%20xfo.png
But that's just my opinion...
![Soldering Iron Kit, 120W LED Digital Advanced Solder Iron Soldering Gun kit, 110V Welding Tools, Smart Temperature Control [356℉-932℉], Extra 5pcs Tips, Auto Sleep, Temp Calibration, Orange](https://m.media-amazon.com/images/I/51sFKu9SdeL._SL500_.jpg)
