11-11-2023, 11:10 AM
Actual circuits
Fig 1 is typical electret microphone with a battery.
Fig 2 is my electret microphones before I swapped AA for 2x N/Lady/LR1 cells.
All the circuits I find using search on web are variations of fig1. The fig 2, even without increasing the 680 Ω, allows twice the signal using two cells rather than one cell. The fig1 circuit doesn't have a capacitor across the battery. A mic powered by laptop/soundcard/phone/tablet simply has the resistor and capacitor of fig1 inside the device. Some laptops seem to have noise on the electret supply, so my son changed to a lapel mic with it's own cell and inexplicably it was less noisy (the existing mic had battery, resistor and capacitor inserted inline on the cable using an old "quartz" clock case as battery holder and space for resistor and capacitor where mechanism was.
So a fig1 will have maybe at best 1/2 the battery noise of the fig2 circuit. However the FET noise is loudest noise source, followed by actual electret membrane. The noise from one or two alkaline cells LR1 to AA size) at 150 μA to 600 μA (typical range) is insignificant. At least it's a JFET, not a MOSFET and you aren't likely to find one much quieter, so changing it is pointless. I wonder what the noise of an 1j24b, EM10 or 1j42A is in comparison? However the 1.5V consumption is then 25 to 60 higher. I think I'd only bother with a valve with a true condenser capsule.
(Sketches done on a Kobo Sage, as even with export and connecting to USB it's faster than photo/scan of paper or using CAD (i've Eagle and KiCad). The png file uploaded direct from USB mass storage.)
You can see that the FET Vds is the same in fig1 and fig2 if the resistor was the same. I'd guess the 680 Ω in the Altai mic end cap across output cable is to make it look more like 600 Ω. Though the DC resistance of FET looks like 2.8V / 0.3mA = approx 9K Ohms, the AC impedance is higher because the DC current doesn't change much with Vds (it's a JFET, not a VFET).
Fig 1 is typical electret microphone with a battery.
Fig 2 is my electret microphones before I swapped AA for 2x N/Lady/LR1 cells.
All the circuits I find using search on web are variations of fig1. The fig 2, even without increasing the 680 Ω, allows twice the signal using two cells rather than one cell. The fig1 circuit doesn't have a capacitor across the battery. A mic powered by laptop/soundcard/phone/tablet simply has the resistor and capacitor of fig1 inside the device. Some laptops seem to have noise on the electret supply, so my son changed to a lapel mic with it's own cell and inexplicably it was less noisy (the existing mic had battery, resistor and capacitor inserted inline on the cable using an old "quartz" clock case as battery holder and space for resistor and capacitor where mechanism was.
So a fig1 will have maybe at best 1/2 the battery noise of the fig2 circuit. However the FET noise is loudest noise source, followed by actual electret membrane. The noise from one or two alkaline cells LR1 to AA size) at 150 μA to 600 μA (typical range) is insignificant. At least it's a JFET, not a MOSFET and you aren't likely to find one much quieter, so changing it is pointless. I wonder what the noise of an 1j24b, EM10 or 1j42A is in comparison? However the 1.5V consumption is then 25 to 60 higher. I think I'd only bother with a valve with a true condenser capsule.
(Sketches done on a Kobo Sage, as even with export and connecting to USB it's faster than photo/scan of paper or using CAD (i've Eagle and KiCad). The png file uploaded direct from USB mass storage.)
You can see that the FET Vds is the same in fig1 and fig2 if the resistor was the same. I'd guess the 680 Ω in the Altai mic end cap across output cable is to make it look more like 600 Ω. Though the DC resistance of FET looks like 2.8V / 0.3mA = approx 9K Ohms, the AC impedance is higher because the DC current doesn't change much with Vds (it's a JFET, not a VFET).
