Any simple mods for lowering the noise floor of microphones?

[jp008]

Again I'm terrible at this, don't risk wasting your time on this assuming it works right

Very interesting work, nonetheless. I'm assuming we're still talking about a circuit that's being applied BEFORE plugging it into a preamp? Since that was my initial inquiry, which seems to be almost non-existent, at least, on the internet.

BTW.. I see you're also a plugin developer. Very interesting plugin work as well, but what I'd really like to try is a version for the old DirectX format because the program I use to record vocals is very old and does not support VST.

[aciddose]

There should be a VST to DX wrapper somewhere. I believe the DX plug-in interface is already obsolete and replaced by other interfaces in modern DX under modern OSes.

Here's a further improved version with some changes (100k input impedance, 20k output = useful in a modular synth or for power saving: 1/2 mA usual peak current, stand-by ~400 uA.)

Code: Select all

Version 4
SHEET 1 880 680
WIRE -272 -16 -272 -32
WIRE -144 -16 -144 -32
WIRE -144 80 -144 64
WIRE -144 80 -160 80
WIRE -16 80 -144 80
WIRE 48 80 -16 80
WIRE 176 80 48 80
WIRE -272 144 -272 64
WIRE -272 144 -288 144
WIRE -144 144 -144 80
WIRE -144 144 -192 144
WIRE -16 160 -16 80
WIRE 48 160 48 80
WIRE -144 176 -144 144
WIRE -240 192 -192 144
WIRE -448 224 -464 224
WIRE -432 224 -448 224
WIRE -272 224 -272 144
WIRE -272 224 -288 224
WIRE -208 224 -272 224
WIRE 176 240 176 80
WIRE -240 256 -240 192
WIRE -208 288 -240 256
WIRE -144 288 -144 272
WIRE -144 288 -160 288
WIRE -80 288 -144 288
WIRE -64 288 -80 288
WIRE 48 288 48 240
WIRE 48 288 32 288
WIRE 112 288 48 288
WIRE 144 288 112 288
WIRE -448 336 -448 224
WIRE -16 352 -16 240
WIRE -16 352 -32 352
WIRE 48 352 -16 352
WIRE 176 352 176 336
WIRE 176 352 160 352
WIRE 208 352 176 352
WIRE 304 352 272 352
WIRE 336 352 304 352
WIRE -272 384 -272 224
WIRE -208 384 -208 288
WIRE -144 384 -144 288
WIRE -80 384 -80 288
WIRE -16 384 -16 352
WIRE 48 384 48 352
WIRE 112 384 112 288
WIRE 176 384 176 352
WIRE 304 384 304 352
WIRE -448 480 -448 416
WIRE -272 480 -272 448
WIRE -272 480 -448 480
WIRE -208 480 -208 448
WIRE -208 480 -272 480
WIRE -144 480 -144 464
WIRE -144 480 -208 480
WIRE -80 480 -80 448
WIRE -80 480 -144 480
WIRE -16 480 -16 464
WIRE -16 480 -80 480
WIRE 48 480 48 448
WIRE 48 480 -16 480
WIRE 112 480 112 448
WIRE 112 480 48 480
WIRE 176 480 176 464
WIRE 176 480 112 480
WIRE 304 480 304 464
WIRE 304 480 176 480
WIRE 304 496 304 480
FLAG -464 224 input
IOPIN -464 224 In
FLAG 336 352 output
IOPIN 336 352 Out
FLAG -144 -32 b+
IOPIN -144 -32 In
FLAG 304 496 0
FLAG -160 288 follow
FLAG -32 352 base
FLAG 144 288 amp
FLAG -272 -32 bias
IOPIN -272 -32 In
FLAG 160 352 ofollow
FLAG -288 144 i_bias
FLAG -160 80 supply
SYMBOL npn -208 176 R0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL npn 112 240 R0
SYMATTR InstName Q3
SYMATTR Value 2N3904
SYMBOL npn -64 352 R270
SYMATTR InstName Q2
SYMATTR Value 2N3904
SYMBOL res -288 -32 R0
SYMATTR InstName R1
SYMATTR Value 100k
SYMBOL cap -368 208 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C1
SYMATTR Value 1µ
SYMBOL cap 272 336 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C3
SYMATTR Value 1µ
SYMBOL res 160 368 R0
SYMATTR InstName R6
SYMATTR Value 20k
SYMBOL res 288 368 R0
SYMATTR InstName R7
SYMATTR Value 100k
SYMBOL res 32 144 R0
SYMATTR InstName R5
SYMATTR Value 220k
SYMBOL res -160 368 R0
SYMATTR InstName R2
SYMATTR Value 10k
SYMBOL res -32 368 R0
SYMATTR InstName R4
SYMATTR Value 20k
SYMBOL default\\voltage -448 320 R0
WINDOW 3 -63 183 Left 2
WINDOW 123 24 32 Left 2
WINDOW 39 0 0 Left 0
SYMATTR Value SINE(0 100m 1k)
SYMATTR Value2 AC 1 0
SYMATTR InstName VG
SYMBOL res -32 144 R0
SYMATTR InstName R3
SYMATTR Value 220k
SYMBOL cap 32 384 R0
SYMATTR InstName C2
SYMATTR Value 1µ
SYMBOL res -272 208 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName Ri1
SYMATTR Value 1k
SYMBOL res -160 -32 R0
SYMATTR InstName Rbattery_ref
SYMATTR Value 1
SYMBOL cap -288 384 R0
SYMATTR InstName Cb1
SYMATTR Value 3.3n
SYMBOL cap -96 384 R0
SYMATTR InstName Cb3
SYMATTR Value 3.3n
SYMBOL cap 96 384 R0
SYMATTR InstName Cb4
SYMATTR Value 4.7p
SYMBOL cap -224 384 R0
SYMATTR InstName Cb2
SYMATTR Value 100n
TEXT -128 -48 Left 2 !V1 b+ 0 DC 9
TEXT 272 328 Left 2 !.tran 0 200m 198m
TEXT -400 -48 Left 2 !V2 bias 0 DC 730m

schematic:

io:


Usually the "bias" input would be from a 1 meg trimmer or panel mounted pot if you wanted to use it for timbre adjustment. (That's my preference in my implementations of the circuit.) You can vary the input impedance by adjusting R1 but without a FET or darlington buffer you won't get super high Z anyway.

I've added a lot of bypass capacitors although I'm not sure they're perfect, the result is a very nice band above -3.01 dB from 9.5 Hz to 25 kHz. It might also be useful to bypass the power supply and use an inductor input (LC filter) there to minimize noise from a regulated supply.

I should really document the circuit more thoroughly... but you'll have to figure out how to tweak things mostly on your own. I'll just provide this rough list (consider this draft quality):

1) input impedance = Ri1 + R1 + Cb1 + Q1 (mostly tweak R1 for example 1 meg is a common value.)
This is simple enough, but depending upon what input you have you'll want to set the impedance right to get the results you need. 100k is very low and the 1k + 3.3n low-pass filter should sit near 25k cutoff to remove radio-frequency signals from the input before the Q1 follower.

2) gain = Ri1 + R1 + R2 + R5 (mostly increase Ri1 and reduce R2 to decrease gain.)
By adjusting input/output current and the ratio of Q2 base current and voltage you can control the gain within certain limits. It's almost always going to be better to avoid using signal-path resistance to form a voltage divider or feedback path to control gain since they'll have a negative influence on noise level and the intentional non-linearity.

3) output impedance = R5 + R6 + C3 + R7 (mostly just R6 sets the output impedance.)
You always need enough supply current from Q2 and R5 for the Q3 follower to operate and drive R6, C3 and R7 correctly. That said the existing configuration should work fine if R6 is 10k or 4.7k instead, which are more common values for output impedance.

4) Q2 common-base base bias + filter = R3 + R4 + C2 (Mostly just C2 controls the low-frequency handling of the circuit.)
You can adjust the amplifier bias current or level to change gain or maximize headroom, trim symmetry (odd vs. even harmonic) and adjust the low-frequency amplification. C2 might typically be larger to pass sub-bass frequencies. In a mic pre you generally might even decrease it significantly to increase the high-pass cutoff. It's possible to supply R3 via a PNP current source rather than connecting it to the supply rail to allow for voltage control, but this while cool probably won't be very useful since the biasing circuit (R1) would need to be more complicated to maintain symmetry. That said, you can get variable voltage-controlled gain a little bit from this simple circuit too.

That was a lot more info than I planned on. Pretty much covered everything except RFI bypassing which can be summed up as "Cb#" capacitors form RC low-pass filters to filter away ultrasonic frequency content.