Hi
Does anybody know what is better to choose for a High Pass Filter (Mic Preamp):
Big Capacitor value, small Resistor value?
or Small Capacitor value, big Resistoe value?
Or use as special Resistor value und adjust the Capacitor for the needed Frequency?
I don't have the Tools to check it by myself at home. Otherwise I would try and measure by myself ;-).
Best regards
Lukas
Does anybody know what is better to choose for a High Pass Filter (Mic Preamp):
Big Capacitor value, small Resistor value?
or Small Capacitor value, big Resistoe value?
Or use as special Resistor value und adjust the Capacitor for the needed Frequency?
I don't have the Tools to check it by myself at home. Otherwise I would try and measure by myself ;-).
Best regards
Lukas
Thas difficult, because I have aself designed Mic Preamp which after I build it I have some question why it worked 🙂.
I use an ls489b jfet, 1 Giga Ohm Gate to GND Resistor, RD 4906 Ohm, RS 293 Ohm. I combine the couple Condenser with the HPF.
How can I calculate / measure the Impedance?
Best regards
Lukas
I use an ls489b jfet, 1 Giga Ohm Gate to GND Resistor, RD 4906 Ohm, RS 293 Ohm. I combine the couple Condenser with the HPF.
How can I calculate / measure the Impedance?
Best regards
Lukas
Is the signal source a condenser microphone capsule without built-in JFET? If not, what kind of microphone is it?
Do you mean a coupling capacitor between the drain of the LS489B and whatever load it drives? Or between the microphone capsule and the gate?
Do you mean a coupling capacitor between the drain of the LS489B and whatever load it drives? Or between the microphone capsule and the gate?
it's only the 38mm Big Membrane Condenser which is polarized by 48 V on the Backplate.
The Signal goes without Capacitor into the Gate of the JFET.
I have just a Capacitor on the Drain.
The Signal goes without Capacitor into the Gate of the JFET.
I have just a Capacitor on the Drain.
Good, as trying to put a high-pass filter between the microphone and the gate could not possibly work well.
Do you know the input impedance of whatever will be connected to nodes OUT+ and OUT-?
Do you know the input impedance of whatever will be connected to nodes OUT+ and OUT-?
mostly a Focusrite Scarlet Solo Gen 4 or a DI-Box.
Chinch to Jack 6.35 mm.
The Impedance of a standart Line Input I think from 100 Ohms to 600 Ohms.
Chinch to Jack 6.35 mm.
The Impedance of a standart Line Input I think from 100 Ohms to 600 Ohms.
Standard line inputs are 10k to 50k normally. You can change the 1M resistor to implement a high-pass on the output with the 68nF. For 100Hz cutoff try 22k
The manuals are at https://downloads.focusrite.com/focusrite/scarlett-4th-gen/scarlett-solo-4th-gen
According to page 36 of https://fael-downloads-prod.focusri...carlett_solo_4th_gen_user_guide_v4-pdf-en.pdf :
3 kΩ microphone input
60 kΩ line input
1 MΩ instrument input
If I understand you correctly, you use the line input, so 60 kΩ nominal. I would then reduce the 1 MΩ resistor to something like 27 kΩ, well above the 4906 Ω drain resistor, yet smaller than the load impedance, so you get less sensitive to the accuracy of that 60 kΩ (or to the change in cut-off frequency when you connect it to other equipment). Pretending that the JFET is a perfect current source, you get a cut-off frequency of
f = 1/(2 π (4906 Ω + (27 kΩ // 60 kΩ)) Ccouple)
where 27 kΩ // 60 kΩ means 27 kΩ in parallel with 60 kΩ.
According to page 36 of https://fael-downloads-prod.focusri...carlett_solo_4th_gen_user_guide_v4-pdf-en.pdf :
3 kΩ microphone input
60 kΩ line input
1 MΩ instrument input
If I understand you correctly, you use the line input, so 60 kΩ nominal. I would then reduce the 1 MΩ resistor to something like 27 kΩ, well above the 4906 Ω drain resistor, yet smaller than the load impedance, so you get less sensitive to the accuracy of that 60 kΩ (or to the change in cut-off frequency when you connect it to other equipment). Pretending that the JFET is a perfect current source, you get a cut-off frequency of
f = 1/(2 π (4906 Ω + (27 kΩ // 60 kΩ)) Ccouple)
where 27 kΩ // 60 kΩ means 27 kΩ in parallel with 60 kΩ.
Thank you very much for your Help!
There are still two Questions:
So I have to select a Resistor smaller than the expected Impedance of the following Device?
And if so, why?
Bes regards
Lukas
There are still two Questions:
So I have to select a Resistor smaller than the expected Impedance of the following Device?
And if so, why?
Bes regards
Lukas
In audio you normally have source impedance << load impedance, so that attenuation is minimized, and signal fan-out is not an issue.
so the conclusion is:
That it isn't the question about what is better, big Capacitorvalue or big Resistorvalue.
I have to get the Impedance of the Cirquit on the Signal-Out and based on the needed low cut Frequency I have to use the Capacitorvalue based on the Result of the Equation.
Did I get that correct?
That it isn't the question about what is better, big Capacitorvalue or big Resistorvalue.
I have to get the Impedance of the Cirquit on the Signal-Out and based on the needed low cut Frequency I have to use the Capacitorvalue based on the Result of the Equation.
Did I get that correct?
If the impedance of the filter is too low, it will reduce the gain of your microphone preamplifier, as the preamplifier output is essentially a current source (the drain of the JFET) with a resistor of 4906 Ω in parallel. The amplifier's output impedance is therefore almost 4906 Ω.
I assume that you had some good reason for choosing the gain you have now, and that reducing it is therefore undesired. Hence, the filter impedance has to be large compared to 4906 Ω.
One could just use the 60 kΩ ADC input impedance as the filter resistor, I guess that is still low enough to make the filter thermal noise negligible compared to the microphone and preamplifier noise. We don't have any information about how accurate that 60 kΩ is, though, and whether or not it depends on the gain setting, for example. You also indicated that you sometimes connect other equipment, which may have a different input resistance. I therefore suggested reducing the 1 MΩ resistor to some value well below 60 kΩ and well above 4906 Ω.