Help in design LDC jfet+mosfet

[And]

Hello everyone!
I would like to design a circuit for a condenser microphone, but I have more experience with electronics for guitar pedals, so I'm afraid of approaching it wrong or not considering important aspects in the design of condenser mic circuits...I was thinking of developing it this way:Capsule (polarized via phantom power) connected to a JFET in a buffer configuration (to take advantage of its high input impedance and low noise characteristics but without the bias and polarization issues of FETs, which are less critical in a buffer stage) and then enter a MOSFET amplification stage (whose characteristics are much more repeatable). Exiting the MOSFET circuit, enter a step-down transformer and then to the XLR connector.I have studied various schematics but have never found MOSFETs on this type of board, so I'm afraid it might not be suitable... Could anyone give me some guidance?Sorry if my English is not very good...
Thanks to everyone

 

[kingkorg]

Hello everyone!
I would like to design a circuit for a condenser microphone, but I have more experience with electronics for guitar pedals, so I'm afraid of approaching it wrong or not considering important aspects in the design of condenser mic circuits...I was thinking of developing it this way:Capsule (polarized via phantom power) connected to a JFET in a buffer configuration (to take advantage of its high input impedance and low noise characteristics but without the bias and polarization issues of FETs, which are less critical in a buffer stage) and then enter a MOSFET amplification stage (whose characteristics are much more repeatable). Exiting the MOSFET circuit, enter a step-down transformer and then to the XLR connector.I have studied various schematics but have never found MOSFETs on this type of board, so I'm afraid it might not be suitable... Could anyone give me some guidance?Sorry if my English is not very good...
Thanks to everyone

You don't need amplification in mics. It's mostly about impedance conversion. Capsule has enough output.

 

[mhelin]

You don't need amplification in mics. It's mostly about impedance conversion. Capsule has enough output.

Maybe he needs because of the step-down transformer used as output transformer (for color maybe), but not so much anyway... Also there are many kind of MOSFET's like depletion mode vs. enhancement mode MOSFETs to start with, and generally they are quite sensitive devices for external charges and have strict limits for gate-source voltages (startup conditions must be considered well).

 

[And]

Thank you for the responses!

Yes, you're right kingkorg, what's needed is an impedance converter. However, if I'm not mistaken, in many circuits I've tried to study (such as KM84, U87), the JFET circuit introduces amplification... Is it just because a transformer with a significant reduction ratio is used later? So, do we need to boost the signal before reducing it? (as mhelin mentioned).

I'd like to understand the microphone electronics a bit more, to try to give my product some character rather than just copying an existing one... Ideally, I'm fond of the transformer-based topology (although I'll also try solutions without a transformer), so I wanted to understand why I've never seen MOSFETs in the circuits I've studied (they are widely used in other audio applications).

 

[kingkorg]

Thank you for the responses!

Yes, you're right kingkorg, what's needed is an impedance converter. However, if I'm not mistaken, in many circuits I've tried to study (such as KM84, U87), the JFET circuit introduces amplification... Is it just because a transformer with a significant reduction ratio is used later? So, do we need to boost the signal before reducing it? (as mhelin mentioned).

I'd like to understand the microphone electronics a bit more, to try to give my product some character rather than just copying an existing one... Ideally, I'm fond of the transformer-based topology (although I'll also try solutions without a transformer), so I wanted to understand why I've never seen MOSFETs in the circuits I've studied (they are widely used in other audio applications).

Drain output (common source) has higher impedance than source output (common drain). Hence it needs step down transformer to reduce impedance to useful level. In both cases transformer is needed to provide balanced signal. Unless you go for impedance balancing, or phase splitter.

https://sound-au.com/articles/jfet-design.htm

 

[Voyager10]

I have briefly experimented with building a preamp using a 2N7000 MOSFET, but found that the input noise was a good 10dB worse than a JFET.

Once you have a source impedance below 100K or so, ordinary bipolar transistors give very good noise performance, so if you're starting with a JFET buffer anyway there's no reason not to use them.

Or, if transformer output is part of the appeal, you could use an OPA1641 (widely discussed on GroupDIY) but configured with a bit of gain. Keep the resistor values low for low noise.