Electronics > Projects, Designs, and Technical Stuff

Fet preamp circuits for dynamic microphones

<< < (3/6) > >>

computerlen:
I just thought that maybe someone else was searching for a GREAT simple circuit to amplify their weak output dynamic mic such as a Shure SM7 that I see being used almost everywhere on the internet. I have that circuit that goes before your board's input preamp circuit. It's completely safe for ribbon mics and dynamic mics. Plus no noticeable noise is added. I am really not looking for help because I have been experimenting for the last few months and tweaking the circuit. I guess what I really am hoping for is to help someone who has searched just about everywhere for the wiring diagram and I will send it to him or her but I want that person to at least write directly to me so that I might find out what he or she thinks of the results when they build it. This is the second time that I have joined an electronics forum only to be criticized for one thing or another.

Zero999:

--- Quote from: computerlen on August 02, 2020, 05:37:09 pm ---Why do I want to a FET input amplifier for a dynamic amplifier?? I think you meant to write 'dynamic mic'. I have wired up about a dozen preamps using fet transistors. I have proven that fets work as low impedance preamps and my circuit blocks the dc from getting to dynamic mics. Low level dynamic and ribbon mics. there is much more that I could write about to someone who is genuinely interested. So much information to share.

--- End quote ---
Yes, that's what I meant. Of course a FET input amplifier will work with a dynamic microphone. It'll just have inferior noise performance, to a BJT input amplifier, assuming everything else is equal. BJTs have much lower voltage noise, than FETs, but higher current noise. A BJT input amplifier is better for low input impedances, such as a dynamic microphone. A J-FET input amplifier, is better for high input impedances, such as a condenser microphone.

--- Quote from: computerlen on August 02, 2020, 08:42:12 pm ---I just thought that maybe someone else was searching for a GREAT simple circuit to amplify their weak output dynamic mic such as a Shure SM7 that I see being used almost everywhere on the internet. I have that circuit that goes before your board's input preamp circuit. It's completely safe for ribbon mics and dynamic mics. Plus no noticeable noise is added. I am really not looking for help because I have been experimenting for the last few months and tweaking the circuit. I guess what I really am hoping for is to help someone who has searched just about everywhere for the wiring diagram and I will send it to him or her but I want that person to at least write directly to me so that I might find out what he or she thinks of the results when they build it. This is the second time that I have joined an electronics forum only to be criticized for one thing or another.

--- End quote ---
Use a low noise op-amp. The NJM2122 looks good and is cheap too. If you want differential, use an instrumentation amplifier.
https://www.njr.com/electronic_device/PDF/NJM2122_E.pdf

fourfathom:

--- Quote from: Zero999 on August 02, 2020, 08:51:56 pm ---A BJT input amplifier is better for low input impedances, such as a dynamic microphone. A J-FET input amplifier, is better for high input impedances, such as a condenser microphone.
--- End quote ---

I'm just curious, how would a common-gate JFET compare in the low-Z case (source is the input, drain the output)?

computerlen:
I have a diagram if you are interested. It works perfectly with no noticeable noise. 4 fets are used. My problem was figuring out how to use the voltage from the mixer's phantom supply to amplify the low level output from a Shure SM7 mic or a ribbon mic and also get the mic's audio 'back into' the mixer's input using pin two of the XLR connector from where the voltage comes from. Plus, of course, somehow blocking the dc from entering the mic. I run an internet music program and have 3 of these preamps in use. I find it easier to send schematics at computerlen@hotmail.com. I won't be coming back to this site for another few weeks.

Marco:

--- Quote from: fourfathom on August 02, 2020, 09:16:33 pm ---I'm just curious, how would a common-gate JFET compare in the low-Z case (source is the input, drain the output)?

--- End quote ---
A BF862 is so low noise in the audio band it simply doesn't matter any more.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod