Fet preamp circuits for dynamic microphones - Page 1
Forum section: electronics / projects

author: computerlen
author_rank: Newbie
timestamp: August 01, 2020, 09:50:44 pm
content: I would like to converse with others on this topic. Presently I am making microphone preamps or boosters [like the cloud lifter and fet head] for low output dynamic microphones such as the Shure SM7 and ribbon mics that operate on the 40 volt phantom power of audio mixers. These preamp circuits work perfectly but I have questions about why does this happen and why does that happen? You might disagree with me but I have found that blue leds operate on the 25-40 volt phantom supplies with 5meg and more resistors in series with the leds. I have even wired 4 blue leds in series with the 2.2MEG OR 5 meg resistors in series with them connected to the phantom voltage on pins 2&3 of the male XLR CONNECTORS. THANKS. LEN. [you are welcome to get in touch with me at computerlen@hotmail.com. This might be faster]

author: Zero999
author_rank: Super Contributor
timestamp: August 01, 2020, 10:33:15 pm
content: A phantom power supply is used for a condenser microphone, not dynamic.Why do you want to use a FET input amplifier for a dynamic amplifier? Dynamic microphones have a low impedance, so a high input impedance FET input amplifier is not necessary and will have a higher voltage noise, than a BJT input amplifier, everything else being equal.

author: Bassman59
author_rank: Super Contributor
timestamp: August 02, 2020, 03:47:06 am
content: Quote from: computerlen on August 01, 2020, 09:50:44 pmWhat is your question, exactly?

author: Whales
author_rank: Super Contributor
timestamp: August 02, 2020, 05:20:59 am
content: Indeed, I don't understand what you're asking.You start with something about pre-amps, but then talk about your surprise that LEDs can be powered by the phantom power supply used for mics? Phantom power is normal electricity, no ghosts involved.Perhaps you're asking for designs of phantom-powered pre-amps?

author: magic
author_rank: Super Contributor
timestamp: August 02, 2020, 09:52:28 am
content: Not sure if OP is a robot or just needs to sober up after Saturday

author: computerlen
author_rank: Newbie
timestamp: August 02, 2020, 05:12:41 pm
content: I guess that I didn't explain myself properly. Is there anyone here who would email me and we can write back and forth about preamp circuits? Someone who is knowledgeable about electronics. I know that phantom power is used traditionally for condenser mics. No one has heard about the Cloud Lifter or the Fet Head? The voltage does not get to the mic's pins. The phantom power is used to amplify the low output level from the mic. And the fets used in my circuits are used in a low impedance configuration. Now this is why I hesitate to post anything on websites. There are always smart asses who know better. Isn't one of the stipulations on this site to NOT PUT DOWN ANOTHER"S COMMENTS?

author: computerlen
author_rank: Newbie
timestamp: August 02, 2020, 05:37:09 pm
content: 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.

author: magic
author_rank: Super Contributor
timestamp: August 02, 2020, 06:21:24 pm
content: Okay, not a robot. Fair enough, phantom powered preamps are possible. I have actually built a simple one for interfacing cheap-ass low voltage electret capsules with standard phantom mic inputs. FET devices are no problem too, although bipolar may have less noise.I'm not sure if anyone will be interested in talking over email, but if you have something that you can post publicly, go ahead.

author: themadhippy
author_rank: Super Contributor
timestamp: August 02, 2020, 06:46:19 pm
content: Quoteribbon mikes and phantom power,a combination that should never meet

author: OM222O
author_rank: Frequent Contributor
timestamp: August 02, 2020, 07:29:35 pm
content: If you want help, you should put up with some nonsense, if you have a question then ask it. No one will bother to go back and forth via emails. there will always be signal AND noise (just like your amplifier circuit) when you ask questions.

author: computerlen
author_rank: Newbie
timestamp: August 02, 2020, 08:42:12 pm
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: August 02, 2020, 08:51:56 pm
content: Quote from: computerlen on August 02, 2020, 05:37:09 pmYes, 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 pmUse 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

author: fourfathom
author_rank: Super Contributor
timestamp: August 02, 2020, 09:16:33 pm
content: Quote from: Zero999 on August 02, 2020, 08:51:56 pmI'm just curious, how would a common-gate JFET compare in the low-Z case (source is the input, drain the output)?

author: computerlen
author_rank: Newbie
timestamp: August 02, 2020, 11:30:09 pm
content: 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.

author: Marco
author_rank: Super Contributor
timestamp: August 02, 2020, 11:53:25 pm
content: Quote from: fourfathom on August 02, 2020, 09:16:33 pmA BF862 is so low noise in the audio band it simply doesn't matter any more.

author: Bassman59
author_rank: Super Contributor
timestamp: August 03, 2020, 03:19:08 am
content: Quote from: computerlen on August 02, 2020, 11:30:09 pmAsk here, engage here. I don’t know why anyone would want to continue this as a private discussion.

author: Zero999
author_rank: Super Contributor
timestamp: August 05, 2020, 08:27:58 am
content: Quote from: fourfathom on August 02, 2020, 09:16:33 pmSince the voltage noise is between the gate and souce, it won't make any difference to the noise voltage, whether it's common gate or common source.

author: computerlen
author_rank: Newbie
timestamp: August 07, 2020, 10:39:00 pm
content: I would like to converse via my email address about my circuit design. I can more easily send the wiring diagrams back and forth to the more knowledgeable tech about my ideas of improving the circuit. Also, if that tech is is willing to go to the trouble of writing to me directly, I am happy to share the schematic. I really do not want too many people to see this diagram.

author: magic
author_rank: Super Contributor
timestamp: August 08, 2020, 06:29:14 am
content: Quote from: Marco on August 02, 2020, 11:53:25 pmThat might be true.Just the Johnson noise of a 150~200Ω dynamic mic is more than 1.5nV/rtHz.Do they make refrigerated mics to cut it down?

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2020, 10:55:51 am
content: Quote from: computerlen on August 07, 2020, 10:39:00 pmThe downside is no one else can see any of the schematics and make suggestions/corrections. Suppose you and I discuss this via email and I make a few mistakes in my responses, you probably won't spot the. Now consider the same scenario, but on the forum, where everyone else can read and correct my errors.

author: arno
author_rank: Contributor
timestamp: August 08, 2020, 04:41:20 pm
content: Hi,a source for really good microphone preamp schematics was Studer's FTP server (http://ftp.studer.ch/). It went down several years ago, but you can find a backup athttps://www.reeltoreel.nl/studer/Public/Products/73 de Arno, DL2SSBEdit: Please, take a look at the 990 input circuit. There is no FET, instead a BC327.https://www.reeltoreel.nl/studer/Public/Products/Mixing_Analog/990/Manuals/

author: magic
author_rank: Super Contributor
timestamp: August 08, 2020, 07:45:35 pm
content: Nice avatarBut seriously, how manymegabytes to get those few PDFs?Quote from: Zero999 on August 08, 2020, 10:55:51 amOP has a working circuit and is looking for guinea pigs, not for suggestions. I semi-expect that you have just triggered him to come back and post yet another boring post with all the same stuff about "I would prefer email" and why he doesn't care about your concerns in the slightest. So don't bother with trickery and just admit that you are a principled exhibitionist who can't stand a bit of privacy.OP, maybe try DIYAudio. As you see, not much response here. Truth is, we all are exhibitionists, terminally lazy and upvote whores. We only read threads for pictures (you have posted none) and write answers for those "thanked" counters on our profiles (nothing like that by email)

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2020, 09:18:08 pm
content: Quote from: magic on August 08, 2020, 07:45:35 pmHe hasn't actually posted any schematics. If he starts to be a problem here, then I'm sure the moderators will ban him soon.

author: magic
author_rank: Super Contributor
timestamp: August 08, 2020, 09:25:05 pm
content: Quote from: Zero999 on August 08, 2020, 09:18:08 pmAnd never willQuote from: computerlen on August 02, 2020, 08:42:12 pm

author: computerlen
author_rank: Newbie
timestamp: August 08, 2020, 11:34:36 pm
content: I just made contact with a tech who I sent the wiring diagram to. That is all I wanted. Someone to communicate with back and forth about the diagram. Someone who is showing genuine interest. No more of these chat rooms for me. Bye.

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2020, 02:15:06 pm
content: Quote from: computerlen on August 08, 2020, 11:34:36 pmThe whole point of a chat room is to communicate with many people simultaneously. If you don't want to do that, then this is not the place for you.

author: David Hess
author_rank: Super Contributor
timestamp: August 09, 2020, 11:12:25 pm
content: Quote from: magic on August 08, 2020, 06:29:14 amLinear Integrated Systems and Interfet also make some suitable low frequency low noise JFETs, but they have a higher flicker noise and higher capacitance than a less expensive bipolar transistor. The JFETs would be more suitable for a "universal" design because they could be used with both low and high impedance sources if their input capacitance was bootstrapped.