Help with design considerations of a proper microphone preamp - Page 1
Forum section: electronics / projects

author: t1d
author_rank: Super Contributor
timestamp: July 27, 2022, 09:19:47 am
content: Background = I am a hobbyist. I have nice lab equipment that is smarter than I am.Design GoalsI want to DIY a microphone amp for a Sennheiser e835 mic that will work with audio systems that are Balanced, Low Impedance and Line Level. I want to use a Differential Op Amp arrangement, to take advantage of its noise canceling abilities, with a Constant Current Source added on its Tail. (I am just now learning about all of these terms and I may not have a clear understanding of them.)I have LM386 Op Amps in stock. They are left over from building a simple 20W, 9V Battery Powered, 8 Ohm Speaker Amplifier, as a proof of concept. I will run the new preamp into this speaker amp, if it is appropriate for the new preamp. (This speaker amp was built as a Proof of Concept. It needs some minor corrections, but let’s address those in a separate thread, at some later point, unless the new amp will simply not work with it.)This is the speaker amp schematic. Not the new preamp under consideration.The preamp will be powered at 9V, because it is convenient.Questions1) The LM386 has a Current Mirror on the output of its Differential Pair of Transistors. Will this serve the same function as a separate Constant Current Source on the output, such as in the arrangement shown in JohnAudioTech’s design?2) The LM386 has an internal resistor divider to create a virtual ground at the midpoint of Vsupply and its Diff Amp arrangement is referenced to this ground. Does that somehow violate the goal for a Balanced grounding system?3) As I understand it, adding gain within an op amp can lead to greater noise. The LM386 is configured to have a gain of 20, if no external adaptations are made. The Sennheiser microphone has an output sensitivity (free field, at 1 kHz) of 2.7 mV/Pa.This is an area that I am not understanding well. If the math is [20 Gain X 2.7mV = 54mV] and “Line Level” is say 1Vpp, then don’t I need to configure the gain to [1000mV/2.7mV = 370 Gain]?The maximum gain of the LM386 is 200. So, if a gain of 370 is needed, I will need to add additional amplification stages. I would think that I would want to keep the gain of the first amplification, which would contain the noise canceling differential amplifier, to a minimum and do the heavier lifting in another amplifier downstream.4) As I understand it, op amps have high impedance inputs and low impedance outputs. The LM386 has 50K termination resistors on its inputs. This is repeated, of course, in the main amplifier, because it uses the same op amp. Therefore, I would think that there is not any mismatch in impedance between the proposed preamp and main speaker amp.I appreciate your comments and help.

author: david77
author_rank: Frequent Contributor
timestamp: July 27, 2022, 09:26:12 am
content: I have a feeling the LM386 may not be the best choice for a mic preamp. You want something that is very low noise and high gain in this application. Have a look athttps://sound-au.com/.I

author: jonpaul
author_rank: Super Contributor
timestamp: July 27, 2022, 10:50:49 am
content: Bonjour cher MonsieurA dynamic mic may be specified at ~one mv rms output into rated Zo, eg 600 Ohms, at standard sound pressure level SPL, thus you need a preamplifier that can have low noise relatif to uVs.Most dynamic mic preamplifiers use a Zo matching transformer for best CMRR rejection, and no opamp.The transistors noise figure, mic and amp impédance and other characteristics are important.Suggest you study the huge selection of app notes, articles and books on low noise microphone preamplifiers.Check AES, studio sound and pro audio forums and sites.Mic transformers were made by old line transformers firms, nowadays try Jensen Transformers or Lundahl.Bon chancejon

author: CaptDon
author_rank: Super Contributor
timestamp: July 27, 2022, 01:19:04 pm
content: You do not mute a mic by going open circuit!!! You drop a short across pin 2 and 3 of the XLR connector. Phantom mic power is often provided by supplying +48vdc through a pair of 6.8K resistors one to pin 2 and one to pin 3 of the XLR. The return is through the shield. The LM386 you chose is total crap for your application. That is so '40 years ago' in mic preamp design. There are far better choices today, the OPA series for instance and even ultra low noise instrumentation amplifier I.C.'s

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 27, 2022, 02:49:11 pm
content: As t1d has described the project and why two 386 are being used, as input as well as output, what is the need for such negative criticism?Sennheiser e 835 gives -52 dBV @ 1Pa into 1000 OhmSo the optional increased gain of one of the 386 will be neededI have a Shure SV100 here with similar specs for the teenage rock band.Its mixer channel is using a transformer input , I just tried plugging and unplugging the mic XLR at normal gain.Then moved mic xlr to a higher impedance guitar channel.There is no problem with either of these XLR channels being unplugged to open circuit , a bit more noise clicks on the high Z channel.I also see that the Stratocaster selector switch does open circuit the various pickups when they are not used.The Telecaster has two pickup sets and its switch actually open circuits both in the mid position.So contrary to exclamations "Do Not.." we have 80 years and probably millions of examples of audio inputs being open circuited during live performance.

author: TimFox
author_rank: Super Contributor
timestamp: July 27, 2022, 03:38:17 pm
content: For low noise in a microphone preamplifier, you may consider the THAT Corp series 1510, 1512, 1580, and 1583 devices.http://www.thatcorp.com/THAT_IC_Selection_Guide.shtml#Preamplifiers

author: Jaye
author_rank: Newbie
timestamp: July 27, 2022, 04:31:15 pm
content: Well,The current mirror, basically is an internal function of the LM386 that, in this case can be ignored. The DC blocking capacitor at the output fairly indicates you are not intending to us any substantial current going into the power amplification stage so no additional circuitry, such as the current mirror you mention, would be needed.Stepping aside from the issues of noise ( noise floor hiss, etc ) mentioned already, what you have in mind should work as shown. Because you are new at this, just put the circuit together and start making measurements. It should pass audio and right there you have something to work with. It all begins with making a circuit work. Followed with measurements to see the baseline performance of your microphone amplifier.Thank you.Jaye

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 27, 2022, 04:50:51 pm
content: THAT data for the prog preamps is shown as around -105 dB for Rs = 150 Ohm.Now t1d can't have Rin that low although his circuit's Rin can be reduced from presently shown values of around 10k.NF for 386 is not on data sheet as far as I read.If the Rin can be reduced to around 2 k then ERin ~ -123 dBVas "ASSume" ordinary transistor is used in 386 diff amp, typical NF at 2 k Rin is about < 4dB ( eg 3904 data Fig 10 for example)I think the 386 will be quite OK noise wise for the $100 microhone with -52 dBV.Side note: Notice how the professional performers get real close to the microphone.

author: t1d
author_rank: Super Contributor
timestamp: July 27, 2022, 05:28:10 pm
content: Thank you so much, to each and everyone of you, for your help and comments! You are greatly appreciated!@ JonPaul - Most dynamic mic preamplifiers use a Zo matching transformer for best CMRR rejection, and no opamp.I have heard about using a transformer for CMRR rejection, but it is an area I need to study.@CaptDon - You do not mute a mic by going open circuit!!! You drop a short across pin 2 and 3 of the XLR connector. Phantom mic power is often provided by supplying +48vdc through a pair of 6.8K resistors one to pin 2 and one to pin 3 of the XLR. The return is through the shield. The LM386 you chose is total crap for your application. That is so '40 years ago' in mic preamp design. There are far better choices today, the OPA series for instance and even ultra low noise instrumentation amplifier I.C.'s- Excellent point on the mute switch; I will make corrections.- I am still rather confused about needing to supply +48V for my particular mic = e835. The data sheet is quite abbreviated. See attached- Better choices than the LM386 = See below.@mag_therm - Thank you for your support.@TimFox - Thank you for pointing out the other models in the THAT series. I had looked for the 1510 at Mouser, but they did not have it. I did not know of the others; 1512/1580/1583. Mouser does have these and I am ordering several of each to play with and learn from.@Jaye - The current mirror, basically is an internal function of the LM386 that, in this case can be ignored. The DC blocking capacitor at the output fairly indicates you are not intending to us any substantial current going into the power amplification stage so no additional circuitry, such as the current mirror you mention, would be needed.- Hmm... JohnAudioTech's experiments show a significant improvement in CNRR with the use of a CCS. Are you saying that the CCS is only helpful, if the circuit is starved for current, and it is not likely that my use will be starving the circuit for current? Here's the link to the JAT video.Stepping aside from the issues of noise ( noise floor hiss, etc ) mentioned already, what you have in mind should work as shown. Because you are new at this, just put the circuit together and start making measurements. It should pass audio and right there you have something to work with. It all begins with making a circuit work. Followed with measurements to see the baseline performance of your microphone amplifier.- I greatly like your approach. There is lots to learn through experimentation and play. And, this board is small enough to make OSH Park the better deal on getting the board manufactured = 3 Boards/$10.21USD/Shipped = Cheap Enough.@mag_therm - THAT data for the prog preamps is shown as around -105 dB for Rs = 150 Ohm.Now t1d can't have Rin that low although his circuit's Rin can be reduced from presently shown values of around 10k.NF for 386 is not on data sheet as far as I read.If the Rin can be reduced to around 2 k then ERin ~ -123 dBVas "ASSume" ordinary transistor is used in 386 diff amp, typical NF at 2 k Rin is about < 4dB ( eg 3904 data Fig 10 for example)I think the 386 will be quite OK noise wise for the $100 microhone with -52 dBV.Side note: Notice how the professional performers get real close to the microphone.- Agreed. I will play with LM386, now, and have the THAT 1512/80/83 for future explorations.Thank you all, again!!! What a great community we have in EEVBlog/Forum!

author: Zero999
author_rank: Super Contributor
timestamp: July 27, 2022, 05:50:41 pm
content: You're better off with a cheap op-amp, such as the NE5532 or NE5534. The 5534 is single, but slightly lower noise and is a little less common than the 5532. Two channels of the 5532 can be connected in parallel to reduce the noise

author: t1d
author_rank: Super Contributor
timestamp: July 27, 2022, 06:22:58 pm
content: Quote from: Zero999 on July 27, 2022, 05:50:41 pmThanks! I am pretty sure that I have the 5532 in stock. I will check.

author: MikeK
author_rank: Super Contributor
timestamp: July 27, 2022, 07:54:00 pm
content: OPA2134 is a pretty good audio op-amp, I must say.

author: Martian Tech
author_rank: Regular Contributor
timestamp: July 28, 2022, 02:35:55 am
content: Or use one of these:https://www.ti.com/lit/ds/symlink/ina217.pdf(hint: use 2 9V batteries to provide +9V and -9V power)