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Is it possible to make a mic preamp that only works with sound card dc output

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Fluffyboii

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Hi,
A year ago or so I made a microphone preamp with tl071 when I wasn't very good at making electronic circuits. It used a lithium ion battery, a boost converter to boost that up to 30V or something like that and feed that to the op amp which was audioguru's design which had a middle point so that op amp could function with a single sided supply. It was pain to get it working without noise that was generated from the boost converter and everyone was telling me to use one or two 9V batteries but I didn't want it because 9V batteries suck and they are expensive. But at the end, my rechargeable setup use to last 1 week with a single charge and it was working fine. I used it for a long time with both electret and dynamic microphones. Unfortunately my father lost it while "organizing" my stuff while I was at college so I don't know where it is.

Anyway at the dorm I use a single electret microphone that is directly connected to a USB-C to 3.5mm jack converter with an extension cable like this for the sake of easy usage when I rarely need it:
1671237492992.png

This works because sound card inside the converter provides some DC voltage about 2.1V to the microphone and it's internal amplifier has barely enough gain to make it sound acceptable when sensitivity is full on my computer without software boosting. Of course not all sound cards like this setup. From my experience any USB to 3.5mm sound card works with something like this but integrated ones that are in desktops and laptops have low gain so additional software boost is required which makes it sound awful. But all sound cards I tested had 2-3V bias in them so if there was a way to use that voltage for an amplifier that went between the mic and the sound card while biasing the mic as well it could be improved greatly.

For example the extension cable I use picks up some AC signal from my desk lamp because we still use flourecent lamps at dorms. If I used a differential amplifier I would be technically able to get rid of it while giving me extra gain and possibly sound quality. And this idea unfortunately got stuck in my head.

I don't want to look for low voltage op amps since they are usually work as low as 2.5V but I did not come across something with 2V min working voltage and with the single sided supply it would be even worse. So I started looking for a full discrete design.

When I checked the output of this capsule It was peaking about 100mV. Since I technically don't need additional gain with my sound card I can keep the gain low on the diff amp and focus on getting good CMMR. I immediately realized it wouldn't be possible to amplify 100mV signal with a amplifier that had no feedback since it would clip to a nice square wave. So I used some feedback to both increase input resistance and decrease the gain to something about 4 with 20K input resistors. If I find a 22k double pot for example I can make the gain adjustable on both inputs and maybe even make it be useful for dynamic mics that require much more gain. I do have 1M double pots so maybe tapering (putting parallel resistor with pot) could work?
For now with no input resistors to decrease gain it gets about 35 times max gain. CMMR is about 50dB which is more than enough. Current consumption is quite low about 250uA at 2V so hopefully sound card output will be able to feed it. I messed around with values a little and found something like this is ideal.

1671239455524.png


It works well with different input voltages. I am aware that I can replace bottom right transistor with a voltage divider and if I build this in real life I may make it with a trimpot for better adjustment. Load resistors can be replaced with PNP transistors to perhaps increase open loop gain but as it is I find it satisfactory.

Two important questions I have right now before trying this on a breadboard or something like that.
How will I get differential output from electret mic.
1671239226758.png

I saw this and I wonder if I can pick a resistor value to get something I can apply at the two inputs. Or can I use two electret microphones to get two similar signals. But if I used two mics that gave the same output it would be cancelled by the differential amplifier so one of them would need a 180 degree phase shift. Can these problems be relieved?

Second is getting the output. Should I use one of the differential outputs like a single ended output. Or should I have another circuit to convert differential to single ended output? I think I am only losing gain if I do the first one.
I think for making this work with a dynamic mic which requires about 200 times gain I would need PNP transistors or another stage too boost the gain. At that point I would have litteraly made a bad op amp that tolerates low voltages.
Even if I can't make this directly work without any batteries from the sound card I can always connect 2AA batteries to it I guess.

LM386 looks similar at the input side ignoring the darlington pairs. I started seeing differential amplifiers everywhere after learning about it. Are they they useful?
1671242379360.png
 

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Firstly, normal PC sound card mic connections have been intended for the direct connection of an electret mic capsule, since PC sound cards first appeared.

How well they work depends on the quality of the electret capsule - some are absolutely abysmal, others are excellent.

This appears to be one of the classic ones from decades ago, and they were superb!

The standard mic feed was 470 Ohms from 5V to the mic socket ring, with tip being the electret capsule output. The two were normally just linked.


I do not understand what you intend to do with a differential signal?

The PC inputs input are single-ended, so there is nowhere to connect the inverse feed.
Bad screening or bad cable that has significant resistance in the ground conductor may pick up hum. Good cable should not.

It should be possible to make a single transistor inline preamp - it's something I have considered doing, but it's never been high on my project priority list.

The simplest phase splitter is just an appropriately biased transistor with equal emitter and collector load resistors - the same voltage is developed across both, but one inverted relative to the other.

See the circuits in this document - the phase splitter is Q3:

The full circuits are build options for a mic board to feed balanced XLR microphone inputs, using different types of capsules and options.
 
Firstly, normal PC sound card mic connections have been intended for the direct connection of an electret mic capsule, since PC sound cards first appeared.

How well they work depends on the quality of the electret capsule - some are absolutely abysmal, others are excellent.

This appears to be one of the classic ones from decades ago, and they were superb!

The standard mic feed was 470 Ohms from 5V to the mic socket ring, with tip being the electret capsule output. The two were normally just linked.


I do not understand what you intend to do with a differential signal?

The PC inputs input are single-ended, so there is nowhere to connect the inverse feed.
Bad screening or bad cable that has significant resistance in the ground conductor may pick up hum. Good cable should not.

It should be possible to make a single transistor inline preamp - it's something I have considered doing, but it's never been high on my project priority list.

The simplest phase splitter is just an appropriately biased transistor with equal emitter and collector load resistors - the same voltage is developed across both, but one inverted relative to the other.

See the circuits in this document - the phase splitter is Q3:

The full circuits are build options for a mic board to feed balanced XLR microphone inputs, using different types of capsules and options.
Very informative. I knew sound cards were supposed to feed 5V but never actually measured that much. Makes sense when thinking they have a resistor integrated. I guess there is some voltage drop. I could feed the differential signal to an op amp for making it single ended. I mean maybe I should just make it with some TL074 op amps and ditch the low voltage idea. I want to know if there are analog circuits that use multiple electret microphones to decrease noise instead of using one large capsule.

LMV1090 seems to do what I wanted.
 

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It turns out I can't even get a single transistor amplifier work on this line voltage without the voltage dropping so external power seems to be necessary anyway.
 
It turns out I can't even get a single transistor amplifier work on this line voltage without the voltage dropping so external power seems to be necessary anyway.
As already explained, it's just a simple resistor to feed power to an electret microphone, it's basically a 'pull-up' and a load resistor. It's a very old system, and has worked perfectly for decades -if you connect the correct type of electret mike, then it will work fine - no preamp required.
 
As already explained, it's just a simple resistor to feed power to an electret microphone, it's basically a 'pull-up' and a load resistor. It's a very old system, and has worked perfectly for decades -if you connect the correct type of electret mike, then it will work fine - no preamp required.
After messing around a little bit I come up with this. Electret mics are connected with 1K resistors both to VCC and ground. This way I can have two inverted signals like in the LMV1090 datasheet. adding emitter resistor to the jfet inside the mics drops it's gain. For that reason I have about 100 gain at the diff amp. This way if the inputs are both connected to one mic, common mode signal is rejected. I used one op amp for getting a middle voltage so I can use it with my small lead acid battery. It works very nicely. Maybe I can have some gain at the last stage as well to not lose too much bandwidth since CMMR is good enough. I did not put too much thought to capacitor values. Maybe two caps can be added parallel to last op amps feedback to reduce high freq noise. What you think?
I don't need two mics for this, does it improve signal to noise ratio to use two. I think it sounds good and noise is much lower. If I solder it permanently and have it at the laptops input it should get rid of even more noise.

1K is bit too low for 12V perhaps, changed with 2.2K

Using one mic sucks, introduces immense noise.

Does using metal resistors really decrease noise, interesting.
 

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This works, and gives a decent amount of gain - it is directly connected to my laptop mic input, no external power.

The signal level indication in Audacity is roughly 20db higher than with the capsule directly connected to the cable.

PC_Mic_Preamp.jpg



It's just a BC550, as that was already in the breadboard; emitter to ground/screen (bare tinned wire), collector to the mic plug tip and ring (red & black) , with a 220K resistor collector to base for bias.

The collector has 5K (~4K7, but the 10Ks were in the board) to a 47uF cap to 0V.
That then has 4K7 / 5K to the electret capsule positive. That connects to the base via a 0.47uF

All values are just bits from the previous stuff in the breadboard.

The green wire links the 47uF decoupling cap negative to the lower ground strip.

If it distorts with your mic capsule, try slightly reducing the 4K7s, eg. 3K3, and use a larger decoupling cap.

It does take a few seconds to stabilise and start working properly when connected.
 
This works, and gives a decent amount of gain - it is directly connected to my laptop mic input, no external power.

The signal level indication in Audacity is roughly 20db higher than with the capsule directly connected to the cable.

View attachment 139654


It's just a BC550, as that was already in the breadboard; emitter to ground/screen (bare tinned wire), collector to the mic plug tip and ring (red & black) , with a 220K resistor collector to base for bias.

The collector has 5K (~4K7, but the 10Ks were in the board) to a 47uF cap to 0V.
That then has 4K7 / 5K to the electret capsule positive. That connects to the base via a 0.47uF

All values are just bits from the previous stuff in the breadboard.

The green wire links the 47uF decoupling cap negative to the lower ground strip.

If it distorts with your mic capsule, try slightly reducing the 4K7s, eg. 3K3, and use a larger decoupling cap.

It does take a few seconds to stabilise and start working properly when connected.
Nice. I tried same thing without using collector resistor and it didn't get me any noticeable gain, which makes sense. What you think about the circuit I made on breadboard. It requires external power and consumes about 8mA at 12V but performs nicely.
Just using the circuit you made then decreasing the laptop sensitivity would improve signal enough I guess so I will give it a try.
 
Gave it a quick test here. For some reason it did not work with that 22u filtering capacitor. When I removed it it started working. I can solder it on a jack I guess.
 

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The tip is supposed to connect to the transistor collector, not the decoupling capacitor?
Note that the tip and ring must be connected together in the plug.
 
1671322490805.png

This is the circuit I have in mind for making a pre-amplifier to replace my old one. I am not sure if I should move some of the gain to the last op amp. As it is it has 500 max gain and about 50 minimum. Normally electret mics don't require much gain but because of the setup to get differential signal from them biasing causes their inner gain to drop significantly so I think it is a good gain range. I want to know if it is a logical circuit to make a pcb for.
audioguru Since you made a electret mic amplifier in the past I want to ask you if this is a viable way to increase signal to noise ratio. I did not put a RC filter to supply since I still don't understand how that works but I would use a linear voltage converter to have clean input anyway if I were to make it work from the wall. On breadboard it works nicely with 12V lead acid battery.

If I use 2 electrets I am planning to use the GND for just grounding an wire microphones to Left and Right channel and use Left as ground for audio as seen in schematic. This should decrease noise right?
 
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TL074 opamps have FET inputs that use no current. Then the virtual ground opamp (that has its output shorted with a capacitor) is not needed.

The differential circuit also is not needed if the mic is close to the preamp and has a half-decent shielded audio cable and the circuit also feeds a good shielded output cable.

The circuit has a huge voltage gain then it should be compact and housed in a grounded (not earthed) metal box.
 

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TL074 opamps have FET inputs that use no current. Then the virtual ground opamp (that has its output shorted with a capacitor) is not needed.

The differential circuit also is not needed if the mic is close to the preamp and has a half-decent shielded audio cable and the circuit also feeds a good shielded output cable.

The circuit has a huge voltage gain then it should be compact and housed in a grounded (not earthed) metal box.
I see, so aside from those there are no major flaws. 10uF cap was something I added to output of the refference after seeing it from another circuit but it creates a current spike at the startup right? What would you recommend to do with the unused op amp in the quad package, should I use it as an output buffer even though it is not needed for sake of using it? Unfortunately I don't have quality cables. Differential amp would help to mitigate weird noises I get from long dynamic mic cable.
 
An opamp probably oscillates with a capacitor to ground shorting its output. The opamp's max output current in the capacitor when there is a signal or when it is oscillating will also heat the opamp and quickly drain the battery.
The datasheet of most opamps show that it can drive a 47pF load on its output. A series resistor of at least 100 ohms allows an opamp to drive the 100pF of a long output cable.

To disable an unused opamp, bias its + input at the half supply (REF) voltage, connect its - input to its output and do not use its output.
 
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