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How to eliminate ground loop noise from USB for microphone circuit

Fluffyboii

Active Member
Hi,
I have this problem for a long time now. Whenever I power something from USB that has analog circuitry inside, high pitched annoying random noise invades the circuit. I was able to eliminate some of it by using a BJT capacitance multiplier but some high pitched random stuff still appears on the output. I learned that this is most likely from ground loop issue and there are some devices that can be bought that are especially build to eliminate this issue. Only problem is that these devices are usually expensive and hard to find.
It is also said that this issue can be eliminated by having the circuit galvanic isolated.
For example DiyPerks electret mic circuit runs from USB power. But his diy microphone has no annoying noises at the end. Only logical reason can be that the NMA0515 isolated boost converter is eliminating it. Finding this device locally is a nightmare though. I can only buy it from china and unknown sources. He also has 4400uF capacitance connected directly to USB power. Wouldn't this cause sparks at the USB port when you plug it in and ruin it overtime.
1674606553885.png

Regular non isolated boost converters are cheap but they don't fix the issue. For example this is the one I got which directly converts 5V USB up to 26V. I soldered two pole low pass capacitance multiplier with 2000uF and 2.6K resistors (3.3K parallel with 12K because 12K was too much and caps were taking too long to charge) which doesn't really help. Seems like generally adding low pass filters to the circuit that will be powered doesn't really work to fight against this noise.
MVIMG_20230125_033239.jpg

I have multiple transformer cores so I can wind a transformer for making a galvanic isolated boost converter. Any simple topologies for making a boost converter that will eliminate USB noise? Most of the galvanic isolated DC-DC boost circuits are too complex for my liking. If it can go up to 48V it would be nice to make a USB powered Phantom Power supply. But a lower voltage like 25V would work as well. My objective is making a low noise USB power supply that I can use for my future USB powered projects.

For lowest noise possible I found out that using a simple transformer to lower mains voltage, full bridge rectify it and use something like 7809 regulator with lots of filtering works really well. But I don't really want to make it work from mains and I need higher voltage.

I also found out that when using 2 pin electret capsules with build in JFETs and powering the preamplifier circuit from batteries and shielding the circuit with metal case some noise still appears when gain is increased. This noise doesn't change with changing the op amp from generic TL081 to something else. Which only leaves the JFET itself as the only possible noise source. The capsule I tested was this head part from some random microphone I found for cheap:
1673900665946-png.139959

audiogrus pinned electret preamp circuit. All metal resistors, additional two pole input low pass with 10ohm resistor and 100uF capacitors, decoupling caps + one directly at the op amp inputs. As I said I put it inside a metal contained and grounded both the electret capsule shield and the container itself. Still did not get low noise signal out of it.
IMG_20230125_040701.jpg

rjenkinsgb made this simple amplifier that can run off the mic line voltage and it performs much better with a cheap two pin small capsule. It basically has no noise compared with the one on top at similar output levels. I am not sure how can it works so well with just one active element.
pc_mic_preamp-jpg.139654


I also just purchased a 34mm bare electret capsule accidentally thinking it was a true condenser one (they look very similar). Default configuration on sale page is this:
1674608042671.png

Can I configure it as a 3 terminal electret instead (is this technically 3 terminal). From my understanding and testing in real life when I have some negative feedback at the internal JFET (degeneration resistor) the gain of the capsule decreases but SNR ratio increases. Then I can amplify it with a regular Op Amp and have much better Gain/Noise ratio since Op Amp performs better than a single JFET amplifying most of the signal. Looks like the picture at the bottom is how people usually wire bare electret capsules. JFET directly connected to the capsule. He then connects it to the circuit at the first image having resistors both at the drain and source.
1674608401434.png


I also want to add that I wanted to use this capsule with rjenkinsgb's microphone upgrade circuit. He took the trouble of sending one to me and I really appreciate it. I am sure it works really well but the fact that I accidentally bought an electret capsule and me having no XLR USB interface with Phantom supply makes it complicated.
Can I leave the red circled area unoccupied on the PCB and directly connect the middle positive plate of the bare electret to Gate of Q4 JFET and ground its negative plate to circuit's ground.
1674609751047.png

Sorry for posting so many different thing in one topic. I just can't get this out of my head.
 
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Fluffyboii

Active Member
Not all caps have equal bypass performance for the same capacitance :

iu


OSCON is polymer tantalum.


Regards, Dana.
I used both electrolitic and ceramic caps to filter input voltages. I am aware electrolitics don't work well at high frequencies but high capacitance ceramic or poly caps are usually rated for high voltages and very large. Maybe I should buy some Tantalum ones. They go up to 100uF at online store.

This is the noise that haunts me:
 
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danadak

Well-Known Member
Most Helpful Member
Your construction technique not exactly a low noise construction method,
especially with regards a "stiff" ground. Manhattan technique :

iu


This is not so good at RF as islands create unwanted C stray, but low freq stuff
very good.

Ferrite beads can help : https://www.murata.com/en-us/products/emc/emifil/overview/lineup/bl

Also : https://www.eternaltools.com/blog/manhattan-printed-circuit-board-project-by-will-beattie



Regards, Dana.
 
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Fluffyboii

Active Member
Your construction technique not exactly a low noise construction method,
especially with regards a "stiff" ground. Manhattan technique :

iu


This is not so good at RF as islands create unwanted C stray, but low freq stuff
very good.

Regards, Dana.
This is how I constructed AM radio circuits. Only problem was that I did not have those small island pad things so everything was floating except the ground. What they are called, I would like to buy some.
Although if I put the whole circuit inside a grounded chassis wouldn't that do a good job at eliminating interference as well. The noise in my case mostly comes from the supply so shielding the circuit doesn't really help.
 

audioguru

Well-Known Member
Most Helpful Member
The Murata voltage booster has perfect isolation between its USB input and its outputs and ground then there is no ground loop. Do you hear 60Hz or 120Hz hum or high frequency hiss? 60Hz hum is picked up through the air when the connecting cable or circuit wiring picks it up line antennas do.

It looks like the gate of your Jfet is not biased to 0VDC with an extremely high resistor resistance, but is floating at any voltage.
 

Fluffyboii

Active Member
The Murata voltage booster has perfect isolation between its USB input and its outputs and ground then there is no ground loop. Do you hear 60Hz or 120Hz hum or high frequency hiss? 60Hz hum is picked up through the air when the connecting cable or circuit wiring picks it up line antennas do.

It looks like the gate of your Jfet is not biased to 0VDC with an extremely high resistor resistance, but is floating at any voltage.
Some designs point out a 1G resistor to ground thats is connected to the capsule and Gate of Jfet or a Jfet op amp input. Then a voltage appears on the resistor which gets amplified. I have made a ~1G resistor by connecting 20 56M ohm resistors in series, would that work.

My noise problem doesnt sound like 60Hz. It is more like a high pitched annoyance instead of a low freq hum. It also sometimes gets the type of beeps you get from telephone call signal. I had HDD motor sound appearing in my old laptops build in sound card's speaker output and it was very noticable with sensitive in ear monitors so nothing suprise me. If buying that Murata boost converter or something similar like that will fix my issue I can do it.
 

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audioguru

Well-Known Member
Most Helpful Member
Hiss is high frequency random noise. A whistle is a high pitched single tone that might be produced by the Murata voltage boosted and they do not say what frequency it operates at.

When you find a huge expensive microphone like yours for cheap then maybe it is a defective one (electret 48V stored voltage was too low?).
 

Fluffyboii

Active Member
Hiss is high frequency random noise. A whistle is a high pitched single tone that might be produced by the Murata voltage boosted and they do not say what frequency it operates at.

When you find a huge expensive microphone like yours for cheap then maybe it is a defective one (electret 48V stored voltage was too low?).
I do not have the Murata voltage booster. The bare capsule I bought didn't arrive yet.

I have the two terminal electret capsule build into its own head unit with metal mesh (MU-78 VII). I currently have things in 2. 3. and 4. images at #1 and the PCB at the last image. Circuid soldered on perfboard is your electret mic preamplifier with tl081 as non inverting amplifier. I tried powering it with 12V lead acid battery which gave suboptimal gain/noise performance. Powering it with non isolated USB boost converter creates more high frequency noise, hiss. This is not something new because I am using the same circuit for 1 year with my microphones at home. I made another one into a metal Turkish Delight box before. That one was battery powered so it performed nicely but got lost at some point.

When large capsule arrives I will start experimenting with it and see what I can do. by soldering it to JFETs. There are many different circuits to run bare electret capsules from phantom supply or boost converter at first image. I want to utilise rjenkinsgb's PCB. He made a PCB that can be configured in different ways to fit bare condenser capsule, two and three terminal electrets and possibly bare electret capsules. It doesn't necessarily need a phantom supply but designed to work from one.
 
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Fluffyboii

Active Member
It looks like the gate of your Jfet is not biased to 0VDC with an extremely high resistor resistance, but is floating at any voltage.
Somehow small capsule get away with not using the very high value resistor, they must be using parasitic resistances or something like that. Didn't knew high value resistor was mandatory at first. No one sells a 1G resistor here, some say I am making it up and it doesn't exist. Highest one I found was 130M ohm in 1W packade.
 

danadak

Well-Known Member
Most Helpful Member
This is how I constructed AM radio circuits. Only problem was that I did not have those small island pad things so everything was floating except the ground. What they are called, I would like to buy some.
Although if I put the whole circuit inside a grounded chassis wouldn't that do a good job at eliminating interference as well. The noise in my case mostly comes from the supply so shielding the circuit doesn't really help.

https://farcircuits.net/manhatten_pads_list.htm The pics of the pads not loading, I have notified
the company.






Regards, Dana.
 
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Fluffyboii

Active Member
Since I went home from college and get to use my desktop PC I noticed that the Apple USB C dongle I was using with my laptop as sound card was contributing some noise. My PC motherboard sound card mic input is more sensitive and less noisy.
 

Fluffyboii

Active Member
This phantom supply outputs 44V at input port and gives no voltage at the output port, is this normal behavior. My friend gave it to me cus it wasn't working anymore so I am bit puzzled. Looks like it has isolation transformer inside so maybe it doesn't have USB noise issue.
 

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audioguru

Well-Known Member
Most Helpful Member
Somehow small capsule get away with not using the very high value resistor, they must be using parasitic resistances or something like that. Didn't knew high value resistor was mandatory at first. No one sells a 1G resistor here, some say I am making it up and it doesn't exist. Highest one I found was 130M ohm in 1W packade.
A Jfet will not work if its gate-source voltage is "any voltage". The datasheets for most little electret mics show the resistor from gate to source so that Vgs is 0V. They use a Jfet that conducts only 0.5mA maximum. You show an electret mic circuit with a 2N4416 Jfet that conducts 15mA maximum which is 30 times more!
 

Fluffyboii

Active Member
A Jfet will not work if its gate-source voltage is "any voltage". The datasheets for most little electret mics show the resistor from gate to source so that Vgs is 0V. They use a Jfet that conducts only 0.5mA maximum. You show an electret mic circuit with a 2N4416 Jfet that conducts 15mA maximum which is 30 times more!
Interesting. If I can't find a 1/4w 1G resistor, I will have to use the the one I made. But also the image from the sale page of the capsule I bought shows no gate resistor. How those circuits work. Can't the current passing from the JFET be adjusted with resistors. Small current JFETS have smaller PN junction and junction parasitic capacitances so do they perform better? K596 seems to be whats inside common electret capsules which has 1mA max current.

Insides of cheap capsules there are no resistors and just JFET:
1674681336407.png

1674681348104.png

This seems to work somehow but not a legitimate way of doing it I guess.
 

Fluffyboii

Active Member
It arrived and it does work like this connected to the electret mic preamp:
1675287084757.jpg

Purple and green are shorted to ground. 10K is extra I added, there is another 10K feeding it DC inside the preamp.
This is just for testing. Adding 1.5G bias resistor gate to ground creates distortion and noise probably because how sensitive the gate is and no shielding. Acts like regular 2 terminal electret capsule as it is.
Sounds pretty good when I shield it with my hand from fluorescent lamp.
 
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audioguru

Well-Known Member
Most Helpful Member
Microphones must be connected with shielded audio cable that is not an antenna wire picking up interference.
Google finds no information about a 596S Jfet.
Here is how the Jfet inside the electret mic should be biased:
 

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