I ran into something like this. It was an FM band transmitter that had hum in the output no matter how I conditioned the power supply. Even cascading linear regulators and using huge capacitors didn't didn't make much difference. In desperation I powered it from a battery -no hum. Then I connected (only) the ground lead of the power supply -the hum was back!

It turned out to be a well-understood phenomenon. The rectifiers in the power supply were being switched on and off by the AC voltage and as they switched on and off the radiation from the transmitter was changing. It was like putting a 120 Hz switch on the ground plane of an antenna. The two approaches that seemed to help the most were to put ceramic capacitors across the power supply diodes and to put a bifilar choke between the power supply and the transmitter, or quadfilar in the case of feeding audio from an AC powered source.

http://home.computer.net/~pritch/shortwav.htm

 

Did you tried with Metal Box?

 

Since both inputs to U4 are derived from a single point (the output of U1), I see no reason for a fully differential amplifier there. All of the signal damage already has been done. If you want to use a differential amplifier to improve signal quality, it has to be at U1.


ak

My TL431 electret mic booster would do the job with minimum hum pickup. If the cathode load resistor is located at the power amp end; the mic & TL431 can be on the end of pretty much any length of twisted pair.

No differential amp, no hum - simples.

 

Cool thanks for replying and I will test it next week.



Not that I don't want to shield the amp, but I shielding it might be very difficult in my case, but I will discuss with others and look into the possibility of doing it. Or at least try it out in a metal box next week. Thanks for your input.



The electret mic has a 5V regulated supply through 2 1k resistors, 100n and 1u decoupling cap between two 1k resistor. Sorry I didn't draw in my circuit(hense the confusion), instead I just use an voltage source to simulate the mic.



I am not sure how to get an differential signal out from a electric mic, that's why I start with something I can understand.

I started as single ended signal, it work well if I use a separated twisted pairs. But we want to carry all the signal in a cat6 cable. Along with my audio signal, the cat6 also carry two digital line, 12V power, and another two analog signal. And it picked a lot of noise from the two digital line and two analog lines.

That's why I move to a differential pairs. My differential Rx can reject all the common mode noise from the other 6 wires in my cat6, only if I power my mic preamp from a 9v batter.

And we don't want to power our preamp with 9v battery permenantly, that's why I I tested it with 12V plug pack, that leads to the noise problem from my 12 plug pack.

It's the same project I have been working on (I already have two posts asking about how to select electric mic, and switching between electric mic with relays, which you and others have already helped). The requirements has changed, now we want to be able to run the mic up to 20meters (was 5-10m) with a cat6.

We have a VoIP module, we want to make an adaptor/add on board, which can provide up to 6 sets of mic, speaker and other control signals. A user at the far can can control which mic and speaker can be active. And only one set of mic and speaker can be active at one time.

I may have a lot of assumptions here, so if anything is not clear, feel free to ask.

Thanks.

Utp cable is not the best choice for a mic.Try shielded cable and a XLR connector.
Earth the shield.Almost no limit to the length of your cable.
Belden is a good choice in cables.

 

Try connecting the battery AND the Ground from the AC power supply. (leave the + unconnected)
That will tell you what's up

Yes, I tried it. It's all good with battery only, as soon as I connect a Ground from my AC power supply, there is noise.
Here is what I have tried:

• Connect ground only of a Linear DC supply - very noisy
• Connect ground only of a Switch supply - very noisy
• Connect ground only of a Linear DC supply (through isolation transformer) - the noise is about 1-2 hz, it's like two short noise then a longer peroid of quite, then repeat.
• Connect ground only of a Switch supply (through isolation transformer) - the noise is about 1-2 hz, t's like two short noise then a longer peroid of quite, then repeat.

Only if it is the first thing in the door. Your pseudo-differential signal, made by inverting the single-ended mic signal, inverts both the audio and the noise, so there is no cancellation in the diff amp stage....

I haven't tried it yet, I will test it later today after I built the circuit.

I ran into something like this. It was an FM band transmitter that had hum in the output no matter how I conditioned the power supply. Even cascading linear regulators and using huge capacitors didn't didn't make much difference. In desperation I powered it from a battery -no hum. Then I connected (only) the ground lead of the power supply -the hum was back!

http://home.computer.net/~pritch/shortwav.htm ...

This sound exactly like what I have, I will look into this, and will give you an update.

Did you tried with Metal Box?

I put it in a steel cooking pot, noise is still there.

My TL431 electret mic booster would do the job with minimum hum pickup. If the cathode load resistor is located at the power amp end; the mic & TL431 can be on the end of pretty much any length of twisted pair.

Can you share you circuit, I did a search, but didn't find your circuit.

Utp cable is not the best choice for a mic.Try shielded cable and a XLR connector.
Earth the shield.Almost no limit to the length of your cable.
Belden is a good choice in cables.

Using a cat6 can save us a lot of cost on wiring, and a tidy. using a separate shielded cable is my last resource.

 

Solder a wire to OpAmps GND and with other end of the wire touch the metal case of SMPSU.

 

@bug13
Can you show how the mic is hooked up? I'm not clear on how it is powered.

 

Solder a wire to OpAmps GND and with other end of the wire touch the metal case of SMPSU.

Retested with your suggestion, noise is still here.

@bug13
Can you show how the mic is hooked up? I'm not clear on how it is powered.

Please ignore the chip I put on the board, that is just me trying different things. The PCB (without my mod) match the original circuit posted.

 

I think this may be the problem.

Note: this part is powered by a separated switch mode power supply.

It looks like the ground also goes thru the cable.
A ground diagram may shed some light on it. Are there also 2 Vref's?

 

I think this may be the problem.

It looks like the ground also goes thru the cable.
A ground diagram may shed some light on it. Are there also 2 Vref's?

Ground goes thru the cable, it's digital ground.(the preamp doesn't share the same ground) This digital round is joined together with the differential Rx ground at the adapter board.

Yes there are 2 Vrefs, one at the differential Rx, another one at the preamp.

Let me know if it's not clear, or doesn't make sense, I can draw another one.

edited: some mistake on the drawing, fixed now (was wrong ground on the differential Rx)
edited_2: there are 100n and 1u on both Vrefs.

 

.
http://home.computer.net/~pritch/shortwav.htm

My power supply is switch mode, so I am assuming putting bypass on diodes is not an options, so I made a quadfilar myself, don't seem to work, attached is my home made quadfilar, anything wrongs? Do I need more turns?

edit: my quadfilar is about 6 turns, I have also a bifilar in my 12V supply only.

 

Ground goes thru the cable, it's digital ground.(the preamp doesn't share the same ground) This digital round is joined together with the differential Rx ground at the adapter board.

Yes there are 2 Vrefs, one at the differential Rx, another one at the preamp.

Let me know if it's not clear, or doesn't make sense, I can draw another one.

edited: some mistake on the drawing, fixed now (was wrong ground on the differential Rx)
edited_2: there are 100n and 1u on both Vrefs.

Wow. I think you may have a problem. I'm guessing you don't have any pairs left in the cable, but as a test can you power both boards off of a single supply. This would answer the question of the big ground loop between supplies.

 

Wow. I think you may have a problem. I'm guessing you don't have any pairs left in the cable, but as a test can you power both boards off of a single supply. This would answer the question of the big ground loop between supplies.

It works!!!! woohoo!!

 

Hi guys

I finally get it working in a cat6 (it also carries other analog and digital signals). The winner is a bifiler (thanks to DickCappels). My circuit seem to be able reject all the common mode signal also carries by the cat6. And the audio is as good as it was powered by a 9V battery.

Here is what works for me:
My original circuit work, what I do differently is adding a bifiler between my power supply and my preamp. And power my preamp from the same power supply my differential Rx. (Note: there is no bifiler between my power supply and my differential Rx)

Please feel free to suggestion any improvements I need to do/test.

Thank you all who have contributed, all your help is appreciated! Thanks again!
(I am so excited right now!!)

 

Yes, I tried it. It's all good with battery only, as soon as I connect a Ground from my AC power supply, there is noise.
Here is what I have tried:

• Connect ground only of a Linear DC supply - very noisy
• Connect ground only of a Switch supply - very noisy
• Connect ground only of a Linear DC supply (through isolation transformer) - the noise is about 1-2 hz, it's like two short noise then a longer peroid of quite, then repeat.
• Connect ground only of a Switch supply (through isolation transformer) - the noise is about 1-2 hz, t's like two short noise then a longer peroid of quite, then repeat.

I haven't tried it yet, I will test it later today after I built the circuit.



This sound exactly like what I have, I will look into this, and will give you an update.


I put it in a steel cooking pot, noise is still there.


Can you share you circuit, I did a search, but didn't find your circuit.
.

A couple of occasions it was mentioned, someone on this forum posted a link to an online copy of the Elektor article. If I saved it - I can't remember where.

 

Hello,

For a balanced output have a look at figure 7 on the following page:
http://sound.westhost.com/project93.htm

Bertus

 

Hi guys

After excitement gone, now I google it, from what I can understand, it's something to do with common-mode noise in the power supply. A good explanation is here:
http://www.murata.com/~/media/webrenewal/products/emc/emifil/knowhow/26to30.ashx

While I can get it working nicely, but I am still not quite understand why it doesn't work with separated power supply (with bifiler between my preamp and power supply), it only work with same supply (also with bifiler between my preamp and power supply).

However I think I can understand why it works on battery only.

 

Hello,

For a balanced output have a look at figure 7 on the following page:
http://sound.westhost.com/project93.htm

Bertus

I have a look at the circuit you linked in, I don't understand why it need to derive the DC from the signal line??

 

It is neither common mode nor differential mode noise by itself. It is RF getting to the diodes on the input of your power supply which then modulate the RF. These are the diodes in the switching power supply that convert the incoming mains voltage to DC, prior to switching.

You should gain some insight from the article at the URL below.

http://home.computer.net/~pritch/shortwav.htm

 

I have a look at the circuit you linked in, I don't understand why it need to derive the DC from the signal line??

It is called "Phantom Power". Condenser and electret microphone elements need a DC voltage to operate. The microphone input in a mixing console or other high end audio device has a DC voltage (usually +48 Vdc) impressed on the two signal lines through resistors (usually 6.8 K). There is not much current available, but it is enough to run a small preamp to boost the audio signal before its long journey through the cable to the console. Balanced audio techniaues eliminate a lot of induces crud, but nothing is better than having a loud, low output impedance signal out at the source.

https://en.wikipedia.org/wiki/Phantom_power

ak