Audio baluns for sound card input?

[Joerg]

Hello Rich,

So, it's not so much that the power supply itself is providing the
noise, just that the ground is so long that it makes a nice high
impedance path for the common mode noise to excite?

Well, the PS is the source. I believe these things have gotten a lot
worse over the years. After hooking up a new printer a couple months ago
I couldn't even hear our local AM station anymore. Blew me away. All you
heard was hash. Until I cut down on it with a good dose of #43 ferrites.
Now we have more #43 toroids in the supply bins than other people have
aspirins. Luckily that stuff is cheap.

Regards, Joerg

 

[Joerg]

Hello Graham,

I got one of my protoype switchers to *hiss*. It seemed to happen as it
transitioned from non-continuous to continuous mode. I fitted a new core
and the noise went away IIRC. Probably due to indifferent assembly of the
transformer.

Could be but that printer thing is glued shut. Sometimes when I have to
look at switchers I get the impression that there are designers who
don't even know the difference between CCM and DCM anymore. Let alone
the RHP issue.

Regards, Joerg

 

[Joerg]

Hello Tony,

I've used common mode chokes successfully in this situation, and also
in the serial port connection, since this too seemed to be necessary.
Compared with an isolating transformer, at very low frequencies, the
transformer has great isolation but poor response, whereas the CM
choke has no isolation but great response, and this seems to be more
appropriate. They are similar at HF, but as the load impedance can be
quite high, there should be no real problem at HF. Try anything that's
available, eg I've recently come across some very cheap CM chokes for
ethernet applications that are 4.7mH with 4 windings. Others are up to
47mH (better still).

That's a good idea. Actually I have CM chokes with several mH.

Regards, Joerg

 

[Joerg]

Hello Bob,

this unit is ideal for this use stereo 2 isolating transformers in one
box, used for car
audio.http://www.maplin.co.uk/Module.aspx?ModuleNo=33172&TabID=1&source=15&WorldID=&doy=1m3
availible from radio shack i think

Yes, that looks very similar to the RS version, except that they also
throw in 3.5mm to RCA adapters which is nice for laptops.

Regards, Joerg

 

[Joerg]

Hello John,

I don't know exactly what you are doing (e.g. how many microphones and
how long are the cables), but I think you need to do some tests before
you assume that the power supply is the problem. While these things emit
into the mains supply, they don't normally emit enough radiation to
cause problems with audio unless the audio and power supply cables are
much too close together.

Well, when run on the battery all noise is gone.

How are you proposing to use 'baluns'?

They are only called baluns by the audio people. It's just an isolation
transformer that can also be used for balun situations such as a
symmetrical mike into a grounded amp input etc.

Regards, Joerg

 

[Pooh Bear]

Hello John,


Well, when run on the battery all noise is gone.


They are only called baluns by the audio people.

Really ? I thought it was an RF term ! Been in audio from hobbyist level
onwards for over 35 yrs and never heard an audio type use the term.

Funnily enough I was just about to go to dictionary.com to see if I could find
an explanation for the term.

Aha - BALUN Balanced/Unbalanced

http://www.acronymfinder.com/af-query.asp?p=dict&String=exact&Acronym=BALUN

It's just an isolation
transformer that can also be used for balun situations such as a
symmetrical mike into a grounded amp input etc.

Yup, I'd call it an isolation transformer too. You really need an UNBAL !


Graham

 

[Ted Edwards]

When someone at our church tried to record audio into a laptop this
caused a serious racket if line powered. Probably this is due to the
laptop switch mode power supply. Only running the laptop off its battery
produces a decent recording. Is there a simple balun transformer that
you could recommend?

What brand of laptop? Does any of "racket" get back into the rest of
the church audio system? If you hold a small portable radio close to
the line operated power supply or the computer, do you pick up a lot of
noise? (I'll be surprised if you do! The FCC get pretty upset about
these things creating a lot of RFI.)

From your sig, I assume you have easy access to a decent oscilloscope.
Try looking at what's on the lines with the laptop operating of line
power.

Personally, I suspect a lot of 60Hz, etc. pickup. If the garbage is
only a few volts or less, you could easily build a differential
amplifier out of three transistors, a few small capacitors and a 9V
battery. The third transistor is for a current source to supply the
emitter current for the diff-amp. This will give very good common mode
rejection.

Ted

 

[J M Noeding]

......... and practical experience in solving problems helps

 

[Michael A. Terrell]

........ and practical experience in solving problems helps

The last adapter I built cost less than $6 for the parts. I always
have a small pile of empty project boxes and the line matching
transformers so all I had to buy was the two pots, a knob and one
connector. It takes about 30 minutes to drill the holes, mount the
parts and solder all the wires. I still have one I made 20 years ago
with a R.L. Drake tuning knob on the box. I built it and used it to
record radio spots from a satellite feed and sent tapes out to a number
of radio stations every month. No one ever said a word about the audio
quality. Since John is so worried abut stepping the voltage up less
than a 1 to 3 ratio you can turn it round and lose a couple dB up front.

 

[John Woodgate]

I read in sci.electronics.design that Joerg

Hello John,


Maybe, but it's not my laptop so I'll have to ask. Some "modern" power
supplies seem weird to me. One in a new printer works but produces lots
of EMI and makes inconsistent hissing noises. Like a whining bearing.
Dell said that's "normal".

What Dell says and what is true are not the same. I suspect it will fail
rather soon.

 

[John Woodgate]

I read in sci.electronics.design that Joerg

They are only called baluns by the audio people. It's just an isolation
transformer that can also be used for balun situations such as a
symmetrical mike into a grounded amp input etc.

AFAIK, they are only called baluns by **RF** people, not audio people.

 

[Jim Gregory]

Hear, hear, we always called them 600 ohm:600 ohm telephone-line repeaters
or isolating xformers.
Baluns (RF) are often added to a TV/radio receiver to convert 300 ohm ribbon
antenna to 75 ohm coax input.

 

[J M Noeding]

Since John is so worried abut stepping the voltage up less
than a 1 to 3 ratio you can turn it round and lose a couple dB up front.

stepping up from loadspeaker o/p to line impedance.... By the way, is
your experience with posting on newsgroup about the same level as your
missed technical experience?

 

[Michael A. Terrell]

stepping up from loadspeaker o/p to line impedance.... By the way, is
your experience with posting on newsgroup about the same level as your
missed technical experience?

How is your track record in putting mission critical systems back on
line and keeping them operating while meeting all specifications? As
far as newsgroups I've been online for at least five years. BTW, the
special DC powered Microdyne 700 series receiver I worked on for NASA is
still in use aboard the International Space Station. Now stick your
nose back up your ass and piss off.

 

[Joerg]

Hello Fred,

Run the laptop off the battery and run a small jumper from laptop case
ground to the power supply safety ground while it is plugged in. If the
noise is present then a small isolation transformer, in conjunction with
screened twisted pair mic cable, should improve the situation and will
be the best you can do. If no noise is present with the jumper, then you
are wasting your time, the power supply is corrupting the sound card
circuitry itself- nothing you can do except go after that power supply
to increase attenuation of high frequency components by cutting cable
and interposing small box with multiple LC lowpass chain and ferrite
chokes.

That's a good idea to test a ground loop w/o the power supply. I suspect
it'll be clean though because we also use other gear there, such as a
cassette deck for recording. Those are grounded and work fine.

The laptop is said to work ok with its power supply and a mike, just not
when connected to the other (grounded) equipment.

Regards, Joerg

 

[Joerg]

Hello Graham,

To cut a long stort short.... Can you afford $30 ? It's small, portable and
does what you want.

Pro audio guys call it a DI box.

http://www.behringer.com/DI20/index.cfm?lang=ENG

Thanks. That is not very expensive at all. We'll see what the RS
transformer does. It should at least improve things noticably but if it
leaves noise then, yes, it will be time for a DI box. But maybe a
passive one because the Ultra-DI needs phantom power (which we have but
it increases the number of cables) or a 9V battery.

Regards, Joerg

 

[John Woodgate]

I read in sci.electronics.design that Joerg

The laptop is said to work ok with its power supply and a mike, just
not when connected to the other (grounded) equipment.

B*****d computer people, never did understand audio and never will!

 

[Joerg]

Hello Ted,

What brand of laptop? Does any of "racket" get back into the rest of
the church audio system? If you hold a small portable radio close to
the line operated power supply or the computer, do you pick up a lot of
noise? (I'll be surprised if you do! The FCC get pretty upset about
these things creating a lot of RFI.)

Dell. I don't know about that one but I just held an AM radio to the DC
cable of my Inspiron 2500. If closer than a few inches I even lost local
AM stations which were replaces by the typical "wheeeeshhhhh".

From your sig, I assume you have easy access to a decent oscilloscope.
Try looking at what's on the lines with the laptop operating of line
power.

I hope to avoid having to schlepp a big HP or Tek to church but will if
needed. This audio room is very packed and just squeezing in there with
a scope will be a challenge.

only a few volts or less, you could easily build a differential
amplifier out of three transistors, a few small capacitors and a 9V
battery. The third transistor is for a current source to supply the
emitter current for the diff-amp. This will give very good common mode
rejection.

It was higher frequency stuff, the typical switcher noises. I'll try
passives first because anything with batteries is a hassle. Murphy's law
says they'll exhaust in the middle of the sermon. That has already been
a big problem with our wireless mikes until we found nifty uC controlled
charge stations and 9V NiMH that can pack 250mAh.

Regards, Joerg

 

[J M Noeding]

How is your track record in putting mission critical systems back on
line and keeping them operating while meeting all specifications? As
far as newsgroups I've been online for at least five years.

then you might have learned how to follow the thread

 

[Joerg]

Hello John,

B*****d computer people, never did understand audio and never will!

Well, yeah, they could hire some pros to take care of this stuff. But
nowadays I have the impression that computers are pieced together using
existing designs. And pieced together in quite a hurry. The MB comes
from this mfg, the drives from that, the sound stuff from another and
the PS from yet another. Slap it all together and you've got a PC. Sort
of. As long as it does video games nicely then 95% or more of the
customers are going to be happy campers.

Heck, even our answering machine (from a BIG US mfg) is full of noise
when the broadband connection runs. Some day I'll fix that, too, but
sometimes I am getting sick of having to polish off the designs that
others were supposed to finish properly. Unless I am paid for it, then
I'll do it with gladness.

Regards, Joerg