Cell phone noise into opamps

[Jan Panteltje]

Phil aside, low Z dynamic microphones have maximum outputs on the
order of 10s of mV, magnetic phonograph pickups have full amplitude
ratings of a few mV. This means if there is a quiet passage the
output is in the 10's of uV range, and the noise floor is at least 40
dB below that (fractional uV). This is one of the reasons that high
futility (Hi-Fi) used to be very hard.

Yes true, especially if you are in the Australian outback with
a dynamic mike listening for ants moving ;-)

As to the dynamic mike, I remember getting into an argument with
somebody, but what it came down to was: the person told me the dynamic mikes
were mainly used for on stage vocals, where it is used close to the
food opening, and the membrane is a bit heavy, so it probably will not
react a lot to the ant noises, and should not pick up anything further
away.
Capacitor mikes, and electrets, have a much lighter membrane, and are more
suitable to pick up those ant noises.
There were also ceramic mikes, usin gsome crystal, Ronette is
what I remember, those had quite a bit of output voltage too.
Same difficult to move membrane.
4.5mV/µb at 1KHz
http://www.xs4all.nl/~odemar/microphones/ronette-mics.htm

I have used the Ronettes

 

[Fred Bartoli]

Joerg a écrit :

Digikey: Only 31 OPA627 left, in the classy TO can. Nice for a retro
project but don't count on building more than 31 units. Sometimes I have
the impression the time you were talking about has come. Lots of BB
parts shortages.

The OPA656 is around 80nV/rtHz at 10Hz or as Archie Bunker would have
put it, lousay.

Oh, you can find consolation by looking at some CMOS opamp datasheet.

 

[Fred Bartoli]

Jan Panteltje a écrit :

Yes true, especially if you are in the Australian outback with
a dynamic mike listening for ants moving ;-)

As to the dynamic mike, I remember getting into an argument with
somebody, but what it came down to was: the person told me the dynamic mikes
were mainly used for on stage vocals, where it is used close to the
food opening,

Which end?

and the membrane is a bit heavy, so it probably will not
react a lot to the ant noises,

Hope there aren't any ant there!

and should not pick up anything further

Hope so too

 

[Bob Woodward]

Hello Folks,

In a really sensitive sub-audio application we are experiencing cell
phone pickup.

Hi Joerg

Does the circuit run on a single supply ?
Try dual ! and separate 0V from shielding ground by a few Ohms.

Robert

 

[Joerg]

Hi Joerg

Does the circuit run on a single supply ?
Try dual ! and separate 0V from shielding ground by a few Ohms.

It has dual supplies. Uncoupling anyb of the grounds makes things a lot
worse, pretty much what I've always seen. That's usually because the
source cannot be truly differential.

 

[Robert Latest]

["Followup-To:" header set to sci.electronics.design.]

I did build audio gear with tubes, back when even Ge transistors were
expensive. Pre-amps, compressors, bigger amps, and one really big amp.
That one had six color-TV H-tubes in it. You could make the plaster rain
down from the ceiling with a few cords on the E-guitar.

Yeah, I used to have a dozen of those PL519's, too, but never got around to
doing anything with them. But they were impressive beasts, and ambundant in
Sperrmüll TV sets.

I've never seen a hobby electronics magazine pick up on the idea of putting
some of those to good use, but maybe I just haven't looked hard enough.

robert

 

[Jan Panteltje]

["Followup-To:" header set to sci.electronics.design.]

I did build audio gear with tubes, back when even Ge transistors were
expensive. Pre-amps, compressors, bigger amps, and one really big amp.
That one had six color-TV H-tubes in it. You could make the plaster rain
down from the ceiling with a few cords on the E-guitar.

Yeah, I used to have a dozen of those PL519's, too, but never got around to
doing anything with them. But they were impressive beasts, and ambundant in
Sperrmüll TV sets.

I've never seen a hobby electronics magazine pick up on the idea of putting
some of those to good use, but maybe I just haven't looked hard enough.

robert

I know some guy who used PL519s on 27MHz.
He was far away but came in S9.
All of the sudden he was gone.

 

[[email protected]]

Hello Folks,

In a really sensitive sub-audio application we are experiencing cell
phone pickup. It got a lot worse when switching from the LT1112 to the
LM833 in order to lower the sub-audio noise (and yeah, reduce cost...).
Anyhow, this surprised me because the LM833 doesn't even have the input
protectors like the LT1112 so the chances of something clipping and
demodulating should be lower. How could this be? Older process with
larger patterns and thus more pickup loop area?

The challenge here is that bypassing is all nice and dandy but
capacitors always have a residual inductance and at cell phone
frequencies that begins to matter. Cell phones are particularly nasty
because anything that gets AM demodulated produces this rat-tat-tat
pulse pattern between phone and cell tower. Shielding is going to be a
major pain in the neck in this application.

In case anyone wants to look at the datasheets:http://www.linear.com/pc/downloadDo...64,...http://www.national.com/ds/LM/LM833.pdf

Just wondering, are the cell phones in question GSM T-Mobile or
Cingular/AT&T phones?

Michael

 

[Joerg]

Just wondering, are the cell phones in question GSM T-Mobile or
Cingular/AT&T phones?

We tried a lot of them and it was all over the map. Mostly CDMA I guess,
at least none was from T-Mobile. Mine (Nokia 2115i on VirginMobile which
uses the Sprint Net) was actually the most docile. A Verizon one really
hit hard, so did a BlackBerry. But it'll depend on how far the next
tower is and thus the ouput power it sets itself to.

 

[[email protected]]

We tried a lot of them and it was all over the map. Mostly CDMA I guess,
at least none was from T-Mobile. Mine (Nokia 2115i on VirginMobile which
uses the Sprint Net) was actually the most docile. A Verizon one really
hit hard, so did a BlackBerry. But it'll depend on how far the next
tower is and thus the ouput power it sets itself to.

Ah, ok. I thought the interference was predominantly from GSM phones.

http://en.wikipedia.org/wiki/Gsm

"It is a common occurrence for a nearby GSM handset to induce a "dit,
dit di-dit, dit di-dit, dit di-dit" output on PA's, wireless
microphones, home stereo systems, televisions, computers, cordless
phones, and personal music devices."

I had thought this was our car stereo going bad; knowing this now, it
was most likely my wife's GSM phone.

Regards,

Michael

 

[Joerg]

Ah, ok. I thought the interference was predominantly from GSM phones.

http://en.wikipedia.org/wiki/Gsm

"It is a common occurrence for a nearby GSM handset to induce a "dit,
dit di-dit, dit di-dit, dit di-dit" output on PA's, wireless
microphones, home stereo systems, televisions, computers, cordless
phones, and personal music devices."

I had thought this was our car stereo going bad; knowing this now, it
was most likely my wife's GSM phone.

Yep, just had a thread in a German NG. Dits and grumbles in the stereo
ever hour or so. One of the guys asked whether they had plopped the cell
phone down anywhere close. Answer: Uhm, ah, well, yes! Problem solved.

 

[Fred Abse]

Yep, just had a thread in a German NG. Dits and grumbles in the stereo
ever hour or so. One of the guys asked whether they had plopped the cell
phone down anywhere close. Answer: Uhm, ah, well, yes! Problem solved.

I'd have thought *everyone* over there knew about that. They've been
exclusively GSM for years in Europe.

 

[Joerg]

I'd have thought *everyone* over there knew about that. They've been
exclusively GSM for years in Europe.

Ahem, this happened in Germany