Parabolic microphone.

[Deleted]

Hi, everybody,

My circuit of parabolic microphone have a problem: it's capturing local radio signal's and amplifying. Does anybody knows how do I fix this?

Thanks!

[Gary Lecomte]

Mar 12, 2009 7:56:44 GMT -8 @fabaum said:

Hi, everybody,

My circuit of parabolic microphone have a problem: it's capturing local radio signal's and amplifying. Does anybody knows how do I fix this?

Thanks!

1) You need to Properly SHIELD the wires from your Microphone and any others that are Sensitive to those signals.

2) Also Possibly put the Preamp and other components into a METAL BOX, with the battery negative connected to the box, thus acting as a ground shield.

Gary

[Deleted]

Hi, Gary,

The cable (that have 3.3 feet) of the speaker is shielded, but the box is plastic.
The circuit was assembled in just one board (2 x 4 pol.).
I was thinking to put a 10nF ceramic capacitor in parallel with the 47K resistor at the "IN" of the pre-amp, maybe this night. Do you think it could solve the problem?

[Deleted]

I forgot to say: the interference occurs just when I connect the cable of the speaker in the box. The electronic parts of the "speaker as a phone" are assembled direct in the circuit board, so the speaker don't have any component directly around its.
This is the reason becouse I was thinking to put the capacitor at the "IN" of the pre-amp...
What do you think?

[Gary Lecomte]

Mar 12, 2009 17:42:08 GMT -8 @fabaum said:

Hi, Gary, The cable (that have 3.3 feet) of the speaker is shielded, but the box is plastic.
The circuit was assembled in just one board (2 x 4 pol.).
I was thinking to put a 10nF ceramic capacitor in parallel with the 47K resistor at the "IN" of the pre-amp, maybe this night. Do you think it could solve the problem?

I forgot to say: the interference occurs just when I connect the cable of the speaker in the box. The electronic parts of the "speaker as a phone" are assembled direct in the circuit board, so the speaker don't have any component directly around its.
This is the reason becouse I was thinking to put the capacitor at the "IN" of the pre-amp...
What do you think?

That 10n Cap may help.
But a Shielded box is also a good idea.
You could Possibly LINE the Inside of your Plastic box with Aluminum Foil and Connect it to Battey Negative.

Gary

[Deleted]

Gary,
The 10n Cap didn't work.
I tried to use a short cable, but I still having problems.
At the last tentative, I placed the circuit inside a aluminum pan, but the interference still...
What do you suggest?

[Gary Lecomte]

Mar 17, 2009 3:26:07 GMT -8 @fabaum said:

Gary,
The 10n Cap didn't work.
I tried to use a short cable, but I still having problems.
At the last tentative, I placed the circuit inside a aluminum pan, but the interference still...
What do you suggest?

connect the speakers metal frame to circuit ground.

[Deleted]

Also problem is microphone and amplifier impedance , high impedance make trouble, on my hand amateur radio I was use external microphone , when I try low impedance microphone, work good even with simple non shielded telephone cable, but high impedance doesn't work even if I was use short but good shielded cable. So when I was using capacitors shot ended all contacts between and in serial add suppressor choke even high impedance microphone work ok in strong RF field and I was use electret condenser microphone with integrated FET preamplifier.
In this case if I understand good is amplifier very sensitive and high impedance, all inside box must be blocked for RF using choke and capacitors too, then must work. I was have source RF on only 50 cm from microphone , you have no local radio such close microphone.

[Gary Lecomte]

Apr 13, 2009 16:09:05 GMT -8 @1oskar said:

Also problem is microphone and amplifier impedance , high impedance make trouble, on my hand amateur radio I was use external microphone , when I try low impedance microphone, work good even with simple non shielded telephone cable, but high impedance doesn't work even if I was use short but good shielded cable. So when I was using capacitors shot ended all contacts between and in serial add suppressor choke even high impedance microphone work ok in strong RF field and I was use electret condenser microphone with integrated FET preamplifier.
In this case if I understand good is amplifier very sensitive and high impedance, all inside box must be blocked for RF using choke and capacitors too, then must work. I was have source RF on only 50 cm from microphone , you have no local radio such close microphone.

Maybe you need to shield the Microphone?
Or Ferrite Beads on signal and power supply wires.

I have never had any problem like that.

[Deleted]

I was solve my problem , and also try to give idea to other people.
If I make so , maybe not impossible to make this , capacitors between any contact , and in serial in any contact small ferrite choke.
In my case 3 contact from microphone I was need 3 capacitor and 3 choke, capacitor are to small to make problems on low frequency, and choke also not interfere on audio, good idea is all prevent on this way, power supply, external speaker.
Must work.

P.S. Gary you not have this trouble because you proper design audio circuits.

This look like maybe too much components for nothing.
But before 25 year I was try to repair 200 Wat audio amplifier (professional product) and I was wonder "why on any connector I was found capacitors in pF measure , also many ferrite choke.") So when I was repair amplifier, I was remove "unnecessary components" ;D And turn audio amplifier to big radio receiver , OK not real radio receiver ,maybe like detector , but big one, and on end the line speakers are maybe more than 800 meter far a way. When I was all "unnecessary parts" back again in amplifier no more radio entertainment. ;D
OK shielded is elementary , some time double shield. I agree too.

[Gary Lecomte]

For Limiting Radio Interference, Typically you use 100pF Caps.
Because they are Low Capacitance, Normally they will not Reduce the Audio Frequency Response.

And in Conjunction with "Ferrite Beads, These small caps Produce a Better RF Filter.

[Deleted]

Posted by fabaum on Mar 13, 2009, 3:42am
I forgot to say: the interference occurs just when I connect the cable of the speaker in the box. The electronic parts of the "speaker as a phone" are assembled direct in the circuit board, so the speaker don't have any component directly around its.
This is the reason becouse I was thinking to put the capacitor at the "IN" of the pre-amp...
What do you think?

You see Gary , we booth tell many information, and from two opposite approach(your professional and my little amateur) solve problem instead fabaum
Old good capacitors and ferrite beads , if I good see law of physic are stay same all this years.