headphone amp output to shop bought power amplifier

[SimonW]

Dear Friends,

I made the below circuit from a kit which I bought from

**broken link removed**

(both mics are electret types)

I know the circuit is working good when I connect the output with a headphone.


when I try connect output to a household shelf Power amplifier I get a noise on speakers.....

the noise is present even if I turn RV1 A/B up or low


I think it is got something to do with impedance matching.... which is set by
the R8+R13 and R7+R11??

I read somewhere that an L pad circuit might help?

Hope someone can help!

Thanks

Simonw

 

[kubeek]

Do you power the thing with a battery?
Also what kind of noise? Hiss, rumble, or high pitched squeal?

 

[Nigel Goodwin]

I think it is got something to do with impedance matching.... which is set by
the R8+R13 and R7+R11??

No, it's nothing to do with impedances - which is set by the output impedance of the opamp, plus the 8.2 ohm in series.

 

[audioguru]

Of course the "noise" is acoustical feedback howling or squealing because the microphones hear the speakers and the amplified sounds go around and around.

To eliminate feedback then the mics and speakers should be in separate enclosed buildings far apart.
Or you can record the sounds then play them back later.

The "noise" can also be mains hum if non-shielded cables were used.

 

[kubeek]

To eliminate feedback then the mics and speakers should be in separate enclosed buildings far apart.

Must be fun even with loud open headphones

 

[SimonW]

Many think for your replies,

I am using a el cheapo wall adaptor converting to 4.5v DC

what is the minimum distance needed between this circuit and the power amp.... in inches, cm,mm

I think that it might be main hum, the power cable is very close to the audio cables

I have recorded the noise onto a *.wav file..... it is in the noise.zip
(rest assured there is no virus, trojan etc..... as I am in the need of help)

 

[ericgibbs]

Many think for your replies,

I have recorded the noise onto a *.wav file..... it is in the noise.zip
(rest assured there is no virus, trojan etc..... as I am in the need of help)

hi Simon,
Change the wave file extension to .txt and you can upload to your post as an attachment

 

[SimonW]

Thanks Eric..... the noise.zip is really *.wav file.....

 

[ericgibbs]

Thanks Eric..... the noise.zip is really *.wav file.....

Its 50Hz mains Hum, you have a ground loop signal break thru

E

EDIT:
Do you have a 4.5V battery you could use in place of the 4.5V wall wart, for test only?

 

[audioguru]

Where is a description of the "noise" (the attachments do not work)?
Is it acoustical feedback howling or mains hum?

 

[SimonW]

**broken link removed**

 

[audioguru]

The low frequency noise is mains hum.
Maybe your power supply output is AC instead of DC?
Maybe your AC to DC power supply has a filter capacitor value that is too small?
Maybe the cable feeding the power amp or preamp is not a shielded audio cable?
Maybe you built the circuit on a breadboard and all the contact strips and very long connecting wires pickup mains hum?