Eliminate Feedback Oscillations in PA Audio Circuit

HI,
i need any suggestion of a circuit to put between microphone and power amp , that can eliminate auto oscillation. The audio system is set up in a church built in 1130 and i can't see any other way to eliminate positive feedback due to the acoustic of the interior .

Feedback tones ("auto oscillations") are stoked up if at certain frequencies the feed back signal that arrives at the microphone from the speaker is stronger than the original signal.

The best means to avoid it are a good directional microphone and well conbsidered speaker placements. In case that the speaker and/or the room properties are boosting certain frequencies, equalizers can be used to reduce the gain at these frequencies.

If a microphone can hear the output from its speakers then you have acoustical feedback howling. Record it and play it back later when the mic is turned off, or "eat" the mic (very close to your mouth), turn the mic away from the speakers and turn down the volume. Frequently a cheap speaker is much more sensitive at one or a few frequencies (peaks) that can be notched by an audio equalizer to a lower level. Some "feedback cancellers" change the output frequencies a little but the sound heard by the person speaking or singing into the mic sounds weird.

Simply, move the mic closer to the person speaking and point the mic away from the speakers, mount many speakers close to the people listening and turn down the volume.

Years ago when echo cancellers were invented for telephone systems and were used for telephone speakerphones I thought the echo canceller could be used to acoustical eliminate feedback but I never pursued trying it. The echo canceller builds a model of the acoustics and cancels any sound from the speakers modified by the speakers and by the room acoustics that is the same as the sound from the microphone. There are echo canceller ICs available that might be able to do it and there might be a sound system manufacturer who uses it in a product.

Maybe this fairly old document describes it better than me: https://ccrma.stanford.edu/~eberdahl/Projects/FeedbackCancellation/FeedbackCancellation.pdf

With a parametric (graphic) equalizer you can eliminate feedback at a particular frequency, by reducing the gain. In other words, a notch filter.

Try to get one with 10 or 15 bands. Also an led grid, to indicate soft or loud frequencies.

I purchased a similar equalizer for 10-15 dollars on Ebay (used Radio Shack model). If you wish, you can build your own notch filter. Some types make the frequency adjustable.

Once you eliminate feedback at one frequency, the tendency is to increase volume until you get feedback at another frequency. So you can fiddle with the controls for a while, customize the sound. Etc.

Hi,

You may try to invert the signal by turning the speakers connection, or only if some speakers.
While with him I it is a no go to turn just one speakers polarity, it may be useful for speakers or speaker pairs in a church.

On other approach, often used in studios is to delay the signal to the speakers.
Maybe in the range of some tens up to some hundreds of milliseconds.
But if it is too big delay the one who speaks may recognise it and may be irritated.

Klaus

a) use the best noise cancelling mike... Sound more than a few cm should be > 40dB
b) if too far away, move mic closer to corner of lips.
c) if not avail, not possible, use twin mikes in differential mode.
d) after this they make sure speaker phasing is correct and not in phase but out of phase with mike.

Professional equipment can use tunable notch filters for room resonances, but should not be required here.
usually b) cheap mics work best. with mic close to cheek and corner of lips with over the ear of adjustable boom.

If not noise cancelling type, but magnetic cardioid type.
you need two in differential mode .

hi ,
i took a closer look to the ecquipment i found ,
this is the type of one of the microphones https://www.monacor.ro/product/show/3435/ECM-500
the other one is "PHANTOM 48V" microphone .
The reaction does happen only if i speak a little louder in the mic . Could an automatic level control circuit help ? something like this https://www.electroschematics.com/9400/audio-compressor-agc/

Hi,

I don't think so. I rather think - caused by the variable gain - there is a higher chance for resonance.

Klaus

I'm confused about the Phantom 48 V mic. I found that it should be supplied with 48 Vdc . The amp is PA-900 MONACOR type it has a switch in the back that selects PHANTOM mic supply 21 V.

O.K., phantom supply is a different point, not related to the feedback problem.

Phantom supply is classically using 48V, the high voltage is hardly required by any recent microphone, so 21V will be fine for 99% of microphones needing phantom supply (which is a smaller percentage of available microphones, mostly electret and other capacitor microphones).

It's not clear why feedback tones start when you are talking, could be the speaker near the microphone reflecting some sound. A dynamic compressor is unlikely to be helpful, as said.

I see that PA-900 has RCA jacks that can be used to loop in an equalizer after the pre-amp https://www.monacor.de/de/FLE/PA900.pdf

I would put-in a "grahic" third octave or a parametric equalizer.

The feedback is just slightly under one, so oscillations don't build up spontaneously. But any exciting signal, like talking, will make it 'ring'. Up the gain a little bit and it howls as expected.

i measured that 21 V , it's only 18 V . Something is wrong . I unmounted the whole thing & i will take things step by step starting with the amp . It has a faulty potentiometer also ...

The phase of a speaker and room reflections especially from the floor make the phase on the speaker and mic all over the place but you might be lucky to cancel one room resonance or a speaker resonance by inverting the phase of the speakers or mic.

If the feedback is a low frequency when you talk loudly then it is probably caused by a cheap boomy speaker that resonates too much at the boomy frequency, or a room resonance. Turn down the bass tone control or notch it. It the feedback is a high frequency when you talk loudly then it might be excited by a peak in the speaker frequency response excited by the amplifier clipping and producing distortion. harmonics. Turn down the level.

i measured that 21 V , it's only 18 V . Something is wrong .

Click to expand...

Not necessarily wrong. 18V can be still fine Did you check the microphone specification for required phantom voltage minimum?

In any case, that's a different problem, not directly related to the thread topic.

A condenser microphone needs a 21V to 48V bias on its moving membrane then a Fet or vacuum tube preamp can amplify the capacitive voltage divider created by the moving membrane and a close fixed membrane. Sometimes the moving membrane is grounded and the bias voltage is on the close fixed membrane.
An electret microphone is a condenser microphone with the 48V permanently charged in the electret material and has a Jfet to reduce the extremely high impedance of the capacitive voltage divider. The Jfet must be powered, usually from a few volts through its load resistor with a current of about 0.5mA. Some electret mics have three wires where the Jfet is a source follower then the sound level can be very high like insider a drum or piano.

Electret microphones for professional applications have usually a wide range phantom power supply for the preamplifier. E.g. Sennheiser microphones have 12 - 48V supply range.

FvM said:

Electret microphones for professional applications have usually a wide range phantom power supply for the preamplifier. E.g. Sennheiser microphones have 12 - 48V supply range.

Click to expand...

That is so that their electret mic can be used with a preamp that supplies 48V to a true condenser mic.

To stop feedback you can use a pitch changer, this raises the frequency of the amplified sound very slightly from the sound entering the mic. Some thing to consider is that the acoustics of the hall will change a lot when its full of people wearing soft clothes. You will need to run a lot more power out of the loudspeakers and the echoes will be much lower. It might be OK!
Frank

thanks for all replays,
I found that the amp (preamp included) mixes up 4 input channels which can, with a hidden switch, select between phantom type microphone and other random microphone . The selector switch when pressed, selects all 4 channels with 21V supply. They used both microphones phantom and monacor type at same time. Someone modified the monacor type microphone pinout so that it grounded the 21V . By opening up the case i broke some thin wires :lol: , so after i finish with the amp, i will look for ECM -500 Monacore desktop microphone schematic . It's a simple pcb inside with only 1 tranzistor . I tried to draw the circuit with the remaining wires that i did not break, but it makes no sense. Someone modified that also. I wonder if the tranzistor could be in common base connection , than it would make sense (something about impedances , i see 600 ohm on the case ) .

PS: the speakers are about 20 ... 30 m away from the microphones. I did not get to inspect them yet . One seems to have troubles also since it makes distortions (maybe mice damaged the membrane :-D )

chuckey said:

To stop feedback you can use a pitch changer

Click to expand...

I mentioned it in post #3.