Eliminate Feedback Oscillations in PA Audio Circuit

[zsolt1]

what is a pitch changer ? there is the language barrier again :-D

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hehe , i wonder what people did in 1130 when no 120w amp existed . Maybe priests where 120 w loud :laugh:
near the church is the famous Banffy castle , in restoration now https://www.google.ro/imgres?imgurl=http://transilvaniareporter.ro/wp-content/uploads/2013/11/castelul-banffy-2.jpg&imgrefurl=http://transilvaniareporter.ro/esential/extraordinarele-aventuri-ale-contelui-banffy-miklos-povestite-de-batranii-satului-pentru-copii/&h=677&w=1002&tbnid=woJr825UeZS_sM:&tbnh=160&tbnw=237&usg=__C8v4O2mfR3kkExtj6fdJ4LYF0BE=&docid=PFfpzd1UTTJE_M&itg=1

 

[D.A.(Tony)Stewart]

An audio processor that uses a different sampling rate for output than input is said to shift the pitch thereby preventing feedback of same frequency for regnerative gtain >1 which is the Barkhausen criteria for oscillation.

Darth Vader would be an extreme example of pitch shift. But a 1% pitch shift may be enough to suppress systems close to unity feedback gain at 0 degree loop phase.


If you wish the KISS approach, use my methods.

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what is a pitch changer ? there is the language barrier again :-D

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hehe , i wonder what people did in 1130 when no 120w amp existed . Maybe priests where 120 w loud :laugh:

They used the acoustic wave focal properties of the DOME to amplify their voice and then expected delays and echo were normal if out of position.

In England Cathedrals were designed by Masons with 3 tools, a compass, a square and a triangle to accomplish this and more. ( See whispering corridors)

 

[zsolt1]

darth vader :laugh:
keep it simple is my motto also so i will not deal with audio processor .. thous i could use a laptop's input/output jacks and a little program ... :thinker:

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An audio processor that uses a different sampling rate for output than input is said to shift the pitch thereby preventing feedback of same frequency for regnerative gtain >1 which is the Barkhausen criteria for oscillation.

Darth Vader would be an extreme example of pitch shift. But a 1% pitch shift may be enough to suppress systems close to unity feedback gain at 0 degree loop phase.


If you wish the KISS approach, use my methods.

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They used the acoustic wave focal properties of the DOME to amplify their voice and then expected delays and echo were normal if out of position.

In England Cathedrals were designed by Masons with 3 tools, a compass, a square and a triangle to accomplish this and more. ( See whispering corridors)

good to know , maybe can use those 3 tools to design audio boxes also . I never found relevant info about how to build audio boxes

 

[FvM]

I won't seriously suggest a pitch changer for a church. It sounds ugly enough with speech and is completely unsuitable for singing.

 

[Audioguru]

The pitch of a sound is its frequency. If you change the pitch a little (maybe make it a 10Hz higher than the original) then feedback cannot occur but it sounds weird to the person speaking or singing and to audience people who can hear both the original and the pitch-changed output.

 

[FvM]

The usual "feedback control" pitch changers (e.g. one offered by Monacor) are single side band modulators, shifting the audio signal by a fixed (3 - 15 Hz) amount. As Audioguru explained, it's very confusing for the speaker. Musical harmonies are even distorted.

A pitch changer of the harmonizer type, as mentioned by SunnySkyguide sounds better, but still confuses the speaker if he/she isn't used to it.

 

[zsolt1]

what is dis telephone circuit doing ?

 

[Audioguru]

The telephone circuit (it is called a "hybrid network" and used a hybrid transformer long ago) is used in a speakerphone to keep the signal from your mic from coming out your speaker which would cause feedback. Then your mic signal goes out onto the telephone line to the other end and the signal from the mic at the other end comes out your speaker.

But if both ends have the same circuit then you will have long distance feedback or very loud echoes: Your mic signal goes to the other end, through the air into its mic and back to you. Their mic signal comes out of your speaker, through the air to your mic and out their speaker. An "echo canceller circuit" is used in modern speakerphones (look at Polycom) to prevent long distance feedback.

This circuit cannot be used in a PA system because the distance from the mic to the speaker and sound reflections change the phase at different frequencies too much. An acoustic echo canceller circuit can make a model of the phase changes then feedback might be prevented. The echo canceller circuits I worked with adjusted the model actively because of doors opening or mics being moved changed the phase of sounds a little.

 

[zsolt1]

How to use twin mikes in differential mode ? how it is donne

I repaired the amp , works like charm . I tested the phantom microphone , incredible how good that microphone is . I had to put it out in the yard in order to avoid auto oscillation at full volume :grin: . It is such sensible that i could hear cristal clear people talking outside.

 

[Audioguru]

I tested high quality speaker systems outside to avoid room reflections. The speaker system was laying on the ground pointing up to the mic that was hanging above it.
Did you reduce the feedback in the church? What did you do?

 

[zsolt1]

not , yet . Ill deal with it in the weekend . I'm thinking to change the speakers . Now they where only 2 speakers in about the middle .What if i spread a lot of little speakers until i get the total 120w installed ?

 

[FvM]

I presume you have "100-V" speakers with built-in transformer.

Using multiple speakers in a church is the usual way to achieve uniform sound level.

 

[Audioguru]

A speaker close to a listener will use low power and will not be heard by the microphone so feedback will not happen. Maybe many speakers on the backs of the seats?
In a multiple speaker PA system each speaker needs a transformer and the amplifier also needs an output transformer. The transformers for the speakers usually have a few inputs marked with how much power it uses. Each speaker can play to 5 people. Maybe 1/2W or 1W each?

 

[zsolt1]

the amp has transformer in it . Actually is the first tranzistor PA that i see wth transformer. Good to know i wanted to connect 3~10 W 4 ohm speakers in the far back . I wonder about the labeling of the output of transformer , the secondary has a common wire and the other ones labeled like so : 4ohm ; 70V ; 100V. The datasheet also shows transformers . I wanted to connect about 10 speakers so that the echivalent impedance results 4 ohms :-D

The persons who set up the system have no technical knowledge so this will be great help.

 

[FvM]

You can connect 10 speakers in series to the 70V output, matches well for 120W. Parallel-series won't work good for 10 speakers.

The regular way would be a 70 or 100V transformer at each speaker and a party line connecting all in parallel.
https://en.wikipedia.org/wiki/Constant-voltage_speaker_system

 

[zsolt1]

i think i'll do so

 

[Audioguru]

I think 10 speakers in series will have no damping and will cause boomy sound at the resonance of the speakers, making feedback worse. A speaker connected to the output of an amplifier or with transformers has its resonance damped very well by the extremely low output impedance of a modern amplifier.

 

[KlausST]

Hi,

I agree with audioguru.

I don´t like the idea of connecting speakers in series.
Every impedance mismatch will influence the other speakers.
And every speaker has it´s own impedance curve.

With paralleling there is (about) no influence of one speaker to the other.

I always thought that the 100V system is especially made for paralleling the speakers (with their transformers).
But to be true I am not experienced with the 100V systems. I just have my doubts...

Klaus

 

[zsolt1]

probably we will buy same speakers , i heard about this kind of system from the past .It was a so called radiofication system .Radio was played and the signal was distributed in half town .People had to pay for this service and buy the equipment . I found such vintage equipment, it was a box containing a 750 ohm speaker , a transformer and a potentiometer. The signal came from a 10 w lamp amplifier for every street . I think this was like cable tv :lol:

 

[Audioguru]

Every large store or school has a transformer on each of its many speakers. The transformers are all in parallel and driven from the transformer on the output of the PA amplifier.

A speaker resonates like a bongo drum when it is not damped by the extremely low impedance of a modern amplifier. Speakers that are the same all resonate at the same frequency and at resonance their impedance is much higher. In series, they all prevent damping from the amplifier, bong, bong, bong.