microphone mixer preamplifier howling

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technofreak

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Hello,

Modified Karaoke circuit, microphone input bass/treble input stage for removing microphonic effect on original PCB.

Issue was around Bass Treble and Echo mixer circuit. removed op-amp and related components for bass treble, replaced with circuit made on Stripboard/Veroboard

Schematic based on jrc4558 pre-amplifier circuit published in componentsinfo (Not posting URL)

posting original schematic and modified one. (water mark is there in image).

Modified values (based on articles online) marked in red.

Tried both Inverting and on-inverting circuit, Inverting op-amp circuit has more distortion , hum noise when connected, this does not happen with Non-Inverted op-amp circuit.

Input 1 and 2 is from another op-amp from microhphone UHF receiver circuit.

Modification removed microphonic effect.

Issue faced: when one of the mic (single mic) is used it works good, turned up Mic-A or Mic-B (one of the mic) volume pot to full, no distortion.

Switched on scond mic-B, turn volume up, when pot position reaching around half way howling starts, increases in volume by itself, when one of the mic volume is turned down howling stops, when both mic volume pots are at less than half no howling, but mic volume is too low.

accidentally touched pre-amp resistor on pin 2 / 6 reduced/stopped howling sound (touching causes other distortion but stops howling)

will it help if 33pf capacitor is connected in parallel across 100K resistor (feedback) ?

Requesting help to make circuit stable.

Image 1 : Refrence circuit,


Image 2: modified circuit for mixing two mic.
 
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technofreak said:
Mics are not near speakers, they are far away on couch, not facing speaker.
Click to expand...
If they are in the same room, then it's VERY likely to cause feedback -if you're trying to do karaoke?, then you want the microphones to be BEHIND the forward facing speakers, the microphones also need to be suitable uni-directional types, and you need to keep the volume levels low enough to keep below the feedback threshold.

When doing PA for bands, potential feedback is something you have to work hard to prevent - this is why you sing extremely close to the microphone, so you can keep the gain down as far as possible.
 
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Many microphones have an omni-directional (all directions) pickup pattern. Speakers in a room produce reflection sounds from the floor, ceiling and walls. Then the mic can hear the speaker and howling feedback is produced.

Performers "eat the mic" (very close to their mouth) then their voice is produced loudly but the amplifier's gain is turned down to avoid howling feedback.

It is easy to increase the gain of your preamp without adding another stage.
 
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Worked on drawing schematic from component layout on PCB.
Attached schematic is first stage, this circuit gets input from UHF Microphone IC KT0651.

I will make next stage using mixer circuit from danadak , mix input from two microphones mixed output connects to PT2399 (echo circuit).

Mic input Schematic on PCB (reverse enginneered from PCB traces).
 
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I have DSO, connected channel 1 probe, Auto measure output of capacitor C1, microphone preamp schematic below


some one help in interpreting DSO readings , advice me steps for checking further ?
 
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The opamps have a gain of only 1 but the output is massive 50Hz of mains hum.
Poor shielding of the input cables, the entire circuit and the output cables?
 
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audioguru said:
The opamps have a gain of only 1 but the output is massive 50Hz of mains hum.
Poor shielding of the input cables, the entire circuit and the output cables?
Click to expand...
I Karaoke mixer online, uses 12VDC adapter, using transformer type (not SMPS), in PCB no circuit for filtering 12V DC , DSO Measured directly on PCB at output capacitor (Photo attached)
 
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audioguru said:
The opamps have a gain of only 1 but the output is massive 50Hz of mains hum.
Poor shielding of the input cables, the entire circuit and the output cables?
Click to expand...

Does not have Audio input cables, only output cable is connected to Speaker (IKEA). Power is not filtered.
Looking online for power filter circuits,
I will make preamplifier PCB, use only UHF microphone receiver from existing PCB.
 
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All AC to DC power supplies have a large filter capacitor on their output. But in your circuit it is only 100uF with C2 which is too small for a filter but is good if a battery is used.
R10 feeds all power supply noise directly into the input of the first opamp.
Is the 50Hz interference gone when a 9V battery is used?
 
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audioguru said:
All AC to DC power supplies have a large filter capacitor on their output. But in your circuit it is only 100uF with C2 which is too small for a filter but is good if a battery is used.
Click to expand...
Thank you , was reading about this yesterday night.

audioguru said:
R10 feeds all power supply noise directly into the input of the first opamp.
Is the 50Hz interference gone when a 9V battery is used?
Click to expand...
Yesterday discovered source of 50Hz, connected DSO probe on GND plane on board, showed 50Hz waveform, then saw board was not powered on, I had removed adapter forgot to plug it back, did not suspect speaker because its from IKEA, disconnected speaker cable, placed probe on GND tab of RCA, showed 50Hz waveform. Finally found source of 50Hz noise.

Another accidental discovery microphone howling stopped, it occurred only when microphone faced speaker or very close to speaker, mic volume is ok, usable.
Upon checking pre-amp observed I had mixed up capacitor resistor values.

Used 10k and 10k resistors instead of 10k and 1k, connected 2.2uf capacitor at output.

did not understand purpose of R6 and C1, as most articles explained (R1 is feedback) calculations, and for me math formulas are less interesting.
some circuits has only resistor or capacitor connected to ground from inverting pin,

my understating is capacitor C1 changes frequency response, higher capacitance can handle lowest frequency?



Working circuit.


Next will read about filter circuits to eliminate 50Hz at input.
Later will work on PCB layout

Thank you verymuch all for your time and support.
(had posted in DIY Audio, no one replied there.. !)
Good Day.
 
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technofreak said:
Another accidental discovery microphone howling stopped, it occurred only when microphone faced speaker or very close to speaker, mic volume is ok, usable.
Click to expand...

As we've told you repeatedly throughout this thread


Yes, along with C1 - but your frequency response is already far lower than needed (about 1.5Hz)
 
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technofreak said:
Should I change C1 to lower value may be like 1uf or 0.47uf ?
Click to expand...
Only if you wanted to, you could also drop C6 to 0.1uF - but leaving them as they are isn't an issue.
technofreak said:
Yes.. sorry , I trusted speaker brand is Ikea ..
Click to expand...
It's got nothing to do with the speaker, you'll get feedback with any speaker if the mike is too close and the volume set too high. I wasn't even aware Ikea did speakers?.
 
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Sorry, I am not able to explain here clearly.

what I describe as howling, some sort of distortion which started after few minutes of switching on second mic (one of two mic), to investigate further I moved per-amplifer to make some space on table, observed touching pre-amp enclosure produced sound in speaker (microphonics) , to trouble shoot opened enclosure removed PCB place on table when switched on both mic howling distortion happened only when I was sitting close by, when I touched table heard sound in speaker, then touched PCB (with mic off ) this produced sound in speaker, sound was louder when tapped near bass treble circuit.

Two op-amp inputs (3, 5) were connected in parallel, output of one op-amp fed LED level, another used as bass-treble.
Two microphones inputs were mixed using ceramic capacitor and resistor in series for each mic, this was connected to parallel inputs(pin 5 and 3)

I removed op-amp and all components of bass treble, damaged traces when removing smd IC, so I had to use available parts to make one circuit (untidy hack). modification resolved microphonics but increased distortion, then I posted for which all of you responded...
I remembered DSO (which I hardly used), finally used to find out about noise.

now exessive humming or howling does not start, only when mic is facing speaker sort of horn sound starts ( which is due to feedback.).

yes IKEA has speakers two brands ENEBY and SONOS, these are amplified speakers with Bluetooth and aux in.
 
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We've just bought a Xenyx 1222 mixer. When setting the gain, we're not getting much past 40% on the mic channel before we start getting feedback . The mixer has Feedback detection on it but none of the equalizers are indicating any feedback . We do use the Hall FX on the mic channel but the feedback occurs without speaking into the mic. But this did also occur when the FX were off I've seen a youtube review that states using the compressor can can cause feedback . Does it? Any suggestions on what the cause could be and how to eliminate it are welcome
 
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The biggest issue are the microphones, they MUST be uni-directional types - next would be placement - you want the speakers on the front of the stage (facing forwards) and the mikes further back on the stage facing backwards. It's all mostly common sense.

Disconnect, or turn off, all processing and effects - and turn down all mike levels - then try turning the mike levels up one channel at a time and checking how loud it is. Once you get it to the volume you want, move on to the next mike channel.

Once you've got all mikes working as loud as you wish, without feedback, then you can start adding any effects or processing, again one at a time, so if you suddenly get feedback you know what's caused it.

I've got a Behringer mixer/amp, and use Behringer XM1800 microphones, which I've found excellent - just checked, they are £45 for a set of three - they were £18 for three when I bought mine

I've also got an XM8500, which is £29 now, but I can't hear any difference from the XM1800's, so I never bought any more.
 
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The opamp circuit in post #31 has a voltage gain of (1 + R1/R6)= only 2 times.
If you filter out 50Hz then you also filter out many bass sounds. Prevent 50Hz at the input instead.
 

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audioguru said:
The opamp circuit in post #31 has a voltage gain of (1 + R1/R6)= only 2 times.
If you filter out 50Hz then you also filter out many bass sounds. Prevent 50Hz at the input instead.
Click to expand...
Yes , I am working on this..



I am working on re-designing PCB, will use only UHF receiver portion from existing PCB, in new (customized PCB) will use low gain op-amp circuit to filter at input level,
I am looking at possible passive circuit like inductor at input GND or use op-amp filter circuit at input.


Came across this , Passive Audio input isolator.
 
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