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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With a little stray C at INV input phase margin starts dropping.
If you drop 47K and 100K fdbk R's to 1/10th their values that will
help. You can see the onset of ringing in output sim below.

Also you have DC bias set wrong, see below values for better choice.
Consider scaling them as well by a factor of 10. That may compromise
mic G, due to mic loading, if so then raise fdbk G.



Also you are below min supply recommended operating conditions, eg. min is 10V to meet
its specs. Although datsheet shows at least on one graph operating below that....inconsistencies.

The bypass cap should be placed close to power pins, and a .1 uF ceramic disk also
added in parallel to that.

Lastly mixing the way you are doing it is a compromise, but works in a pinch.
Generally speaking a real summer would be better.




Regards, Dana.
 
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What is a "microphonic effect"?
What type of microphone?
 
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When using opamps for small signal audio amplification, a common pitfall in circuits using a single supply, (meaning that it shares the audio signal ground with the power supply ground), is positive feedback thru ground loops.
Vero boards are very difficult to achieve a good separation between the signal and power paths in the common ground. The result is howling and squealing at the slightest provocation.

Ask me how I know.
 
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danadak said:
Generally speaking a real summer would be better.
Click to expand...
Thank You Dana

Used summer , that had more distortion when both mic inputs were connected, so changed circuit to use two op-amp.

With a little stray C at INV input phase margin starts dropping.
If you drop 47K and 100K fdbk R's to 1/10th their values that will
help. You can see the onset of ringing in output sim below.
Click to expand...

I will replace these and check..
 
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danadak said:
Post a top and bottom image of veroboard prototype.
Click to expand...
Currently worknig in drawing schematic from component layout on PCB (reverse eng.)

Attached schematic, middle one is prototype, other parts schematic is not completed, need to trace tracks going through via (will work on this in two days. )

Schematic from PCB components (Incomplete)


These inputs are from UHF Microphone receivers.


Photos of veroboard.
 
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eTech said:
What is a "microphonic effect"?
What type of microphone?
Click to expand...
Microphonic , where component or PCB of amplifier/preamp behaves like microphone, then touched or tapped response sound heard from speakers attached. Eg. Tube pre-amplifiers, tubes are commonly known for microphonic effect, I came across LA4440 which produced high frequency tweeter sound can hear mild.

Microphone: UHF Dynamic microphone, not directly connected to prototype, UHF receiver output stage (pre-amp) is connected to prototype board which mixed two mircphones, connects to next stage PT2399 echo circuit.
 
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Well that's NOT Veroboard - and despite what 'schmitt trigger' suggested Veroboard is perfectly fine for audio projects, but just like a PCB you have to pay some attention to the layout. Long ago I built a full 12 channel mixer on Veroboard, for use with a band I used to do the PA for.
 
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I agree, it was fast untidy soldering work, will make another clean prototype during weekend.
(as circuit on product is unknown, Idea was to test prototype once schematic is finalised will work on PCB Layout )
 
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When you did summer thinking bias and or G was set wrong and you got clipping.

As my sim showed, just R mixing, your bias is off, needed correction. The 100K to gnd,
200K to Vbatt, produced the right offset, 4.5V with 9V battery.

From vero like board, I do not see any cap across 2344 supply pins ? A combination
of small electrolytic and .1 uF. Proto not pretty but otherwise probably OK as I do
not see any direct C loading on outputs of any significance due to your high valued
series R with each cap. But still with the caution high valued fdbk R's along with stray
C recipe for phase margin reduction. Also the high valued Rs used in bias network
makes the NINV inputs susceptible to + fdbk, when input pot is high value, so I think
those should be reduced into the K range. Side effect of that is Pdiss will go up of circuit.


Regards, Dana.
 
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I also have had mixed results when working with this type of construction, highly
dependent on OpAmps used, layout and bypass caps used, stray L..... I used to do
some stuff with LM318's, they were a pain in that type of construction. Even uA709's
with feed forward compensation asking for trouble.


Regards, Dana.
 
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Nigel;
I didn’t exactly say that a Vero Board was not suitable. Rather I mentioned that it is difficult to achieve a low impedance grounding which doesn’t couple the noise from the current loops into the input.
Specifically for someone who is still learning.
 
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I remember the cheap (cheapest stereo opamp) antique (1974 ) JRC4558 dual opamp that was used in many guitar pedals. It was an "improvement" of the old 741 opamp. It is OK in a guitar overdrive circuit but is too noisy for a hifi audio circuit.

Audio howling and squealing is caused when the mic can hear the speaker. It is mostly at frequencies that have a peak in the frequency response made by the mic, amplifier or speaker.
 
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Howling I am facing is not related to this, howling or distortion occurs when when mics are switched on idle, mic volume is raised, and mics not facing speaker, this does happen when any one of two mic is switched on.
 
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danadak said:
With a little stray C at INV input phase margin starts dropping.
If you drop 47K and 100K fdbk R's to 1/10th their values that will
help. You can see the onset of ringing in output sim below.
Click to expand...
OP-AMP used JRC4580

Changed circuit, resistors used 1k and 10k (this is the least I had), later will check with 4.7k and 470 ohms.
Testing with two types of mixers,

Mixing with resistor at inputs (in series with capaciror)
Mixing with two parallel capacitors of 1uf MLCC

Will update results later, Thank you all

Changed made:


Mixing - two parallel capacitor at input.


Mixing - two parallel resistors 10K
 
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None of those are a proper 'mixer', and won't work properly, a real mixer is really a very basic and simple design,and one of the basic building blocks of opamps.

Have a look here, or countless other sites found by google:

Op Amp Summing Amplifier: Virtual Earth Mixer » Electronics Notes

The op amp circuit for an inverting amplifier can be used to create a simple high performance linear virtual earth mixer or summing amplifier.
www.electronics-notes.com
 
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Of course you will have howling feedback when the mic can hear the speaker, especially when the mic volume is raised.
Record the sounds with the speaker turned off then playback with the mic turned off. Then no howling.
 
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