microphone mixer preamplifier howling

[technofreak]

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.

 

[technofreak]

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.

found it was due to microphonics at bass treble circuit and hum(mains noise) from speaker input.
will re-build circuit and PCB with improvements,

 

[technofreak]

Regards, Dana.

Just wondering.. Dana from X Files ?

Thank You for your time, help, support and schematics, I will be using these for modifications.

 

[danadak]

Sim of mixer post # 19

Regards, Dana.

 

[technofreak]

Sim of mixer post # 19


Regards, Dana.

Thank You very much Dana...

 

[audioguru]

The loss at low frequencies is because the load is too low at only 1k ohms. The datasheet for the TL071 shows much better performance with a 10k load. 1uF feeding 10k ohms gives -3dB at 16Hz.

 

[danadak]

Input is 10K Sine, 1K square, and sine swept from 1 Hz to 10 Mhz
to generate bode plot. Sine and square settings were done for a
transient plot, not shown prior.

1K and 10K loads :

Regards, Dana.