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Need sugestions about virtual ground or symetric supply for OPAMP


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Hi audioguru,

I dont know if "cracking" was the correct name of that but if your hear only the right channel you will listen something "cracky" or "cranky" distortion, i dont know the exact name, sorry.

The strange is that distortion is coming from R5 and R22, looks like a "Feedback Noise"?

Yes, they are ambience channels, one is L-R (L channel) and the other is R-L (R channel), when this two are connected + the standard L and R channels inside the car it gives the effect of 360 degrees rotation if you build a "stereo rotation effect" in Adobe Audition, if you record a train passing by and apply the stereo rotation effect you will have the impression that the train running in a complete circle inside the car, very nice. Together phase analysis show a -180 degree phase shift.

But what is very nice about it is that when you turn it on, since we are getting a -180 degree out of phase  from the original sound, they cancel each other right? If you put a CD with empty tracks and play it inside the car you can feel a really strange silence, is good to help relax when you are trapped on heavy trafic.

The resistors was added just to do signal attenuation, the buffers IC3B/C (ambience channel) was added and so the IC2C/D (voice channel) because inside the car there is a 6 meter long RCA cable from the instalation point to the power amp so they are capable of driving long cables and capacitive loads from the power amp.

What simulation program that you use?

Thanks

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Hi audioguru,

Did a few more experiences with the circuit and have found a document that show me part of the solution for the problem:
http://tangentsoft.net/audio/hs-opamp.html

Paying close attention to the distortion it has to do with the bass of the sound, removing R5/R22 and replacing with a 120K trimpot to adjust the fase and gain, as i reduce the effect the sound was becoming more clear and the distortion was the booming of the bass. The problem is that if i remove the distortion, the effect got killed but if i increase it so the distortion.

Analyzing the bass part of the song the problem was a frequency between 52Hz and 71Hz, the solution was to add a simple R/C high pass filter at the input. I set the filter to 72Hz with a 220nF with a 10K resistor grounded, that gives me a 6db per octave, calculations gives a 72.34Hz cut and a error of -0.07.

It worked fine, did not kill all the distortion but is much better than before.

In respect of audioguru golden ears i choose a real music, with lots of melody and high notes to test and hear if there is too much changes at the output and how the notes above that get affected. The music is "The Winner Takes It All" from ABBA.
http://www.sendspace.com/file/iwlms4

Is the same deal, left channel is voice and right channel is ambience, play with the balance to hear what each channel is doing.

I change the configuration of IC3B/C, for now it has the gain of 1, latter i will try a active bass boost at that frequency range and made a few more tests to see if it is necessary bost the bass or not.

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Hi Wellington,
Vinyl records and tapes had terrible high frequency phase balance between channels.
It causes distortion when you use a circuit like you have that extracts the difference (ambience) between channels.

Reducing the value of the input capacitors killed the deep bass. Each 10k resistor is in parallel with two 162k and also in parallel with 62k, resulting in a -3dB (half-power) frequency of 93.5Hz. It is flat down to about 470Hz, then the bass begins to roll off.

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Hi audioguru,

This circuit was design to be used in my car, so i think i am safe from vinil and tapes  ;D

The idea is to kill thouse deep bass because it does no good over the ambience channel, i did generate a 0db sine wave from 50Hz to 100Hz and in the ambience channel it get only distortion, i did not go any further with this tests but around 560Hz i can completly remove any distortion over the ambience channel.

If in you simulation it get's a flat response down to about 470Hz sounds good to me, because in my car i have a pair of 2-way components system from Diamond (D661a) and a pair of 125W JL subwoofers in a sealed box, the head unit has 4 outputs 2 output for front and 2 output for rear channels and a stereo subwoofer output, so inside the car the subwoofers will take care of thouse deep bass.

This circuit is installed to work with the head unit rear channel only, the front channels are connected in a standard way so i can hear the effect behind me, in the voice channel i use a Alpine SBS-05DC.

When all that stuf is set it gives a incredible sound inside the car, if you hear a live recorded CD it gives the effect that you are there, you can feel it.

I'm open to sugestions, what can be done to increase the cut, for example, -12db or -24db and tweek this a bit more.

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The ambience circuits produce the difference between channels.
Mono is cancelled because it is the same in both channels, no difference.
Bass is usually in mono. So the bass is cancelled which leaves nothing but its harmonics which is distortion.

A Butterworth 2nd-order filter is 12dB/octave, a 3rd-order is 18dB/octave and a 4th-order is 24dB/octave. Sallen and Key filters are the easiest.

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