Baxandall filter with LM358 op-amps

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67553

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#1
Hey guys, I just put together my active 2-band baxandall filter with LM358 Op-amps, basically I used this circuit, but added another op-amp at the input so it would be easier for the input to go in:

baxandale+amp.gif


The problem is I get LOADS of trash sound if the volume goes up :/ The amp is TDA7377, the sound without the baxandall is really clear. The amp circuit is below, it is the same circuit from the datasheet 🙂

And the supply voltage is ~12V DC with 0.8A which I think is enough, correct me please if I am wrong 🙂

Soooo any help would be great 😀
 

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#4
That could indeed be a problem - maybe you are getting feedback to the tone control circuit from the amplifier over the common power rail? I suggest isolating the preamp supply from the amplifier's - you could use a low-dropout regulator or a shunt regulator for your LM358.
 
#5
Most modern sources such as CD, etc, (you don't state which you are using), give around 2vrms output maximum. That is 5.7V peak to peak for your music peaks. If you boost that by 6dB (approx. twice) you have 11.4V peak to peak at which point your opamp output is clipping (hitting supply and 0v alternately). This would manifest itself as "trash sound". You will need to give the circuit larger supply rails (check your capacitors are adequately rated) for better handling.
 
#7
If you insist on using an LM358 op amp, hang a 2k resistor from the op amps output to ground. Don't ask why, just do it. See the JRC4558 thread (two down from this one this morning). Also, add a 10k or so resistor in series with the + input. Running the power supply up to 24V or so wouldn't be a bad idea either.
 
#9
I say well done and thank you to abraxalito 😀 after adding the shunt regulator circuit with TL431, the quality increased dramatically 🙂 It still has a little noise but they are less than nothing compared to what was before 😀
 
#11
I am sorry I have misconnected some wires so there was very little power going to the op-amps so the sound got much quieter and less noisier. Once I fixed it the problems were back with very distorted sounds coming from the speakers :/

But then I tried giving -12V to the LM358 (instead of going for Ground) and it got Better again 😀 The sound is very clear until low frequencies appear, then everything sounds like through the water, the bass is overwhelming everything :/ Have any ideas? 🙂

At first I thought it was the baxandall working but twisting the pots didn't change anything :/ Tommorow I'll try adding those 2Kohm back 🙂 I am adding my current circuit (I did the full circuit for one channel because everything is the same for the second one) 🙂
 

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#12
You appear to have confused a passive tone circuit with an active one. The values of C7 , C8 sbould be identical. The values of C9 and C10 should be identical. .... same again for R1, R2 this would most likely explain your lopsided frequency response... Your first circuit U3 150 ohms input imedance is too low. for a gain of 1 and a good input impedance use R10=R11=10K or whatever input impedance you require - also pin 3 on U3 needs to go to ground, not -vee..... pin 3 on the tone control opamp U2 should also go to 0V not the divider you have. That divider is only needed if you are *not* using a -ve supply in the circuit. Also if the -ve supply is used, C12 should be turned round. C2 should be removed as it is superfluous anyway.
 
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#16
blu_glo said:
You appear to have confused a passive tone circuit with an active one. The values of C7 , C8 sbould be identical. The values of C9 and C10 should be identical. .... same again for R1, R2 this would most likely explain your lopsided frequency response.
Click to expand...

And use linear taper pots. Those passive versions which used unequal cap values used audio (semi-log) taper pots. You don't want those with the feedback-type circuit.
 
#17
If the power supply also powers the TDA7377, you really need more current. Also try a quieter op amp. These car BTL amps often have high gain and you will get the extra hiss from a noisy op amp. You may want to add a level control at the input of the power amp. Add a 1000uf cap across the supply (if not already there) if the PSU main filter is distant (I suppose it is if your using a wall wart).
 
#18
Okay here is an update on the circuit, didn't have much time to work on it, but made a few changes 🙂 I completely remade the filter, changed up the input 🙂 Here is the new circuit (added below) 🙂 Sound honestly is now clearer, unfortunately there still are problems occuring.

-It seems that if there is low frequency sound with bigger amplitude it starts making noise :/

-The overall sound volume is really low. I had some pots as volume control but even if I take them out completely it does not help 😀

I'll try to get my hands on some theory, if I'll find the solution, I'll post it here (because my previous source proved to be a load of crap) 🙂

P.S. I don't know if I mentioned it, but my new source has 8A, so I doubt the problems are caused by current shortage. And I am not sure what kind of pots I am using (got the from a friend), I will post a picture as soon as possible 🙂
 

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