TL072 and Non-inverter small signal amplifying

[Papabravo]

It is my circuit schematic

View attachment 144292

I have distortion problem in Audio_R1 and Audio_L1 when I turn the double volume which is To_back_vol_R more than ~70K .

Why output of this opamp affect on input of it ?!

The answer is – feedback!!

 

[danadak]

If your supplies had been +/- 15 on OpAmp you could have
gone to at least 140K on pot. A G of ~ 65. Note for input of 100 mV.

Regards, Dana.

 

[Tony Stewart]

The input C of OpAmp inputs quite small, and a small amount added due to stray C due to layout.
No need for the 68 ohm R's.

Your G thru OpAmp is 1 + Rvol_control (K) / 2.2K. Any chance at extreme end of volume control
it goes open, very high Z. Which would cause severe distortion. Same could happen on the 0 ohm
end if real cheap pot. If thats problem obviously get quality pots, or use an additional fixed R from
output to OpAmp inv input to set max gain.

As Tony mentions regarding output load its way to low for this OpAmp, look at the datasheet charts
load current versus output swing. Its obvious your load way to large (= Rload way too small).

Laslty note you have to handle load dump in vehicle, eg. excess V, to keep from frying components.
Like bypass caps, semiconductor in general.

https://www.vishay.com/docs/49748/49748.pdf

https://m.littelfuse.com/~/media/electronics/application_notes/littelfuse_tvs_diode_meet_automotive_load_dump_standard_application_note.pdf.pdf

https://www.vishay.com/docs/88490/tvs.pdf

Notice the effect of a 50 ohm versus 1K ohm load on output :

View attachment 144295

Feedback Rfeedback of 140K, clipping evry evident :

View attachment 144296


Regards, Dana.

This is not the cause of the reported problem, nor was my illustration intended to be a solution. Rather it only shows the lowest load for 2Vpp.

I suspect that what has not been shown in the actual layout must be a problem or that higher output with clipping was used without stating the required details as what kind of distortion exists.

Is it amplitude sensitive? or not?
How far is the pot located?

What value was it when distorted? 70K? that is too much gain for 2k2 input for a gain of 25. It should be 55k Max. You can increase 2k2 to 4k ( or 3k9) to reduce the gain to 100k/4k = 25 rather than 55k/2k2 = 25 (iff your pot is 100k)

More details in future speeds up problem solving.
Any questions?

 

[danadak]

Op post :

I have distortion problem in Audio_R1 and Audio_L1 when I turn the double volume which is To_back_vol_R more than ~70K .

Sure sounds like he has too much G. As supported by the sims and the low supply V's.

 

[arvinfx]

Sorry for late...
I had some investigation on my circuit based on your kind informations ...

First: I have found that load ( which is slave amplifier ) doesn't have any affect on distortion. I disconnect the "To_car_amp" and nothing change! Master Amplifier still has distortion on its sound when Gain is near maximum !

Second: I injected 15Hz-22KHz sinusoidal wave and checked the output on "To_car_amp" .In some frequencies (around 49Hz-70Hz) I saw cut-off like this:

The VPP is 4.7V on this frequency I think it is because of DSP (equalizer) of master amplifier.

I tried to reduce the Gain till this distortion disappear. Problem solved! With reducing enough gain both Master and Slave amplifiers work fine! But Master amp sound volume is near 3 time more than Slave! and it is not my target!.

So what should I do to have more Gain and also no any distortion?!

Still I confused about affect of my circuit on master amplifier!

 

[Nigel Goodwin]

Sorry for late...
I had some investigation on my circuit based on your kind informations ...

First: I have found that load ( which is slave amplifier ) doesn't have any affect on distortion. I disconnect the "To_car_amp" and nothing change! Master Amplifier still has distortion on its sound when Gain is near maximum !

Second: I injected 15Hz-22KHz sinusoidal wave and checked the output on "To_car_amp" .In some frequencies (around 49Hz-70Hz) I saw cut-off like this:

View attachment 144322

The VPP is 4.7V on this frequency I think it is because of DSP (equalizer) of master amplifier.

I tried to reduce the Gain till this distortion disappear. Problem solved! With reducing enough gain both Master and Slave amplifiers work fine! But Master amp sound volume is near 3 time more than Slave! and it is not my target!.

So what should I do to have more Gain and also no any distortion?!

Still I confused about affect of my circuit on master amplifier!

The answer is simple, leave the master gain HIGH, if not fully up. You've never provided any schematics yet, so we've no idea how you're connecting things, but a power amplifier is usually designed for full output with about 1V input, with the volume turned right up. You can't increase your gain to exceed the 4.7V or so you've got, that's a limitation of the power supply, and there's no need to, and no desire to - the master gain full up is how it's designed.

Have you added coupling capacitors to your preamp yet, it's really something you should have.

 

[danadak]

Do you need AGC applied to the input signal path ?

Regards, Dana.

 

[Tony Stewart]

Is bass boost in DSP the source of problem? Perhaps you need woofers with lower impedance to get more power at lower voltage.

 

[ZipZapOuch]

Put the scope across the op amp supply rails (pins 8 and pin 4) to check the voltage stability and DC voltage when you apply a 49hz to 70hz signal. Looks to me like your power supply may be dropping out at low frequencies (you may need larger capacitors across your power supply or expect less power from your bass.

 

[arvinfx]

The answer is simple, leave the master gain HIGH, if not fully up. You've never provided any schematics yet, so we've no idea how you're connecting things, but a power amplifier is usually designed for full output with about 1V input, with the volume turned right up. You can't increase your gain to exceed the 4.7V or so you've got, that's a limitation of the power supply, and there's no need to, and no desire to - the master gain full up is how it's designed.

I have found the Master schematic full PDF is in first post.
There is the spots that I got my wires from Master AMP.

PAGE 39 of PDF in first post of topic :

Have you added coupling capacitors to your preamp yet, it's really something you should have.

I put a cap but found out that this cap make phase-shift on slave and it is not desire for me.

 

[arvinfx]

Do you need AGC applied to the input signal path ?

Regards, Dana.

I think it is not necessary.

 

[arvinfx]

Is bass boost in DSP the source of problem? Perhaps you need woofers with lower impedance to get more power at lower voltage.

Might be . But as I have told It is not exactly my issue. the main problem is when Gain on TL072 exceed than 25 I have distortion on Master Amplifier witch is non-inverting pin of TL072! in this topic I found one problem that is High-Gain.
But still I can not understand how to overcome on it.

also I tried something , I disconnected speakers from Master Amp and has no any difference .

 

[rjenkinsgb]

I put a cap but found out that this cap make phase-shift on slave and it is not desire for me.

The "master amp" shows a DC path to a what appears to be a 2.2V bias voltage on its outputs.

You MUST have a series capacitor between that and the opamp + input, as Nigel said much earlier on.
Try eg. 10uF with cap + to the master amp side.

You then need a resistor (100k?) from the opamp + input to 0V to prevent the input floating.

(If you saw a phase shift, that sound like you put a cap from somewhere to ground??)

 

[arvinfx]

Put the scope across the op amp supply rails (pins 8 and pin 4) to check the voltage stability and DC voltage when you apply a 49hz to 70hz signal. Looks to me like your power supply may be dropping out at low frequencies (you may need larger capacitors across your power supply or expect less power from your bass.

Checked, 7+ and 7- is smooth and fine .

 

[ZipZapOuch]

I think the problem is that you are confusing gain and volume.
For gain, you are adjusting the feedback resistance and, even though they are labelled "50k", they have a +/-20% tolerance (typically) so, if they are off on the high side, you are over amplifying and causing clipping. Moreover, you are using a potentiometer (voltage divider) as a variable resistor (rheostat).

To do it properly (in my book), I'd set the feedback resistance to the maximum value without distortion using fixed resistors and then use a potentiometer wired as a potentiometer for an output volume control (like most circuits use). That way, at max volume, you simply be sending most of your signal to the downstream power amp as an undistorted signal.

 

[ZipZapOuch]

arvinfx
If you do use the volume control as I described, use a 10k log potentiometer (also known as an audio potentiometer). It would be wired so volume increases as you turn it clockwise.

 

[ZipZapOuch]

And, lastly (hopefully), if your audio source's output is capacitively coupled to your non-inverting input, you'll need a high-ish value resistor from the non-inverting input to ground.

The capacitor is likely inside your audio source so, if you get a signal that seems to attenuate for 1-second to 1-minute, your likely seeing a charge build-up in the non-inverting input. This jFEt input is essentially a brick wall insulator and no charge will dissipate without this resistor. In any case, a 100k resistor from the non-inverting input to ground will not hurt in either case, just to make sure.

 

[arvinfx]

And, lastly (hopefully), if your audio source's output is capacitively coupled to your non-inverting input, you'll need a high-ish value resistor from the non-inverting input to ground.

The capacitor is likely inside your audio source so, if you get a signal that seems to attenuate for 1-second to 1-minute, your likely seeing a charge build-up in the non-inverting input. This jFEt input is essentially a brick wall insulator and no charge will dissipate without this resistor. In any case, a 100k resistor from the non-inverting input to ground will not hurt in either case, just to make sure.

I have checked deeply the schematic and saw this:

Actually both capacitor and that resistor is available in Master-Amp. I have checked again all information and circuit . I think problem is ±7V power supply of non-Rail to rail opamp that I used . I am trying to find a rail-to-rail op-amp for my project . Any result I will release it here friends.

 

[arvinfx]

I have changed the supply voltage to ±10.5V ( to much problem to provide low noise and clean voltage ) instead of changing op-amp to rail-to-rail .
Now the distortion of output of TL072 is around ±9V instead of 6.5V when supply voltage of TL072 was ±7.

But still I have noise on Master Amplifier (and also Slave amplifier) when G > ~30 ! Thus the problem is negative feedback .

Is this problem solved if I put a buffer between Master amplifier output and TL072 ?

 

[Nigel Goodwin]

I have changed the supply voltage to ±10.5V ( to much problem to provide low noise and clean voltage ) instead of changing op-amp to rail-to-rail .
Now the distortion of output of TL072 is around ±9V instead of 6.5V when supply voltage of TL072 was ±7.

But still I have noise on Master Amplifier (and also Slave amplifier) when G > ~30 ! Thus the problem is negative feedback .

Is this problem solved if I put a buffer between Master amplifier output and TL072 ?

As I've said all along, you don't need all that signal, nor rail-to-rail opamps - power amplifiers pretty well universally only need 1V signal for full output. You're also still ignoring the requirements for capacitors in the circuit, although we have little idea of what you're actually using, as you never post any full circuits.

Audio amplifiers are for AC amplification, they don't need DC capability - and making a DC amplifier causes problems using it for audio, and is completely pointless.