designing a stereo Booster using TL072 - help needed

[dschwartz]

I think the 100k input resistors should be connected to Vref.
As drawn, is connected to the feedback shunt resistor on the wrong side.

[antonis]

I think the 100k input resistors should be connected to Vref.
As drawn, is connected to the feedback shunt resistor on the wrong side.

But they are connected to V_REF, via 1k gain resistors, Daniel..
(clearly for bootstrapping reasons..)
Particular bias resistors configuration aims to equal signal voltage swing across 100k resistor(s) (negligible voltage drop, hence practically no current flowing through them, resulting into "infinite" apparent value..)

[half_smith]

Interesting, can you explain why you made those changes?

If I rebuild my working prototype with these changes what would I expect to see as a result?

[antonis]

Interesting, can you explain why you made those changes?

Actually, only for raising 1M (100k on last mod scheme) bias resistors apparent value by bootstrapping them..

If I rebuild my working prototype with these changes what would I expect to see as a result?

To be honest, almost nothing..
(unless you deal with high impedance audio sources..)

https://www.soundonsound.com/techniques/understanding-impedance

[half_smith]

does "bootstrapping" in this context mean, that instead of 1M you run the 100k, 1k, and 10k in series?
I'm not sure I will ever understand all of this. I didn't go to engineering school, just figuring it out along the way.

[antonis]

does "bootstrapping" in this context mean, that instead of 1M you run the 100k, 1k, and 10k in series?

No..

In breef:
Both op-amp inputs tend to appear equal voltage level (via negative feedback configuration) at any instant so any input signal voltage "swing" on non-inverting input (+), also present on one bias resistor leg, is almost identical with the one on inverting input (-), also present on the bias resistor other leg (via 10μF cap)..
So, there isn't any substantial voltage drop across bias resistor hence almost no current flows through it..
But the apparent value of a resistor with no current flowing across it is infite..
No signal leaks through bias resistors hence no bias configuration loading effect..

1k resistor serves here (interchanged places with 10μF cap) to connect non-inverting input (via bias resistor) with half the supply voltage (formed by 10k/10k voltage divider)..

[half_smith]

I've decided to remove/jumper the Pots and just go full open. I was running them both at 10 anyway, plus I had a problem with them getting bent when jamming into the enclosure and broke a joint on the board.

Having the thing on the workbench again, I went back to reread this comment and am considering swapping the 1M for 100k as suggested in your final schem. You refer to this as "raising" it's apparent value? Do you mean that by having less resistance it has greater effect (in this context)?

Actually, only for raising 1M (100k on last mod scheme) bias resistors apparent value by bootstrapping them..

and the enigmatic "bias current to be reckoned with..."

P.S.1
You can maintain 1M bias resistors for TL072 (or other FET input op-amp) for even higher input impedance..
100k above are intended mostly for bipolar input op-amps of input bias current to be reckoned with..

The relationship/equation between the resistors in the FB loop adjust the gain of the op-amp right? But I'm still working out how the bootstrapping effect functions.
In any case, will changing the 1M to 100k seriously increase my gain, therefore I will require the Pots?

[antonis]

100k bias resistors are commonly used only for bipolar input op-amps where input bias current causes a "significant" voltage drop across bias resistor..
(you didn't read careffully my comments..)

You can use any value between 100k - 1M..
(but you have to study a bit about "bootstrapping"..)
https://electronics.stackexchange.com/questions/586839/explanation-about-op-amp-bias-resistor-and-bootstrapping

>will changing the 1M to 100k seriously increase my gain, therefore I will require the Pots?<

Not at all..!!

[half_smith]

so I removed the pots and just jumped them right to the 10uf cap + side.


it sounds pretty good, but I want it even louder, what would I change to get hotter output? - I'm still diming the console main volume
Av=1 + R2/R1 for gain right? Did removing the Pots make R2=0 ?
or was the 10k pot R2 and R1 was 11k (1k +10k=11k), so Av=1+ 10/11, Av=1.9 ? 
Is raising R2 what I want?
Have I mucked it all up?


Lastly, I'm getting some pretty bad radio interference. Is shielded cable or metal enclosure my best option to reduce?

[antonis]

so I removed the pots and just jumped them right to the 10uf cap + side.

So you've result into a fantastic buffer..!! 
(a voltage follower of very high input impedance and UNITY gain..)

If you wish to remove the pots, first set them where you like, measure resistance between lugs 1 & 2 and then replace them with a resistor of measured value..

P.S.
Unity gain can be obtained  by making either R2 zero (short) or R1 infinite (open)..

[half_smith]

funny! ok so my suspicion was correct. By removing the 10k Pots I made R2=0, no added gain.
I had to increase the value of R2 to get more gain.

I put a 47K resistor (on each channel/side) in place of the Pot and now it is REALLY LOUD.

I remember Antonis suggested the Pot placement in the FB Loop in order to be able to control clipping, but it seems to be loud and clean (not clipping).
perhaps because I am running it at 12v off the Car, additional headroom.

Ok, thanks again! hopefully it is good to go now. 

[antonis]

I remember Antonis suggested the Pot placement in the FB Loop in order to be able to control clipping, but it seems to be loud and clean (not clipping).

Under normal circumstances, I shouldn't dare to speak on behalf of that ridiculous person, but..

Pot in the NFB loop controls GAIN ..!!
Clipping might (or might not) occur and is solely dependent on power supply margins together with bias point..
(always talking for small signals, in the mean of input signal amplitude in relation with headroom..)

[half_smith]

P.S.2
Next project will be a further modification of the above with an input "guard ring" (for huge input impedance) and some kind of Tone control (for a self respecting pre-amp)..

I suppose some shielding, a Tone control, and a "guard ring" would be the finishing touches.

I will continue to disassociate and fall asleep reading about "bootstrapping" and "noiseless biasing" and "guard rings" and "bias configuration loading effects"...
usually I slowly digest this stuff over a decade or two.

[half_smith]

As drawn, if the input level is too high,  it will clip the opamp and no matter how you set the level, it will keep clipping.
If you change the gain by varying the feedback resistance, you can lower the gain and avoid clipping.

My apologies, I attributed the Pot in the NFB to Antonis, it was dschwartz