virtual ground unknown behaviour

Thread Starter

jacopo1919

Joined Apr 12, 2020
41
i have a single supply voltage circuit ( rat distorsion ) which powers the IC (lm308) with +9V and through a voltage divider, it creates a virtual ground of half the voltage in order to equally clip the signal.

First at all i’m asking myself why this is done since voltage dividers are usually not suited for this application.. but anyway if i recreate the same situation with +12V, i don’t get the expected +6V. (i get 7.7V?!)
i don’t get what i’m missing here and it appears strange to me!

any hint is appreciated
 

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Thread Starter

jacopo1919

Joined Apr 12, 2020
41
the clipping circuit is the same but the whole distorsion is part of a bigger circuit with is using other 2 tl072 and a lm13700.
 

Thread Starter

jacopo1919

Joined Apr 12, 2020
41
thoise are correct. after some measurement, i get +6V correctly instead of +4.5V before the 1M resistor.
This is supposed to DC offset the signal of +6V over which, the audio signal will ride (no offset before as shown in the picture).
weired is... that i measure the signal riding only over +2V offset.

the virtual ground should make the IC to equally distorce the negative and the positive polarity of signal-
when measuring the output with the oscilloscope , i see that the lower polarity clips much before the upper one. And this means that the virtual ground is not half of the IC supplied voltage but less ( i believe only +2V)
 

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AlbertHall

Joined Jun 4, 2014
11,309
What voltge do you get on pin 6 of the LM308?
If it isn't 6V then disconnect at least one end of the 1uF capacitor connected to pin 2 of the LM308 and measure pin 6 again.
 

DickCappels

Joined Aug 21, 2008
7,432
The + supply and the - supply appear to not be equally loaded. Something like this, below should help.
1608774442251.png

A complimentary pair of transistors can extend the range of current imbalance that can be corrected.

Choose an opamp that can work well with the output at 1/2 of the power supply. The last time I did this was 18 years ago using half of an LM358 to split a 10V supply to ±5V.

1608774839109.png
Try to ignore the TL431 circuits - they are there for over-voltage protection -there were some hard to replace chips in the circuits being powered.
 

Audioguru again

Joined Oct 21, 2019
3,186
The schematics show the extremely old LM308 opamp with its input pin numbers 2 and 3 backwards.
It is shown with positive feedback instead of negative feedback.

It probably does not matter if the clipping of the opamp is not perfectly symmetrical since the diodes cause symmetrical clipping at a much lower level.
 
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