Audio Pre Amplifier

Also, look at the upper portion: the assembly of the middle point with an operational amplifier: Ain't it a beauty. Instead of a lousy zener.

Zeners are 5% components in their real form. There are 1% and 0.1% and, probably 0.01%. There are reference components based on a zener which are extremely accurate (but also low power sinking). These are not general components.

Zeners have a NON LINEAR voltage current characteristics as well as the diodes. This, combined with the 5% may give much more offset the a 5% divider where the 5% gets compensated to some extend, statistically speaking.

Assume you have a 9V power supply which bounces from 7.5 to 13 or more as depends on the load. One can't take advantage of this non regulation with a fixed zener. One can with an operational amplifier.

But even with ideally regulated supply. Evaluate the zener behaviour only. Assume 9V. There ain't 4.5% zenner in the 5%. One must use 4.7. Assume worst case of an error of 5% up. 4.7 + 5% of 4.7 = 4.935. Assume the non linearity of the U(I) goes up. I do NOT want to get to read a zener chart but I am certain the voltage would go higher than 5V. You look at the chart and comment. So, 9V supply, 5V middle point and the desired one is 4.5. We lost 0.5 from the upper portion. TL084 gets anothe 1.5V of the upper portion (can't swing all the way up), so we lost 2V and we have only 2.5V maximum amplitude.

True, the incorrect middle point will not affect the operation of high performance low signal electronics. (36V power supply, 1V amplitude or less).

Inexpensive electronics may be affected. However, inexpensive is more than enough in most of the cases.
 
Another application would be to cut the unwanted frequencies and to increase the wanted with a filter at the pre amplifier and supply the amplifier input with only the wanted ones thus allowing fo higher amplification of the wanted frequencies. For example, at the beginning of the equipment, a base singer would rather cut the treble and increase the base sky high to the maximum of the equipment. A bit of cheating as far as singing is concerned but what do we care.
 
Another application would be a booster to a power subwoofer in case the subwoofer is separate (connected in parallel to the speakers). Then, the stereo signal from the source can be taken from the source, given to the pre amplifier, then a simple stereo passive filter can be easily made, then the signal goes to the subwoofer. The same effect can be achieved without the pre amplifier but there would not be base boosting just filtering and a bit of attenuation but, then, the subwoofer is supposed to have a filter anyways so additional filtering and then amplifying more by the subwoofer's amplifier may not make so much difference.
 
Howdy, y'all. What's goin' on with Good Ol' UK today? I got 700 viewings for a simple publication of the noninverting operational amplifier which is so basic and simple I was to be stoned 18 years ago (when last seen in the UK) just to think to publish such even (in) case I had invented the noninverting amplifier?

Yes, I know, they got no way of knowin' what this **** is prior to viewin' and they just click but ain't reading and ain't interested.

🙂
 
Mistake: when doing the amplifier, the 10uF capacitors at the end must not be put because these would filter audio frequencies when coupled with the 4, 8, 16 Ohm resistance of the load.

Theoretically, this value should be 2mF for 4 Ohm to cut at 20Hz. Practically, this should not be done not so much because of oscillations but because these capacitors are mot available unless many are parallelled.

I have seen 1F capacitors used in cars as big as the pistons and, most likely, electrolitic. The size was approximately 20" by 5".

For the pre amplifier, the capacitors are OK due to the high input resistance of the amplifier. PROBLEM: In case the preamplifier is to be put in a microphone input with 1K resistor in parallel, then the cut off frequency is ~160Hz. Not nice. Then the capacitor must not be installed or be 100uF. Haven't done the phase analysis but believe the amplifier would not oscilate. 100uF is either not generally available or huge in size.

Although the Internet cites microphone amplifiers with 1K input, these are highly rare. The mostly cited ones are 10K, 20K and up. Even at 10K, the cut off frequency is 16Hz.

Of coarse, one may not put a capacitor at all.

Another problem is when the pre amplifier input(s) is(are) connected to an amplifier whose output is decoupled by a capacitor (in case such exist because lower in size capacitors are lower in capacitance and would filter audio frequencies). The problem is due to the decoupled (DC cut) positive input of the operational amplifier. This would cut the necessary current (DC or very slowly changing) to the base of the transistor in the differential input of the operational amplifier. A resistor of a relatively high value of 1M must be put across the input of the preamplifier to allow for the tiny current. This may introduce noise although AC connected to the low output impedance of the sourcing amplifier and a capacitor accross might be a good idea but is not recomended because there might be a decoupling capacitor at the output of the amplifier.