opamps

[mettula]

Hello everybody

I have a difficulty. On the attahced .jpg file one can see 2 symbols of an opamp. But one opamp has 2+ instead of 1+ and 1-. Does this means that the oparational amplifier is used as not as an non inverting amplifier.?? Also how can I make it without any gain??

Also when I have to build this circuit, those amplifiers have to have no gain. so for the normal non inverting opamps, I do not do the feedback resistors??
??? ??? ???
Thanks for your help and patience

View attachment 40204

 

[audioguru2]

An opamp has a non-inverting input (+) and an inverting input (-).
A non-inverting circuit can have no gain (a gain of 1.0) by connecting its inverting input to its output.
An inverting circuit can have no gain (a gain of 1.0) by using the same value for its input and feedback resistors.

 

[mettula]

Thanks AudioGuru; but what about that schematic of the opamp with 2 +ives as the inputs - what do those mean

thanks

 

[audioguru2]

Your schematic is wrong. Opamps don't have two "+" inputs.
Attach the entire schematic and we will show you which one is "+" and which one is "-".

 

[mettula]

ok audio guru, here is it. thanks

View attachment 40209

 

[audioguru2]

Hi Mettula,
They are not opamps. They are "matrix adding and subtracting circuits". Opamps can be used to make them but the details are not shown.

 

[mettula]

Audioguru, I don not know about adding or subtracting matrices; where can I learn, any ideas of any IC's which I can use???

Thanks

 

[mettula]

if I do something like in the picture; will it work?? - any suggestions for the IC's?? will 741 work? I prefer somthing of very good quality - so any ideas.
Many thanks

View attachment 40210

 

[audioguru2]

1) The microphones have a very low level so each one needs a preamplifier. The preamplifiers have very low output impedances so the value of the 10M resistors is way too high. Replace the 10M resistors with 10k.
2) Very few good opamps work with a supply voltage of only + and -2.5V. Use + and -10V.
3) The old 741 opamp is noisy and has a bandwidth to only 9kHz at full output. Use an audio opamp like the TL071 that is low noise and has a bandwidth to 100kHz.

 

[mettula]

thanks audioguru. I'll try it; maybe it'll melt down knowing my luck!! lol but very thanks for evrything I appreciate your patience and help

Mettula

 

[mettula]

hi again,

If i do not do the feedback resistor and the resistor on the input, whislt working with OPA134 and the input being at line level, will the IC have 0 gain? or will it just wont work??

 

[audioguru2]

Opamps don't work properly without negative feedback.
Look at the datasheet for the opamp. Without a negative feedback resistor, the voltage gain is 1,000,000 at DC and at low frequencies, then the output would be saturated against a supply rail due to the opamp amplifying its own input offset voltage.

With line level input levels then the voltage gain must be reduced to about 1 with negative feedback.

 

[mettula]

I think I made my point not clear, I wanted to know if the output of the apamp would be connected to the -ive input of the opamp without any resistor, and the positive input of the opamp alsoi without a resistor working at line level, will the opamp work coorectly and with 0 gain??

 

[audioguru2]

An opamp can have a non-inverting gain of 1. It has a high input impedance and a very low output impedance like these ones:

View attachment 40249