Basic question on audio mixer circuit

[donturner]

Hopefully this is a simple one: What is the purpose of the 27k resistor at the opamp's non-inverting input? Why wouldn't you just connect directly to ground?

[danadak]

Its the parallel equivalent of the 3 input Rs and is used to minimize
offset due to bias current the OpAmp inputs need. If left out you would
see a DC offset at output of OpAmp.


http://www.analog.com/media/en/training-seminars/tutorials/MT-038.pdf


Regards, Dana.

[helius]

Op amps drive their output in whatever manner causes the two inputs to be equal. If you tied one of the inputs directly to ground, the opamp would be unable to balance its inputs; the input bias current on the inverting input creates a voltage on the three 100K resistors, but there isn't any voltage on the non-inverting input, because it would be tied directly to the ground rail.
This circuit is a "inverting summer". See page A-24 of Texas Instruments' Op Amps For Everyone.

[Audioguru]

The design of the 741 opamp is 48 years old and it was never intended to produce good audio like modern audio opamps do.

[helius]

The design of the 741 opamp is 48 years old and it was never intended to produce good audio like modern audio opamps do.

The NE5534A is 40 years old and has been used continually during that time in mixer equipment. Age doesn't mean much.

[b_force]

The design of the 741 opamp is 48 years old and it was never intended to produce good audio like modern audio opamps do.

The NE5534A is 40 years old and has been used continually during that time in mixer equipment. Age doesn't mean much.

Well, the 5532 or 5534 wouldn't my personal choice for new designs, unless the budget is exmtremely tight.
There are enough (audio) opamps out there with better specs and most of all, less Iq.

[JS]

When noise is critical a cap could (should) be added bypassing that 27k resistor, so you eliminate the thermal noise of that resistor.

In big mixers summing amps are one of the critical noise points, a ton of noise gain to get unity signal gain is calling for problems, so any noise in that non inverting input really gets amplified. For this same reason using as low resistance values as possible is a good practice and is very important the routing of such PCB, specially where that non inverting input is connected to.

JS

[neil t]

danadak said it,  bias current offset