Choice of op.amp. model

[autir]

Hello

[audioguru]

I would like someone to recommend an operational amplifier IC.
Definitely NOT the 741 with its offset null pins and its dual power supply. I want something with a single V+ and GND and which does not need external adjustments to be calibrated.

All single opamps have offset null pins. Their max input offset voltage is as low as the distorted, noisy, narrow-band old 741 opamp and is only 6mV so doesn't matter in an AC circuit.
All opamps work fine with a single V+ and GND. They just need to be biased correctly when using a single supply. See my sketch.

Of course it has to be popular (easy to find in Europe, even in the future).
Decent quality in audio frequencies, the ability to drive 4 Ohm loads and a reasonable price would be considered a plus.

I don't know about Europe, but the TL071 single, TL072 dual and TL074 quad opamps are popular and inexpensive in Canada. They have very low distortion, fairly low noise and wide bandwidth. They have JFET inputs that are useful when needed in very high impedance circuits. Like most other JFET-input opamps, their output inverts if an input exceeds the negative common-mode range (about 1.5V above the negative supply voltage, or above ground in a single supply circuit). This only happens if you turn-up-the-volume way too high and sounds cool if its gain is low so it doesn't clip. On a 'scope it looks like the output suddenly turns inside-out.

Opamps can't drive 4 ohms, a power amplifier is needed. Most opamps can drive a minimum load of 2k ohms, a few can drive 600 ohms. I try not to load an opamp with less than 10k ohms. ;D

[autir]

Opamps can't drive 4 ohms, a power amplifier is needed.

Can you propose a power amplifier IC family? (1- and 2- input).

Also, can you please explain how can we calculate the value of the voltage-dividing resistors in your sketch?

And why all these capacitors between V+, V- and GND?

[autir]

All single opamps have offset null pins.

Just downloaded the TL071, TL072 and TL074 datasheets from ST.

In TL071's datasheet the pinout diagram shows the existence of the two offset null pins. There is also the scematic displaying the proper calibration.
In TL074 there are no such pins.
In TL072's datasheet the output from the second opamp is correctly designated as "Output 2", while the output from the first is designated as "Offset null 1".

[autir]

And the third...

[audioguru]

In TL071's datasheet the pinout diagram shows the existence of the two offset null pins. There is also the schematic displaying the proper calibration.
In TL074 there are no such pins.

A 14 pin package is standard for ICs. 22 pins is not. If you need to null the input offset voltage then use a single opamp.

In TL072's datasheet the output from the second opamp is correctly designated as "Output 2", while the output from the first is designated as "Offset null 1".

[audioguru]

Can you propose a power amplifier IC family?

The choice of a power amp IC depends on how much power you need and what is the impedance of your speaker. National Semiconductor, Philips and ST-Micro make hundreds of them.

Also, can you please explain how can we calculate the value of the voltage-dividing resistors in your sketch?

A few tutorials about opamps are in our Articles section.

And why all these capacitors between V+, V- and GND?

The positive and negative supplies need a bypass capacitor which is part of their power supply and are needed if batteries are used. Input and output capacitors are needed to pass AC but block DC. The opamp's reference voltage terminal needs a capacitor to ground. ;D

[gillyne]

Hi.

I am currently working on FTV projects. I am using TC9400 to convert frequency to voltage and TL071 to get rid of ripple and ADC output.

I am using single supply for both case.
May I know how should I modify the TL071 to be 10v-GND?
Is there any additional circuit? ???

Attach on left is the circuits that I am working on, on right is the inverting single supply opamp taken from audioguru.

Thanks in advance. :)

[audioguru]

Hi Gillyne,
Welcome to our forum. ;D
I have converted the filter circuit to a single supply voltage:

[gillyne]

Thanks for your help.  ;)

Somehow, I face another problem dealing with my TC9400 :-\. It is a frequency to voltage IC from MICROCHIP. I had inputed the frequency supply at pin 11, and the result get at pin12 is very low.
The frequency input is 4k, 10k, 20k, 40k; I get more or less around 1.23v.
Is the diagram wrong?

Thanks.

[audioguru]

Your circuit is from their datasheet so should work if it is correct. Maybe a parts value of yours is wrong, the diode is reversed or the IC is bad.