Re: TL071 Offset Voltage

[SSDIY]

Re: TL071 Offset Voltage

Hello Everyone

I just finished building a TL071 line pre-amp for my cd player. But the output (Pin #6) of the op-amp measured before the coupling capacitor (2.2uF) has a voltage offset of 0.1v for one channel and 0.7v for the other channel. DC Voltage measurement for both channels after the coupling capacitors are 0v.

Would anyone please tell me what caused this problem and is there any way I can fix it?

Thank you

 

[Mooly]

We need to see the circuit to advise.

What are the voltages on pins 2, 3, 4, 6 and 7?

Pins 1 and 8 should be floating.

 

[Mark Tillotson]

TL071 have worse case input offset of 10mV, so a gain of 10 makes the output offset 0.1V worst case. If the preamp circuit has gain (perhaps before the volume pot), then that could easily explain 0.1V offset. 0.7V offset sounds wrong though - are these opamps socketed?

 

[SSDIY]

Hi Mooly

Attached please find the schematic of the TL071 Pre-amp.
I measured the following voltages on each pin of the TL071 OP Amp:

Channel A
Pin 1: -14.5v
Pin 2: 0v
Pin 3: 0v
Pin 4: -15v
Pin 5: -15v
Pin 6: -0.78v
Pin 7: +15v
Pin 8: 0v

Channel B
Pin 1: -14.5v
Pin 2: 0v
Pin 3: 0v
Pin 4: -15v
Pin 5: -15v
Pin 6: -0.01v
Pin 7: +15v
Pin 8: 0v

Please comment.
Cheers

 

[SSDIY]

Schematic is here:

 

[SSDIY]

Hi Mark

Yes, the op-amps are socketed.
There is a 10k volume pot before the 100 Ohm resistor at the input.

 

[jaycee]

As it's a JFET opamp, the capacitors C102 and C202 are unneeded. The DC block capacitors C101 and C201 are in the wrong place.

 

[jan.didden]

Schematic is here:

This schematic is wrong. There is no DC path for the inverting input, therefor you get huge offset.

Check a standard non-inverting circuit, that will make it clear.

Jan

 

[Mark Tillotson]

Schematic is here:

Bad circuit, no DC bias to the opamp input, so it latches up.

 

[diyralf]

There are thousands of examples of correct op wiring. You have to copy correctly.

 

[Mooly]

Hi Mooly

Attached please find the schematic of the TL071 Pre-amp.
I measured the following voltages on each pin of the TL071 OP Amp.............

It is as the others have commented...

Here are two ways to fix it. The second way which retains the feedback cap would be best because it keeps the gain at DC equal to 1. Both of the caps could usefully be a bit bigger in value and be of much lower voltage. Try a 10 or 20uF electrolytic.

The input (which you say is from a pot) must have a DC path to ground. As long as you no series coupling cap that you have not shown then it should be fine.

 

[Fast Eddie D]

I don't think you can get offset like that with the circuit you posted. I think the output would go to one rail or another.

I don't think you copied the circuit correctly.

 

[jan.didden]

Agree F/Eddie.
OTOH, isn't that a FET opamp? The cap leakage may work some DC current, and when measuring the DC impedance of the multimeter would draw the input to gnd.

Jan

 

[steveu]

Schematic is here:

Oops, that's not going to work! Two problems:
1. There is no DC feedback through the caps in series with the negative inputs.
2. There is no DC reference unless the input is shorted or connected to an external DC path to ground.

An op-amp does not bias itself. You have to arrange the input networks so that the input difference is zero DC when the output is where you want it, and that can be (almost) anywhere you like between the rail voltages.

 

[Fast Eddie D]

Agree F/Eddie.
OTOH, isn't that a FET opamp? The cap leakage may work some DC current, and when measuring the DC impedance of the multimeter would draw the input to gnd.

Jan

If you could stabilize the bias, then the AC performance of the circuit would be usable.

My only experience with open loop op amps was when I tried to get some linear op amps to work as a comparator. That didn't work so I ended up buying a batch of NE555 "timers." That was a very good idea because you can do a million things with them.

 

[Mark Tillotson]

Basically using an opamp as a comparator is pretty bogus at the best of times, and many opamps are not suitable for open loop at all. If you want a cheap but reasonable comparator, LM339 is the go-to-device. I'd suggest always choosing CMOS versions of the 555, they are much better behaved (no 150mA spikes on the supply for one thing!) 7555 is an example.

 

[Osvaldo de Banfield]

I remember a small interface made with a TL081 open loop, powered from the RS232 port in my old 486, and that receive audio from a spk in a short wave radio, and inputs those signals squared into the port, and a program that ran in DOS called HamComm (Or the like) to read RTTY's in the 90´s. I read many RTTY's during several months in the night using a tube radio, U series and that I added a carrier oscillator at 465KHz and elementarily decode SSB, and a large aerial wire. Reception was very decent by those era.

 

[SSDIY]

Hello Everyone

First of all, I want to say thank you for all your information and comments about the circuit. The reason why I want to build this circuit is because I had built it (one channel) before when I was a student long time ago as an experiment, and when I finished, I hooked it up with a cd player and it sounded normally.

Anyway, I searched on the internet over the weekend tried to look for some information to fix the offset problem, but I find the attached pre-amp circuit and it looks great to me. Once again, please review the schematic and your comments are highly appreciated.

Cheers

 

[rayma]

Add a 1M resistor across each input, to keep the coupling capacitor discharged.
Should work ok if the amp input is 100k or higher.

 

[jackinnj]

btw, TL071 pins 1 and 8 can be connected to a 100k potentiometer. use a 1k resistor to connect Vee to the swiper. It's all there in the datasheet.

Read the prospectus carefully before you invest.