Bloody frustrating opamps wont work

I have a number of dual opamps, ST LM358N being one of them.

Three times I have tried to implement an ac coupled adjustable amplifier, according to the datashseet with two channels to amplify the audio of an ipod.

And 3 *** damned times the same side (pins 1,2,3,4) of the opamp fail to work. The sound is either scratchy and fades in an out or it is non-existent.

I can't think of any other reason other than the pin out specified in the datasheet is wrong for pins 1,2,3,4.

What other explanation is there for this consistent failure?

boylesg said:

What other explanation is there for this consistent failure?

Click to expand...

wrong circuit implementation?

I don't think the chips have a different pinout from the one shown in the datasheet (it would be a huge surprise) so you must be doing something wrong but you have to show your circuit and tell us how you are using it (for example trying to drive a speaker?)

Are you using a dip case version?

alexan_e said:

wrong circuit implementation?

I don't think the chips have a different pinout from the one shown in the datasheet (it would be a huge surprise) so you must be doing something wrong but you have to show your circuit and tell us how you are using it (for example trying to drive a speaker?)

Are you using a dip case version?

Click to expand...

In the latest case I was trying to drive a pair of ear buds.

I am trying to implement identical circuits on both sides of the opamp. Is there somthing wrong with doing this?

Each side is an independent opamp so the same circuit should work no matter which side you use or both sides at the same time.

So opamp2 works fine but as soon as you try to use opamp1 at the same time you get problems?
If you try with opamp1 only what happens?

alexan_e said:

Each side is an independent opamp so the same circuit should work no matter which side you use or both sides at the same time.

So opamp2 works fine but as soon as you try to use opamp1 at the same time you get problems?
If you try with opamp1 only what happens?

Click to expand...

Opamp (1,2,3,4) doesn't work properly under any circumstances, whether or not the other opamp is connected to the speakers and ipod or not.

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boylesg said:

Opamp (1,2,3,4) doesn't work properly under any circumstances, whether or not the other opamp is connected to the speakers and ipod or not.

Click to expand...

I have tried plugging different makes of opamp into the socket with the same results.

I will recheck my circuitry but surely I could not make the same mistake 3 times.

Are the two opamps totally electrically isolated from each other?

So one side works fine in several opamps and the other one is not, the two opamps are independent so this doesn't make sense and I can't imagine that one of the opamps has been damages in several different chips.

Opamp (1,2,3,4) doesn't work properly under any circumstances

Click to expand...

The opamp is just 1,2,3. Pin 4 in the ground.

Could you upload an image of the circuit, maybe we can see something that you don't see.
Is it on a breadboard?

It sounds like a wiring problem but there may be another explanation that accounts for the 'fading': is it possible the combined gain of the two amplifiers close to each other is making it unstable? It might work with one amp OK but not when both are running.

Sorry if I'm pointing out the obvious, but do you have decoupling capacitors very close to the supply pins?

Brian.

In the latest case I was trying to drive a pair of ear buds.

Click to expand...

Opamp (1,2,3,4) doesn't work properly under any circumstances, whether or not the other opamp is connected to the speakers and ipod or not.

Click to expand...

It's mostly guessing without a schematic, but quite obviously LM358 hasn't enough output current to drive a usual earphone (with e.g. 32 ohm impedance) to acceptable volume. The Ipod is equipped with an efficient small power amplifier in contrast.

boylesg said:

Opamp (1,2,3,4) doesn't work properly under any circumstances..... I have tried plugging different makes of opamp into the socket with the same results.

Click to expand...

The fact that the amplifier on pins 5,6 and 7 works with several i.cs points to a problem associated with the socket and/or the wiring to it.

@boylesg : Please post a complete schematic and, if possible, a picture of the physical layout. Otherwise, those attempting to help you are all shooting in the dark.

Pjdd said:

@boylesg : Please post a complete schematic and, if possible, a picture of the physical layout. Otherwise, those attempting to help you are all shooting in the dark.

Click to expand...

The schematic I am following is from the LM358N datasheet and is identical to this one:
It works fine with opamp 5,6,7 but it just wont work with opamp 2,3,4.
I am using 6.8k for RB as I don't have any 6.2K resistors on hand.
Cin is 10u and Cout is 100u
RL is 100k

**broken link removed**

boylesg said:

It works fine with opamp 5,6,7 but it just wont work with opamp 2,3,4.

Click to expand...

The opamps are on pins 5,6,7 and 1,2,3 NOT 2,3,4.

^ That's right. Most 8-pin dual opamps use this pin configuration. This diagram will make it clearer -

godfreyl said:

The opamps are on pins 5,6,7 and 1,2,3 NOT 2,3,4.

Click to expand...

Appolgies, I meant 1,2,3.

I have just checked my wiring and pin 4 goes straight to ground.

I still can't see any mistakes in the wiring for opamp 1,2,3

I made a simulation of your schematic in post #11.

Screenshot:



This configuration is meant for a single polarity supply. I used 12V, not knowing what your supply is.

The gain is 11. If your input is line level (say 2V pk-to-pk) then the output is at risk of exceeding your supply, with distorted audio output as the result.

Any abrupt change requires several seconds for levels to settle. This means that your input and output capacitors could be reduced in value.

as many have already written, it is not known what it is you want to do ?

ipod output is ~ 2v pk-pk i think. with gain of 11 it gives 22v pk-pk.
what is your Vs voltage ?

do you have single R3/R4/R5 in your circuit or have you made separate phantom voltage to each opamp ? if it is single R4/R5, then you have to at least duplicate R3 for 2nd opamp

also output current of lM358 is only ~ 20-30ma. In some place you are using headphone - very low impedance, in other circuit diagram you are using 6.2K and 100K ??

please clarify these points

kripacharya said:

as many have already written, it is not known what it is you want to do ?

ipod output is ~ 2v pk-pk i think. with gain of 11 it gives 22v pk-pk.
what is your Vs voltage ?

Click to expand...

24V, but I could make it 36V if I wish.

kripacharya said:

do you have single R3/R4/R5 in your circuit or have you made separate phantom voltage to each opamp ? if it is single R4/R5, then you have to at least duplicate R3 for 2nd opamp

Click to expand...

No they are both totally independant circuits with their own set of all resistors and capacitors.

kripacharya said:

also output current of lM358 is only ~ 20-30ma. In some place you are using headphone - very low impedance, in other circuit diagram you are using 6.2K and 100K ??

please clarify these points

Click to expand...

Is that what the problem is likely to be, i.e. that the the opamp can't sustain the mA required for both channels hence one side just mysteriously fails to work?

Perhaps then I need to add a simple common collector amplifier to the output?

But then this does not account for the fact that I can disconnect opamp 5,6,7 from one ear bud channel and opamp 1,2,3 still fails to work on the still connected ear bud channel.

On the opamp outputs I have the series capacitor. Between he cap and the opamp a 6.8k resistor goes to ground and after the capacitor a 10k resistor goes to ground......as per the specs in the datasheet. That is in addition to the resistor that feeds back to the inverting input.

Have I misunderstood anything about the spec?

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BradtheRad said:

I made a simulation of your schematic in post #11.

Screenshot:



This configuration is meant for a single polarity supply. I used 12V, not knowing what your supply is.

The gain is 11. If your input is line level (say 2V pk-to-pk) then the output is at risk of exceeding your supply, with distorted audio output as the result.

Any abrupt change requires several seconds for levels to settle. This means that your input and output capacitors could be reduced in value.

Click to expand...

Vcc or Vss or what ever yoiu call it is 24V.

Also R1 is 5.6k and R2 is a 100k pot so that I can vary the gain up to x20. Opamp 5,6,7 works brilliantly all the way up to x20. Opamp 1,2,3 jut does not work at all.

boylesg said:

But then this does not account for the fact that I can disconnect opamp 5,6,7 from one ear bud channel and opamp 1,2,3 still fails to work on the still connected ear bud channel.

Click to expand...

if everything as you say, then problem is not with the opamp.

other areas to check are --

is the ipod output coming correctly for both channels ? try switching the input lines. make sure your shield/ earth is connected to common gnd point.

maybe your earbud one channel is bad. try switching this next.

if both of these separate tests give exactly same result - i.e. opamp 1/2/3 doesn't work - then it HAS to be a wiring problem, or maybe a bad or incorrect component in the 1/2/3 side. sometimes it is worh it to remove everything and then place it again. you have not mentioned if this is on breadboard or pcb ? if it's on breadboard, then definitely worth trying to re assemble the entire 1/2/3 circuit from scratch.

boylesg said:

Opamp 1,2,3 jut does not work at all.

Click to expand...

What does this mean: ...doesn`t work at all ? Output at supply rail?

Did you try already to reduce R3 (dc offset effects) ?

LvW said:

What does this mean: ...doesn`t work at all ? Output at supply rail?

Did you try already to reduce R3 (dc offset effects) ?

Click to expand...

Doesn't work as in no sound at all or very faint and scratchy audio. In previous attempts it was very faint and scratchy audio but in my latest attempt there doesn't seem to be any audio at all.

DC offset effects? Would you mind explaining this?

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kripacharya said:

if everything as you say, then problem is not with the opamp.

other areas to check are --

is the ipod output coming correctly for both channels ? try switching the input lines. make sure your shield/ earth is connected to common gnd point.

maybe your earbud one channel is bad. try switching this next.

if both of these separate tests give exactly same result - i.e. opamp 1/2/3 doesn't work - then it HAS to be a wiring problem, or maybe a bad or incorrect component in the 1/2/3 side. sometimes it is worh it to remove everything and then place it again. you have not mentioned if this is on breadboard or pcb ? if it's on breadboard, then definitely worth trying to re assemble the entire 1/2/3 circuit from scratch.

Click to expand...

At the input capacitor of opamp 5,6,7, on the opamp side, I can measure about 2V peak with my multimeter while the ipod is playing.

At the input capacitor of opamp 1,2,3, on the opamp side, I can measure less than 1V peak with my multimeter while the ipod is playing.

Swapping ipod channels at the inputs makes no difference, however I will double check this.

There is nothing wrong with the audio coming from the ipod when you plug the ear buds directly into it.

I have a plug with bare wires at the other end. I can connect these to some 8R speakers and hear audio on them for both channels.

During my latest attempt to solder this circuit I did check each component with my multimeter and found no problems with them.

A wiring problem......I wish I could spot it because I have checked it a couple of times now and can't see any mistakes. I have compared the wiring of opamp 123 with the wiring of opamp 567 a stage at a time, while refering to the pinout of the LM358N, and still can't see any differences bewteen the way I have soldered up each circuit.