Bloody frustrating opamps wont work

I've got to ask the question: What are you trying to achieve? If you have a 'high' power amplifier producing 2V into a low impedance load, why are you following it with an op-amp that is incapable of high power or driving low impedance loads.

Sounds a bit like you have a Ferrari pushing a bicycle and you are wondering why it's brakes don't slow it down.

Brian.

betwixt said:

Sounds a bit like you have a Ferrari pushing a bicycle and you are wondering why it's brakes don't slow it down.

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we all know this, but are still pushing ahead for the fun of it

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boylesg said:

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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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.

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based on all your experiments and what you have written, it is definitely an implementation problem. in other words - trash your pcb and make a new one

kripacharya said:

- trash your pcb and make a new one

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Already done this twice before. I don't see any point in doing it again unless I can figure out what the frig I am doing wrong with opamp 123.

I am only using the ear buds to test the thing. The ultimate aim is to feed the amplified audio into a 555.

Any suggestions on another way to test it that does not amount to driving a ferari with a 2 stroke engine?

boylesg said:

Already done this twice before. I don't see any point in doing it again unless I can figure out what the frig I am doing wrong with opamp 123.

I am only using the ear buds to test the thing. The ultimate aim is to feed the amplified audio into a 555.

Any suggestions on another way to test it that does not amount to driving a ferari with a 2 stroke engine?

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you have actually re-designed your pcb 3 times ? that is impressive.
can you post a pic of your setup here. this might help us solve the problem.

also, what will the 555 do with the audio? where will you connect this output on 555? what is your real ultimate aim ?

It has been mentioned in several posts, but you are apparently still ignoring an obvious problem of your test setup: An OP like LM358 can't drive the low impedance of an earphone with sufficient output current. This doesn't answer why you seem to have problems with only one of both channels, but it demands cautiousness when judging the results.

kripacharya said:

you have actually re-designed your pcb 3 times ? that is impressive.
can you post a pic of your setup here. this might help us solve the problem.

also, what will the 555 do with the audio? where will you connect this output on 555? what is your real ultimate aim ?

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I tried it again earlier on and I actually got some audio from opamp 123 this time and it wasn't far the audio I got from opamp 567. Previously the audio from opamp 567 was quite a bit stronger.

I am now thinking there might be some subtle physical differences between the two opamps, I mean along the lines of variations in transistors of the same model, that is resulting in the differences and unreliability of audio on the two sides. Along the lines that you have been suggesting of overloarding the opamps.

Perhaps I can assume that both sides are actually working and the only way to really test them is to run them through the 555 and then connect the speakers up to the output of this instead.

Here is the circuit anyway:

I have short lengths of magnet wire soldered to the opamp related pins and then spaced apart on the PCB. I then have all my resistors etc connected to those points on the PCB. I find that with this arrangement it is easier to spot mistakes and soldering and corrections are a great deal easier.

There is a ground rail soldered beneath the board at the top in this photo and a Vcc rail at the bottom of the board.

The heat sink has a 24V voltage regulator and the board was originally designed to take the 36V that I was running my flyback transformer driver with - I have just left it that way.

[image replaced with smaller version attached top the post]

boylesg said:

I tried it again earlier on and I actually got some audio from opamp 123 this time and it wasn't far the audio I got from opamp 567. Previously the audio from opamp 567 was quite a bit stronger.

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excellent !! i remember from one of your earlier posts that the input signal from one ipod input was less than from the other. maybe this accounts for the difference in output ?

I am now thinking there might be some subtle physical differences between the two opamps, I mean along the lines of variations in transistors of the same model, that is resulting in the differences and unreliability of audio on the two sides. Along the lines that you have been suggesting of overloarding the opamps.

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overloading is one thing which is possible with your setup. but even if there are subtle differences in the 2 opamps, they will be too minor to show up in the results you are getting. i think you should discount such differences.

Here is the circuit anyway:

I have short lengths of magnet wire soldered to the opamp related pins and then spaced apart on the PCB. I then have all my resistors etc connected to those points on the PCB. I find that with this arrangement it is easier to spot mistakes and soldering and corrections are a great deal easier.

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i just hope your magnet wire has its insulation in place everywhere.

whats the dual-gang potentiometer for ? I don't remember you mentioning this. sometimes the 2 sides of a dual gang are not well matched. you should try replacing this with a simple voltage divider, or removing it entirely

I'm glad it's working but to be honest, a layout like that is asking for trouble. The whole thing should be built in the space of four of those white squares to keep the wiring to a minimum and all the signal wires to and from the board should be screened. I suspect the reason one amplifier doesn't work is instability, it's probably oscillating at a frequency above hearing range. Not having decoupling capacitors close the IC will make matters worse.

Incidentally, if you post pictures, can you compress them first please. That one is 2.7Mb and on my internet connection took 4 minutes to download!

Brian.

kripacharya said:

excellent !! i remember from one of your earlier posts that the input signal from one ipod input was less than from the other. maybe this accounts for the difference in output ?

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No. That difference in peak voltage was measured on the opamp side of the input capacitor. So I assume the voltage peaks were related to what the opamp is doing rather than what the ipod is doing

I couldn't actually measure any voltage on the ipod side of the input capacitors on either side. Not sure why this is, perhaps you could enlighten me.

But anyway there is nothing wrong, at least audibly, with the output on both channels from the ipod.

kripacharya said:

i just hope your magnet wire has its insulation in place everywhere.

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I have tried to minimise contact between magnet wire and components, particularly near the ends where I have stripped the insulation.

kripacharya said:

whats the dual-gang potentiometer for ? I don't remember you mentioning this. sometimes the 2 sides of a dual gang are not well matched. you should try replacing this with a simple voltage divider, or removing it entirely

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So that I can vary the voltage gain and get the best sound quality coming through the HV arc. I did have a fixed common emitter amplifier but the sound coming through the HV arc seemed a bit scratchy and I suspected the gain of my amplifier was too high.

I know that using magnet wire like this is far from ideal re quality electronics, but I am not trying to make quality electronics and it makes the soldering and spotting mistakes a lot easier.

Ideally I should create custom etched circuit boards where the components are closer together, to reduce parasitic inductance and capacitance and all that, but I have not gotten into all that yet. The matrix boards and magnet wire will do me for now.