Dual opamp incompatibility, why?

[Gabriel Simoes]

Sorry, two posts for almost the same thing, but not that same.
Here I'd like to know why a project would reject some opamps and accept others with same pinout.
In a dual opamp with the rc4558 pinout, in the second opamp (5678), signal gets in using 5 (posivite in) 6 (negative in) and out using leg number 7 (out).
I dont have an ideia why in a circuit projected for this pinout, a signal would reach the pin n. 6, pin 5 is connected to ground, and no sound gets out from pin 7, using some different cis for testing: tl072, rc4559, rc 4580, etc ...  I even got no sound trying to direct replace it using a tle2142
And in the same circuit, changing the ci for a tlc2262, tchanan! the sound gets out from pin number 7.
And using all opamps everything works fine with the first half opamp...
What could cause this ? is there any kind of explanation ? I'm happy I used sockets in this circuit ... so I can try to figure out the problem ... and to me it's not a voltage thing, since the tlc2262 works just right ....
Please give me a hand here, lets try to figure out what could cause this, maybe tomorrow you will face the same problem!
and thanks for your attention,
Gabriel

[gez]

Don't fully understand your post, and I didn't pay any attention to the codes of the amps, but if they really are direct replacements (you have checked the data sheets yeah?) then they should work, unless you're using them for something other than bog-standard audio (some circuitry like LFOs etc will only work with certain chips).

Helps if you tell us what the amps are doing (schematic even?).  Like I said, I didn't pay any attention to the chip codes you posted but it might be that they're not direct replacements.

[Gabriel Simoes]

Sorry for my bad english, maybe the post is not that well written ....
I'm using the opamps in a bassballs clone, and in two different boards from 2 different pcbs I got the same problem ....
When I change the ic for tl072, rc4559, rc4580 I get no sweep because the second half (pins 5, 6, 7) do not amplify the signal, but I tracked that and it is reaching pin 6 (negative in), and pin 5 is connected to ground, so it's been used as an inversor amplifier for gain only ...
Like a magic, if I change the opamp, and use a tlc2262, the signal flows from this second half of the opamp, but I think it's not amplifying very well because I need to play strong to hear the sweep of the envelope ...

So, it's been used for audio ... and by what I saw at ti.com they are direct replacement .... and that could be confirmed as the original project asked for 1458 (dual 741, that I did not use to get away of the 741 classic noise) ....

I hope someone can help me, I could just change the diode and the effect would work, but now it's a question of honor, I want to figure out what is happening.

I will read the voltage in the pins and write it here ...
Thanks
Gabriel

[gez]

I'm using the opamps in a bassballs clone,

I've just had a look at your other post.  I take it the amp you're having problems with is the one used for the envelope control?  The amp used for the rectifier has its + input tied to ground.  Many amps don't like having their inputs pulled close to the rails, let alone biased from one or the other.  

Unfortunately you have to scour data sheets/try various amps until you find one that works.  This is one of those occasions where subbing in any old thing doesn't necessarily work.

[petemoore]

Sounds like an 'opamp on the edge' of operational functions.
 Perhaps actually taking voltages might shed light from a different angle on this, I'm not all too familiar with the Baseballs, what Gez Sez sounds correct...possibly there's another tweek that would cause more OA's to function on your board, and still retain the BB effect, such as bringing the bias a bit farther from the rail....just typing nonsense that might make sense, maybe someone else can help me too !

[Gabriel Simoes]

Please take a look at this post

http://diystompboxes.com/sboxforum/viewtopic.php?t=35294

I've written the voltages there, the opamp is the one used to amplify the signal before the diode rectify it ...
I do not know if it should be like this but the in voltage on the problematic opamp is almost 0, and the amplified is just 0.1 at max 0.2....
But the fist opamp is like sending more than 3 volts ....
So I don't know if it should be like this or if I'm loosing the signal somewhere ...but in both boards I made (from 2 different pcbs) I had the same problem .... so could you guys take a look at the other post and see if the values are ok or if this is a simptome of loosing signal before it gets amplified ?
Thanks
Gabriel

[gez]

Sounds like an 'opamp on the edge' of operational functions

Yup, I'd say that's the case.  Never built/breadboarded this circuit, but the voltages look about right.  

...possibly there's another tweek that would cause more OA's to function on your board, and still retain the BB effect, such as bringing the bias a bit farther from the rail....just typing nonsense that might make sense, maybe someone else can help me too !

Not nonsense Pete, look at Jack Orman's version of the Dr Q and you'll see how he tackled this problem.  Not much you can do if you've got a PCB done though Gabriel other than find an amp that works I recon.  ICL7621 should work perfectly if you can get hold of one.  Mouser sell them and there are a few places in the UK & Europe where you can get hold of them. I'm sure there are 'plenty' of other amps that could do the job but my brain is fried at the moment so can't think...

[Gabriel Simoes]

I do not understand how do this 0v in the second negative in (pin 6) is right, to me I would mean no signal or a signal with small amplitude ... am I wrong ?
I guesses that because the first half of the opamp send 4.36 to the sensitivity pot, and at max sensitivity it would send 4.36 to the resistors and cap before the negative in ...

[gez]

I do not understand how do this 0v in the second negative in (pin 6) is right, to me I would mean no signal or a signal with small amplitude ... am I wrong ?
I guesses that because the first half of the opamp send 4.36 to the sensitivity pot, and at max sensitivity it would send 4.36 to the resistors and cap before the negative in ...

The first amp is acting as a follower so the signal fed to the amp doing the rectifying isn't that great.  I'm assuming that the 4k7 limits current on negative swings.  That asside, the virtual earth effect should prevent this happening (like I said, my brain is fried so I reserve the right to change my mind tommorow morning!).  

The inputs of the second amp are both referenced to ground so you need to use an amp that's happy with this setup.

[StephenGiles]

The inputs of the second amp are both referenced to ground so you need to use an amp that's happy with this setup.

Then use the quoted opamp - cheap and cheerful!
Stephen

[gez]

The inputs of the second amp are both referenced to ground so you need to use an amp that's happy with this setup.

Then use the quoted opamp - cheap and cheerful!
Stephen

Agreed!