Having trouble getting an opamp to 'op'

[tdkrause]

I've been trying to prototype a simple overdrive pedal and so far I haven't had any luck.  I'm using TLO72 op amps.  The circuit I currently have on the breadboard is based loosely on a Marshall Bluesbreaker pedal, which uses a TLO72.  I just built a new power supply and carefully matched the resistors for the voltage divider.  With a 9v power adapter feeding it, my power supply measures 4.49v- and 4.5v+ relative to virtual ground.  The spec sheet for the TLO72 says that it needs a higher minimum voltage, but it works (for other people).  General guitar gadgets uses the same op amp with their Bluesbreaker kit with no more than a 9v battery.  I've only got the circuit built up to the first gain stage for testing purposes.  I'm using a 100k pot in series with a 2k resistor in the feedback loop, with a 47 ohm resistor to ground.  This is the same as stage 1 on a Carl Martin Plexitone.  But here's my problem that I can't figure out.  With the volume all the way up, if I hit a chord really hard I can just barely hear a fuzzy note coming through.  This makes me think that it needs more voltage because it sounds like it's already clipping at it's 'turn on' point.  The Plexitone uses a +/- 12v supply, so my first thought was that I needed to pull back the gain, since there is 15v less rail to rail than the Plexitone.  However, if that were the case, then turning down the volume would help and I only just start to hear the signal coming through with the volume all the way up.  Why does it seem the exact same power supply / op amp combo is working for other people and not for me?  Could it be a dead op amp?

[R.G.]

Why does it seem the exact same power supply / op amp combo is working for other people and not for me?  Could it be a dead op amp?

It *could* be a dead opamp. But that is absolutely the last thing to worry about.

Since this is your first post, I'm guessing you're a new reader here. I'm tired of typing the "Mother Nature" notes, so I'll just use tough love.

It's almost certainly not working because you don't have it hooked up correctly.  Every single connection, part value, pinout, everything has to be correct or you'll get the result you're seeing. Even ONE thing not as intended will make it not work. This isn't as rigid as that sounds, because there are only a relative handful of things that have to be exactly right. You mention breadboard. If your *breadboard* has dirty/old contacts, that could do it.

The reverse of this is also true - if you have everything hooked up correctly, it *will* work. Your problem is that you do not yet have the experience to tell if you have it correct or not. It's unfortunately true that there is just some amount of dinking with it and thinking about what's done and not done that you're going to have to live through.

I suggest reading "What to do when it doesn't work" - you know, that stickie that you skip past when you start reading the forum. It's interesting stuff for the peculiar condition of having some circuit you've made just not work.

Lest you think I'm putting you down - I'm not. This is just the straight information. We've all been there. I've been there personally. Inhale. Take a break. Come back to it fresh with a patient attitude of nailing down one by one all the things that could be mis- or un-connected or crossed up. It will work, and you'll have learned something in a way that will stick with you.

[PRR]

EDIT: R.G. types fast. He is quite correct.


> carefully matched the resistors for the voltage divider

Not necessary. You only need to be "roughly centered".

> The spec sheet for the TLO72 says that it needs a higher minimum voltage

When new, +/-15V supplies were everywhere, also it works pretty good there. TI is coy about saying how low you can go. However Figure 10 shows it making significant output (2.5V peak) at +/-3.5V supply (7V total). Fig 15 shows supply current dang-flat to +/-3.5V (ie it isn't starving). And as you observed, EVERYbody runs them on 9V.

> Could it be a dead op amp?

Unlikely.

Much more likely that there is an error in building.

> I've only got the circuit built up to.....

When you cut-off a circuit, you need to know how and where to cut, and if any patch-up is needed. Even less certain when the implementation is "based loosely".

I don't care if your Vref is 4V or 5V. Whatever it is, most of the non-power pins will be at that voltage. This fact comes from the assumption that "most" single-supply audio opamp amplifiers have a blocking cap in the NFB, so for DC they follow the + input, which is "usually" at Vref; and the basic ideas of op-amps.

From the symptom, I bet you find output stuck at near-zero or near-9V.

[newfish]

Hi,  Daft question - what happens if you use a bigger pot in your feedback loop?

From meagre experience with op-amps, I've found that a 1M pot is better - the higher resistance giving higher 'gain'.

Thinking about it, it your feedback signal is too big (i.e. with only a small resitance limiting it in the feedback loop), there'll be less room for the op-amp to operate (no pun intended), as the signal won't have any room to swing - so increasing the resistance of the feedback loop may well give your op-amp a fighting chance at amplifying.

If your're feeding the same size signal (or nearly the same) *back* into the amp, it can only do so much before it runs out of headroom to work with.

Do you have a schematic you could post - or a link to one?

[R.G.]

Hi,  Daft question - what happens if you use a bigger pot in your feedback loop?
...
From meagre experience with op-amps, I've found that a 1M pot is better - the higher resistance giving higher 'gain'.

From his description, what he has is a connection problem, not a gain problem. The distortion he describes is a good description of misbiasing.

[tdkrause]

Regarding the minimum operating voltage for the TLO72, I double checked and apparently it needs 7v rail to rail, which has me even more perplexed.

[tdkrause]

Since this is your first post, I'm guessing you're a new reader here. I'm tired of typing the "Mother Nature" notes, so I'll just use tough love.

Lest you think I'm putting you down - I'm not. This is just the straight information. We've all been there. I've been there personally. Inhale. Take a break. Come back to it fresh with a patient attitude of nailing down one by one all the things that could be mis- or un-connected or crossed up. It will work, and you'll have learned something in a way that will stick with you.

Don't worry.  I don't get offended that easily.  I appreciate people who can be straight forward with me.  I'll double check the connections. 

[caress]

if it's based on the schematic, just post it and i'm sure the forum can help you figure out whether or not the circuit or the wiring is your problem...

[R.G.]

Regarding the minimum operating voltage for the TLO72, I double checked and apparently it needs 7v rail to rail, which has me even more perplexed.

Have no fear, TL072s work just fine on 9Vdc. I've put literally thousands of these into use on 9V. That's not a problem.