What dual opamp to use

[seven]

I'm repairing a damaged Fulltone Plimsoul. It is working now but the original IC was burnt for some reason and when I try with JRC4558 or TL072 gain gets way too high. I don't know what opamp they are using since they have removed the product code. Any suggestions on what they might use, or proposal what I should look for?
I have asked Fulltone if they can sell the opamp but they don't do it.

//Sven

[Kesh]

Op amp shouldn't really affect gain as it's set by other components around it. Maybe some of those are damaged. Trying one of each type of op amp (bjt and fet) as you have done covers all bases, unless they are using something a bit more exotic.

[duck_arse]

there isn't a lot about the plimsoul on this site, but it seems it's a dual op-amp, diode clipper type affair. check yr diodes, if they are not diode-ing properly, you will get much more volume than you might want

[Mark Hammer]

I gather the original does not use a socket, but solders the chip to the board.

So, if the chip has been removed and replaced with a socket, or other chips have been soldered to the board and then unsoldered, there is a possibility that something has happened during that process.  If, for example, the feedback path of an op-amp is interrupted, that becomes equivalent to a VERY high feedback resistance, which will instantly increase the gain of that op-amp.

[seven]

Thanks for the advices.
I have tested the other components and I cannot find any other broken parts. The reason why I suspect the opamp is that if you are familiar with the schematic you can see that the design consist of two gain stages where gain stage no2 is variable, in the second gain stage there is a led that will lit up the more you turn it up. What I have noticed now is that the clipping starts much earlier than it used to do, for me it is indicating that the voltage level is slightly higher in the clipping circuit with the opamp I'm currently using.  Also the Fulltone team has removed all data  on the opamp so they are using something exotic

/7sven

[Mark Hammer]

No, I don't think they are using anything "exotic".  They probably just want people to think what you are thinking right now: that it IS something exotic.

Whatever influence the specific op-amp might have, is small, compared to four other aspects of the circuit:

1) It clips the signal twice
2) The first clipping stage is asymmetrical
3) The second clipping stage uses LEDs and adjusts a resistance in series with the LEDs in conjunction with op-amp gain
4) The first stage uses a RAT strategy to provide more gain for mid and higher frequencies than for bass and lower mids

There is NO difference between op-amps that would be strong enough to have a clear impact over and above all of that.

Use a 4558, relax, and have fun tweakng the controls.

[seven]

I can definately hear a difference between opamps so something is going on. I have installed a holder so I can easiliy swap.
I will test a few other variants and try to find out what they are using.

[EATyourGuitar]

If, for example, the feedback path of an op-amp is interrupted, that becomes equivalent to a VERY high feedback resistance, which will instantly increase the gain of that op-amp.

^this. two opamps means TWO feedback resistors that could be disconnected. your problem could be that the first opamp is giving you maximum gain with no negative feedback while your looking really hard at opamp #2. I would probe it after opamp #1 and just play some guitar through it to see what it sounds like.

[slacker]

How did it get damaged? What ever killed the opamp might have damaged other things, it might be worth replacing any electrolytic caps and the transistors.

[brett]

Hi

but the original IC was **burnt** for some reason

I'd guess that's your basic problem - the burning.
If the circuit is good, then as everyone is saying, all op-amps are more-or-less the same. They are built to conform to a simple conceptual model.
Once the circuit is fixed, put in any dual opamp. Some (e.g. tl072) are quieter than a 4558.
cheers
*It is plausible that the different overload protection systems of op-amps is causing a difference because of a fault. e.g. different circuits have different current-limiting systems in the output stage that protect the chip in the case of a short circuit. Old chips might not even have this protection??? I'm no expert.

[seven]

My son borrowed the pedal, and when I got it back it did not work, when I opened it up I saw that the two diodes in the dc-input section was burnt, and the PCB had a black stains on it, so somehow he has managed to create an internal short-circuit. I replaced the two diodes and then checked other components and the only other I could find to be damaged was the opamp, however I did not troubleshoot other parts thoroughly when I got it working after the opamp change. I do not suspect any other part to be damaged since the sound out from the pedal is great, other then mentioned that I get a higher gain level when I turn on the second stage.

[R.G.]

My son borrowed the pedal, and when I got it back it did not work, when I opened it up I saw that the two diodes in the dc-input section was burnt, and the PCB had a black stains on it, so somehow he has managed to create an internal short-circuit. I replaced the two diodes and then checked other components and the only other I could find to be damaged was the opamp, however I did not troubleshoot other parts thoroughly when I got it working after the opamp change. I do not suspect any other part to be damaged since the sound out from the pedal is great, other then mentioned that I get a higher gain level when I turn on the second stage.

It is highly likely that he plugged an AC-output power adapter into it. This is very easy to do, as the plugs and jacks are usually identical, and easy to mix up. Fishing around for a black plastic DC plug on the bottom of a pedalboard is ripe for errors. Unfortunately, after an incident like this, you cannot trust any component which expects a polarized voltage, as the power supply has been alternating positive and negative for many cycles.

It is also difficult to say whether a distortion pedal is "fixed" or not. It may be damaged in a way that happens to sound good to you. If it were mine, I'd replace every polarized component in it - opamp, transistors, electro caps, and diodes.

And the earlier advice is correct. The whole reason for being for opamps is that they have little life of their own. The idea behind opamps is to make them do whatever the stuff around them tells them to. A typical opamp has a gain of 100,000 or more, cut back down to the desired gain by the feedback parts. If an opamp has too much gain, it's not the opamp's fault - it's the parts around it.

[seven]

Even though my son denies I think he hooked up the pedal to the power supply he uses for the POD, I think that one is AC.
I probably have to bite the bullet and change the parts. I will report back once I have done ( it may take some time!!) hopefully that will finally restore the pedal

[R.G.]

It is very possible that your son does not believe that he did. The plugs look identical, molded black plastic, and fit the same socket. That's the real problem - it's so very, very easy to make a mistake.

I like the idea of painting some bright-yellow color onto the AC plugs with some of the tool-handle dip stuff from hardware stores. That at least lets you know which ones are different and possibly dangerous to the pedals.

[duck_arse]

you might now want to fit supply protection diodes to all your pedals before son gets hold of them, too.

or fit diodes to son.

[R.G.]

The problem with using an AC adapter is that it will *fry* the most common type of protection diode, the reverse-shunt diode from ground to input power.

First it kills the diode, sometimes scorching the PCB under it, sometimes just heating it till it shorts, and then it kills the circuit. A series diode will stop this, but it subtracts the diode's forward voltage from the available power, so a series Schottky diode, like a 1N5817/18/19 is a good choice for this. It minimizes the series diode loss.

[Paul Marossy]

It is very possible that your son does not believe that he did. The plugs look identical, molded black plastic, and fit the same socket. That's the real problem - it's so very, very easy to make a mistake.

Yeah, it is super easy to do. It should be more like a 220V AC outlet vs. a 120V AC outlet. It's impossible to ever confuse the two.

The problem with using an AC adapter is that it will *fry* the most common type of protection diode, the reverse-shunt diode from ground to input power.

First it kills the diode, sometimes scorching the PCB under it, sometimes just heating it till it shorts, and then it kills the circuit.

AC power is very mean under these circumstances.