Question about opamp envelope follower circuit

[.Mike]

I have run into a gap in my knowledge that I would like to fill. I pulled the envelope section off the "Meat" envelope pedals for hacking into my Crybaby. I don't have an LM1458, so I am using an NE5532. The results are poor, as expected. I'm ordering the proper opamp, and am confident it will work. In the mean time, I've been having fun trying different things to get it to work with the wrong opamp.

Here is my schematic:



Everything to the right of VR1 is copied, pretty much. I take the signal right off the bypass switch, so I meant for U1 to have high input impedance, and add a bit of gain.

As drawn, the envelope is really choppy. When I turn the sensitivity knob down all the way, the LED is dark when built as drawn.

I tried a bipolar supply, lifting pin 4 of the NE5532 and connecting it to -9V using test leads and a MAX1044. Works great-- nice, smooth envelope. BUT, the Sensitivity knob doesn't work right. All the way to the left, the LED lights up. It gets dimmer towards the middle of the rotation, where it turns off, and then it gets progressively brighter as I turn to the right.

I also tried putting a 100k resistor from V+ to pin 3 of the NE5532, creating a voltage divider and lifting the voltage similar what is done with the LED in the Dr. Quack. Same interaction between the Sensitivity knob and the LED as running it on a bipolar supply.

This is what is stumping me. What is causing this interaction to occur?



Mike

[PRR]

U2 is "biased" very strangely. It just may be true that "only" a 1458 (maybe a '741) will work that way.

However some of your observations suggest it isn't really wired as shown. "to the left, the LED lights up. ...dimmer towards the middle... off, and then ...brighter as I turn to the right." This smells like a problem near C2 or C3.

[.Mike]

Thanks Paul. I'm going to investigate it. I thought all my soldering looked clean, but triple checking is in order.

I think I copied it pretty much exactly from available schematics from the McMeat/MeatSphere, which are both based on the Meatball. Everything I've read indicates that it requires the LM1458 to work properly.

I am just wondering what is happening for curiosity's sake.

Mike

[Gurner]

I'm less petrubed about the biasing of U2 (only positive parts of the signal will be acted upon as it's +ve pin is referenced to ground) ....but wrt U1, the way its configured seems a bit pointless (it's biased at 1/2 VCC to let the positive & negative swings through to U2...the negative swings are then ignored by U2!).

I reckon a gain of 10,000 is a bit excessive for U2 too! (in reality that'll give you only two conditions ...full on or full off)

Also, with zero input signal  there'll be 0V DC on the output pin 7 of your final opamp ...the led won't even start illuminating until about 1.5V (red led)  ...therefore between 0->1.5V is a dead zone ....personally I'd arrange it so with no signal the opto led is *just* biased off.

[PRR]

> less petrubed about the biasing of U2 .... +ve pin is referenced to ground
> there'll be 0V DC on the output pin 7 of your final opamp

Look at LM1458 schematic.



The NPN inputs certainly can't turn-on until 0.6V above the supply rail; actually over 1.5V to get the second stage active. Likewise the output can't pull-down all the way to the rail.

I think it is jammed into an "invalid" (though safe) condition, and the way a signal brings it out of that condition into conduction may be very specific to the internal topology. i.e. it may only work with 1458 (or '741) chips.

Normally I say "the opamp hardly matters" but in this case I suspect it does.

> I'd arrange it so with no signal the opto led is *just* biased off.

That would be the difference between a soft-knee compressor or a hard-knee limiter. Some of my designs use the 1.6V dead-zone of an LED so that signals zero to 1.5V are untouched, anything over 1.7V is stepped-on. OTOH sometimes you do want to follow the FULL envelope, which I think is the intent here (?), and then the 1.2V and 1.6V dead-zones on opamp and LED are "flaws".

[.Mike]

I just want to let you guys know that, a month and a half later, I finally got this going.

I got the right opamp for the circuit (LM1458). The release was smoother, but I was still having some strange problems with the LED lighting up at certain settings when not playing.

I had debugged the crap out of it, and couldn't figure it out. Everything looked good. The envelope circuit was a direct copy of a known working circuit. The gain/buffer stage was tested to work (and based on a known working circuit). I wasn't reinventing the wheel, just trying to hack together a little auto-wah.

I flexed the board a bit with my finger, and the problem went away briefly. After probing everything bamboo skewer, I figured it out: The opamp did not have good contact with the socket. I scrapped the socket and soldered the opamp directly, and it works great now.

I'll record a video of it in the next couple of weeks when I get my recording setup put back together.

Thanks!

Mike