Pearl OD-5 zener for Vref? And has anyone ever seen a zener like this?

[aion]

Schematic for reference: http://i1249.photobucket.com/albums/hh506/aionelec/PearlOD5old.gif~original

First of all, this is what's in D2's place:



Going by the factory schematic, it's a 5.1v zener diode. The trace by Build Your Own Clone lists it as a 3.3v zener diode. Any way to tell by the markings which one it is? The lighting on the photo isn't the best but it's a teal/aqua color. The two smaller striped are brown and the bigger band is dark green. Never seen anything like this - it looks much more like a resistor and it's about the size of a 1/8W.

Second: assuming the schematic is correct, this zener creates the reference voltage along with a 3.3k resistor from +V. I have an original unit and the reference voltage measures at 4.65v. However, +V is at exactly 7.5 volts using a 9.6-volt wall supply, even measuring it at the DC jack input. Does the zener diode drop +V as well, with the 3.3k resistor raising the threshold to 7.5v for V+?

Third: what in the world are they trying to accomplish here? Is it bad engineering or does it serve a purpose? This is just a normal op-amp circuit so I would expect to see a resistor in D2's place of equivalent value to R28, giving 9V and 4.5V. But it seems like if Vref is 4.65v and V+ is only 7.5v, the op-amp could very easily clip on one side (the 4.65~7.5v swing) which wouldn't be good. This may be why it's been reported that diodes with higher clipping thresholds (e.g. LEDs) don't sound good in this circuit. Any reason not to swap this out with a resistor for a more standard Vref configuration?

[aion]

A couple of followup things to add just for future posterity.

1) It's a 5.1V zener diode marked using resistor color codes. Green is 5, brown is 1.

2) The zener diode method of getting Vref is most likely just taken directly from other Pearl circuits in the same product line such as the phaser which require a constant Vref (as in a Phase 90). It works, but it's mainly just lazy engineering and this circuit would be better off with a traditional voltage-divider setup.

3) The 7.5v supply issue is caused by R29, a 220R resistor connecting power ground to signal ground, which is only in the circuit when using an external supply. I haven't tested the circuit with a 9V battery yet, but I would assume I'd get full voltage, whereas with an external supply it drops down to 7.5v, which is a factor of the overall current consumption of the circuit, by my understanding. The zener diode was lazy engineering but this is just plain bad—especially in conjunction with the zener diode, meaning the voltage swing is less than 2 volts on one half of the wave.

Anyway: in conclusion, jumper R29, increase R28 to 10k or more, and replace the D2 zener diode with an equivalent-value resistor.

[duck_arse]

aion, I read this thread a few days ago, and it's been bothering me since. today I think I can shed some light for you (at least till an expert arrives).

I've been messing with the lm358 driven to max clipping, and then into more gain, and it was causing me troubles further down the line clocking cmos dividers. today I tried all the other type opamps I had (the cmos rail-to-rail was the best, but not strictly needed). next best turned out to be the lm1458, as crummy an opamp as you could wish, but it clips hard clean, and solves my clocking problem.

that's the preamble. looking on the oscilloscope at the output of the lm1458 with max clipping (and feeding a schmitt trigger for the other half), the ic's limitations are apparent, as stated in the datasheet. the output won't swing down to ground, or up to the V+ rail, by some volts each way. and, with the Vref at half the supply, the waveform clips one side before the other. this sorta stuff always buggs me, because I let it, so I fiddled the divider resistors to get as near as symmetric clipp onset, and guess what my final voltage for Vref was.

go on, guess. so now I'm tossing the equal value resistor divider pair and subbing in a 3k3 to V+ and a 5V1 zener to ground. this be handy for up/down led drive later, as well. and I know that a zener stabilised Vref will stay at 5V1 as the battery voltage drops, Vref won't blindly follow a dying battery. this may also be a good thing. I hope this helps some, somehow.

[slacker]

220R across 9 Volts is only 40mA, most regulated pedal power supplies will supply that easily so wouldn't drop any voltage, you sure there isn't something else wrong. Presumably it's there because the recommended  power supply is unregulated and the resistor provides enough load to drop the Voltage to 9 Volts.