Testing (suspected) fake opamps - Page 1
Forum section: electronics / projects

author: Buriedcode
author_rank: Super Contributor
timestamp: December 11, 2024, 10:27:41 pm
content: Hi,Recently whilst building a few guitar pedals that have been on my list for a while I picked up some NOS chips - bucket brigade delays, and I added an LM308N to the order for a Rat clone.Now, I'm aware that theres a lot of bullsh*t about certain opamps in certain pedals, and as such, some parts have inflated prices or just relabelled/faked. I knew this before purchasing so I thought, if its genuine, fine, if it isn't, its cheap enough to be worth it to satisfy curiosity to find out what opamp it really is. I'm not sure I'd call it a "fun" endevaour, perhaps somewhat sad, but we all need a hobby..So,1) a first simple test was current consumption - all LM308 datasheets I've seen say between 0.3- 0.8mA. Measured 2.8 - 3.0 with a +/-10V supply, voltage follower and non inverting to 0V.2) Slew rate - this was about 0.2V/us, which is actually a bit slower than the 308's 0.3V/usClearly not a LM308, but any other measurements I can do to narrow down the exact part number? GBWP? Common mode input limits? Testing with quirks like phase reversal, crossover distortion? Ideally not a test that requires a complicated circuit setup, but something one can do in 10 minutes with a scope.The slew rate actually makes it a good fit for a Rat clone since thats probably the only important factor in that pedal, but whilst I'm at it I'd like to close the door on this so I have a fairly good idea what it is. I imediately thought it was a 741 or TL071 since they seem to use these in fakes of other opamps, but the low slew rate and high Iq threw me. I just don't know that many opamps to know what it could be.

author: moffy
author_rank: Super Contributor
timestamp: December 11, 2024, 11:36:08 pm
content: There are vast numbers of generic opamp types, impossible to know which specific one it is but you might be able to determine if it is JFET or bipolar input or even CMOS by the input bias current. Might be hard to distinguish between CMOS and JFET input just from a bias current measurement. Bipolar will be nA or above, JFET/CMOS should be in the pA level at room temperature.

author: magic
author_rank: Super Contributor
timestamp: December 12, 2024, 10:43:32 am
content: What's the voltage on pins 1, 5, 8 during operation?Does any of them track the output voltage? (On LM308, pin 8 is supposed to.)Which of them are NC or have internal resistors to either VEE or VCC?

author: Buriedcode
author_rank: Super Contributor
timestamp: December 12, 2024, 11:30:36 pm
content: Quote from: magic on December 12, 2024, 10:43:32 amPin 5 seems to have NC. Can't find any resistance/forward voltage to any of the other pins.Neither pin8 or pin1 tracks with output voltage. Test with +/-9.5V supply, voltage follower, non inverting input from -3 to +3V. Both pin 1 and 8 sit at roughly +V.Comparing it to a modern OP177, and a NOS (genuine, colleague had in storage) OP07 the input bias current for the LM308 is a lot lower: -0.6 to -0.7nA, compared to the OP07's -2.0 and -1.9nA. Admittedly, it wasn't particularly accurate, just using 5% 1Meg resistors, but thats why I'm comparing it to known opamps with the same setup.The plot thickens.

author: magic
author_rank: Super Contributor
timestamp: December 13, 2024, 12:07:36 am
content: Right, the slow speed and pin 1, 8 offset trimming to VCC looks like some OP-07 family part.Current consumption is a little high at 3mA, but not outside specified limits.One thing about OP-07 is that datasheets show a fairly strong dependence of supply current on voltage.Input bias is canceled by those PNP current mirrors on input pins (see any schematic), so a "lucky" chip could be closer to zero than others.