opamp problem on guitar preamplifier - Page 1
Forum section: electronics / projects

author: hozone
author_rank: Regular Contributor
timestamp: November 04, 2019, 11:13:21 pm
content: Hello,I've an issue with an opamp and unluckly I'm not able to solve that.I'm building a guitar amp, amp board is based on TDA2030, no problem with that.Supply it's single rail, +30V for the TDA2030, +9V for the preamp.The preamp board is where I have some problems.Stage 1 (PreStage) is a Tillman based FET preamp, build using J201.Stage 2 (ToneStage) there is simple Fender "brownface" tone stack, a FET preamp to restore the signal, build using J201.Stage 3 (DistorsionStage) an Electra Distorsion based pedal, somthing like the COT50 pedal, this is activate through a bypass DPDT switch.Stage 4 (ReverbStage) a PT2399 based delay pedal, design based on the SmallTime from Valve WizardThen signal goes to a volume pot then to Amp board.The problem I'm encountering is on the opamp of the delay pedal.Let's suppose a test signal of 100mV 500Hz, out of stage 3 vpp is 200mV almost, out of stage 4 is 300mV almost, it depends on the gain.Out of ReverbStage signal is still ok, no problem with clean (DistorsionStage disabled).Out of ReverbStage signal went down to 10mV, DistorsionStage disabled, but - that is stange to my poor analog electronic experience - if i shortcuts for just a second, even by putting hands on, the last opamp out resitor (R27) with the +9V signal, the signal is now to 300mV, now i can switch back to clean and distorted signal with no problem.What I've notice is that, if i use a LM358 instead of TL072 and 10k instead of 100 input opamp resistor (R5), things get's better, output from the opamp exists, but "sometimes" signal collapse again.On the breadboard I've used to check the prototype I've not find this issue, but there was noise everywhere there. I've double check wiring, to me it seems all ok, the weird things is that the signal collapse not always, I'm not an "analog" guy, so I need your help here.Did you have any clue on this?Thank you!

author: magic
author_rank: Super Contributor
timestamp: November 05, 2019, 07:59:51 am
content: Check average DC levels of the signal, make sure they stay within the common mode input and output ranges of those chips.LM358 has a wider CMIR and it works down to 0V, TL072 works better with inputs close to VCC.

author: hozone
author_rank: Regular Contributor
timestamp: November 05, 2019, 11:12:35 am
content: Thank you!Indeed is something I was thinking about last night after reading this:https://www.analog.com/en/analog-dialogue/articles/avoiding-op-amp-instability-problems.htmlI will check it back from office.In case it was due to DC levels, do you think the mod (rail is splitted) in the schematic attached here can help?I move the rail before the opamp, always after a 100n, removed the R7 resistor, and change the R9, R10, and R5 to 100k.Better use 10k instead of 100k?I don't understand the R7 resistor, pin + (#5) of second stage opamp can not be simply VCC/2?Excuse my "analog" ignorance

author: hozone
author_rank: Regular Contributor
timestamp: November 06, 2019, 11:04:10 pm
content: Found it!!!It was a wrong soldering! Damn!Now it's working, just a few mods and it's done