Problems with a simple OpAmp audio preamplifier - Page 1
Forum section: electronics / beginners

author: Nei
author_rank: Newbie
timestamp: August 22, 2024, 08:35:54 pm
content: Hi all,I'm struggling with a basic circuit : a simple audio preamplifier based on an op amp LF356N (with single power supply, i.e. V- is connected to ground).Here is the circuit : Figure_OK_low_amp_sch.pngThe aim is to make an amplifier (gain adjust), the output is connected to an ADC (RPi pico)As an input (instead of ISIN), I use audio output of my phone.Output is between C4 and R9 (to ADC)R5 and R6 are in fact a potentiometer.Here is the thing.With a low amplification (R5 and R6 as in the schematic), I get :Figure_OK_low_amp.png(red plot is the input of V+ of op amp, the green one is the output)If I use a sighlty higher gain (R5=14.1k and R6=8.1k), output is distorted :Figure_KO_low_amp.pngwhereas simulation is KiCad shows :Figure_KO_low_amp_sim.pngWith the same gain, if the opamp is supplied with 18.V, I get a correct result :Figure_KO_low_amp_OK_18.5V.pngbut as soon as I increase gain, distortion is back again :Figure_KO_amp_18.5V.pngNote : my scope is a Raspberry Pi pico, so as soon as input is >=3.3v, I have to disconnect it si I don't damage the board.I don't understand why real circuit does not behave like the simulation.Does anybody can help me ?Thanks a lot in advance !Nei

author: PGPG
author_rank: Frequent Contributor
timestamp: August 22, 2024, 09:57:47 pm
content: Where in your opinion the current from current source should go?For me it looks that 170mA * 100k should generate about 17000V at 100k resistor.

author: Konkedout
author_rank: Frequent Contributor
timestamp: August 23, 2024, 03:08:59 am
content: Not sure; but you could be getting "crossover distortion." A quick look at the LF356 datasheet does not turn up that term, but I also see nothing about it being recommended for audio. I suggest that you try using a resistor to load the op amp output to either + or - supply (a few mA) and see whether that changes the output waveform.https://en.wikipedia.org/wiki/Crossover_distortion

author: Circlotron
author_rank: Super Contributor
timestamp: August 23, 2024, 05:45:14 am
content: Might be something to do with the long time constant of R2 charging up C2, so the half rail voltage takes a long time to come up. I suggest making both R2 and R3 10K and C2 10uF.Also, you can take R9 straight to ground if you want. No need to take it to half rail.

author: magic
author_rank: Super Contributor
timestamp: August 23, 2024, 06:27:29 am
content: My first thought was that LF356 isn't really well suited for low volage operation and its output stage can't reliably get close to ground. You would be better off with just about anything, even TL071.But the waveform at 18V with higher gain looks like slew limiting, which makes me wonder if this is even a true LF356 or a rebadged LM358 sourced from China...If you have any other opamps, try them instead and see if it helps.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 23, 2024, 07:00:09 am
content: Quote from: Nei on August 22, 2024, 08:35:54 pmGeez, Stop with your audio project and first make an input stage for your measurement system. Something with a few resistors (switchable?) an opamp for buffering and amplification, some protection diodes to limit the voltage.For range switching you can either use a multi position (rotary) switch to do it manually. If you want to control it with software, you can use some small signal relays or analog switches like 74HC4051.

author: Xena E
author_rank: Frequent Contributor
timestamp: August 23, 2024, 07:03:26 am
content: Too many unnecessary time constants.Start with this:Many good books and video tutorials available.Get a copy of 'The Art of Electronics' and watch EEVBlog tutorials on OP Amps.

author: ArdWar
author_rank: Frequent Contributor
timestamp: August 23, 2024, 07:52:29 am
content: The excessive AC coupling and level shifting used by OP is kind of necessary due to their constraints.RP2040 ADC is 3v3 only and I'm pretty sure phone output is also at that level or lower (and AC coupled too) but LF356 can't swing any closer than 2V (1v5) into supply, hence it needs to be biased somewhere higher.I'm also agree that it doesn't look like what a reasonable fast opamp would behave...

author: wasedadoc
author_rank: Super Contributor
timestamp: August 23, 2024, 08:43:48 am
content: Quote from: Konkedout on August 23, 2024, 03:08:59 amYou can ignore that post. If Konkedout had actually read and understood the wikipedia page, he would not have written that post.

author: Nei
author_rank: Newbie
timestamp: August 23, 2024, 05:20:38 pm
content: Thank you for your schematics and links.I simulated this circuit, and it appears that I need to add extra components to get an output centered on 3.3V/2 (mid value of ADC range).I will try to add the same group of C and R (in lower right hand corner of my first schematics, from C4 to V2)Nei

author: Nei
author_rank: Newbie
timestamp: August 23, 2024, 05:23:31 pm
content: Thank you,I can't connect R9 to ground, as I need output to be centered on 3.3V/2 (mid value od ADC range).Nei

author: Xena E
author_rank: Frequent Contributor
timestamp: August 23, 2024, 06:01:11 pm
content: Quote from: Nei on August 23, 2024, 05:23:31 pmWhere are you taking the output of your circuit from? It's not plain:You can level shift to whatever level you like, but as the amplifier is AC coupled could it be powered fom a more convenient supply.You will have to select a different op amp if you're constrained to the supply you show in your diagram.Edit:My go to op amp for very low supply voltages and that have input and outputs that will swing to within milivolts of the supply rails is ICL 7611 family, just set the divider on the non inverting input to give the required centre operating voltage at the output.

author: Nei
author_rank: Newbie
timestamp: August 23, 2024, 07:45:25 pm
content: HiYou are right, output is not realy visible. It s taken between C4 and R9.What do you mean "a convenient power supply" ?Basically i d like to use USB power supply (5V), but if requiered i could add a voltage regulator (some simple buck/boost DC converter. This is how I generate the 19V shown in my schematics).RegardsNei

author: Xena E
author_rank: Frequent Contributor
timestamp: August 23, 2024, 08:31:24 pm
content: A 12 volt supply as a suggested minimum would work with most common op amps.The use of such op amps does mean that you would have to level shift your output, as you are attempting to do in your schematic.In the schematic I posted Using a low supply voltage amplifier like the 7611 series, (others available) that has outputs that can swing to supply and ground you can specify the output level by the ratio of the resistive divider on the non inverting input and then dc couple into your ADC, and power it from the same supply voltage.The resistive divider on a mosfet input amplifier can use higher values to increase the input impedance if requiredThe 7611 will work with supplies down to around 2 volts.It can be arranged the way you have it with an inferior op amp but you will have to use the higher supply than 6 volts to allow adequate output swing. Also the level shifting time constants will have to be chosen carefully.Using a boost converter is OK if you're happy with that with the later arrangement... I didn't see a 19v supply on your diagram.

author: Nei
author_rank: Newbie
timestamp: September 02, 2024, 08:09:46 pm
content: Hi all,You're going to laugh... I didn't double check my electronic assembly, and I had left a 100µF capacitance between the output of the operational amplifier and the ground. Hence the very large time constant which gave the signal the appearance of a triangle.I'm so sorry for that, shame on me...Finally, my op amp is not faulty.However, thanks a lot for all of your replies.@Xena E, your design seems compact, I will study it for sure.@Doctorandus_P, make a better scope is planned. I want to make an input stage to use an OpAmp with a +/-12V power supply, and shift it to 0/3.3V. I actually have to build 3 of them, for the input of the pico.Our mistakes make us grow !CheersNei

author: Circlotron
author_rank: Super Contributor
timestamp: September 03, 2024, 02:40:55 am
content: Quote from: Nei on September 02, 2024, 08:09:46 pmNope. No shame in that. Everybody that actually tries new stuff makes mistakes sometimes.Shame would be if many others pointed out this mistake to you and you refused to listen and argued with everybody.

author: Xena E
author_rank: Frequent Contributor
timestamp: September 03, 2024, 04:56:22 am
content: Yes! And thanks for reporting back, (often there's no conclusion).Regards,X