audio opamp oscilatring working as a FM radio in audio amplifier - Page 1
Forum section: electronics / beginners

author: Herschel
author_rank: Regular Contributor
timestamp: March 22, 2024, 04:08:13 pm
content: i have made a simple pre amplifier and audio poweramp using njm4565. the problem is, the zober network in the poweramp section getting hot. the opamp is oscilating at maybe a 101.1 mhz. is there any way to prevent this?how i found it oscilating at 101.1mhz? here is the full story:I have accidentaly created a radio... After replacing the circuit with jrc4565 i changed the 220k resistor in the first circuit to 56k resistor and replaced the 2k preset with 10k according to my needs and after turning it on, the zober resistor started to get little bit warm and the opamp started oscilating.I accidently touched the input capacitor AND I COULD HEAR MY LOCAL RADIO which is tuned to around 101mhz.I think my opamp is oscilating at 101mhz.how to prevent this? i have never seen something like this before... can someone plz explain this?(coppied from my previous post)i have also attached the original pre amp circuit from eleccircuit, the last image...

author: Herschel
author_rank: Regular Contributor
timestamp: March 22, 2024, 05:01:32 pm
content: how should i correct this circuits to run it properly?

author: Manul
author_rank: Super Contributor
timestamp: March 22, 2024, 05:03:32 pm
content: I doubt that it's 101 Mhz, because that is a rather slow op amp. Probably it oscillates at lower frequency, but there are higher harmonics present. Anyway, that does not matter.If you want to have stable op amp, look for these things:1. Proper decoupling! (I see no supply decoupling capacitors at all in your circuit);2. Minimize any parasitic capacitace at inverting input;3. Careful when there is capacitive load at the output, usually some series resistance is required for stability;4. General layout stuff - minimize parasitics, no long traces;That should do it, in case there is still instability:5. Cut excessive bandwidth by including a small capacitor in parallel with feedback resistor (R4 in your schematic), don't cut too much that it will affect your signal bandwidth;

author: Herschel
author_rank: Regular Contributor
timestamp: March 23, 2024, 03:30:53 am
content: replacing the 10k prest with 2k preset did the job, no zober network heating. i have already placed the decoupling caps under the circuit..

author: MarkT
author_rank: Frequent Contributor
timestamp: March 23, 2024, 09:01:19 am
content: [ Its a Zobel network, named after Otto Zobel of Bell Labs, lots of recognizable names from that source, including Black, Nyquist, Johnson, Shannon, Bardeen+Shockley+Brattain, Hamming, Karnaugh, Kernighan+Richie+Thompson...]You should add decoupling if its not present, otherwise you are likely to be on the cliff-edge of instability by just reducing that trimmer impedance a bit you haven't necessarily moved clear of the danger zone enough. Decoupling is not considered optional(!)

author: Herschel
author_rank: Regular Contributor
timestamp: March 23, 2024, 05:27:12 pm
content: i thing i solved the issue... I just connected the negative of the speaker to the ground of the 12v powersupply, before the step up converter.Quote from: MarkT on March 23, 2024, 09:01:19 ami have already added the decoupling capacitors. they are actually soldered onto the other side of the board(below the amplifier and the opamp)Quote from: MarkT on March 23, 2024, 09:01:19 amyeah i already had that problem. but i have prevented it by replacing the 10k trimmer at the output of the opamp with a 2K one which have solved the issue.but if i touched the audio input(without the phone connected), the zobel resistor may get warm. I think, a small feedback from the speaker is still getting into the opamp.

author: Herschel
author_rank: Regular Contributor
timestamp: March 23, 2024, 05:30:44 pm
content: But why it picked up FM signals which lies in 101.1 Mhz? i have never seen something like that before... also the picked up signal was pretty clear... i forgot to take a video of it happeningalso it only worked at that time

author: Zero999
author_rank: Super Contributor
timestamp: March 24, 2024, 09:51:40 am
content: Quote from: Herschel on March 23, 2024, 05:30:44 pmThe op-amp doesn't need to oscillate at 101MHz to pick up an FM station.What's happening is there's a tuned circuit somewhere, which is resonating at 101MHz. This is being rectified by a PN (diode) junction in your circuit and amplified by the op-amp.It's an amplified crystal radio. Normally crystal radios are known for AM (Amplitude Modulation) but they can work with FM (Frequency Modulation). In your case, the tuned circuit will have a sharp peak, so the amplitude of the resonance will change, as the frequency of the signal changes, resulting in amplitude modulation, which is being rectified.Here's an article about FM crystal radio, if you're interested.https://electronbunker.ca/eb/FMCrystalSet.htmlGoing back to your problem. It's probably lack of decoupling and poor layout. The low level signals should use screened, preferably twisted pairs. Also try adding ferrite beads to the outputs of the power supplies.The schematic of your output stage also isn't clear to me. Please use proper symbols, rather than just boxes.

author: coppercone2
author_rank: Super Contributor
timestamp: March 24, 2024, 10:17:53 am
content: I like the idea to rename zobel to zober networkunfortunately its named after a individual so it cant be done

author: radiolistener
author_rank: Super Contributor
timestamp: March 24, 2024, 12:38:17 pm
content: try to reduce amplifier gain

author: Herschel
author_rank: Regular Contributor
timestamp: March 29, 2024, 02:46:59 pm
content: Quote from: Zero999 on March 24, 2024, 09:51:40 amyeah i also figured it out. it was the power amplifier not the opamp. I was actually making a 30 watt class ab amplifier that could run on single supply. but the problem is my phone cannot run it. needs a pre amp. single transistor preamps don't work with this poweramp. actually the power amp circuit is coppied from a random youtube video. also it was the only circuit that could be any loud otherthan the overheating ones. also the video claimed it could give out 50 watts which i dont think so.. on testing it that circuit could only give out 5 watts...

author: Terry Bites
author_rank: Super Contributor
timestamp: March 31, 2024, 03:07:55 pm
content: I've been zober for 5 days now. Do I get a star?