weird opamp configuration - Page 1
Forum section: electronics / projects

author: stojke
author_rank: Regular Contributor
timestamp: January 18, 2019, 10:02:59 pm
content: Can anybody help me find which opamp actually works in this configuration?In the original design the output should be a lot higher (max , ~16V), but none of the opamps I have tried worked as they all acted as an voltage follower taking the whatever is on the input with out any amplification directly to the output.The schematic tells its an NJM4580D-D, but an NJM4580D I tried does not work.

author: wraper
author_rank: Unknown
timestamp: January 18, 2019, 10:08:28 pm
content: Quote from: stojke on January 18, 2019, 10:02:59 pmOf course they do. Because gain is 1. It's exactly how it should operate.http://www.learningaboutelectronics.com/Articles/Voltage-follower

author: stojke
author_rank: Regular Contributor
timestamp: January 18, 2019, 10:18:57 pm
content: Trust me, it should not be working as an voltage follower, it is the last opamp stage before the main amplifier. In original design it "should be" 16V on the output.

author: wraper
author_rank: Unknown
timestamp: January 18, 2019, 10:23:08 pm
content: Quote from: stojke on January 18, 2019, 10:18:57 pmDo you see inverting input directly connected to the output? That is called voltage follower. There is a link in my previous post, and if you don't trust it, google "voltage follower".

author: mk_
author_rank: Regular Contributor
timestamp: January 18, 2019, 10:25:57 pm
content: Quote from: stojke on January 18, 2019, 10:18:57 pmThis is a voltage follower with gain=1, show the whole schematic to verify your hypothesis

author: stojke
author_rank: Regular Contributor
timestamp: January 18, 2019, 10:38:58 pm
content: Yes I know that is an voltage follower, I have read the datasheets, but the circuit is not amplifying my signal. That Op amp is just before the volume pot and the amplifier circuit. I have replaced all op amps but it wont amplify, it is always just pushing out what is on the input. (eg. 200mV input jack -> op amp -> volume max -> 200mV on amplifier -> average loudness on the output)Edit - I have modified an SMD to Trough hole mount to fit an SMD NJM4580 onto it with resistors and tied it to the ground and got an proper amplification model but it also introduced sound distortion. I am lost at what the do. The om amp it self has no markings on it and was almost eaten away by electrolyte acid from two failed caps.

author: glarsson
author_rank: Frequent Contributor
timestamp: January 18, 2019, 10:49:49 pm
content: The op amps have +16V and -16V as their power supply. The output should absolutely not be 16V. The output should follow the + input and its DC level is held at 0V by the resistor to ground.An emitter follower is just a buffer, it will not amplify the signal.

author: stojke
author_rank: Regular Contributor
timestamp: January 18, 2019, 10:55:52 pm
content: From all that IC3 should be the one to amply if than, but it isnt.This amplifier failed in a way that the output capacitors C15 and C16 completely failed, original parts being rated only 16V implies , at least to me, that the voltage trough them was past 16V.

author: wraper
author_rank: Unknown
timestamp: January 18, 2019, 10:58:43 pm
content: 16V on output is some stupid nonsense. No way this circuit is supposed to output 16V. And it simply cannot do it because there is not enough voltage on power rails to begin with. And it is completely normal this circuit has no gain. Only first stage has some slight gain (a little above 1). But it's "eaten up" right at at it's output.

author: ArthurDent
author_rank: Super Contributor
timestamp: January 18, 2019, 11:05:00 pm
content: Stojke - "Yes I know that is an voltage follower, I have read the datasheets, but the circuit is not amplifying my signal."If you know it is a voltage follower, why do you expect it to act otherwise and amplify? One more time for emphasis.

author: wraper
author_rank: Unknown
timestamp: January 18, 2019, 11:06:08 pm
content: The only thing where you could see a voltage swing above a few volts is inside a power amplifier stage. But there is none on this schematic.

author: stojke
author_rank: Regular Contributor
timestamp: January 18, 2019, 11:10:28 pm
content: I am no electronics expert, all I know is from practice, but this amplifier should sound a lot louder and at a lot lesser turn of the volume pot. The owner confirmed this, some reseller, by owning multiple units where the others had a greater amplification than this one.Still , it makes no sense for an amplifier not to amplify the input signal , whats the deal with that?It only made sense that due to failed capacitors the voltage should be higher, and also an parallel from an Akai amplifier that amplifies the input signals via an opamp setup to almost 15x on maximum volume. (In Akai case volume pot being before the opamp)

author: wraper
author_rank: Unknown
timestamp: January 18, 2019, 11:19:48 pm
content: Quote from: stojke on January 18, 2019, 11:10:28 pmIf both channels have the same volume, I would assume likely there is no failure. As I already said, what you have measured on this part of the circuit is completely fine and expectable. It might be that what you are using as signal source has too low voltage swing. As it seems, this circuit is supposed to have around 1 Vrms nominal at it's input.

author: ArthurDent
author_rank: Super Contributor
timestamp: January 18, 2019, 11:34:23 pm
content: Stojke - "Still , it makes no sense for an amplifier not to amplify the input signal , whats the deal with that?"Voltage followers by definition have avoltagegain of one. They are used for impedance matching and current gain. You could have a power amplifier (not a preamp) with a gain of one. Even though the input and and outputvoltageof a unity gain power amp could be the same, there is no way you could eliminate the power amp and drive speakers directly from a preamp. You have to realize there is a difference between voltage gain and current gain. Read and understand the following. Google is your friend.https://www.electronicshub.org/power-amplifier/

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: January 19, 2019, 01:52:03 am
content: Quote from: stojke on January 18, 2019, 11:10:28 pmAre you sure, you don´t have a short in the input of the power amplifier sector? Are you sure that the op-amp you have placed there can drive the input stage? (although there is a resistor so it shouldn´t be the case). Cause this is a buffer / voltage follower. This part does NOT amplify, unless it uses an op-amp with internal gain. But a normal op-amp would simply NOT amplify the signal, it only acts as a buffer between filter stages most probably, but I bet my degree on this, this is NOT an amplitude amplifier. Probably you have a problem elsewhere and you don´t know it. Have you checked the supply rails also? The problem could also be there...

author: xavier60
author_rank: Super Contributor
timestamp: January 19, 2019, 03:19:57 am
content: Quote from: stojke on January 18, 2019, 11:10:28 pmCMOS analog selector switches tend to have non-linear channel resistance with signal swing.It's common for them to feed into high input impedance buffers to minimize loading and therefore distortion.Show us the power amplifier section so that we can calculate its voltage gain.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 20, 2019, 01:39:28 am
content: Trust me:You've shown the classical circuit diagram for an opamp in voltage follower configuration.Whenever there is a direct connection between the output and the inverting input of an opamp you have a voltage follower.(With some exceptions, for example there must be a proper power supply and common mode voltages have to be met, and if the opamp is not unity gain stable, you might have an oscillator)The rest of the schematics is utter bollocks and irrelevant, so it does not matter that it is not shown.If you want to find out why there is no 30V on the output of the circuit of which you don't show the schematics is a complete mistery to the rest of the world, and probably your favourite secret.

author: stojke
author_rank: Regular Contributor
timestamp: January 20, 2019, 10:44:09 pm
content: Sorry for the delay, I didn´t expect to see many people interested in my problem.I cant upload the entire schematic due to file limits, and the scan is completely broken, I have edited the images so it fits on one sheet.

author: xavier60
author_rank: Super Contributor
timestamp: January 21, 2019, 12:05:38 am
content: The gain of the power amp is calculated in the same way as a non-inverting op-amp based amplifier.The resistors that set the gain for one of the channels are R174, R20 and R16. If I'm reading the values correctly, the gain calculates to 161.The is unusually high by my experience.If the power amplifier sections are responsible for the loss of gain, C16 and the 1uF input capacitor would be highly suspect.

author: stojke
author_rank: Regular Contributor
timestamp: January 21, 2019, 09:51:13 pm
content: I dont really understand but what I am getting is the following:CD input : 440Hz test tone @ 0.207mV AC > Passes trough the Tone board : 0.207mV AC at maximum volume on output ( @ CN1 )If I insert my (home made tm) Op Amp modification with applied gain before the volume pot I get around ~10V AC ( @ CN1 ) at max volume and the output sounds nice and loud as it should but with distortion.Because I have* no practical nor theoretical knowledge of precise operation of amplifier circuits I had no other choice but to modify the area that seemed the most logical to be the origin of the problem.

author: wraper
author_rank: Unknown
timestamp: January 21, 2019, 10:29:53 pm
content: Quote from: stojke on January 21, 2019, 09:51:13 pmModifying what is not faulty and making crap out of that. No way there should be 10V AC on the output. If there is any fault then somewhere in power amplifier, not in the part of the circuit where you are making stupid experiments.

author: xavier60
author_rank: Super Contributor
timestamp: January 21, 2019, 11:03:13 pm
content: What were the signal levels at the input and output of the power amplifiers?

author: stojke
author_rank: Regular Contributor
timestamp: January 21, 2019, 11:08:05 pm
content: CN1 207mV , the same as at input jacks , output I did not measure (it was loud , but not as loud as it should be. If we were to compare villager style it was like an average 30-40W amplifier).

author: dmills
author_rank: Super Contributor
timestamp: January 21, 2019, 11:25:33 pm
content: Preamp/Tone board is fine, it is not supposed to have meaningful gain, it is there for tone control, and some filtering for the sub.I would be eyeballing the power amp and possibly IC6 on the input selector board, but really, stick about 1V up it from a signal generator and work your way thru it with a 'scope, where the signal vanishes you have your problem.What are you measuring 0.207mV with? That seems awful low for the output of a CD player, they usually swing a volt or so....Regards, Dan.

author: stojke
author_rank: Regular Contributor
timestamp: January 21, 2019, 11:32:12 pm
content: Thanks for the diagnosis dan (and xavier60!) I will try that, it seems I am really off the track from all the replies Ive read.I am using my phone as an signal generator with some signal generator app because the voltage swing in CD player playback is trowing me off most of the time.I am lacking some proper testing equipment at the time beeing.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 22, 2019, 11:10:12 pm
content: Hi,I would suggest to read some Art of Electronics by Horowitz and Hill. A life with no theoretical skills is a hard life, because you live in a world of unexplainable things. A life without some test equipment (multimeter, oscilloscope, signal generator) is also not funny.Anyway, learning and experimenting is a lot of fun. But dont guess - understand what you are doing. Success comes much faster.

author: wraper
author_rank: Unknown
timestamp: January 22, 2019, 11:28:08 pm
content: Smartphones have too low output voltage to drive majority of amplifiers at max power. You'd better measure output voltage coming to speakers. Then we could at least assume if overall gain is fine or not.