LM358 VCO Problem

Thread Starter

xavseg

Joined Mar 26, 2020
9
Hi everybody !!

Well, I recently made the famous LM358-based VCO as in this article : https://www.allaboutcircuits.com/projects/diy-synth-series-vco/
Everything seems to work fine except those buffers which I can't get to work at all and I just can't understand why ...

I've already tried putting a 20 ohm load between GND and Buffer's LM358's 1IN+, which resulted as a signal coming from 1OUT (I was quite satisfied) BUT It was still not buffering well : things made after that buffer (Low pass filter, VCA) are still affecting my VCO's pitch...

I didn't put those caps at the LM358s Vcc pins and I'm using a 500mA 5V from an Arduino UNO as voltage source

Any ideas ??
 

Thread Starter

xavseg

Joined Mar 26, 2020
9
@Wolframore I simply don't since I use a simple pot to create Vc haha

@DickCappels I'm speaking about both U4A and U4B, let me explain, when I plug my speaker between U3B output and Ground, I have a signal coming out (I also have an oscilloscope wired, no signal without the speaker plugged)

The problem is that at the output of both buffers, I have nothing except if i put a load between for example 5 and GND of U4B, in which case I have a signal coming out from my opamp but the VCO frequency is still affected by things after that buffer (RC filters, VCA and so on)

I tested it with a bunch of different LM358 (I have a bag of those), it always work the same way...
 

Wolframore

Joined Jan 21, 2019
1,743
It won’t work with just 5v... for one the 358 is not rail to rail. You need a dual supply, the instructions suggested 2 9v batteries if you don’t have anything else... two 2S lipo would work also, I’ve done that in the past.
 

Thread Starter

xavseg

Joined Mar 26, 2020
9
Well the LM358 datasheet (http://www.ti.com/lit/ds/symlink/lm358-n.pdf) tells us "SingleSupply: 3V to 32V Or DualSupplies:±1.5Vto ±16V"

I just tested the buffer schematic with a pot on +, and a led on out (itself wired to -) and it works, so it means that the problem rather comes from my VCO, I'm first gonna check contuinity all arround my VCO and try with some other voltage as You said yep
 

Wolframore

Joined Jan 21, 2019
1,743
The circuit needs dual supply

the opamp can work on single supply but it only goes to about 1.5v of VCC so on 5v and ground... it will limit output to 3.5v and almost 0v. You need dual supply and atleast +/- 7v to work correctly. Follow the recommendations on the instructions.
 

Audioguru again

Joined Oct 21, 2019
1,309
Why are you overloading the output of an opamp with a 20 ohm resistor or a speaker??
The datasheet of a low power LM358 shows that it is overloaded if its load is about 5V/15mA= 333 ohms.

You did not show your schematic of a lowpass filter and a VCA that might be overloading the buffers.
 

Thread Starter

xavseg

Joined Mar 26, 2020
9
Why are you overloading the output of an opamp with a 20 ohm resistor or a speaker??
The datasheet of a low power LM358 shows that it is overloaded if its load is about 5V/15mA= 333 ohms.

You did not show your schematic of a lowpass filter and a VCA that might be overloading the buffers.
Well i load it with a speaker to hear the square/triangle wave, since I'm making it for audio purpose haha
Concerning the 20ohm resistor, I was simply trying to find why the current doesn't seem to "flow" though my 358 :/

Im actually confused by the instructions:

if the VCO is using 358's with 5V, how are you getting 5V outout?
Yep and I looked for people having that one (or the LM358 datasheet version) realised on Youtube and they mostly seem to use 5V as Vcc

Otherwise, I controlled all of the contuinities of my "PCB" to check if there wasn't a short or a bad connection, everything's OK for the VCO part, I even tried with other transistors and other LM358 (in case the first one was bad) but still, I can't get any signal out of those buffers...
 

Wolframore

Joined Jan 21, 2019
1,743
OK I got it to work... this is where the circuit is from:

PLEASE NOTE you can't get 0-5V out of this using 5V source.
1585335409192.png
1585335655310.png


1585335565310.png
 
Last edited:

Thread Starter

xavseg

Joined Mar 26, 2020
9
OK I got it to work... this is where the circuit is from:

PLEASE NOTE you can't get 0-5V out of this using 5V source.
View attachment 202594
View attachment 202597


View attachment 202596
Yep exactly it's that one, so when I plug on my 8 ohms speaker between for instance U2 OUT and GND, I have a signal (same one my oscilloscope), when I remove my speaker the signal also disappears on my oscilloscope (thing that I linked as the fact there's no load, I guess it's false) but when I put my buffer(U3) at the output of U2, nothing's getting out from it, here's my problem
 

Audioguru again

Joined Oct 21, 2019
1,309
An LM386 is a little audio power amplifier. With a 9V supply it can produce 0.5W into an 8 ohm speaker and sound as loud as a cheap clock radio. It will need a coupling capacitor feeding a volume control on its input.
 

DickCappels

Joined Aug 21, 2008
6,317
  1. One important fact that Audioguru again is trying to get you to realize is that the LM358 cannot drive an 8 ohm speaker because the speaker looks like a short circuit to your poor little LM358. That is why he is suggesting the LM386 which was designed to drive a speaker.

Since you have an oscilloscope you can confirm the operation of the circuit without the speaker connected until you can connect a suitable audio amplifier to drive the circuit, such as the LM386.
 

Thread Starter

xavseg

Joined Mar 26, 2020
9
Ok I hear You all, I can't plug a speaker that way but as I said : My issue is that, as showed on pics below, my circuit (VCO alone without buffer) doesn't output anything without the speaker plugged... Same if I add a buffer which is not driven since there's nothing coming on its + pin

Here are some scopes I just made :
>2.5V CV, speaker not wired, nothing outputing from LM358 pin 7 (2OUT / SQUARE OUTPUT in my case)
IMG_20200328_155115.jpg
As we can see, there's just a steady 3.9V, no oscillation

>Same situation, but with a way longer sec/div to control it just doesn't oscillate with a really low frequency
IMG_20200328_155203.jpg
A longer timebase to control if it's just a idontknowhowmuchbutreallylow Hz oscillation, still a steady 3.9V ://

>2.5V CV, speaker wired between LM358 pin 7 (2OUT / SQUARE OUTPUT in my case) and GND
IMG_20200328_155124.jpg
A 700Hz square wave signal (that ghosty blurry waveshape is due to my camera)

I checked all my connections, tried with 5 different LM358, different caps and different BC548, but still not working...

Any idea guys ??
 

DickCappels

Joined Aug 21, 2008
6,317
The only thing that comes to mind is that unless all of your LM358's are broken or fake (it happens to the best of us) there might be a problem with the wiring, the way Vcc/2 is made, or could it be that you need to add a bypass capacitor to your power supply pins?

See post #13 where @Wolframore got it to work.
 

Audioguru again

Joined Oct 21, 2019
1,309
An LM358 cannot produce 0.77V into 8 ohms (96mA) and it certainly cannot produce an output of -0.6V into 8 ohms (-75mV) without having a negative supply.

Your Vref has a high impedance that causes your R13 to appear as 106k then opamp U3B does not work properly. Try adding a capacitor with a few hundred uF to ground at Vref.
A few of your resistor values are not the same as the datasheet circuit.
 

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