Why is my sawtooth oscillator not oscillating?

[petterg]

Supply is rated 15V, 530mA

DC voltmeter:
Measured supply voltage unloaded: -15.1 to -15.3 and +16.2 to +16.7
Measured supply voltage loaded with oscillator: -15.1 and +16.3
Measured supply voltage loaded with oscillator and amp.gain=2: -15.1 and +15.3
Measured supply voltage loaded with oscillator and amp.gain=20: +13.5 and +13.6

AC voltmeter:
Measured supply voltage unloaded: 0.0V and 36V
Measured supply voltage loaded with oscillator: 0.0V and 36V
Measured supply voltage loaded with oscillator and amp.gain=20: 0.2V and 40V

oscilloscope:
Supply voltage loaded with oscillator and amp.gain=20, negative rail: Between -14.9V and -15.1V. Variations looks like random noise.

Supply voltage loaded with oscillator and amp.gain=20, positive rail: Between +16.1V and +16.3V. Variations looks like random noise.

Supply voltage loaded with oscillator and amp.gain=20, negative rail: -15V, with some triangle shaped pulse dips (about 100 pr second) down to 11,5V. (Living in a country where electricity has 50Hz I suspect these dips have some relation to that.)

Supply voltage loaded with oscillator and amp.gain=20, positive rail: Between +38V and +52V. Variations looks like random noise.


How can the DC voltmeter say 13.5V (loaded, positive rail) when the oscilloscope show variations between 38 and 52? Just reading the oscilloscope I'd say there is a DC of 45V + random AC.

I guess I need to find another power supply. Could two laptop AC/DC adapters be OK? If I make sure they are not connected to grounded outlet I could connect + from one and - from the other to my circuits ground, right?

 

[petterg]

Now I've also tested with a dual 12V rated for 1A. Voltage drop to -10.4V / + 11,1V once the lm1875 is connected.
(C2 removed, C3 shortened)

I've replaced the lm1875 now. That didn't make any changes.

Here is the mystery: I remove R9, then the lm1875 has nothing connected to its output - problem still occurs.
I remove R7 and R8 - problem still occurs.
I place a R=10k between pin 1 and 2 on the lm1875 (its inputs) - problem goes away.
Now, with that 10k resistor in place I reinsert R7 and R8 - basically grounding the inputs - problem reoccurs! How can that be?

 

[jim hardy]

okay you're making progress

those odd power supply readings suggest to me that your first supply is somehow not well isolated.
Scopes are usually Earth grounded through power cord and when you 'scoped your supply you saw lots of AC.
I don't completely understand it yet but there's something there.

Since it CAN be stable with your second supply let's continue with it...

So now you have made another good observation, the 1875 seems to like its inputs close together. Not surprising, it is an audio amp so expects only about a volt of signal.

Be aware that R9-R8 is the feedback, so removing R9 leaves the amp at wide open gain. If there's any coupling between output pin 6 and +input pin 1 it'll likely oscillate - look for wires run close together...

You noted when inputs are connected together the amp quiets down... another good observation.
Beginning to sound like stability could be an issue ...

I would , for the duration of testing, connect the inputs together by two diodes from pin 1 to pin 2, one pointing each way. That holds the two inputs within 0.6 volts of each other, and we call that a 'clamp'.
Then replace R8 & R9, and a C3 that's closer to what datasheet shows - several microfarads.

How's she look ?

Then insert R7. change ?

Lastly, the power supply bypass capacitors C 3, 4, 6 & 7 in datasheet aren't on your schematic.
If your power wires are more than a few inches long twist them together so as to minimize area they encircle - that reduces their inductance.If it still cuts up then, let us look into stability. Those problems can be elusive so it's important to be methodical.
Here's the direction i'd suggest we take:
Read the section "stability" in datasheet

When designing a different layout, it is important to return the load ground, the output compensation ground, and the low level (feedback and input) grounds to the circuit board ground point through separate paths. Otherwise, large currents flowing along a ground conductor will generate voltages on the conductor which can effectively act as signals at the nput, resulting in high frequency oscillation or excessive distortion.

It is advisable to keep the output compensation components and the 0.1 μF supply decoupling capacitors as close as possible to the LM1875 to reduce the effects of PCB trace resistance and inductance.

For the same reason, the ground return paths for these components should be as short as possible.

I don't see the output compensation on your schematic - see datasheet 'typical application' drawing top of page 2 C5 & R5 .

And I have no idea how you routed your 'grounds' (I call them commons or returns to distinguish from Earth grounds).
They are telling us to keep the signal returns separated from the power and output returns so that the input pins don't see voltage drop along traces carrying those currents.
You see, if something is tied right to bottom of R7, any current flowing down that return wire from there to common causes a voltage drop that adds to input signal.

Edison said "1% inspiration, 99% perspiration." Welcome , you're doing fine !

 

[petterg]

those odd power supply readings suggest to me that your first supply is somehow not well isolated. Scopes are usually Earth grounded through power cord and when you 'scoped your supply you saw lots of AC. I don't completely understand it yet but there's something there.

My scope runs on battery. I have only one cable connected to it, the probe, so the scopes grounding is not the cause of the odd results. (And the outlets in my living room, where I play with this, is not grounded either, so ground on the supply is kind of a virtual ground, not earth.)

Be aware that R9-R8 is the feedback, so removing R9 leaves the amp at wide open gain. If there's any coupling between output pin 6 and +input pin 1 it'll likely oscillate - look for wires run close together...

Are you pointing to the output of oscillator (pin 6 on ua741) or output of the lm1875 (pin 4)?
When there is a 20k resistor from output to inverting input, and a resistor from both inputs to ground, and nothing else connected to output/inputs, shouldn't the opamp stay at the lowest possible power consumption? This seems to not be the case with the lm1875.

You noted when inputs are connected together the amp quiets down... another good observation.
Beginning to sound like stability could be an issue ...

Yes, but I don't understand why it makes a difference once they are connected to ground while they are connected together. Could it be the resistors that picks up noise?

I would , for the duration of testing, connect the inputs together by two diodes from pin 1 to pin 2, one pointing each way. That holds the two inputs within 0.6 volts of each other, and we call that a 'clamp'.
Then replace R8 & R9, and a C3 that's closer to what datasheet shows - several microfarads.

You've got so many genius ideas, Jim!

Diodes is a good idea. 0.6V might be a bit much tough?

The largest capacitor I've got is 100nF. A bunch of larger capacitors is in the mail somewhere, hopefully not too far away.
Does those capacitors (on signal/output) serve any other purpose than wide band filter? I think they are large to not interfere with the frequency response. Again my intuition say they should be as small as possible without without interfering with the frequency range the amplifier will work in.
Other circuits I've googled doesn't use the capacitors when they know the signal source will not send DC.

Cable roundtrip from supply, trough the breadboard and back to supply/0V is only 15cm (6 inches).

I think I'll put R7 and C3 on separate ground cables on the todolist too.

I don't see the output compensation on your schematic

I thought of them like load and figured starting with no load would be a good idea. That might have been one more mistake.

A thing I don't like about the lm1875 is that the heatsink seems to be directly connected to pin3.

I'm out of town for volunteer work for two nights now. I'll get started on the todolist Thursday morning.

 

[petterg]

Separate ground cables to every object that has connection to ground did not make any difference.

A new observation:
When setting a small capacitor over the inputs of 1875, C2 and R7 removed, R8=1k, C3 shortened, the voltage on inverting input is stable 545mV or -545mV (on scoop). I saw it swap once, but the normal seems to be that it decides when powered on if it will go to the positive or negative side. Most times it chooses the negative side.
Then once I remove the short over C3, the voltage goes to near supply voltage.
The same happens if I insert R7 (no matter if C3 is shorted or not).

Using two diodes (actually two transistors with base and collector put together) insted of the capacitor between pin1 and 2 did not affect much, just that voltage on pin1 increased when pin 2 increased.
Diodes and capacitor in parallel worked the same way.

Unplugging R7 while the power is on does not make any difference. Power has to be switched off and back on to start the oscillator.

Another experiment:
I replaced the short cables to the power supply with some longer (30cm each), and twisted them. That made the oscillator (without amplifier connected) create sine waves at 9.6kHz rather than sawtooth at 4.9kHz.

The obvious explanation for what's happening is that some voltage difference between inputs is significantly amplified once pin1 get connected to anything that doesn't follow pin2.
As the voltage is so stable with C3 shorted, it's really no surprise that removing the short makes gain go nuts.

Is it really needed to put a small resistor from output to ground? That would make quite a lot of wasted current.

I started to think maybe the two inputs were swapped on my samples on lm1875. But creating a circuit with one lm1875 that made square wave worked perfectly.

 

[jim hardy]

Separate ground cables to every object that has connection to ground did not make any difference.

Good. I trust they're short wires.

A new observation:
When setting a small capacitor over the inputs of 1875, C2 and R7 removed, R8=1k, C3 shortened, the voltage on inverting input is stable 545mV or -545mV (on scoop).

Is that with R9 in place? If so, that says output is around ten volts, R9-R8 divides it down by 20.
If that's with R9 open, that sort of says the 1875 is trying to force a half milliamp out through its input leads.

I saw it swap once, but the normal seems to be that it decides when powered on if it will go to the positive or negative side. Most times it chooses the negative side.

That sure sounds like a latchup from positive feedback.

Then once I remove the short over C3, the voltage goes to near supply voltage.
The same happens if I insert R7 (no matter if C3 is shorted or not).

Possibly being overdriven.

Using two diodes (actually two transistors with base and collector put together) insted of the capacitor between pin1 and 2 did not affect much, just that voltage on pin1 increased when pin 2 increased.
Diodes and capacitor in parallel worked the same way.

okay - 0.6 volt clamp on input amplified by 20 is still >10 volts output. But we are no longer prying the inputs ten volts away from each other.

Unplugging R7 while the power is on does not make any difference. Power has to be switched off and back on to start the oscillator.

Still sounds like a latchup.

Another experiment:
I replaced the short cables to the power supply with some longer (30cm each), and twisted them. That made the oscillator (without amplifier connected) create sine waves at 9.6kHz rather than sawtooth at 4.9kHz.

So your oscillator itself is that sensitive to power supply lead length? Great observation.
Do you have capacitors on both the negative and positive power supply leads physically close, like within a few inches of U1 and U2?
Opamps need those. Sometimes you get away without them but it's not good practice.
Your power amp really needs them, observe its datasheet shows a 100uf and a 0.1 in parallel. Both are important and here's why:
Big electrolytics are made by rolling up a long strip of aluminum and stuffing it into that cylindrical container. That makes a coil. So at high frequency, the inductance of that coil overwhelms its capacitance and it no longer provides filtering. So they add a small capacitor with less inductance to do the high frequency work.

So make sure they're present and accounted for.

The obvious explanation for what's happening is that some voltage difference between inputs is significantly amplified once pin1 get connected to anything that doesn't follow pin2.

That's what amplifiers do... agreed . Where's the phantom input coming from is our mystery.

As the voltage is so stable with C3 shorted, it's really no surprise that removing the short makes gain go nuts.

C3 should be large enough that it looks like a short circuit for AC. Did you get a large one with your shipment?

Is it really needed to put a small resistor from output to ground? That would make quite a lot of wasted current.

Observe in datasheet that it's connected through a 0.1 uf capacitor which is greater than ten ohms for all frequency below about 100khz. So there's no power getting to it unless amplifier tries to oscillate at very high frequency.
That is its job - to overload the amplifier at high frequency way beyond audio, where it shouldn't be in the first place.

I started to think maybe the two inputs were swapped on my samples on lm1875. But creating a circuit with one lm1875 that made square wave worked perfectly.

I was wondering about that too.
What I think is happening is the 1875 is breaking into oscillation because of stability problems.
When it does that, it overloads the power supply.
When power supply goes haywire, everything looks confusing.

So - you had one working with square waves?
What was its gain?
Try feeding it a 0.1 volt sawtooth wave. That, at gain of 20, should give 2 volts out.
That'll prove your oscillator and power amp CAn work together.

This is the schematic I'm working from - is it right?

Tweak your R4::R3 ratio to get about a quarter volt sawtooth from U1.
That'll both not overdrive Mr 1875, AND let a slow opamp keep up with the sawtooth's slope.

So for starters:

Continue on improving stability by adding bypass caps to power supply and power amp output.

Get oscillator working good by itself with ~ 1/4 volt output at U1 pin6.

Make C3 big enough to do its job of shorting out signal, then you won't need to short it anymore.

When 1875 is stable with C2 out, it should accept a small sawtooth okay. Be sure sawtooth amplitude X gain < power supply volts, that opamp seems to not like being overdriven.
I think overdriving it makes it yank your power supply's tail.

Your perseverance is commendable, sir. I see progress. It looks darkest just before dawn.

The important thing is to take small steps, learning from each one.

Hang in there !

old jim

 

[petterg]

IT WORKS!
(by accident)

Two things:
I reread the datasheet and noticed the sentence telling the lm1875 is intended for gain between 10 and 20. Having R9=20.04k and R8=998. That makes gain of more than 20. So I replaced R8 with a 1.2k. Then the oscillator seemed to spend a little bit of time (like half a second) before it stabilized on no oscillation.
Then I figured I'd remove the short over C3. While I was doing that I accidentally pulled out the potentiometer that is part of R1. (R1 was a 47k resistor + a pot.) As I built this as tiny as possible there was no way I would be able to the pot back with my fat fingers without removing other components. So I measured the value of the pot and inserted a 12k resistor instead. All of a sudden the oscillation worked! (And I forgot about removing the short over C3)

Now I connected what is C2 on the sketch, but I used a 100k resistor instead. And I got sawtooth output from the lm1875!
(The choice of 100k came from that I'm not able to turn output of U1 lower than +/-0.9V without increasing the frequency significantly. And with R7=20k, that I'll experiment with higher voltage out of U1 as I don't like to be close to limits.)

Just to try this out: Reinserting the R8=998 kills signal.
Reinserting the pot also kills signal.

So it seems like the pot was affected by noise.
And the maximum gain of 20 for lm1875 seems to be absolute. (Why do they make example for "typical application" in the datasheet that is SO on the limit?)


(I'm currently running with 4 short (4cm) cables and 8 long (30cm) twisted cables from the power supply. One ground wire is connected to every screw on the supply as well as the ground connectors.)

Big electrolytics are made by rolling up a long strip of aluminum and stuffing it into that cylindrical container. That makes a coil. So at high frequency, the inductance of that coil overwhelms its capacitance and it no longer provides filtering. So they add a small capacitor with less inductance to do the high frequency work.

The wisdom will come handy! I've never though that drawback with large capacitors.

C3 should be large enough that it looks like a short circuit for AC. Did you get a large one with your shipment?

I still haven't got the shipment with larger capacitors. But I realized why I hadn't thrown away that old cisco dslam - it was full of various capacitors. I spent the evening pulling capacitors out of it with the intention to increase size of C3. I never got to the point of replacing C3 because of the accident that solved the problem.


Jim, I've learned a lot from your postings. You've been such a great help. Without your help I wouldn't have a clue of what's going on. This would probably have taken months to figure out.


Next challenge: make a amplifier/filter that can remove a very slow (less than 1Hz) sine wave and amplify the pulse signal (3-10kHz) that is mixed with the sine - positive or negative pulses. That will be a new thread.

 

[jim hardy]

Glad you "got 'er done"... I learned too .
I suspect your potentiometer in previous post was an antenna - a teeny capacitor from pin 6 to pin 2.
You ought to put C3 into avoid DC offset in output. And study all you went through - you'll be the Jedi Master of LM1875's .

Filter, eh ?
Look into "Bi-Quad Filter". TI's LM324 datasheet has one, so does LM3900.
I built the one in fig 40 of this application note for a sharp filter at ~25khz and it worked excellent. Actually I wanted to separate two signals, one at 22 kHz and one at 28 kHz.
http://www.ti.com/lit/an/snoa653/snoa653.pdf
The amp is fast enough you have to be careful with layout.

Filters are the most fun, I think, because they directly apply math..

Thanks for the kind words, and
Good luck !

 

[petterg]

That link had lots of great examples!

I don't really see how regular filters apply for pulse signals. Pulses, from a filter point of view consists of quite a wide range of frequencies.

New thread created for the filtering here:
https://www.physicsforums.com/showthread.php?p=4495116#post4495116