petterg said:
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