i'm thinking you're exactly right.What is the advangtages / disadvangtages of using a small vs large C1?
I'm thinking small C1 makes it possible with large R1, which will make less power consume and less load on the opamps (= more stability)
Correct. Time of the up slope devided by time of the down slope (or the other way around).What do you mean by " rise/fall factor " ? Up vs down slopes of sawtooth ? Is that what your 160K R5 adjusts?
My target is between 4kHz and 6kHz. Should be within range for 741 and 324. Although the sawtooth requires faster change in output in a fraction of the cycle than what a symmetric triangle requires at 10kHz.Looks like this would operate in the 10khz region which is about as far as you want to push a 741.
that's from its datasheetThe LM1875 is designed to be stable when operated at a closed-loop gain of 10 or greater, but, as with any
other high-current amplifier, the LM1875 can be made to oscillate under certain conditions. These usually involve
printed circuit board layout or output/input coupling.
Let's focus on that a minuteIt doesn't make sense that the powersupply can't drive one unloaded lm1875
I don't see the output compensation on your schematic - see datasheet 'typical application' drawing top of page 2 C5 & R5 .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.
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.)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.
Are you pointing to the output of oscillator (pin 6 on ua741) or output of the lm1875 (pin 4)?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....
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?You noted when inputs are connected together the amp quiets down... another good observation.
Beginning to sound like stability could be an issue .......
You've got so many genius ideas, Jim!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.
I thought of them like load and figured starting with no load would be a good idea. That might have been one more mistake.I don't see the output compensation on your schematic
Good. I trust they're short wires.Separate ground cables to every object that has connection to ground did not make any difference.
Is that with R9 in place? If so, that says output is around ten volts, R9-R8 divides it down by 20.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).
That sure sounds like a latchup from positive feedback.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.
Possibly being overdriven.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).
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.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.
Still sounds like a latchup.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.
So your oscillator itself is that sensitive to power supply lead length? Great observation.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.
That's what amplifiers do.... agreed . Where's the phantom input coming from is our mystery.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.
C3 should be large enough that it looks like a short circuit for AC. Did you get a large one with your shipment?As the voltage is so stable with C3 shorted, it's really no surprise that removing the short makes gain go nuts.
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.Is it really needed to put a small resistor from output to ground? That would make quite a lot of wasted current.
I was wondering about that too.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.
The wisdom will come handy! I've never though that drawback with large capacitors.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.
C3 should be large enough that it looks like a short circuit for AC. Did you get a large one with your shipment?