LM380 guitar amp has a pulsating sound at high volume level only, how can I fix?

[Jasonmatthew911]

Hi, I recently made an LM380 mini guitar amp with a 4580 IC pre-amp section (Volume, Tone, Gain knobs only) running on 18V, 1A adapter...It works almost perfect, except that when the volume gets to about 3:00 it starts pulsating a little like a tremolo effect, other than that, if I turn the Gain all the way up it has minor oscillation in the background...On the power section of the 4580 there is a .1uf cap to ground, then a 10 ohm resistor to an electrolytic 100uf to ground, then 1N4001 diode to ground, to 18V Zener diode to ground, then that goes into a .1 cap to ground, to a 10 ohms resistor connected parallel with a ferrite bead into a 2200uf electrolytic cap into the pin of the LM380 chip...I'm pretty sure I have to add something else to the power section either for the 4580 or LM380, but I'm not sure what else to put....Maybe more resistors could help....Would not having all the wires shielded where audio passes through, affect in this way causing the pulsating at high volume levels?...I would appreciate any help I can get, as I spent a lot of time on this, and would like to perfect the circuit so it works normally at any volume level...I also know that my preamp section and power section can work perfectly together because I had already tried them together but feeding off of separate power supplies, but both @ 18V....Now I put them together to feed off of 1 power supply, so the issue is in the power section for sure....Can anyone help me?...Let me know if you need more info about the circuit.

[PRR]

> a 10 ohms resistor .... cap into the pin of the LM380 chip

Change that 10 ohms to 470 ohms. See if preamp supply voltage falls much. See if it motoboats less.

[jimbeaux]

This link might help - It's a LM380 with a preamp by Mark Hammer (sounds similar to your project - except for the supply voltage).

http://hammer.ampage.org/?cmd=lt&xid=&fid=&ex=&pg=3

Also you're running at close to max voltage - if you haven't used a heat sink - might be a good idea to add one.

[Gurner]

PRR has hit the nail on the head....motorboating is the problem (google it - nautical hits aside, there are a hole heap of articles on how to cure it)

Bottom line you need to ensure an ripple on the PSU rail caused by your output stage .....doesn't find its way back into your preamp stage (ie via the same PSU rail) - else you've got a positive feedback loop going on (eg  output stage draws large current at higher settings, ripple on supply rail increases, this is seen by the preamp psu rail (since it shares the same rail as the output stage), this psu ripple then 'bleeds' into the audio signal coming out of the preamp, which then gets amplified by the preamplifier itself & then the power stage again.....ripple increase on PSU rinse, reapeat.)

a Low pass filter between your output stage PSU rail and your preamp PSU rail is the likely solution....but if you want to get quick and dirty.....ramp up those 100uf & 2,200uf caps to the largest you can lay your hands on!

[Jasonmatthew911]

Yes, motorboating is the issue here for sure...I will try and change the 10 ohm resistor to the larger 470...Also the LM380 has the staver V7 copper heatsink on it as well, I also had added a clip on heat sink to the 4580 chip just in case...Here is exactly what I have  in the supply right now, starting from where the power goes into the pre-amp:
.1uf cap to GND, 10 ohm resistor w/ ferrite in parallel, 470uf electrolytic to GND, 1N4001 to GND, 18V Zener diode to GND, then goes into 4580 IC and continues to power section, starting with .1uf cap to GND, 10 ohm resistor w/ ferrite in parallel, 2200uf electrolytic to GND, then goes directly into LM380....I also have a 2200uf electrolytic on the output to the speaker.....Any recomendations on what exactly I should change or add to the power supply section?....I know when it has to do with motorboating, I need to isolate these 2 circuits as much as possible...Thanks for the help, I know we're on the right track now...

[Jasonmatthew911]

PRR has hit the nail on the head....motorboating is the problem (google it - nautical hits aside, there are a hole heap of articles on how to cure it)

Bottom line you need to ensure an ripple on the PSU rail caused by your output stage .....doesn't find its way back into your preamp stage (ie via the same PSU rail) - else you've got a positive feedback loop going on (eg  output stage draws large current at higher settings, ripple on supply rail increases, this is seen by the preamp psu rail (since it shares the same rail as the output stage), this psu ripple then 'bleeds' into the audio signal coming out of the preamp, which then gets amplified by the preamplifier itself & then the power stage again.....ripple increase on PSU rinse, reapeat.)

a Low pass filter between your output stage PSU rail and your preamp PSU rail is the likely solution....but if you want to get quick and dirty.....ramp up those 100uf & 2,200uf caps to the largest you can lay your hands on!

Yes, motorboating is the issue here for sure...I will try and change the 10 ohm resistor to the larger 470 that was recommended...Also the LM380 has the staver V7 copper heatsink on it as well, I also had added a clip on heat sink to the 4580 chip just in case...Here is exactly what I have  in the supply right now, starting from where the power goes into the pre-amp:
.1uf cap to GND, 10 ohm resistor w/ ferrite in parallel, 470uf electrolytic to GND, 1N4001 to GND, 18V Zener diode to GND, then goes into 4580 IC and continues to power section, starting with .1uf cap to GND, 10 ohm resistor w/ ferrite in parallel, 2200uf electrolytic to GND, then goes directly into LM380....I also have a 2200uf electrolytic on the output to the speaker.....Any recommendations on what exactly I should change or add to the power supply section?.....Do you think I should make the 470uf electrolytic for the pre amp stage larger even?....If I try the low pass filter, what values should I be using for it?...Thanks for the help, I know we're on the right track now...

[Jasonmatthew911]

> a 10 ohms resistor .... cap into the pin of the LM380 chip

Change that 10 ohms to 470 ohms. See if preamp supply voltage falls much. See if it motoboats less.

I still have to try this, I'll let you know what happens as soon as I do, thanks....Can you tell me what values I should try if I was to put a low-pass filter between the pre-amp and power-amp supply voltage as was recommended as well?

[Jasonmatthew911]

Ok, I raised the 470uf cap to  1,200uf then even to 2,200uf, but the motorboating and occasional high pitch oscillating feedback at higher volumes persists, then I also tried changing the 10 ohm resistor in parallel to a ferrite with a 1K, since I didn't have a 470 ohm at the moment, but I figured the 1K would tell me if raising this resistor would help, but unfortunately this didn't fix it either...Would a 1K instead of 470 resistor make much of a difference in this case?...Lastly I tried the Low-pass filter between the pre-amp and power amp, though I wasn't sure what values to use, I tried a 10K resistor to a .047uf cap to GND, but this didn't help either...There is one thing that I forgot to do, and don't know if this would affect the LM380 or cause the motorboating, but I didn't put any heatsink compound between the copper heatsink and the LM380 IC...Could this be a cause maybe?....Or should I be adding more components to the supply rail, like more little pf caps and resistors in between?

[Jasonmatthew911]

What about putting a 10 ohm resistor in parallel with a ferrite at the output of the power amp, after the 2,200uf cap that goes to the speaker, could this help maybe?

[Jasonmatthew911]

What about putting a 10 ohm resistor in parallel with a ferrite at the output of the power amp, after the 2,200uf cap that goes to the speaker, could this help maybe?

Ok, I tried this and it didn't fix it either....I noticed a 1N4001 Diode in the link schematic at the beginning of the supply, which I don't have on mine, what would that Diode do?....I just have the 1N4001 going to GND in case the wrong supply is used....Can anyone please help me any further with this problem?

[Jasonmatthew911]

I'm starting to think that the problem could be the fact that I used no thermal compound between LM380 and the copper heatsink....Can anyone tell me if I could be right, before I de-solder the heatsink from the LM380?

[Jasonmatthew911]

> a 10 ohms resistor .... cap into the pin of the LM380 chip

Change that 10 ohms to 470 ohms. See if preamp supply voltage falls much. See if it motoboats less.

Hey PRR, I know you know a lot more about electronics than I do...Can you help me any further with this???...I've tried many of the things you and Gurner recommended, and now I'm down to thinking that the fact I didn't add any thermal compound between LM380 and copper heatsink could be causing the issue at the higher volume levels...Could I be right or not?...Would be nice to know before I go and de-solder the heatsink from LM380...Also, I was wondering what the 1N4001 diode at the beginning of the supply rail on the LM380 miniamp Link that jimbeaux posted is actually for???


ANYTHING YOU COULD RECOMMEND MAY HELP A LOT...THANKS AGAIN.

[Gurner]

I've just had a look at the datasheet (I'm not familiar with the LM380)...

"The output is short circuit proof with internal thermal limiting."

...what that means is, if the chip starts getting to hot, the chip will self protect - your heatsink arrangement is not the source of your problems (nor would I be going and adding ferrites to the output of the amp as per your post a couple back)

I don't know what more to say...clearly the problem is motorboating...the only way to solve it is to revisit your circuit's PSU rail configuration. When I mention an LPF earlier, I meant something like this...

http://www.techlib.com/electronics/audioamps.html (look at the diagram called "4-Transistor Amplifier for Small Speaker Applications" - the 470 Ohm resistor & the 10uf cap to the left of it are an LPF ...the purpose is to take out any ripple generated by the high current draw of the output stage finding it's way back into the preamplifier stage.

[Jasonmatthew911]

I've just had a look at the datasheet (I'm not familiar with the LM380)...

"The output is short circuit proof with internal thermal limiting."

...what that means is, if the chip starts getting to hot, the chip will self protect - your heatsink arrangement is not the source of your problems (nor would I be going and adding ferrites to the output of the amp as per your post a couple back)

I don't know what more to say...clearly the problem is motorboating...the only way to solve it is to revisit your circuit's PSU rail configuration. When I mention an LPF earlier, I meant something like this...

http://www.techlib.com/electronics/audioamps.html (look at the diagram called "4-Transistor Amplifier for Small Speaker Applications" - the 470 Ohm resistor & the 10uf cap to the left of it are an LPF ...the purpose is to take out any ripple generated by the high current draw of the output stage finding it's way back into the preamplifier stage.

Ok, I tried the 470 ohm resistor on the power supply rail between the pre-amp stage and going into the power amp stage, the other leg of the resistor going into the (+) of the 10uf cap, and the (-) to GND...I'm not sure if this is how it was to be connected, but this way didn't do anything for the motorboating....However I managed to get rid of the motorboating by connecting a 250K resistor from lug 1 to lug 3 of my 500K Gain pot, basically attenuating it, cuz I think it gets to full gain before the 500K, after half way it would just add in all kinds of noise and motorboating, maybe I'm pushing the 4580 IC too much with the 500K Gain pot and running on 18V....What do you think about this, attenuating the Gain pot?...Is this a solution, or do you think my audio may not be as clean and clear as it can be, for the fact that maybe the supply isn't properly isolated?...It's all still odd too me, the fact that when I ran the pre-amp and power amp stages on different power supplies, motorboating wasn't an issue at all....I thought there could be a quick little circuit of a few components to add between the 2 stages that would isolate them from each other....I don't know how much this matters, but I originally connected everything so that the power goes through the pre-amps power components first and then connected in series it goes into the LM380's power supply components....Or do you think they should be in parallel, so the power goes through both at the same time?

[thedefog]

What does your grounding/shielding configuration look like?

[Jasonmatthew911]

I've just had a look at the datasheet (I'm not familiar with the LM380)...

"The output is short circuit proof with internal thermal limiting."

...what that means is, if the chip starts getting to hot, the chip will self protect - your heatsink arrangement is not the source of your problems (nor would I be going and adding ferrites to the output of the amp as per your post a couple back)

I don't know what more to say...clearly the problem is motorboating...the only way to solve it is to revisit your circuit's PSU rail configuration. When I mention an LPF earlier, I meant something like this...

http://www.techlib.com/electronics/audioamps.html (look at the diagram called "4-Transistor Amplifier for Small Speaker Applications" - the 470 Ohm resistor & the 10uf cap to the left of it are an LPF ...the purpose is to take out any ripple generated by the high current draw of the output stage finding it's way back into the preamplifier stage.

Ok, I tried the 470 ohm resistor on the power supply rail between the pre-amp stage and going into the power amp stage, the other leg of the resistor going into the (+) of the 10uf cap, and the (-) to GND...I'm not sure if this is how it was to be connected, but this way didn't do anything for the motorboating....However I managed to get rid of the motorboating by connecting a 250K resistor from lug 1 to lug 3 of my 500K Gain pot, basically attenuating it, cuz I think it gets to full gain before the 500K, after half way it would just add in all kinds of noise and motorboating, maybe I'm pushing the 4580 IC too much with the 500K Gain pot and running on 18V....What do you think about this, attenuating the Gain pot?...Is this a solution, or do you think my audio may not be as clean and clear as it can be, for the fact that maybe the supply isn't properly isolated?...It's all still odd too me, the fact that when I ran the pre-amp and power amp stages on different power supplies, motorboating wasn't an issue at all....I thought there could be a quick little circuit of a few components to add between the 2 stages that would isolate them from each other....I don't know how much this matters, but I originally connected everything so that the power goes through the pre-amps power components first and then connected in series it goes into the LM380's power supply components....Or do you think they should be in parallel, so the power goes through both at the same time?

Sorry Gurner, I was wrong, the 250K resistor on my 500K Gain pot didn't get rid of the motorboating, I was deceived for a minute....But I did Audio probe the whole circuit and the signal is clean all the way to the input pin of the LM380...The oscillating and motorboating actually starts right at the output pin of the LM380...Does this help anyone to know where or what the problem is???

Also, everything is properly grounded, I'm using shielded cable from input jack to the input of 4580 IC, and from the output of 4580 (Lug 2 of 100K Volume pot) to the input of the LM380....After audio probing the signal path, the motorboating doesn't start until the output of the LM380, it is clean all the way past the input components of the LM380, problem starts at the output of LM380....What exactly do you mean as to what my grounding/shielding looks like?

[thedefog]

I was just wondering if you were using a star ground and had some sort of shielding going on with your build. I had a similiar problem with my Tiny Giant build that was due to a poor grounding scheme at the power supply input that cleared up after I redid the ground network.

[Jasonmatthew911]

I was just wondering if you were using a star ground and had some sort of shielding going on with your build. I had a similiar problem with my Tiny Giant build that was due to a poor grounding scheme at the power supply input that cleared up after I redid the ground network.

Ok, I don't have a star GND, I'm using stripboards....Does anyone think that maybe it's the LM380 chip giving problems, since the motorboating only appears to be at the output of the IC?....Or maybe not...What if I replaced the LM380 with LM384, it's basically the same thing, but 5W and can handle higher voltage?

[merlinb]

Here is exactly what I have  in the supply right now, starting from where the power goes into the pre-amp:

... then goes into 4580 IC and continues to power section, starting with .1uf cap to GND, 10 ohm resistor w/ ferrite in parallel, 2200uf electrolytic to GND, then goes directly into LM380....

That is backwards! The power supply should always go to the current-hungry power stage first, then progress on to the preamp. This is almost certainly the source of your problems.

Ok, I tried the 470 ohm resistor on the power supply rail between the pre-amp stage and going into the power amp stage, the other leg of the resistor going into the (+) of the 10uf cap, and the (-) to GND...

You connected a 470 ohm between V+ and ground?? Why would you do that?? PRR's instruction was pretty clear; replace the 10R with 470R. But, per my comment above, I think you need to reverse the other of your PSU first.

[Jasonmatthew911]

Here is exactly what I have  in the supply right now, starting from where the power goes into the pre-amp:

... then goes into 4580 IC and continues to power section, starting with .1uf cap to GND, 10 ohm resistor w/ ferrite in parallel, 2200uf electrolytic to GND, then goes directly into LM380....

That is backwards! The power supply should always go to the current-hungry power stage first, then progress on to the preamp. This is almost certainly the source of your problems.

Ok, I tried the 470 ohm resistor on the power supply rail between the pre-amp stage and going into the power amp stage, the other leg of the resistor going into the (+) of the 10uf cap, and the (-) to GND...

You connected a 470 ohm between V+ and ground?? Why would you do that?? PRR's instruction was pretty clear; replace the 10R with 470R. But, per my comment above, I think you need to reverse the other of your PSU first.

Ok thanks, that makes sense....What about the power supply going to both stages at the same time, in parallel?...Cuz I had tried them in parallel and the motorboating persists, but I haven't tried what you said, first power amp to pre-amp...I'll let you know how that goes...If it still doesn't fix the problem, I'll try to get my schematics up, so you guys can help me out better...Thanks again.