Help me plan my 2W LM386-N3 (power) amp design

Hi,

I have a 20 watts amp with a valve in the preamp and a FX loop that I really like the sounds (Joyo Bantamp Meteor / "orange clone").
Unfortunately 20 watts is far more than enough to piss off the neighbors and shake the windows, heck, even at 9 o'clock the thing is a bit too loud for the living room.
So I'm planning to build a 2 watts poweramp to hook after my delays and bypass the poweramp from the Joyo and be able to crank up the volume from the power section.

It's been many many years since my electronic classes when I was calculating values and knew exactly what I was doing, and now what I am winging it and mixing parts from 2 different projects and kind of hoping it works.

So I made a drawing of the circuit and come here asking for your expertise. I am mixing those 2 projects:
- The 2 watts power section from the little rebel https://www.musikding.de/Little-Rebel-Amp-kit
- The "great sounding" without bass-boss from this page https://www.circuitbasics.com/build-a-great-…from-the-lm386/

Of course I want it to be very low noise and keep it clean and clean from buzzing from the power supply, so I'm trying to apply all the filtering from the circuitbasics page.

Here's what I got:

- power input got 1N4007 + 100u + 0.1uF for filtering
- signal out has the 47nF + 10R (from the lil rebel)
- pin 7 from the LM386 has 10K + 10uF (from the circuitbasics)

- added 0.1uF between 9V and ground (from the circuitbasics)

- use 2x LM386 as per the lil rebel schematic (https://www.musikding.de/docs/musikding…rebelschalt.pdf)

- 10K for volume control

Some open questions:
- I'm sure it needs some filtering on the signal input, it feels like it will get noisy. I see R1,C1,R2,C2 on the lil rebel, but not sure if that's appropriate or how add to here. I found on this eleccircuit pagethis eleccircuit page using an RC with R=47K and C=4.7uF. Is that good?
- I didn't add the 470pF from the circuitbasics page between ground and minus because I'm not sure how it will work on the other IC. I mean on the bottom IC it's from (2) and ground, but not sure on the top IC. Will it be between (3) and ground? It says on that site it's to "filters radio interference picked up by the audio input wires" and I definitely want that on this circuit.
- I want to add the gain control with a small trimming pot to adjust the max output but I'm not sure if it would cause problems. As much as I try, the 2 pots will never be at the exact same resistance value. Will this cause problems? Maybe there's a way to link the 2 and use just 1 trimmer (datasheet schematic for ref)?

- There's a 1000uF on the output to the speaker on the circuitbasics page, is that needed?

I hope someone can help.
Thanks

Zitat von cebersp

• The 10k at pin 7 do not make sense, have a look at the datasheet.
• For Guitar signals the input impedance will be too low. You should have an input buffer, like : https://robrobinette.com/How_the_Ruby_Amp_Works.htm

Hi Christof,

Thanks for the reply.

• As you can see I was just copying from this circuit here. I don't totally understand this part, but I've seen without the 10K in other places, and now with your suggestion, I'll definitely remove it. Thanks!
• This website helps on my learning lot. There's a small difference thou. I'll be connecting this circuit after several pedals, the tube pre-amp from my Joyo amp and some delay/looper on the FX loop. So I believe the impedance wouldn't be low, BUT anyway, it seems to be a great idea to have the input buffer, so I'll add it to the circuit 1.5M + J201 + 3.9K + 47nF before the volume control. I also see on this site showing the output capacitor for coupling, I shall add this too.

Thanks a lot for the comments.

Before finalizing the parts list and order everything I still want to:
- know more about the input filter closer to the signal input. I've been researching on up to now I got a band-pass RC+RC filter cutting on 31Hz and 19KHz.
- From this site there's a decoupling 470pF between LM386 negative (pin 2) and ground (pin 4) and I want to find out if really needed and how to do it when using 2 LM386. Maybe it's not needed if I add the RC input filter.

Zitat von cebersp

Hm, have a close look at the schematics and at the breadboard. C6 470p is between Gnd and Gnd at the negative input. ???

That's what I've seen too. Between pin 2 (negative grounded input) and 4 (power supply ground). According to the author to "filters radio interference picked up by the audio input wires". I don't know how I would to my circuit because one of the LM386 is a positive amplifier and the other is a negative amplifier, so pins 2/3 are inverted.

I'm leaning towards ignore those and hope the RC band pass on the input will clean it nicely.

Updated circuit based on the feedback by cebersp and further research.
- Added the input buffer
- Removed the resistor on pin 7 and changed the value to 100nF matching the Ruby amp
- Quit the idea of adding gain control. I read about unbalance output messing speakers, so unless someone have feedback there, I better just remove it.
- I added a band pass filtering between 31Hz and 19KHz on the corner. I'm not sure where to add to the circuit thou. Before or after the buffer? Before the buffer, the signal might be kind of week, after the buffer it might be already too late to filter it ¯\_(ツ)_/¯. Help me Obi Wan Kenobi.

Anyone got any insights here for me?
I'll start planning the tracks and probably order components by next week.

Zitat von eBernd

Filter? For what reason?

I just finished a pedal kit that gives a whole lot of noise coming from the power supply (high pitched, audibly higher frequency) so I'm probably trying to be a bit of extra cautions. Also on my research I found notes about high frequency radio frequency interference that usually shows up on instrument cables. So my attempt here was to keep all the original guitar signal, normally around 80~120Hz, but add some room for the Octaver and harmonics from distortions.

My original plan was just to solder it on a stripboard, but as I keep on my research/planning on this Sunday, I found some reasonable priced board printing on my area; so I'm changing the plan to just order the extra resistors/capacitors for filters, test it all on solderless protoboard before send it for printing.

Zitat von cebersp

I would add an input capacitor 100nF to make sure that there is no DC voltage.
The 47nF will act as a high pass.

Hi,

here I come with another status update. I ordered the parts and they should arrive on Monday, let construction starts =)
(I also order the filters you suggest on my other thread Help needed with noise on HardOne)

So, I'll show my ignorance and ask another silly question:
This 100nF, is that in series to the buffer, or parallel to R 1.5M to ground? (see attached schematic)

I believe it will be in series. If there's any DC coming from the signal input, the capacitor will fill-up and block that DC from going forward while still allowing the frequencies to pass. But I'm really not sure, so better ask.

Thanks once more for the help

Zitat von cebersp

The capacitor comes first after the input

Once again thank you very very much!

Zitat von cebersp

Frankly speaking: I have got the feeling that designing a new schematic is very advanced for your knowledge.

Oh, I'm sure I'm way over my knowledge level here , but I'm stuck in the house with wife and kid (you know it's a pandemic) I gotta find stuff to keep it me busy and not go crazy. But yeah, I will have to read more on the topic.
But on the other side, I don't think I'm designing my own circuit here, I'm just Frankensteining from several places. DC input filtering from that circuitbasics.com; double LM386 from the rebel amp kit, asked questions here, you pointed me to add the JFET buffer in the signal input, and it becomes my very own Frankenstein monster. Budiustein!

As long as I don't burn the house, it is all good!

Zitat von cebersp

Perhaps it would be good to go on in steps

I just don't want to have to have to order more and more stuff because it's an extra 5 euros for DHL; but now that it's all coming, I can build step-by-step and test on the way.

Thanks again for the all the help, I really appreciate. If/when it works I'll report back here.

Hey,

After a long winter, I come back to report on the success of the project.
Once again thanks a lot @cebersp for all the help.

Here there are a few photos of it:

If anyone wants to build it, here is the Fritzing file I sent to print amp_final.zip
Some minor improvements for consideration:
- the capacitor on the amp output C10 is a bit fatter, so it could use a bit of more space to sit nicely in the board.
- the holes for the PCB stands are too small for those https://www.musikding.de/PCB-standoff-64mm

And here a couple of videos https://photos.app.goo.gl/3PpyZW5httrWoG427
One is the work in progress and the other is the full build.
The full build video first play directly from my computer (no noise whats-so-ever) and then there's a bit of my poor guitar skills. You'll hear first just the power amp, then with the Sunn-T and then added the Tube driver.
So the signal path is Gibson SG -> Tube Driver -> Sunn-T -> My AMp -> Orange 8" cab.

Remember that's all just directly recorded with a mobile phone, so it's just to have an idea.
It really works great for bedroom practice.