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February 19, 2025, 12:28:38 AM

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Ruby amp from LM386M-1 module

Started by blackcorvo, February 09, 2025, 01:09:06 AM

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blackcorvo

Recently I got a handful of those LM386 module boards peeps seem to like turning into fuzz boxes, and got very dissapointed with how they sounded right away, but then I thought "what if I added the input buffer from the Ruby to it?", and went to work.


It's night and day. More gain, better dynamic response, and the "Hiwatt" and "Bassman" mods work as expected.

So, if you have one of these lying around, try adding that buffer to the input and see how it works for you.

For a "full Ruby conversion", on top of adding the input buffer, you can remove C1 on the board and attach a couple wires to it's pads to connect the 1K gain pot, remove the volume trimpot and wire up a 10K volume pot to it's pads, and change C3 from 100uF for a 220uF.
She/they since August 2021

J M Fahey

Just checked your YT video: killer sound  :tu:

Congratulations :)

blackcorvo

#2
I had some free time at work today so I drew this perfboard layout for the buffer, and made it once I got home - worked first try.

Attached are the layout and some pictures of the project so far.

Only difference between the Hiwatt/Bassman mod I drew and the actual build is that I didn't have any 18nF caps (to get 15nF in series with 100nF for the Hiwatt mode), but I do have a bag of 33nF caps, so 2x of those in series gets me in the right ballpark (~14nF) for this mod.
She/they since August 2021

J M Fahey

#3
Good!!! :dbtu:

No need go get *exact* cap values if not available, +/-20% is always close enough.
At least it was close enough for Leo Fender ;)

We are pampered by modern parts availability and precision; but way back then ... not that much ... yet they made killer amps.

Please continue posting your experiments  :tu:

blackcorvo

And here it is, finished!


Although I'm suspecting I should replace the MPF102 because there's some strange underlying distortion I didn't have before. I accidentally had the DC-DC converter board set to 25v and fried the first LM386 amp board, but I'm not sure if the J-Fet didn't also get nuked. I'll do it tomorrow cus it's midnight right now.
She/they since August 2021

J M Fahey

That FET takes 25V with no problem, LM386 does not.

Your first video had better sound, it used a different speaker.

To my ears your amp now is unstable and jumps into oscillation, giving an ugly buzzy sound.

IF you have a scope, check that.

If not, pull board from new one and mount it in the old multimeter one and check.

blackcorvo

I'll try playing it through another speaker, I don't really want to mess with dismantling the build because I used the box from the old multimeter for the finished thing.

Just in case, I'll try swapping the J-Fet for another one today (I also ordered more, cus I only have like 2 of them left).
She/they since August 2021

blackcorvo

I fixed it.


The main issue was the MT3608 DC-DC boost converter I was using. I think it was inducing high frequency oscillation into the circuit. Subbing it for an XL6009 solved it.
I also isolated the input jack from the panel because I think it was causing a ground loop with the output jack.
She/they since August 2021