lm386 amp questions

[mark2]

I built this amp and it works and sounds fine: http://www.circuitbasics.com/build-a-great-sounding-audio-amplifier-with-bass-boost-from-the-lm386/

Using the same ic and speaker, i built this and it's EXTREMELY quiet: http://www.hgamps.com/lm386-overdrive-effect/

Does anyone see any reason that should be the case? To my very amateur eyes one big difference is the latter has a MOSFET input buffer. Could this be  where the problem lies?

Second (less important) question: I'm going to build a Ruby amp (http://www.runoffgroove.com/ruby.html) as soon as I get some JFETs in the mail, so I'm curious to see how that turns out. In the meantime, what changes could/should I make to use a BS170 MOSFET instead of a JFET MPF102 or 2N5457. e.g. add some diodes? change any of the resistors or caps around it?

Thanks in advance! I just got started in this hobby and am obviously in over my head, but learning quickly and loving it.

[mth5044]

Hello! And Welcome!

I believe the second schematic you linked to, the quiet one, is designed to be an overdrive effect, not a power amplifier to power a speaker, like the first one.

For the ruby, the JFET is acting as a buffer. You could replace the two resistors and cap associated with the JFET, and insert a MOSFET boost or buffer schematic. JFETs are known to sound 'tubey', so I'm not sure if the JFET is integral to the tone or not.

[mark2]

Thanks. Any tips on where to start investigating to figure out what precisely makes the former an amplifier? I'd love to recreate the same volume on the overdrive circuit.  Using the same lm386 and the same power source my naive assumption would be the ic would provide the same boost, but clearly it's not so simple.

I tried adding the variable gain circuit from the amp to the overdrive (i.e. replaced the jumper 1-8 pins w/ a pot+cap), but it didn't make a noticeable difference.

Thanks again.

Edit: After staring at this for a while, I think it makes sense to next try removing the tone circuit. I guess it would make sense (again to an extreme amateur) that this is reducing the power. Assuming that works I'll see if I can move the volume before the amplification (since it seems silly to generate power then burn it off), but that's probably going to be pretty tricky for me since I don't know how/if it'll work alongside the drive pot.

[Phoenix]

I built this amp and it works and sounds fine: http://www.circuitbasics.com/build-a-great-sounding-audio-amplifier-with-bass-boost-from-the-lm386/

Using the same ic and speaker, i built this and it's EXTREMELY quiet: http://www.hgamps.com/lm386-overdrive-effect/

Does anyone see any reason that should be the case? To my very amateur eyes one big difference is the latter has a MOSFET input buffer. Could this be  where the problem lies?

That schematic you posted is not a power amplifier, it uses the high gain of the LM386 to generate an overdrive/distortion effect, and will not drive a speaker directly because of the high series impedance of the tone and volume controls between the output of the LM386 and the output of the effect. It's just an effects pedal, and needs an amplifier to drive a speaker.
If you connect its output to the input of the first one you linked, it should sound great.

The mosfet is just an input buffer, it prevents the low input impedance of the LM386 from loading down your guitar and prevents it from sounding muddy. It will sound quite different without it, and I would say worse (though that's down to taste of course).

Second (less important) question: I'm going to build a Ruby amp (http://www.runoffgroove.com/ruby.html) as soon as I get some JFETs in the mail, so I'm curious to see how that turns out. In the meantime, what changes could/should I make to use a BS170 MOSFET instead of a JFET MPF102 or 2N5457. e.g. add some diodes? change any of the resistors or caps around it?

Thanks in advance! I just got started in this hobby and am obviously in over my head, but learning quickly and loving it.

Omit everything in the Ruby up until the 10kA volume control. Replace it with a mosfet buffer from AMZ's basic buffers.

[mark2]

Thank you both. This is starting to click for me.

My current goal for this project is to get a Ruby-like amp with some serious fuzzy overdrive like that second one (http://www.hgamps.com/lm386-overdrive-effect/).  My fallback plan is to just use two LM386s, as you noted, and put the effect in front of the amp.

However, is there a more economical way to do this? e.g. modifications to the Ruby to give it more fuzz? What I'm trying to wrap my head around is what the Ruby (or the 'circuitbasics' amp) is doing with volume + gain and how that differs from what the Overdrive circuit is doing with the drive pot.

[mark2]

By the way, what you both said on using a MOSFET buffer... I get it now! Thank you so much.

[GibsonGM]

Pins 1 + 8 on the 386 set the gain, and you can see where they have used a pot to allow you to set that gain.  So you can get max output from any 386 that does this.   I think you could get what you want by using the "overdrive" in front of the "amp" schematic.   The OD is losing a lot of level due to that tone stack, but with a following drive circuit, it would be made-up.  The difference really looks like just how hard you hit the 386 with input signal....too much, and it fuzzes out. 

I don't think you can get a TON of beautiful tone out of the 386, just my opinion    The Ruby really takes it to the limit, tho some ppl have reported good results really beating on them.    But it seems they work "one OR the other" best...EITHER OD OR as a tiny practice amp.

By all means play with the Rudy & your ideas, that's how you get to know what the different things do... but I think you'd get more bang for the buck with (at least one) opamp gain stage + clipping diodes into a chip amp or 386.   2 opamp stages would give you clipping followed by more level and ability to incorporate a tone control...or 2 mosfet or jefet stages into the 386, etc.... There are many ways to do this!