Questions about the LM380:

[Carcano]

Hi, This is my first post on here with a few questions about a mini amp I'd like to build...inspired by my Smokey Amp.

The Smokeys a lot of fun, esp with a graphic EQ in front, which opens up a huge palette of tones...but, it has two big drawbacks. It sounds terrible through a real speaker (tinny bass response), and it has virtually no clean headroom. Of course, this is be expected from a toy. I'm thinking there are two reasons.

1. It doesnt get enough current from its 9v battery to provide strong clear bass tones.
2. The output cap is too small for good bass, only 47uf.

However, I thought I could ramp up the design by making something similiar with a LM380 chip instead of the Smokey's LM386, and using a power adapter with 470uf caps.

Question is, what power supply exactly...12v...18v...20v?
How will the power supply voltage affect the amps clean headroom?

Any suggestions? Thanks!

[Joecool85]

I first try just adjusting your caps.  Personally my little gem (also a LM386 circuit) sounded too bassy, so I had to use smaller caps.  The little gem uses a 220uF for the output.

[Carcano]

What cap size did you eventually settle on?

[Mark Hammer]

http://hammer.ampage.org/?cmd=lt&xid=&fid=&ex=&pg=2  middle of the page, see "miniamp".

This is 380-based, bloody loud, and decent sounding.  I power it with 8 C-cells (12vdc).

[Carcano]

Hi Mark thanks,, Yes I read about your mini-amp while rummageing in the archives. I want to ask you about the pre-amps you have running the LM380 - what would happen if they were removed, and the amp used the LM380 only?

Is a guitar signal strong enough to drive it?

What does it take to overdrive the LM380?

Ideally, I'd like to get a clean sound up to loud talking levels, and clipping into the loud shouting range.

This is why I need to know about power supplies, input voltages, etc.

[Carcano]

Maybe I can wire up the LM380 the same way as the LM386 in the Smokey (see below). What is the other cap (C1) for...cancelling noise???

http://img134.imageshack.us/img134/1253/smokey2vn3.gif

[Mark Hammer]

All those NatSem LM38x type power amp chips were intended for similar purposes, which was simple low-power applications.  Anderton had several LM380-based "practice amp" designs in his books, using 8-C or D-cells, intended as standalone amps with an input volume control.  No reason why you couldn't just use the design I did with everything preceding the master volume omitted.  Any of the ROG amp simulators could be subbed without any difficulty.  You'd have to bias the FETs for a 6vdc Vref but that shouldn't impede the circuits working.

[Carcano]

Anderton had several LM380-based "practice amp" designs in his books, using 8-C or D-cells, intended as standalone amps with an input volume control.

Ah great...are any of Anderton's schematics for these LM380 amps available online somewhere??? How many volts was he getting out of the battery stack?

I'm going to use a wall adapter of some sort...or maybe rechargeable cells.

[Mark Hammer]

As long as you don't run it full tilt, you can get surprising life out of an octet of C or (especially) D-cells.  A 12vdc adaptor that gives at least 500ma will probably be more than sufficient.

Craig's layouts are not especially convenient, and his "design" is probably not as good as the one I linked you too, since he omits some stabilizing components for a leaner application.  I'm not badmouthing him.  It's just that you won't gain anything from trying his project over another.  Here is a design and PCB layout from Everyday Practical Electronics from a couple of years ago.  Hope the PCB pattern/layout is legible.  You will likely need to scale it properly.

http://img.photobucket.com/albums/v474/mhammer/EPE.png

[Paul Perry (Frostwave)]

The lack of bass response is due entirely to the lack of a bass speaker.
This can easily be proved, by connecting it to a domestic hi-fi speaker.
You will be stunned & amazed.

[Carcano]

The lack of bass response is due entirely to the lack of a bass speaker.
This can easily be proved, by connecting it to a domestic hi-fi speaker.

Yes, this is what I did to discover its 'real' sound. I hooked the Smokey  up to a 12" Roland Jazz Chorus Speaker.

Its apparently a fullrange speaker, which problably doesnt belong in a guitar amp.

[Carcano]

Craig's layouts are not especially convenient, and his "design" is probably not as good as the one I linked you too, since he omits some stabilizing components for a leaner application.  I'm not badmouthing him.  It's just that you won't gain anything from trying his project over another.  Here is a design and PCB layout from Everyday Practical Electronics from a couple of years ago.  Hope the PCB pattern/layout is legible.  You will likely need to scale it properly.
http://img.photobucket.com/albums/v474/mhammer/EPE.png

Thanks, I need all the resources I can find and problably have enough schemes now to get going on it. My only concern was that a tiny guitar signal wouldnt be able to drive it...or it wont overdrive at higher levels. Getting a balance between clean and distorted at the right volumes is the tricky part. The Smokey can't do clean unless its whispering.

The one you linked has a curious GROUNDED (+) input, something I havent seen on others. The notes say its for stability...what does that mean???