My first two LM386 projects

[zener]

I have two LM386-based circuit for my first two DIY projects. So far, here is how they turned out:

My first is the Smash Drive. I built the killer version first. A fading in loud squeal starts when I stop playing while the circuit is still on. I tried the simple version and had the same annoyance. I tried adding a 100K pot in the output as volume control, tried changing the 100ohm resistor with a 150ohm and adding a .001uf in gain lugs 1 and 3. Nothing changed, it squealed like crazy :x . I even accidentally replace the 2.2M with the 150ohm and the squeal changed into loud fading in hiss. The circuit is yet to be placed in an enclosure. Hopefully, I can do so within two days. If putting it in an enclosure still won’t work, then it could be something else and I’ll post it here.

Next is the single circuit Little Gem from runoffgroove. I had it breadboarded and it worked well from the start. This tiny circuit really delivers . I want to replace the gain pot with a switch that will let me select from no (or low) gain, medium and full. What resistors values should I use?

This forum is great.

Thanks for any help. :wink:

[B Tremblay]

I want to replace the gain pot with a switch that will let me select from no (or low) gain, medium and full. What resistors values should I use?

I'm glad you're enjoying the Little Gem.  You could use a 1k2 or 1k5 for low gain, a 680 ohm for medium, and either a 10 ohm or no resistor for full gain.  Alternatively, you could set the existing gain pot to a good "medium" setting, disconnect the power, and measure the gain pot resistance with a multimeter.  Then, use the closest standard value resistor to duplicate that for the medium gain switch position.

[petemoore]

get yer pot set where ya want it.
 take it out without changing it's adjustment.
 measure resistance between wiper and lug 1 and pick a fixed resistor of ~that value, do the same for wiper to other lug.
 Tie the two resistors together.
 the end of the resistor network you just built is an 'end lug' of the pot, where the two meet is the wiper and the other one...the other one is the other 'lug' of your fixed R 'pot'.  
 Build multiple fixed resistor networks and implement a switch to mimic different ot settings...

[zener]

Thanks for the suggestion.

Hey B Tremblay, have you seen the modified Little Gem made by javacody. It was a long thread mostly answered by Peter Snowberg. He put an eq control. Would you recommend it?

[B Tremblay]

I haven't tried it, so I can't offer my opinion of such a setup, but it would probably work well.  Go for it!

[javacody]

If you are going to do that, I would highly recomend another buffer after the tone stack. I would also suggest tweaking the tone stack, as I'm not completely satisfied with it yet.

To be honest, the little Gem sounds awesome all by itself (if you use a good speaker, I got an 8" Jensen for about $20, as I am after a little bit of a champ vibe and I still want something portable). I only did the tone stack for the fun and learning of it. Once I added a JFET buffer in front of the smoky to lower impedance (before adding the tone stack), it sounded really, really good.

[ExpAnonColin]

If you want to get noisier (a la Parallel Universe) try putting capacitors in between the output and bypass pins, coupled with resistors or pots if you'd like.  Voltage starving can also produce very interesting results.  See http://experimentalistsanonymous.com/pedals/

-Colin

[Mark Hammer]

The pin 8-to-pin-1 loop can be used creatively for various combinations of emphasis and gain.  In general, the lower resistance th path between pin 8 and pin 1, the higher the gain, but there is no reason why there has to be a single path.

A recent article in Nuts and Volts on using less than full bandwidth in the feedback loop to produce a TV listening amp for those who are hard of hearing got me thinking about this.

For instance, you can set one RC combination for basic gain, and use a second parallel RC combination with a lower R and C value to provide higher gain for frequencies above a certain rolloff.  The datasheets for LM386 also indicate how to use feedback from pin 5 (output) to pin 1 to produce bass boost.  Between the two you can insert a bit of loudness compensation (high and low boost) to make tiny cabs or tiny speakers sound a little beefier.  

If you play your cars right, you can also make them individually switchable, or make them complement some pre-existing tone-control network.  For instance, any variable treble control might cover a broader range of cut and boost at a lower turnover frequency (e.g., 1khz), but the feedback loop can be used on top of that to goose content above 5khz.

The one thing I'm not so sure about is the extent to which this can be implemented when using a pair of 386's in bridging mode.  I would expect pretty close matching is needed for that sort of you-push-when-I-pull relationships between chips, and 5% resistors and 20% caps just doesn't cut terribly well.

[ExpAnonColin]

The pin 8-to-pin-1 loop can be used creatively for various combinations of emphasis and gain.  In general, the lower resistance th path between pin 8 and pin 1, the higher the gain, but there is no reason why there has to be a single path.

I find a 10uf electrolytic and a 10k pot works well for being simple.. I'll have to try putting other things in the loop.

-Colin