LM386 Tremolo Amp

[RLBJR65]

First of all I've got to say this is a great forum!! Sure wish I'd have found it before starting my first project. Oh well better late than never.

I was playing around with a simple LM386 amp schem. similar to the Smokey Amp, and decided to try and come up with some sort of variable resistor to connect to pins 1 & 8 to get a tremolo effect.

The first thing that I tried was a simple multi vibrator driving an led with an LDR conected to 1 & 8.

It works!  

The first version works ok as is, but could use some tweaking. I'd like any suggestions for mods.


:oops: If someone will tell me how, I'll post the schematic, (Drawn with paint saved as BMP file) or I can Email it.

Thanks

[Transmogrifox]

I still haven't figured out how to post pics in the forum, but you do need to save the schem as a .jpg or .gif.   If you email it to me
transmogrifox@yahoo.com
I'll post it on my geocities page and send you the link if you would like.

That's an interesting use of the 386.  Keep up the tinkering!

[The Tone God]

I'll let the cat out of the bag. I have a mod coming up for the Ruby Tuby amp. Low voltage tube tremolo. It still needs alittle work but sounds great.

Andrew

[puretube]

Transmo:
http://diystompboxes.com/sboxforum/faq.php
:wink:

TTG: worth an xtra thread!

[Transmogrifox]

ok, so I guess I can't upload directly from my computer.

geocities doesn't work well for posting images here.  I just post links.  it works.  I suppose I would need to get my own web domain.
thanks

[spongebob]

You can make a free account at http://www.photobucket.com/ and upload the schematic there.

[RLBJR65]

Setup a page on my Earthlink account it's not pretty but is functioning.

http://home.earthlink.net/~rlbjr65

[Marcos - Munky]

Looks interesting. But I think it will sound better if you use a 1K pot between pins 1 and 8 to control the gain, and put the LDR between the input or output pin to ground to get tremolo sound.

[RLBJR65]

Marcos, thanks I tried that first but only managed to make alot of noise,
might have had somthing wired wrong though i'll try it again.
I'm interested to see what Tone God has come up with for the Ruby.

I really like these small amp designs I've built several for friends, mounted in non amplified PC speaker cabs, pretty decent speakers in most. It makes a nice light, portable and fairly durable practice amp you can drag around almost anywhere. (don't try to get one through airport security though)

[Marcos - Munky]

Yeah, the mini amps are really cool. I need to build another one for me.

[Peter Snowberg]

Very cool.

I love the idea of modulating the gain rather than signal level/impedance.

One thing you might want to try is to put a 10uF or 22uF in series with the LDR element. That will preserve the DC bias on the 386's differential input stage.

[The Tone God]

TTG: worth an xtra thread!

Agreed. Once I finish with it I'll post it with a thread and maybe a quick article to explain what I did. Its kind of an interesting kludge.

Andrew

[RLBJR65]

Thanks Peter,

It sounds a little better that way used a 47uF.

[Transmogrifox]

That's a kind of a cool LFO.  I may have to use it in something.  Is it pretty much a triangle waveform output? ...or is it something weird?

The LM386 has a maximum of 50k input impedance, and it's actually a little lower and changing with the modulation.  This may be creating an interesting effect.  Something you may try is to buffer the input and then stick a resistor in series with the input, play with some different values and see if it does anything different.  The input impedance increases as the gain decreases, so this counter-acts the effect when a high source impedance is seen at the input.  Maybe it would be best to just buffer it to make the driving impedance low.  Play around with it and see if it actually has an effect.  I know the internal feedback network increases the input impedance, so it may not make a big difference to buffer the input.  Just an idea, since you were looking for ideas.

[puretube]

it`s a squarewave generator aka "a-stable multivibrator";
LEDs will lite up alternately;

(the 1st transistor circuit I ever built (w/o the LEDs.. , 40yrs ago))

it`s similar to the "bi-stable multivibrator" you find in B*ss pedals
for bypass switching;

[RLBJR65]

Thanks everyone! lots of usefull tips, thats why I like this forum so much.

Yeah, the led circuit has been around a long time. The original schem. I had used 39K resistors to voltage in place of the 3.3k's and pot. For PNP transistors all you have to do is reverse polarity. It called for 2n3904 npn or 2n3906 pnp, but thats not important, since they are just being used for switching I think you can use about anything you got. Also said to use 330 ohm instead of the 470 ohm resistors if you use green or yellow led's

Thanks again

[RLBJR65]

Ops gotta change the led's around too when using pnp's

I'll post schem. on my web page

[RLBJR65]

Posted new schem. for amp on my web page also the flashing led schem.

Web site is now working correctly.

[teletroy]

I was just wondering if I could get the link to the schematic or website?  The website link didnt seem to work.. thanks

[RLBJR65]

Yep no more web site. Thanks again Aron for the layout gallery!

Working from my original notes this is the schematic.
http://aronnelson.com/gallery/Richard-Boop-RLBJR65/386TremAmp?full=1
I know that I made a few minor adjustments from my original schematic shown and I don't have the original amp anymore to check, but it will work as drawn.

Anyone see any improvements or have some suggestions please let me know.

Richard