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LM386 + 556 = 0 no work

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MrDEB

Well-Known Member
assembled the pocket theremin that shows 2 - 555's
I redid using a 556 and added a LM386 amp.
Not realizing the speaker is connected directly to power (orginal design using 2-555's) so the 555 is sinking not sourcing.
Well I put an LM386 into the mix and realize my error when I have virtually no sound.
Looking at both schematics, how can I insert an LM386 into the circuit??
Thinking about the pot being connected between power and pin 9 then the wiper connected to pin 3 of the LM386??
 

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AllVol

New Member
assembled the pocket theremin that shows 2 - 555's
I redid using a 556 and added a LM386 amp.
Not realizing the speaker is connected directly to power (orginal design using 2-555's) so the 555 is sinking not sourcing.
Well I put an LM386 into the mix and realize my error when I have virtually no sound.
Looking at both schematics, how can I insert an LM386 into the circuit??
Thinking about the pot being connected between power and pin 9 then the wiper connected to pin 3 of the LM386??
Your schematic shows a direct short between 9V and ground in the area near the speaker. Also, there is no need for a 10uF cap on pin 4 of the LM386.
 
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MrDEB

Well-Known Member
The 10uf is from data sheet

short?? if you mean from the negative side of the battery??
I double checked my wiring, found where I failed to make 2 connections.
I also removed the amp section.
The orginal schematic has one major flaw- if you turn the 5k pot to far so you have no resistance - magic smoke from the speaker.
going to wait until I get home so I can diagnoise and get the thing operating.
should be able to connect LED on output to determine if I am getting any signal.
beside I have several speakers that I purchased for a fraction of the cost from Mouser instead of Radio Shack.
 

AllVol

New Member
I double checked my wiring, found where I failed to make 2 connections.
I also removed the amp section.
Some schematics show a 10uf cap on pin 7, not on 4, but if you've removed the amp it becomes moot.

If it is a big thing with the pot, simply place a 1k quarter-watt resistor in series with the pot. That will prevent a situation where there is no resistance at all.

Sometimes the biggest problems turn out to be a missed connection. LOL.
 

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MrDEB

Well-Known Member
I am trying to diagnose in different enviroment

which compounds things.
low light levels, a 4 year old, don't have my magnifying headgear, solder is kinda big, the iron is a chisel tip, etc etc.
just goin to pack it up and get it working when I get home to Id-de-ho
 

sheldonstv

New Member
pin 9 from yr 556 -no coupling cap?.... the lm386 circuit as posted by AllVol should work ok 4 eitha the 555 or 556 version of the circuit
 

audioguru

Well-Known Member
Most Helpful Member
Yes, the input of the LM386 amp expects AC, not the very high level DC pulses from the output of the 556 so an input coupling capacitor is needed to feed the volume control.

Get rid of the 10uF C5 that increases the gain of the LM386 amp to 200. You don't need any gain since the output from the 556 is higher than the output of the LM386.
 
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