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Old 10th October 2005, 03:45 PM   #1
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Join Date: Nov 2004
Location: US
Question Help With Small Guitar Amp??

Hi,

There are 2 guitar practive amps on the link below. The 1st one ("little gem") I made and it works fine. The 2nd ("little gem MII") I made and it does not work. I went over everything 10 times and cannot find anything incorrectly wired according to the schematic, but only a low sound comes from it.

http://www.runoffgroove.com/littlegem.html

For some reason the mk II schematic does not show how to wire a volume pot so I wired one same as on the same at the little gem (see below).

1. Wire from pin 5 of top LM386 to lug 3 of pot. Lug 1 of pot to ground and lug 2 to + of speaker. Negative side of speaker to pin 5 of lower LM386.

Based upon this schematic and what I just explained, can anyone figure out why this amp is not working?? I tested the caps, res's and 386's so for the sake of my question please assume that these are all in good working order.

Thanks in advance for your assitance!
Jeff
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Old 10th October 2005, 05:34 PM   #2
Mark_S is offline Mark_S  United States
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Join Date: Mar 2004
Location: Illinois
It looks like you are only getting the output from one of the lm386's.

In the MkII uses a "floating ground" on the ouputs the two Lm386's are set up to operate in the opposite directions.

one way to fix it would be:

lug 1 the pot at pin5 of the top lm386 in the scematic
lug 2 to the positive terminal on the speaker
lug 3 to the negitive terminal on the speaker and pin 5 of the bottom lm386 in the secmatic.

be sure to choose a pot that can handle at more 2 watts on the output though. When you have the volume turned down all the way all of the amps power will be sent through the pot.

another way would be to wire up the speaker as shown in the scematic then replace the 10k resistor on the ground side of the FET with a 10k pot for a volume control.
lug 1 of the pot to the fet
lug 2 of the pot to the 0.22 cap
lug 3 of the pot to ground

The first way would let you overdrive the LM386's for a more distorted sound. The second way would just get distortion from the overdriven fet.
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Old 10th October 2005, 06:10 PM   #3
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Join Date: Nov 2004
Location: US
Awesome! Forums like this are invaluable. Do you know if any good reference books that speak in laymen's terms about circuits and components? Most of what I have seen are too theoretical. It's definitely good to understand theory, but sometimes you just want info for practical applications.

thanks again
Jeff
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