[Amp Notes]





LM386 Chip LM386 Data Sheet - With circuit examples  

LM386 Pinout
   Beavis Audoio -  LM386 chip page

   Run off Groove -  Ruby Amp 

   Run off Groove -  Grace / Big Daddy

   Run off Groove - Little Gem 





Observations on Smokey Circuit- Dave stork



Other useful [links] 

Beavis Audio - Big Muff Pi Tones tack  

http://www.beavisaudio.com/techpages/HIW/hiw1.gif - How LP-1 Booster works 

http://www.beavisaudio.com/techpages/Buffers/ - How buffers work 


AMZ - LAB BOOK  http://www.muzique.com/lab/main.htm 

Why booster pedals do not all sound alike - Booster/Gain/Distortion http://www.muzique.com/lab/boost.htm



 Practice amp designs 

 Cook your own Distortion 




    (lm386 + more,practice amp schematics with variations)

http://www.generalguitargadgets.com/richardo/distortion/index.html     (cook your own distortuion)  


Stuff worth knowing [][][]

  • The Cap and resistor in the gain loop (to Ground) in a (non inverting) op amp - from a low pass filter
  • Adding 2 clipping diodes in series raises (about doubles) the clipping threshold (will have to be louder before clipping )
  • Low Pass - (Resistor / cap to ground) Passes Lows and Attenuates highs
  • High pass - (Cap / Resistor to ground) Passe Highs and Attenuates Lows


  • 2 Batteries in Series = Double Voltage / Same mah  (Sum of Voltage for each battery added together) 
  • 2 Batteries in Parallel = Same Voltage but double the battery life (mah)


  1. Jfets - Clip softer than BJT's but have less gain
  2. BJT's have a shit load of gain but clip like smashed glass 

If you don't have the right values

Resistors in series = Value of b oth resistors added together

Capacitors in parallel = approx the values added together

Caps block certain frequencies - Caps in a high/low pass reduce certain frequencies in volume (3db per Octave) not block completely ( fact check )



Reverse Polarity Protection


To protect your stompboxes or active guitar wirings from damages caused

by wrong polarity, you only need a simple diode, eg. 1N4148. Insert this
diode in the way shown on the drawing



From - Ryan McPherson


I was just browsing your design blog (love some of the stuff you're doing, very impressed with how neat your work is), and encountered this:
Output volume 470r - 1k [Audio]
  • ok so here the absolute best value to use would be a 470 / 500 ohm [audio log]pot but I can't get hold of any of these for love nor money. Values below 1k seem relatively rare, and if you can get them, getting the right taper and shaft length is a nightmare. I can get the 470 ohm [Linears] but only in a longer shaft length which means cutting the pots down which is time consuming and messy.
So what you can do is, you can put a 1k resistor in parallel with a 1k pot and the result will be slightly non-linear 500r pot. But the tiny amount of non-linearity shouldn't be particularly noticeable.
The same thing would be possible to get you closer to a 2k3 or so pot where you're using a 2k but there's not so much point there.
Just a thought to consider!







LM386N 1             4 -12 V                 0.3 W

LM386N 3             4 -12 V                 0.7 W

LM386N 4             5-18 V                  1      W

Input Resistance is 50k

Input bias Current - V=6v, Pins 2/3 open    (would the input benefit from input / ground biasing ??)

Note 1: All voltages are measured with respect to the ground pin unless otherwise specified




Application Hints


Gain Control

 To make the LM386 a more versatile amplifier, two pins (1and 8) are provided for gain control. With pins 1 and 8 open the [internal] 1.35k resistor sets the gain to 20 (26 dB) If a capacitor [or jumper wire - How much difference to sound does using a cap make ? can you add gain/boost only to certain frequencies using the gain loop ? e.g. mid scoop filter in the gain loop ? Boosted (jfet) mid scoop ? Transistor boost in gain loop] is put from pin 1 to 8, bypassing the 1.35k resistor the gain will go up to 200 (46dB). If a resistor is placed in series with a capacitor, the gain can be set to any value from 20 to 200. Gain control Gain control can also be done by capacitively coupling a resistor (or FET) from pin 1 to ground.


Additional external components can be placed in parallel with the internal feedback resistor to tailor the gain and frequency response for individual applications. For example we can compensate for poor speaker bass response by frequency shaping the feedback path  [See datasheet example - Amp with bass boost]. This is done with a series RC [Resistor / Capacitor] from pin 1 [Gain] to 5 [Output] (paralleling the internal 15k resistor) For 6 dB effective bass boost: R = 15k, (the lowest value for good stable operation is R = 10k if pin 8 is open). IF pins 1 - 8 [gain loop] are bypassed then R as low as 2k can be used. This restriction is because the amplifier is only compensated for closed loop gains greater than 9.


Input Biasing


The schematic shows that both inputs are biased to ground with a 50k resistor [Very low for guitar, can you increase the input Z just by adding another resistor to ground at the input, rather than using a buffer ?]. The base current of the input transistor is about 250 nA, so the inputs are about 12.5 mV when left open [Can the input be biased open to 4.5 Volts with a voltage divider or resistor to ground to give more headroom for guitar signals]. If the DC source resistance driving the LM386 is higher than 250k it will contribute very little additional offset (about 2.5mV at the input, 50mV at the output). If the DC source resistance is less than 10k then shorting the unused input to ground will keep the offset low (about 2.5mV at the input, 50mV at the output). For DC source resistances between these values we can eliminate the offset by putting a resistor in series from the unused input to ground, equal in value to the DC source resistance Of course all the offset problems are eliminated if the input is capacitively coupled. When using the LM386 with higher gains (bypassing the 1.35k resistor between 1 and 8) it is necessary to bypass the unused input, preventing degradation of gain and possible instabilities. This is done with a 0.1uf capacitor or short to ground depending on the DC source resistance on the driven input.



Note 4: Twist [V] Supply and ground leads very tightly

Note 5 Twist speaker lead and ground very tightly 




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Links [][][]



CH guitars - best value guitar stuff on eBay ( now has a website  - http://stg.chguitars.co.uk/)


http://www.geofex.com/article_folders/fuzzface/fffram.htm    (Technology of a fuzz face - Explanation of transistors and values needed for perfect fuzz face)

+ Technology of a tube screamer article  

+ Loads more info on classic effects circuits and distortion overview 



http://www.gmarts.org/index.php?go=217   (explanation of different clipping types + schematics for most well known overdrive and distortion circuits)

 Guitar FX Layouts

http://tagboardeffects.blogspot.co.uk/  100's of vero board layouts fevery kind og guitar pedal and amp you can imagine.  





AMZ blog  




To protect your stompboxes or active guitar wirings from damages caused 
by wrong polarity, you only need a simple diode, eg. 1N4148. Insert this 
diode in the way shown on the drawing