Understanding the cricket

[Dylfish]

Hey Guys,
Im looking at building my first noisy cricket. but first i want to learn whats doing what inside it and how it works.

so far i think ive got a few things fiqured out.

Schematic + Numbered Reference : http://img373.imageshack.us/my.php?image=cricketps6.jpg

1) the JFET at the front, works like a preamp, boosting the regular signal for the lm386 to use later. while r1 protects the circuts against voltage spikes?

2) high pass / low pass filters + tone circut to change EQ before power amp stage.

3) Opamp / Power Amp . Lm386 takes the signal already boosted from the preamp and boosts it further to a stage where it can clip, gain is controlled by a pot between 2 pins on the opamp.

4) no idea how the grit switch works.

can anybody tell me if im thinking along the right lines, or if im misguided completely.

Thanks you
Dylan

[petemoore]

1) the JFET at the front, works like a preamp, boosting the regular signal for the lm386 to use later. while r1 protects the circuts against voltage spikes?
  Picture has a hole right where the 1meg input resistor is, must be that 'is R1 there.
  The 1meg resistor to ground off the gate of the Jfet biases the gate, pulls it toward Gnd.
  The Jfet is a source follower [buffer/impedance booster] and this allows a high input impedance, and low output impedance for the LM386's input to see. The Jfet adds no voltage gain.
2) high pass / low pass filters + tone circut to change EQ before power amp stage.
  The HP filtering is preset as the signal travels through the 47nf [which does most or all of the rolloff], and 100nf caps between the JFet and 386.
  The LP filter is adjustable, one of my favorite types, the 100k pot shunts varying amounts of "HF" [the 100nf 'chooses'] to ground.
3) Opamp / Power Amp . Lm386 takes the signal already boosted from the preamp and boosts it further to a stage where it can clip, gain is controlled by a pot between 2 pins on the opamp.
  The signal isn't voltage boosted, the output of the Jfet is 1/1 with it's input, the LM386 definitely boosts, can clip, run out of PS 'wiggle room' and drive a speaker.
4) no idea how the grit switch works.
  Not sure/haven't tried it, but according to this:
   http://www.diystompboxes.com/smfforum/index.php?action=post;topic=71369.0;num_replies=0
  Pin 7 is bypass, perhaps it 'glitches' the bypass into switching on/off at the frequency of AC input, 'tricked' it into shutting down and back on @frequency of...well whatever it sounds like it's doing...I'd guess it's dependant on some certain signal level to get activated or something.

[Dylfish]

so the jfet just works as a buffer? and a buffer used because the lm386 has such a low input impedance?

sorry im new at this game and im just trying to get a grips with what its all doing rather than just building it.

also, everything is done by the opamp for the sound production?

also what makes the noisy cricket more gainy than the ruby amp? most things look the same, i done see major differenaces apart from some cap values etc



thanks Dylan

[MikeH]

also what makes the noisy cricket more gainy than the ruby amp? most things look the same, i done see major differenaces apart from some cap values etc

Because there are no major differences!  Seriously though; they are very similar, although the "grit" switch adds a bit of a gain boost (and some oscillation too).  I find pretty much all 386-type amps to sound pretty much the same.  Obviously the ruby and the NC have more tweakability than a smokey- but at the end of the day they're all the same animal.

[earthtonesaudio]

Grit switch adds a bootstrap capacitor.

[Dylfish]

so the bootstrap capacitor is activated when the grit switch is on, does it increase the voltage and thus the gain? or was my late night wikipedia a failure haha?
sorry for being such a noob it says its connects from the output to the bias circut, but i dont see that happening, unless im reading in a whole different context
http://en.wikipedia.org/wiki/Bootstrapping_(electronics)#Amplifiers

Cheers Guys!

[demonstar]

Have you looked at the datasheet? There is some good information on there. Third page down is good.

"http://cache.national.com/ds/LM/LM386.pdf"

[earthtonesaudio]

I think I mis-spoke earlier.  The "grit" capacitor is not really a bootstrap capacitor, because it's providing negative feedback, not positive.  I forgot that the Noisy Cricket uses the inverting input.
But anyway...
If you look here:
http://cache.national.com/ds/LM/LM386.pdf
You'll see that pin 7 is in the emitter circuit of the inputs.  Bypassing this to ground (or +V) will raise gain just like any common emitter circuit.
Feeding back the output signal into this point will make the lower of the two 15k resistors appear smaller to the emitters of the input PNP transistors, which is another way to raise gain.

Have you looked at the datasheet? There is some good information on there. Third page down is good.

"http://cache.national.com/ds/LM/LM386.pdf"

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