How does it all work ?


 

How does it all work ?

The Ruby and Noisy Cricket can be separated into two distinct blocks. You have a JFet Transistor Pre-amp block and a 386 chip power amp block.

The jFet pre-amp in both cases is set up as a (unity gain) buffer which means no volume (voltage gain) is added to the signal.

What the buffer does

A standard passive guitar signal is very high impedance, when you run a High impedance signal straight into a Low impedance input such as that of the 386 chip what happens is a lot

is take sluggish, High impedance AC guitar signal and using DC power from the battery makes a low impedance copy of it which has a much easier time traveling up and down the the wires in your circuit and Guitar leads.

In practical terms what this means is the signal will have more treble and clarity.

Pre amp increase the amplitude ? Voltage swing of the signal (will drive headphones)

Power amp gives it enough current or push to be able to drive the magnet in a speaker

  • Explain Volume Master / output confusion
  • Explain VOLTAGE DIVIDER /power filter section of schematic
  • How to add a volume control
  • 386 bcan be used on it's own
Links
  • How an op-amp works
  • How a transistor works

Enter the op-amp based mylk[386]amp. The beauty of using an op-amp is that it can be varied from buffer to screaming booster and anything inbetween by varying the resistance at a single point in the bias network. More resistance = more gain, so the total maximum gain you want can be set by the value of the pot chosen or you can get rid of the pot altogether and use a switch to alternate between buffer operation and any preset level of pre-amp gain.

2.2 uf Cap in 386 gain loop tightens up the gain

Test Caps

In order to tame some of the massive pre-amp gain available which can get a bit abrasive at higher levels if left unchecked, the circuit uses soft clipping diodes in the feedback loop of the op-amp. In buffer operation or at lower levels of boost the clipping diodes aren't really doing anything except clipping of some of the nasty spikes in the signal and providing a touch of compression to smooth out the clean and semi clean tones. As the gain is increased the clipping diodes are engaged more and more providing slightly asymmetrical soft clipping. Essentially the pre-amp turns from a buffer to a booster into a full on overdrive/distortion pedal as the gain is increased which gives you access to a lot more drive than would be possible with a booster alone. You can think of the mylk[386]amp as the business bit of an Ibanez "Tube Screamer" stuck to the back end of a Noisy Cricket.

Voltage Devider
  • Basically the big 470uf cap is just the main power filter cap. And is there to filter noise out of the power supply. It just needs to go between power (9v+) and ground anywhere on the circuit preferably as close to the 386 chip as possible. If you are doing it on a breadboard you can just plug it in right over the chip with the positive leg to pin 6 and the negative leg to pin 3 or 4.
  • The 2 x 33k resistors placed in series between power and ground form a voltage divider. Where the 2 resistors meet in the middle you get 1/2 the supply voltage. So for 9v you get 4.5 volts which is used to bias the op-amp. So like you said the points marked V.ref on the schematic (the end of 1m5 resistor and the 100nf cap) just need connecting to the point marked V.ref on the voltage divider
  • The 47uf cap is another power filter cap and is like the little brother of the 470uf cap above. But this one goes between 4.5 volts (V.ref) and ground.

When pushed hard most solid state power amps chips clip like smashed glass being scraped down a sandpaper chalkboard so the conventional method build a solid state amp is to use the pre-amp to create the distortion and shape the tone, and then use the power amp to cleanly amplify the pre-amp signal. In this configuration there is no need for an output volume/attenuator set up as all the distortion is created solely by the pre-amp. The shitty little 386 chip however clips like an absolute fucking champ which is why it's so popular for d.i.y guitar amp circuits. If you think about the Smokey, Ruby and the noisy cricket circuits all the distortion is coming from the power amp overdriving. No distortion is coming from the pre-amp as it is just a buffer, the smokey doesn't even have a pre-amp. Basically we use the 386 chip much more like a tube amp where we want the power amp to be saturated


Both the amps use a Tube amp style "Master Volume" control which means is that the volume control is placed after the Pre-amp but before the power amp. This is a standard set up for tube amps and is the reason they are notorious for only sounding their best when played at ridiculous volumes. Because the thing that makes a tube amp sound special is the power amp distortion and sag when the power tubes are driving hard.

The thing that makes the 386 chip special and the reason it is so popular for DIY circuits is that unlike most power amp chips it overdrives like an absolute champ

This set up is fine for Hi Fi amplifiers as all you ever want the power amp to do is take the pre-amp signal and amplify it cleanly, but for a guitar amplifier we want distortion.

so when you turn the volume down you are reducing the amount of signal being fed into the 386 chip. Being as both amps rely solely

The truth is, there are lots of modern power amp chips which put out more power, use less external components, and are generally a lot less temperamental than the humble little 386 chip, but when it comes to guitar use the LM386 has an ace up it's sleeve. Where as most solid state power amp chips clip like smashed glass being scraped down a sandpaper chalk board pushed hard, the LM386 chip clips like an absolute champ




If you have been squinting at the image above on he schematic and thinking... "what the fuck does that mean"? Don't panic, it's really not that complicated......... Step this way and I shall explain.

So.... the image above is a voltage divider and it's used for biasing the op-amp or in other words telling the op-amp where it's balance point is. The 2 big electro caps are just power filter caps used for filtering out any stray AC signal noise from the power supply by passing it to ground while blocking DC and sending it on it's merry way to power the circuit. hopefully squeal free.

If we just remove the power filter caps for a sec to make things simple, all you are left with is this, 2 equal value resistors placed in series between power and ground.

When you place 2 equal value resistors in series between a positive signal or voltage and a negative or ground. What you get out of the middle of those 2 resistors is exactly half what you put in. So if you put 9v in the top and connect the bottom to ground what you get out of the middle is 4.5 Volts. This is our Voltage Reference or V.ref which is used to bias the Op-amp.

If your eyes are glazing over at this point and you just want the 'Take home', here it is. Just build this somewhere on your Breadboard or Circuit board and connect the point where the 2 resistors meet in the middle, to the 2 points marked V.ref on the main schematic. The bottom of the 1.5 meg resistor (which sets the input impedance) and the end of the 100nf cap hanging off the side of the Op-amps gain loop (which in conjunction with the 3k3 resistor forms a low pass filter)

The big 470uf cap is the main power filter cap and is really important in any circuit using a 386 chip which are super fucking temperamental when it comes to to the power filtering for some reason. Many a weird a noise can be eliminated just by wiggling the power filter cap about or moving it's position. In theory all the the diagram is telling you to do with the 470uf cap is somewhere on your circuit, place the cap between power and ground.

In practice, when building this or any other 386 circuit, because of the issues mentioned above what you should really do is pace it in your circuit between power and ground as close as humanly possible to the actual chip.

The 47uf cap is just the baby brother of the 470uf cap and is doing exactly the same job. It's filtering out any noise from the DC power signal by passing it to ground only this time it's doing it between the 4.5v rail and ground.

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