The Best way to use this page is to open the schematic in a new window or print it out, and then have it at the side as you scroll down and read through the notes................

 

Op Amp- use any single op-amp with 741 pin outs eg. TL071, TL061, LM741 etc. I usually use a TL071. The TL061 sounds a bit brighter than the others but any will work fine and sound good....even the much maligned LM741, you can always socket the chip and audition different op-amps later. Obviously you can also use 1/2 of a duel op-amp like the TL072 if you just transpose the pinouts.   

Pre-amp gain - Use around a 50k to 500k  preferably audio taper, (particularly for higher values) but linear will work fine. This control is essentially the same as the Drive control on Tube screamer. More resistance = More Gain so if you have been itching for a way to Tap, squeal and dive bomb your way through the "Beat it solo" at a volume that doesn't prompt angry pitchfork wielding mobs to congregate at your bedroom door, go for 500k. If your guitar playing is more SRV than EVH you might wanna go for a 100k or less which will give you less gain, but more control of it over the pots travel. Of course you could always ditch the pot altogether and use a switch and resistors to select between different levels of pre-set boost. 

Clipping diodes I am currently using a 1N4148 and a 1N4448 (Same as a BAW62PA) which sound great and create slightly asymmetrical clipping, but you could really go to town playing with the clipping set up. Different clippers will drastically alter the high gain sound and output level so play around. You could use 3 diodes for full on asymmetrical clipping (like in the tube screamer) or you could move the clippers from the gain loop of the op-amp and place them from the output to ground (after the 1uf cap) for hard clipping. You can also switch out the diodes for LED's and go full on 'RAT' distortion. 

1uf Decoupling cap - The 1uf cap on the output of the op-amp can be adjusting to suit your build and taste. If you are using a high gain op-amp stage (500k pot) or a smaller speaker, 1uf will be perfect. If you are planning to build the circuit with lower gain and a full size 10" or 12" guitar speaker you may want to raise the value a touch (up to around a max of 10uf) which is much better for Bassman style clean and "just breaking up" Jimi Hendrix stuff.

Master Volume (optional) - This one is entirely up to you. A 10k volume pot here will allow you to tailor level of the pre-amp signal being fed into the power amp independent of the amount of overdrive applied to it just like the level control on a pedal or a "Master Volume" control on a tube amp depending on how you look at it.  It can be a useful control if you like tweaking and want the most range possible but you really don't need it to build a decent sounding amp. If you only have finite amount of space in your enclosure and you are wondering which control you can live without, lose this one, you wont miss it. Tone control - This is also the point where you can take advantage of the extra signal on offer and add a tone control or filter like a mid scoop which can be really good fun to play around with although again, you really don't need one, the amp will sound good without.

Addition -  I have found that when building the amp without a master volume control, higher gain pick-ups can cause the amp to bloat and fuzz out as the input of the 386 chip is overloaded. This is particularly noticeable when using the JRC386 over the LM386.  To eliminate this Simply solder a 33k resistor in series between the 1uf cap and the input of the 386 chip) and all should be well.   

 

 

 

Power amp - Use any version of the 386 chip JRC386 or LM386 The chips range from a 1/4 watt to a full 1w. Higher powered versions can be run up to 18v for a bit more clean headroom but don't expect an earth shattering difference. Even the 1/4 watts will go up to 12v but all version will sound awesome running on a 9v battery and thats what matters.  

Power amp gain - Use a  2k2 or 5k linear pot. This gain control works the opposite way to the pre-amp gain with less resistance = more gain. A 1k pot will also work fine but is not enough resistance to allow the minimum (x20) gain to be selected from the 386 chip so may compromise the amount of clean available.  I like to ditch the pot altogether and use a DP3T (on off on) switch to toggle between - closed / open / 1k resistance which gives you Max gain (x200) / Min gain (x20) and a nice mid gain setting. You could also use a DP2T switch for 2 settings or just dispense with the controls altogether and use a resistor or trimmer pot to set and forget. 

Bright Switch Blink and you will miss it....there is a switched 47nf cap in the feedback loop (between pins 5 & 7) of the 386 chip. This is entirely optional but adding this will give you a very usable Top Boost / Bright switch control. This is similar to the "Grit Switch" on the noisy cricket but using a 47nf cap (rather than the 100nf cap on the Cricket) only boosts the higher frequencies. This is great for a bit of extra treble for solos or to compensate for dark sounding humbuckers. You can experiment with the value but in my experience anything above 100nf sounds buzzy and horrible and anything below 47nf is inaudible.

  Output Volume  (essential) This control is exactly like adding an attenuator to tube amp, allowing you to turn down the volume without losing the power amp overdrive. Pot values below 1K work best. A 470 ohms Pot is perfect but they can be harder to get hold of. A Linear 1k  pot will do the job but the volume roll will be quite steep in the first 1/3 of the pots travel.  You can turn a 1k pot into a 500ohm pot by soldering a 1k resistor between the outer legs of the potentiometer. You could also use a 25/50 ohm (1w) rheostat if you can get hold of one and you have enough space in your enclosure.

  The 8 ohm 1w "power resistor" is optional (not needed if  using a rheostat). If you are using the 1/4 watt 386 chip I really wouldn't worry about it. Most pots are rated at a 1/4 watt anyway and I have never experienced any problems but if you are using the 1w chip, I would always add it just to be on the safe side. It's probably overkill but it just provides a load to the amp and a way to dissipate some heat when you have the volume rolled right back. The value isn't too critical anywhere between about 4 - 16 ohms should be dandy as long as it is rated at 1w or above. 

 

 

Power filter cap - The 470uf power filter cap should be placed as close to the 386 chip as possible. When building any 386 circuit, many a mysterious noise can be eliminated just by changing the placement of the Power Filter cap to somewhere nearer the chip. You can use lower values such as a 220uf or even a 100uf but in my experience (and this could all be in my mind) higher values lead to less digitally sounding artifacts in the tone. You could even use a 1000uf if you have one but they are physically fuckin massive.

  

Input Jack - The circuit has a tendency to squeal and run away with itself when no input is present (I should probably look into and try and sort it out) So either use a Switched stereo input jack to turn the power on and off or if  you would rather have a proper on/off switch, you can just wire a switched Mono jack to ground the input when the jack is removed.

  [Build Note] - Keep the signal in/out wires as short as possible Do not Twist the input or output Signal & Ground wires together (particularly the output/Ground). Keep them separate and short. Twisting the wires together can introduce a lot of undesirable noise, particularly with Higher Gain pot values for the pre-amp. 

 

 

 

 

Speaker Selection 

 

Datasheet recommends 8 ohm, In reality anything around  4 - 16 ohm will be dandy. The 386 seems to drive pretty much anything (within reason) without much of a fuss. The little plastic speakers I use in the small pedal size amps are 32 ohm and I've never had any problems. The one thing I will say is please, please don't ruin your build by using a full range/ hi-fi speaker. I know its really tempting to use an old bookshelf speaker or 5in car stereo speaker but please don't unless all you play is crystal clean. They sound fucking horrible for guitar. Speaker distortion plays a massive part of the final sound and Hi-fi speakers do not do that gracefully. Also guitar speakers have quite a specific mid focused frequency range with a dip in the middle that cuts a lot the glassy highs and boomy, farty lows as well as adding some compression to round out the clipping and spikes in the signal [See this GM arts article for a more thorough explanation] When you play a guitar amp through a full range Hi-fi Speaker or Headphones without any speaker cabinet emulation, it sounds glassy and brittle, yet boomy and abrasive. Either use a proper Guitar speaker Cab [Here's how to add speaker in/out jacks to you combo] or if you are building a little mini amp or practice combo, use those little 1w - 46mm and 56mm paper hobby speakers. You might not get loads of clean Volume out of them but they are a lot louder than you think and they overdrive really well. They sound brilliant with a load of gain and are perfect for playing along to the TV, or Stereo. You can also turn the volume right down without losing all the definition and the sound going all woolly like it does with bigger speakers. You can have gain and sustain for days, at a volume you can play along to an iPad.

 

 

 

 



 

Voltage Divider  


If you have been squinting at the image above on the 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. 

What you are looking at is a Voltage divider (used for biasing the op-amp) and 2 power filter caps.

 If we just remove the 2 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.  

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 tell the op-amp where it's middle point 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-amp 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). Capacitors block Direct Current (like power from a battery) and pass Alternating Current (like a guitar signal). What the power filter cap does is filter out any stray AC signal noise from the DC power supply by passing  it straight to ground while blocking the DC and forcing it on it's merry way to power the circuit (hopefully squeal free). In theory all the the diagram is telling you to do with the 470uf cap is place it somewhere on your circuit between power and ground like this.

  

 

 

In practice, when building this or any other 386 based 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. Like in the image below left

 

The little 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 supply by passing it to ground, only this time it's doing it between the 4.5v V.ref rail and ground, instead of the 9v+ Power rail and ground....... Is everybody happy? Now you know how a voltage divider works and what V.ref means you can go and build all those runoffgroove.com circuits you have been avoiding.