![[-]](https://theultimatetone.com/images/collapse.png "[-]")
08-22-2024, 11:43 AM
Hi Guys
Our little cigar box kit amplifier called MICRO can be tricky to assemble. PLUS it has more gain than most players expect.
For the assembly, MICRO uses all through-hole components which are easier to handle than surface-mount components as they are more of a Human-scale size. This also means that component values can be changed in certain circuit areas to affect a custom sound. The circuit itself is pretty standard stuff from the 1970s - nothing clever or surprising - but it is all discreet with no ICs.
To make the unit small overall, we use cordwood construction, which is a common method used in compact electronic equipment - think "8-transistor radio" - haha. With this method, a resistor lead is folded back parallel to the body of the resistor and both leads go through holes in the PCB on a fairly tight spacing. It is important to install one component at a time to assure that the parts are straight and do not foul on each other. Soldering has to be done carefully.
The caps and other parts are selected to have a small footprint on the PCB and the final assembly can be installed in quite a small box.
There is a single jfet gain stage with variable gain (Edge) that can be frequency-focused if the player wishes, just by changing one capacitor value. This is followed by a standard volume control (Retro) allowing output to be dialled to zero. It also allows the input stage to be overdriven on its own if one desires. The power amp has variable gain (Blast) and may have too much gain for many players. So, you have to approach using MICRO in a specific way.
Remember that your electric guitar has a volume control. Set it low to begin with.
Set all of MICRO's controls to zero - fully CCW.
Power up and turn Retro up part way to hear the guitar.
Turn up the guitar volume as desired.
Adjust Edge to hear what it does, then turn it down.
Adjust Blast upward to hear what it does.
If your guitar does not have a level control, you will rely on Retro to control loudness and maybe rely on Edge for basic sensitivity.
Blast has a lot of gain on tap and this may need some taming.
If you flip the board over with controls facing you, the left-hand trace going to Blast can be slashed and a resistor tacked across the gap. Solder the leads to the solder pad for Blast at the front edge of the PCB and to the solder pad for R1, where the slashed trace used to go to. The new resistor works against R44=10k, the PA feedback resistor, to set the maximum gain when Blast is fully CW. The maximum gain is about 600 stock and we can dial this down to <2 with Blast.
Different resistor values for the new resistor yield different maximum gains. You may find you need something in the low range:
10k > 2
4k7 > 3
1k > 11
470 > 22
348 > 30
100 > 100
You may find that you do not need the gain of the input stage, in which case you can change R17=10k to 3k3. This provides unity gain (gain = 1) and Edge can still be used to bump it up. If you want to make Edge more of a treble boost, reduce C8=220n to a lower value.
The jfet input stage has a high input impedance which allows full tonal capture from a guitar with a raw piezo pickup.
MICRO can produce a couple of watts of output depending on the power supply used and whether the output transistors are heat sinked. Through a full-size cabinet this can sound massive. Being solid-state the clean sound can be quite clean and the distortion can be quite metallic if you desire.
Have fun
Our little cigar box kit amplifier called MICRO can be tricky to assemble. PLUS it has more gain than most players expect.
For the assembly, MICRO uses all through-hole components which are easier to handle than surface-mount components as they are more of a Human-scale size. This also means that component values can be changed in certain circuit areas to affect a custom sound. The circuit itself is pretty standard stuff from the 1970s - nothing clever or surprising - but it is all discreet with no ICs.
To make the unit small overall, we use cordwood construction, which is a common method used in compact electronic equipment - think "8-transistor radio" - haha. With this method, a resistor lead is folded back parallel to the body of the resistor and both leads go through holes in the PCB on a fairly tight spacing. It is important to install one component at a time to assure that the parts are straight and do not foul on each other. Soldering has to be done carefully.
The caps and other parts are selected to have a small footprint on the PCB and the final assembly can be installed in quite a small box.
There is a single jfet gain stage with variable gain (Edge) that can be frequency-focused if the player wishes, just by changing one capacitor value. This is followed by a standard volume control (Retro) allowing output to be dialled to zero. It also allows the input stage to be overdriven on its own if one desires. The power amp has variable gain (Blast) and may have too much gain for many players. So, you have to approach using MICRO in a specific way.
Remember that your electric guitar has a volume control. Set it low to begin with.
Set all of MICRO's controls to zero - fully CCW.
Power up and turn Retro up part way to hear the guitar.
Turn up the guitar volume as desired.
Adjust Edge to hear what it does, then turn it down.
Adjust Blast upward to hear what it does.
If your guitar does not have a level control, you will rely on Retro to control loudness and maybe rely on Edge for basic sensitivity.
Blast has a lot of gain on tap and this may need some taming.
If you flip the board over with controls facing you, the left-hand trace going to Blast can be slashed and a resistor tacked across the gap. Solder the leads to the solder pad for Blast at the front edge of the PCB and to the solder pad for R1, where the slashed trace used to go to. The new resistor works against R44=10k, the PA feedback resistor, to set the maximum gain when Blast is fully CW. The maximum gain is about 600 stock and we can dial this down to <2 with Blast.
Different resistor values for the new resistor yield different maximum gains. You may find you need something in the low range:
10k > 2
4k7 > 3
1k > 11
470 > 22
348 > 30
100 > 100
You may find that you do not need the gain of the input stage, in which case you can change R17=10k to 3k3. This provides unity gain (gain = 1) and Edge can still be used to bump it up. If you want to make Edge more of a treble boost, reduce C8=220n to a lower value.
The jfet input stage has a high input impedance which allows full tonal capture from a guitar with a raw piezo pickup.
MICRO can produce a couple of watts of output depending on the power supply used and whether the output transistors are heat sinked. Through a full-size cabinet this can sound massive. Being solid-state the clean sound can be quite clean and the distortion can be quite metallic if you desire.
Have fun
