Faking a "boost pedal"

[filterazonatie]

Is there any reason NOT to make a boost in the following manner...?

I was going to house an Overdrive PCB with a DIY boost (MOSFET or JFET most likely) run them in series, and have a second stomp switch activate the clean boost for solos.

Then I thought, hey this OD has PLENTY of available volume, why not have a toggle switch simply change the volume pot resistance and give me a new volume setting. Much easier, cheaper, and saves space.

Any advantage to building a separate clean boost over this?

Thanks!

[Mark Hammer]

Not really, unless you find there is still too much coloration from the OD itself even at lower volumes.

FWIW, I've made a couple of the AMZ MosFet boosters.  My "bypass" is simply a switch to shunt the drive pot, so that I go between (presumably) unity gain and whatever I set the boost level for.  Let's me do "bypass" and indicator LED with a DPDT switch.

Sometimes simple is better.

[filterazonatie]

Smart! \


Plus, it leaves the circuit in as a high-z buffer, yeah?

[Khas Evets]

Mark,

If you shunt the pot wouldn't that give you full gain, rather than unity?

[Mark Hammer]

You're right, as I've learned since then.   

Fortunately, the theory has little bearing on the method.  If all that needs changing is a single resistance, then you can have a pot set for whatever your desired boost may be in this particular one-transistor design, and use the stompswitch to engage a resistance in parallel with the default resistance.

The AMZ MosFet boost uses a 5k pot to set gain amount, with lower resistances producing more gain.  So, stick a 6.8k resistor between C5 and ground, and make R6 a 25k pot.  When the pot is out of circuit, R6 = 6.8k.  When the switch brings the pot in parallel, the combined resistance is between 5.3k and zero ohms, depending on where you set the pot.  Since the cap always has a path to ground, there should be no popping.  You can also use a stompswitch to select between two different degrees of boost by having two different gain pots in parallel, and selecting between the parallel resistance of the one versus the other.

[Khas Evets]

I was thinking about making a switch selected between full gain and some attenuation. So your original idea should work for me. Bypassing the pot allows for full gain and switching in the pot attenuates it based on the pot setting.

For your example, if you want to switch between unity and the pot (louder), couldn't you use a spdt switch to select a 5K resistor or the 5K pot? Would there be an issue with popping? I guess the cap wouldn't have a path to ground for a very brief time while the switch changes states.

[Mark Hammer]

The change is between a higher vs lower resistance path to ground for the cap.  If you were to switch between paths to ground, yes there would be an interruption in the caps connection to ground and the risk of some very audible popping when you switched (bigger caps store more charge, which rushes through with greater urgency when the connection is restored).  Consequently, the ideal is to use the addition of a parallel resistance so that the path to ground is never interrupted, only changed in its resistance.

On my version of the Anderton Tube Sound Fuzz, I use an arrangement that boosts the gain in the front end and attenuates by a proportional amount on the output, also by means of switching in parallel resistors, except in this case two resistances, rather than just one, are switched.  The first stage is a noninverting op-amp, with gain set by the resistance to ground from the inverting pin.  In the high gain setting, an extra resistor is added in parallel with the existing one to drop the resistance of the ground leg, and increase the gain.  On the output, I have a fixed resistor in series with the volume control.  That extra resistor makes the volume pot act like a higher value one turned down a bit.  When the boost is off, that resistor is shunted by one half of a DPDT switch.  When the boost is on and the parallel resistor is added at the front, the shunt/bypass of the series resistor on the output is disabled so that the volume is turned down to compensate for the increase in level from the boost.  This delivers absolutely popless drive boost (and more hair on the distortion), without the need to adjust the output level.  Recommended!

Incidentally, I'm rather fond of arrangements that deliver "punch-in" functions that can be applied on a riff-wise basis.  Since the changeover from the lowest gain to preset highest gain in a MosFet booster simply requires two contacts to be closed, this is a perfect case for application of a parallel momentary switch in a slightly larger box.  Where a boost would normally require you to step on the stompswitch had enough to get a click, and then step again to disable it, a momentary switch would produce the boost only as long as you held it down, and would revert back to lowest gain the moment you lifted your foot.  Since it is momentary, and in parallel with what the stompswitch does, you have your choice of latched (on until you switch it off) or unlatched (on as long as you hold your foot down) modes.

It may not sound like much UNTIL you get to use it.  Then you start to wonder how you ever got along without it.

[nero1985]

you can actually switch between one passive master volume control and another one with a boost and a volume control so this way u can turn the boost on and also control the volume for it, im building a BSIAB II so im gonna do the same thing, have one volume set for rhythm and the other volume set for leads with the same gain and tone, u might wanna add a treble bleed cap for the volume pot thats gonna at a lower volume just so that u dont lose gain