Booster gain limiter

[speed12]

Hi guys,

I'm looking to build a booster for either a lead boost or to kick a breaking up amp into OD. I will be running it through a Barber Launch Pad and have been informed by david barber that a 6dB boost from unity is about max it could handle before things started to get nasty.

So basically I am looking for a way to limit the overall gain of a booster such as the AMZ MOSFET etc. Obviously just not turning it up as much is the easiest way, but that would then lead the pedal open to accidental knocking and frying of the LP. So is there any other way anyone can think of to limit the gain in the booster? I'm thinking a fixed resisitor or trim pot connected in series with the level pot to effectively pre-attenuate the level pot but would have NO idea what values to use or if that would even work!

Cheers in advance!

[Mark Hammer]

The most readily calculable boost may well be provided by using a single op-amp stage.

[railhead]

1. What are you using the LP for?

2. Use the boost last in the chain.

[petemoore]

  Maybe start with a lower gain booster.
  Use stop resistor on gain pot [between pot and gnd.], reduce gain pot value for better response [maybe fixed R across lugs 1 and 3].
  Use a fixed gain instead of gainpot, this can reduce unneeded gain, add volume pot [variable divider] to the end of the circuit, adjust values to taste more precisely with trim or stop resistors.
 

[amz-fx]

So basically I am looking for a way to limit the overall gain of a booster such as the AMZ MOSFET etc.

Insert a 2.2k resistor in series between C5 and R6. This will limit the gain to a bit over 6db even when the 5k gain pot is set to minimum resistance.

regards, Jack

[slacker]

Obviously just not turning it up as much is the easiest way, but that would then lead the pedal open to accidental knocking and frying of the LP.

I don't think you're going to damage the Launch Pad by feeding it too hot a signal, it will probably just sound bad.

[speed12]

So basically I am looking for a way to limit the overall gain of a booster such as the AMZ MOSFET etc.

Insert a 2.2k resistor in series between C5 and R6. This will limit the gain to a bit over 6db even when the 5k gain pot is set to minimum resistance.

regards, Jack

Spot on, that sounds like what I need....and from the man himself no less! Cheers Jack!

1. What are you using the LP for?

2. Use the boost last in the chain.

I'm going to be using the LP for the 'amp cooking' setup David B suggests. Basically just maximising the input signal to the amp so it has more to work with. This also means that there is a pretty hot signal coming out of the LP (LP is 24V internal) so I can imagine a 9V powered boost placve after it might get a bit frightened of it! I may be wrong though of course!

But the 2.2k resistor sounds like a plan...i don't want a huge boost anyway, so this is more of a safety precaution to make sure nothing untoward happens!

Cheers guys!

[Pushtone]

maybe fixed R across lugs 1 and 3.

 

Just how kosher is this trick?

I've been doing this very thing but with a lot of self confidence.
I built a FTM and its output was way too loud. I could barley turn the VOL up at all.

The resistor across lugs 3 and 1 works great for taming the output 
but I'm worried I messed up the impedance or the pedals ability to get along with other pedals in a chain.

Seems too simple not to have a downside.

(sorry to interject but Petemoore's reply really hits the nail on the head for me)

[amz-fx]

A resistor across lugs 1 and 3 of the pot will change the bias of the transistor.  It also changes the minimum output gain (more not less).  It is not the solution in this case.

Put a resistor across lugs 1 and 2 will do what you want but it changes the taper of the pot. Use a 4.7k or 5.1k resistor if you try this.

Pot numbering:  Looking at the shaft side of the gain pot with the lugs pointed down; from the left, 1 - 2 - 3.   Connect lug 3 to C5 capacitor. Solder the resistor on lugs 1 and 2. Connect lug 1 to ground. 


-Jack

[amz-fx]

correction: I just reread my reply...  it does not modify bias but otherwise is as I stated.

regards, Jack

(I hate not being able to edit comments)