Different way to control Gain of a boost circuit. Which is the best?

[nguitar12]

So I started playing with the BS170 boost circuit. With the first schematic I found that the gain control is not even with the linear pot (yes i don't want to use A&C pot due to the availability) . I have attached another two common way to control the gain (2nd and 3rd schematic). So can someone please tell me the advantages of each?

Thanks

[FiveseveN]

Only the 1^st controls the gain of the amplifier, and you'll probably want to leave the DC part constant as shown here.
The 2^nd controls signal level before amplification. This means the booster's added noise is always present at the output, which might not be an issue but is the worst-case scenario in terms of SNR.
The 3^rd controls signal level after amplification. This means your signal is always limited to the dynamic range available to the booster.
Either option is valid, depending on what you want to achieve. But like I said, you asked about gain and there is only one gain control.

See AMZ's Boosters, Gain and Distortion for more on this.

[antonis]

As FiveseveN said, only the 1st arrangement actualy controlls the amount of boost..

You may add a Source to GND resistor and a bypass capacitor and replace R3 with a pot (wired as variable resistor)..

[nguitar12]

Thanks for all great information. Have a nice day.

[PRR]

I would say they all control the jack-to-jack "gain of the amp".

Only the source resistor changes the gain OF THE AMP, the others work the amp at fixed gain and throw-away gain outside the amp (but inside the box).

I'd say #2 has the wrong connection of the input pot. As-drawn it will short-out the signal source for low settings. This is usually unwanted.

#2 *also* has the input gain pot shunting the MOSFET bias network. As drawn, pot is redundant with R2. You can do it that way, but it smells like a mistake, poor plagiarism.

#2 changes gain INTO the amp. #3 changes gain OUT of the amp. In a stage that is prone to (or intended to) distort, these are very different actions. One sets clipping-level relative to input (guitar effort). The other sets clipping level relative to what comes after (guitar amp).

[antonis]

#2 changes gain INTO the amp. #3 changes gain OUT of the amp. In a stage that is prone to (or intended to) distort, these are very different actions. One sets clipping-level relative to input (guitar effort). The other sets clipping level relative to what comes after (guitar amp).

So, academically speaking, we may consider #3 as a #2 in a 2 stage amplification..

[Gus]

2 and 3 have the MOSFET gain at max and the lowest input resistance at the gate all the time.  R4 is not needed for 3
Notice the drain to gate bias/feedback resistor

IMO using a MOSFET set up like this is ?(two 1meg bias resistors).  You could use a JFET or BJT

[Brisance]

#2 also changes the high pass cutoff point.