Gain control for Linear Power Booster

[YouAre]

So I'm wondering, has anyone ever implemented a "gain control" for the linear power booster by replacing the emitter resistor with a potentiometer? I figured replacing the 390ohm resistor with something lower to be a gain limiter, and adding a 5k pot in series to get it down all the way to almost unity (albeit w/ a bass cut from the input/output caps...) could be a good "gain" control. I'd get rid of the 100k volume control at the end so I can go from "unity" to boost.

I simulated it in spice, and it seems to go from unity gain to about a 35-36db gain boost (again with slight bass cut).

So are there any foreseeable issues with this? What about any experiences with this mod?

[CynicalMan]

That would change the bias of the transistor, lowering the output voltage swing. But, since you're lowering the gain, the headroom still goes up according to Spice. You just might not have as much of an effect as you had hoped for. Worth a try.

[YouAre]

That would change the bias of the transistor, lowering the output voltage swing. But, since you're lowering the gain, the headroom still goes up according to Spice. You just might not have as much of an effect as you had hoped for. Worth a try.

Right. Ideally, I'd like to have a "clean" sound at the bottom end of the dial (higher emitter resistance) and as you turn it up, you get to the stock LPB gain level. For the application I have in mind, I think it'd be better than the stock "gain at full and volume control tacked on at the end" type approach.

[tiges_ tendres]

Have you tried a capacitor to ground off of the emitter?  if you set this up like you see in a fuzz face, you might have a more recognizeable swing in gain.

[Mark Hammer]

Why bother?  It has a volume control, and changes in gain are not intended to provide different tonalities.

[Gus]

http://www.diystompboxes.com/smfforum/index.php?board=6.0

Not a LPB but the beginner project.

[YouAre]

 

Have you tried a capacitor to ground off of the emitter?  if you set this up like you see in a fuzz face, you might have a more recognizeable swing in gain.

I thought about that, but that would give me more gain. I'd like it to be variable.

Why bother?  It has a volume control, and changes in gain are not intended to provide different tonalities.

Trying to improve upon the Musket Fuzz idea. As I understand it, it has a LPB type circuit feeding into the Muff. The volume control of the LPB makes it so that the LPB always has a pretty big effect on the muff, and you can't really dial it out. I figured this might "blend in" the booster to smack the input of the muff, and be more useful.

[YouAre]

http://www.diystompboxes.com/smfforum/index.php?board=6.0

Not a LPB but the beginner project.

Interesting...I'll take a crack at it. Thanks!

[CynicalMan]

Why bother?  It has a volume control, and changes in gain are not intended to provide different tonalities.

Trying to improve upon the Musket Fuzz idea. As I understand it, it has a LPB type circuit feeding into the Muff. The volume control of the LPB makes it so that the LPB always has a pretty big effect on the muff, and you can't really dial it out. I figured this might "blend in" the booster to smack the input of the muff, and be more useful.

So you want less distortion from the LPB? Most of the tone colouration will remain even with the gain control.

[YouAre]

So you want less distortion from the LPB? Most of the tone colouration will remain even with the gain control.

Yeah, that's basically the idea. I want to use it as a preamp to the Muff. Switching it in and out could be cool too. I figure it might be more useful to have it "blendable."

[Derringer]

replace the stock volume control with a 100K resistor and wire a 500K - 1M pot before the input cap and let it serve double duty as an input attenuator as well as an anti-pop pulldown resistor

perhaps add a treble bleed cap as well

[YouAre]

replace the stock volume control with a 100K resistor and wire a 500K - 1M pot before the input cap and let it serve double duty as an input attenuator as well as an anti-pop pulldown resistor

perhaps add a treble bleed cap as well

Won't that just be an input gain, and not really go from "clean" to full boost on the dial though? I appreciate the advice though.

[Derringer]

yes, it's an input gain

but what I'm thinking is this
with the LPB set up stock, the incoming signal is always amplified to the design's maximum rate. The volume off the transistor's collector only controls the volume of the signal coming out of the gain stage.
So if you a/b'd a low output single coil signal with a high output humbucker signal, the humbucker will probably come through sounding a bit more distorted no matter what the output volume is set at (I'm ignoring how the level of the effect may be overdriving the input of the amp that follows it).

so by attenuating the input signal, you should be able to clean up what's coming out of the gain stage. it'll sacrifice some volume for the cleaner settings though

[Mark Hammer]

When one applies fixed gain, and simply attenuates the input signal, then any noise added by the gain stage is essentially fixed, regardless of how much attenuation is applied.  If the attenuation is applied AFTER the gain stage (i.e., between the booster and the distortion circuit to follow), then turning down the output of the booster also has the effect of diminishing the amount of noise contributed.

AFAIC, just throw a 10uf cap across the emitter resistor to ground in the first stage and be done with it.  How on earth you might expect to saturate those clipping stages more than they are is a mystery to me.

[YouAre]

When one applies fixed gain, and simply attenuates the input signal, then any noise added by the gain stage is essentially fixed, regardless of how much attenuation is applied.  If the attenuation is applied AFTER the gain stage (i.e., between the booster and the distortion circuit to follow), then turning down the output of the booster also has the effect of diminishing the amount of noise contributed.

AFAIC, just throw a 10uf cap across the emitter resistor to ground in the first stage and be done with it.  How on earth you might expect to saturate those clipping stages more than they are is a mystery to me.

Heh, the Musket Fuzz seems to be pretty popular, so I guess people like the idea of hitting a big muff with more boost. I figured it wouldn't hurt to give it a shot.

[Mark Hammer]

In the world of fuzzes, there are all sorts of things that are "popular", by virtue of some extreme sound they can produce, but many of those tones that can make them popular actually have very limited applicability.  I'm not dissing it, but you may find that all that effort doesn't really give you something you'd use more than 10 minutes in any given year.

That being said, just about ANY distortion can be contorted into "severe" tones by sticking a booster ahead of it.  The desirability of the tones produced is often a result of carefully shaping the tonal qualities of the boost.  In particular, you may find that trimming some of the top end (in a manner similar to what you might do by turning the guitar tone down) in the boost points it more in the direction you want.

Following that train of thought, a simple LPB1 built into the same box as a BMP, with the standard volume control for the LPB and maybe a toggle or two to shape the tone of it, may work well for you, and better than attempting to vary the gain.

[Gus]

Simple textbook circuit.  Used a 500 ohm linear potentiometer instead of a 390 because you can find them.  Adjusted R3 for the change in the emitter resistor value.  You can adjust R4 or both R4 and R3.  C4 value changes the graph, adjust to taste.  V2, R7, L1, R8, and C3 are a simple guitar and cable sim omit in the build.

[CynicalMan]

But that would only increase the gain past what the stock LPB has. I think he wants to reduce the gain.

[Gus]

CynicalMan yes you are correct it seems the poster wants less gain.

The sim was posted to show what happens with a close to Stock LPB type circuit with a gain control close to the stock 390 ohm value.

If the person wants less gain they can increase the emitter resistor and keep the collector at 10K, however the input divider string will need to be adjusted.  And an issue is the input resistance will change with divider resistance and hfe x emitter resistor changing the input loading and frequency response.  Things can be done to readjust but you need to know what to keep in mind.  There are other thing to note as well

4 resistor type circuit with a 5k emitter gain control not the same as a 390 ohm  500 ohm emitter resistor  I picked the voltage divider values from stock resistor values and thinking about stable bias.  Someone else might pick different resistor values

[YouAre]

CynicalMan yes you are correct it seems the poster wants less gain.

The sim was posted to show what happens with a close to Stock LPB type circuit with a gain control close to the stock 390 ohm value.

If the person wants less gain they can increase the emitter resistor and keep the collector at 10K, however the input divider string will need to be adjusted.  And an issue is the input resistance will change with divider resistance and hfe x emitter resistor changing the input loading and frequency response.  Things can be done to readjust but you need to know what to keep in mind.  There are other thing to note as well

4 resistor type circuit with a 5k emitter gain control not the same as a 390 ohm  500 ohm emitter resistor  I picked the voltage divider values from stock resistor values and thinking about stable bias.  Someone else might pick different resistor values

Ah, my Analog Electronics lessons on biasing a transistor are a bit fuzzy. I totally forgot the need to adjust the base when you alter the emitter current. What are the values of the 2 base resistors? 180k and 68k?

Thank you for the help!