Why is this so quiet?

[FingerBlisters]

Schematic:



Pretty standard stuff here folks. Pregain input, to an input cap switch, then a fuzzface looking thing with a VR on the resistor along the bottom (a la ZVEX wooly mammoth), then a tonestack w/ bypass, and then out through a lowpass filter to remove fizz but I'll change the 47n to a 33n.

Here's the thing: tone stack bypassed and volume is good. Full vol is a bit above unity, which I wanted. However, with the tone stack engaged the volume is about 10% of where it was meaning the actual plink plink acoustic strumming of the guitar is audible in the room.

Now, I know about insertion loss, but there should still be enough juice here to get around that. I have another pedal with the exact same topology (but with a blend pot at the input, and no tone bypass) and it's loud as all hell.

Am I missing something obvious? I thought about putting a boos (LBP1 say) after the tone pot so the switch would bring in the tone stack + boost to hopefully get the gain back, but I'm looking for a cleaner solution and presuming I messed something up somewhere and I'm not seeing it.

Cheers!

[FingerBlisters]

I wonder if it's got anything to do with the switch itself adding some resistance and dropping the signal?

[Vivek]

Tip on manageable sized images

manually edit the IMG tag by adding Width=400

Please see this :

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[img width=400]https://i.imgur.com/9F9UFbc.png[/img]

[jonfoote]

My guess would be switch the 1k resistor in and out.


Mess with the 1k resistor size until switched volumes are the same.

[tonyharker]

When you add a tone network it has an insertion loss.  You need to restore that loss with an amplifier after it, but in the same loop. Just changing the 1k resistor wont help.

[GibsonGM]

What Tony said. If you look at a Big Muff schematic, you'll see how this works...there is a recovery stage after the tone stack to bring the level back up.  The stack eats up quite a bit of signal.

[Mark Hammer]

On many Fuzz Face schematics, the resistance between V+ and the collector of Q2 can be adjusted to yield different output levels.  I'm going to suggest ignoring the recommendation to bias Q2 to 4.5V and simply adjust that 20k "Bias" trimmer for maximum volume.

[antonis]

Copy last Big Muff stage and place a 120k or so resistor into bypass path (between switch lugs 1A & 2A) for equal output levels for both switch settings..

[GibsonGM]

"Output booster", including the 100n cap on the base.

https://www.electrosmash.com/big-muff-pi-analysis

[FingerBlisters]

On many Fuzz Face schematics, the resistance between V+ and the collector of Q2 can be adjusted to yield different output levels.  I'm going to suggest ignoring the recommendation to bias Q2 to 4.5V and simply adjust that 20k "Bias" trimmer for maximum volume.

Hi Mark. I've jiggled with it, but the window of it actually sounding good is pretty slim. Past 5.5v and below around 3.8v sounds a bit gnarly. I think it's at 4.75v now which seems to be a good spot for those particular transistors.

[FingerBlisters]

Copy last Big Muff stage and place a 120k or so resistor into bypass path (between switch lugs 1A & 2A) for equal output levels for both switch settings..

Yeah I'll do this once I've got the tone stack volume back up. If I do that now it'll make both bypass and non super quiet. So same volume, but not enough of it.

[Mark Hammer]

I'll just add that the Q2 resistor shown as 820R is many other lower and occasionally higher values in the various FF schematics I'm sifting through.  Consider use of a 1k trimmer in series with 220R to replace that resistor.

[FingerBlisters]

I'll just add that the Q2 resistor shown as 820R is many other lower and occasionally higher values in the various FF schematics I'm sifting through.  Consider use of a 1k trimmer in series with 220R to replace that resistor.

It's an 820r on the black arts ritual but I get where you're coming from. I'd like to avoid trimmer into trimmer if I can just for ease of use. But I could defo change the 820r to a fixed value, I'm just honestly not sure what that would change given the trimpot helps set the q2 bias?

I added that 1k thinking it would be a stopper resistor for the trimpot, but I doubt it's required.

[zesak]

Maybe Bjt or Jfet adder/mixer to join those two paths, and adjust resistors to wanted gain for each switched path ?

[jonfoote]

When you add a tone network it has an insertion loss.  You need to restore that loss with an amplifier after it, but in the same loop. Just changing the 1k resistor wont help.

if you look at the picture i posted its not just adjusting the 1k resistor as i have rearranged the switching. the larger the resistance between V+ and the 330n cap the louder the output, therefore my idea switches between 2 resistances making the output louder when the tonestack path is in use (if that makes sense).

[antonis]

if you look at the picture i posted its not just adjusting the 1k resistor as i have rearranged the switching. the larger the resistance between V+ and the 330n cap the louder the output, therefore my idea switches between 2 resistances making the output louder when the tonestack path is in use (if that makes sense).

Loundness  has to do with signal amplitude..
Resistance between V+ and 330nF cap has to do with Q3 gain..

Interaction between the two occurs only when Q3 output swing is lower than power supply headroom..

[anotherjim]

What JonFoote suggests is exactly what I'd try. The nearer the output is "resistively" to Q3 collector the louder. Of course, it will vary according to the trimmer setting, but it's simple.