dirt circuit question regarding rolling guitar volume back

[m-theory]

I've noticed this numerous times with several different dirt circuit builds.  Sometimes, a given circuit sounds great with guitar volume cranked.  All pedal knobs function as they should and everything sounds terrific.  Then, I roll guitar volume back, and immediately below "10," the sound get noticeably weak, thin, and completely unimpressive.  From a guitar volume setting of 9 - 9.5 sounds COMPLETELY different than on 10.  All knobs on the pedal still function as before, but it sounds like poo.  It's like there's suddenly some massive loading going on with the guitar pups or something.

I know that this has to be something that I'm doing at times, because there've been times when I've simply started from scratch, and it's come out perfect with the second build.  At this point, I've got a handful of otherwise great-sounding dirt circuits that are unusable, because of this issue.  Can someone point me to what I should be looking at, to correct the ailing builds and try to prevent this from happening again?  Just what is it that's causing this so frequently? 

[Mark Hammer]

Pedals can vary substantially in their input stages.  For example, the Bluesbreaker (and all its derivatives, like the King of Tone) use an inverting op-amp input stage, which will reduce its input impedance, and potentially make it more susceptible (at least as I understand it) to loading.  Of course, if there is something ahead of the pedal (like another pedal that is always on, or that has buffered electronic switching) that provides the appropriate buffering, then you won't encounter any untoward effects of turning the guitar volume down.  But if the guitar goes direct into that pedal, then turning the guitar volume down may well result in unpleasant loading.

Sometimes what sounds great can simultaneously provide very poor input buffering.  I was asked to swap out chips in a buddy's Timmy pedal.  We both agreed that using an LM1458 actually did provide a pleasing smoothing-out of the tone in the pedal, compared to what it came with.  But perhaps that chip diminishes the pedal's "immunity to loading"?

All of this is the long way of saying that some sort of always-on (though bypassable, if necessary) unity-gain buffer is a very handy thing to have on a pedalboard, so that one's guitar volume pot does not take on a second job as tone control.  Some dirt pedals really DO need for the guitar to go direct into them, such that the guitar and cable's loading is an intrinsic part of the resulting sound.  And others can change character in undesirable ways, unless the guitar pot is always up full.  Incidentally, keep in mind that what is a great pot value for loading down the pickups may not always be so great for loading down the pedal.

[m-theory]

Ah, so it actually IS an impedance issue then.  That's what it seems like, and yes, it does shunt off significant amount of high end as well...forgot about that symptom. 

These are all pedal builds where I just plug guitar straight into the circuit, so nothing is going on in front of it.  It sounds like what I should do is build a good buffer pedal to put in front of these "problem children," to  see if that resolves the issue, and if so, simply add that buffer to the input of the troubled circuit, does that sound logical? 

[ashcat_lt]

Can I just ask is there a "treble bleed" cap on your guitar's V pot?

[m-theory]

Yes, and this symptom only arises from SOME dirt circuit builds, not all, and not at all with guitar straight into amp. 

[Mark Hammer]

Ah, so it actually IS an impedance issue then.  That's what it seems like, and yes, it does shunt off significant amount of high end as well...forgot about that symptom. 

These are all pedal builds where I just plug guitar straight into the circuit, so nothing is going on in front of it.  It sounds like what I should do is build a good buffer pedal to put in front of these "problem children," to  see if that resolves the issue, and if so, simply add that buffer to the input of the troubled circuit, does that sound logical? 

Sounds perfectly logical to me.  Although something like an AMZ Mosfet Booster, or SHO clone, or any other super-high input-impedance unit will do fine.  You don't need to use the additional gain/level they provide, but it will be there if you feel like using it.

[m-theory]

Oh sure....I do have a mosfet boost pedal lying around, so I'll try that out.  I'll let you know what I find out, a bit later. 

[ashcat_lt]

So, you're aware that the "treble bleed" cap actually makes the V pot into a pretty effective bass cut control, right?  That action depends on the impedance of the following stage.  As with all filters, the cutoff is inversely proportional to the load, such that a lower in-Z will cut more bass.

[GGBB]

the Bluesbreaker (and all its derivatives, like the King of Tone) use an inverting op-amp input stage

Purely for the record, the Bluesbreaker and spawn use a non-inverting input stage followed by a second inverting stage:

[Mark Hammer]

Gah!  You're absolutely right.  I was focussing so much on the second stage, in my mind, that the input stage was mentally blurred and misremembered.

Never mind!

[Gus]

The guitar and cable are a R s and Ls and Cs network. 

This interacts with the input of the amp effect after it.  The resonance changes with  the volume control and tone and cable cap and pickups and...
If you search this site you might find some post of mine with sims including the guitar/bass cable and the effect

[GibsonGM]

This begs the question of why not breadboard your stuff (at least up to the complexity of a BB), and try it with a buffer out in front, and without - then you could build the buffer INTO the pedal when you make it permanent, if you like it.  You could even customize the input impedance for just the right tone!

I like to have a buffer in some things, but for other work I don't want it on (again, the 'need the cables and parts to color the sound' thing).  For me, worrying  "is that pedal on, and the buffer off?" on stage would be a pain.    I WOULD leave the buffer ON when I want it off, and mess up my sound, ha ha.   None of my stuff is commercial, so there's no Boss pedal sitting there doing the job all the time.   

Just a thought, might be a nice solution for things not yet built.