AMZ Mini Booster question

[italianguy63]

I built a AMZ Mini Booster with the purpose of using it as a clean boost/volume pedal.

I mounted it in a Crybaby pedal, and used the stock crybaby pot.  It (sort of) works.  Of course the volume gets touchy fast because the Crybaby pot is 100K-Log.  Obviously, I need a Linear.

Question is:  To make the pedal go from zero volume to some level of boost, in a pedal type (not full deflection of the pot) environment-- What value pot would be ideal?  Would it be larger (125K?) or smaller (50K?)... or maybe smaller with at fixed resistor in line?  i.e. 50K + 50K? 

Thoughts?
MC

[italianguy63]

Extra thought-- maybe use a 50K trimmer plus the 50K in the pedal, hence you could dial the heal back (trimmer) to match the volume to the non-boost level?

I'm thinking/asking aloud to see if anyone can give guidance based on the circuit design and pedal throw as to what values would be ideal??

MC

[pappasmurfsharem]

I built a AMZ Mini Booster with the purpose of using it as a clean boost/volume pedal.

I mounted it in a Crybaby pedal, and used the stock crybaby pot.  It (sort of) works.  Of course the volume gets touchy fast because the Crybaby pot is 100K-Log.  Obviously, I need a Linear.

Question is:  To make the pedal go from zero volume to some level of boost, in a pedal type (not full deflection of the pot) environment-- What value pot would be ideal?  Would it be larger (125K?) or smaller (50K?)... or maybe smaller with at fixed resistor in line?  i.e. 50K + 50K? 

Thoughts?
MC

If you have the volume at the end of the circuit. I would imagine you would still WANT log, but maybe the rotation of the foot action doesn't coorelate as much to the spin of the knob.

[italianguy63]

Currently it does not do much, then ramps very rapidly to very loud.  It is because the wah pots are a modified log curve... linear would be much more useable.

[italianguy63]

That's actually why I posed the question.  Hoping to make the "sweet spot" coorespond to the actual physical rotation of the pedal throw.  Based on the circuit, it seems the value would need to be on the original side (100K).  Making it smaller for more useable range (50K) may not limit the volume enough?  So, I was thinking the fixed resistor (50K)+ plus the variable resistor (50K) to get the range of travel.  But this solution would mean you would never get the circuit above 1/2 gain-- which may be OK in this case.  I was hoping Jack might comment on the design, and the choice of the size of the volume pot...
MC

[knutolai]

Have you considered building the GeoFex Mu-amp instead? Seems to me like a better, more stable variation.
Check out the article here: http://www.geofex.com/Article_Folders/modmuamp/modmuamp.htm

[amz-fx]

So, I was thinking the fixed resistor (50K)+ plus the variable resistor (50K) to get the range of travel.  But this solution would mean you would never get the circuit above 1/2 gain-- which may be OK in this case.  I was hoping Jack might comment on the design, and the choice of the size of the volume pot...
MC

Like many simple transistor circuits, loading the output will reduce the signal level, so 100k was chosen as a good value to get a lot of gain out of it.

If you want to use less of the available signal range, try a lower value volume pot and see how that "feels".  Use the 50k pot (without the 50k series resistor) and see if that is what you want. If not, go smaller with the pot value.

regards, Jack

[italianguy63]

Thanks Jack, I was wondering if 50K would take the signal down to zero (or near zero)

[italianguy63]

Knutolai-- I am actually building 3.  2 AMZ Mini Boosters, one with older mojo "dirty" components, and one with modern low tolerance "clean" components.  Then, I am also doing a AMZ Mosfet Boost.  I am wanting to listen to them all side-by-side and determine which one I like the sound of best...

[induction]

Any value pot will take the signal to zero. Lower value pots will give you less volume at maximum because more of the signal bleeds to ground through the pot.

Something to think about: Crybaby's are wired so that toe-down rotates them counter-clockwise, so to get them to behave like a volume pedal you have to either wire them backwards or reverse the rotation of the pot (maybe rotate the rack gear so the teeth are facing the other way or make a bracket to mount the pot on the opposite side of the hole). If you reverse the rotation, you get the taper you expect, but if you wire it backwards, the taper will be reversed. If the pot taper is ICAR (revlog on bottom half, log on top half) or linear, the taper will be the same in either direction. But reversing a log taper will give you reverse-log, and reversing a reverse-log taper will give you log. You're describing reverse-log behavior, so I'm assuming you wired the pot backwards and the taper is log (though it's possible that the taper is ICAR and the top half of the rotation is being cut off).

The Mini Booster calls for a log pot, so ideally you'll want a reverse log pot wired backwards to get a good feel. I don't know how feasible it is to reverse the rotation of the pot (I've never tried it) but if you decide to go that way then the log pot will work fine.

A linear pot might do the job well enough. If it's a log pot in there right now, you can test it by putting a 47k resistor between lugs 1 and 2. Wired backward, this will approximate a linear 32k pot. It won't get as much volume at maximum, but you can at least test whether a linear taper will do the job or if you have to find a reverse-log pot that fits in your crybaby.

[italianguy63]

Thank you.. Yes.  wired backwards-- it is acting reverse log.  I kind of figured that.

I picked up a 50K linear in surplus today.. will give it a try.  I also have a 125K linear wah spare I never used.

I'm sure I will figure it out.  Was just hoping to narrow my horizons instead of "parts swapping" a bunch of expensive pots I don't have.

MC

[italianguy63]

Got the pot home and it was 10K.  Won't work.  I had a 50K trimmer I popped in, and I think that will work.

So, Does anybody have a part#/Source for a Linear taper, 50K, D-shaft (for wah) pot.  Hardware/gear and mounting/bezel size to work as a wah pot is wanted!!  I would expect Mouser has it, but trying to navigate their site is impossible.

MC

[smallbearelec]

So, Does anybody have a part#/Source for a Linear taper, 50K, D-shaft (for wah) pot.  Hardware/gear and mounting/bezel size to work as a wah pot is wanted!!  I would expect Mouser has it

Naaaa...pots that are compatible with expression pedals are a specialty item. I don't recall what the taper is on this one:

http://www.smallbearelec.com/servlet/Detail?no=1262

but I will check for you when I'm back in the shop.

SD

[italianguy63]

Thanks Steve-- let me know.  MC

[Jdansti]

Sounds like you might need a reverse log pot. Since it's wired in reverse, wouldn't that make it act like a normal log?

[italianguy63]

I don't think it will matter that much if I can get the right value... I tried the 125K linear I had today.  As expected, it made it worse-- The lesser value makes the range more usable, and yes, I think linear or reverse log will make it less critical too.   If I HAVE to use a log taper, I probably can.

I still haven't tried the prior suggestion of (induction) shunting the 100K pot, which will reduce the resistance and change the slope.  That's probably next when I have a spare minute.  Once I get this actually working to my satisfaction, I'll post the solution--  I personally think it would be pretty cool to have a foot operated clean boost.  I have the one I managed to make work, but I am trying a real attempt at a refined one.

[italianguy63]

Can somebody graph what a 100K log taper pot with a 47K fixed (between pin 1 and 2) would look like across the range?  MC

[arma61]

Hi

reply #8
http://www.diystompboxes.com/smfforum/index.php?topic=84318.msg702768#msg702768

I'm sure there is another Excel file somewhere in the forum about it and IIRC, with more options, have a search.


may help
Ciao

[induction]

Here is an approximation. This is modeled on a log pot with 20% output at a 50% setting. In practice, most log pots are just two linear regions fitted together (as seen in curve 3: "straight line audio taper" in RG's awesome article), so take it with a grain of salt.

As specified, I'm comparing a 100k reverse-wired log pot, a 100k reverse-wired log pot with a 47k between lugs 1 and 2 (effective pot value 32k), and a linear pot:



You can get a better approximation to linear with a smaller resistor (say 28k), but then your effective pot value gets much smaller (20k). You can compensate with a bigger pot to get an identical curve. Here's a 250k log pot with a 68k between lugs 1 and 2 (effective pot value = 54k):


But as I said, the 'log' taper is just an approximation anyway, so there's not much point in tweaking things too much, since the result won't be all that accurately reflected by the sim.

[italianguy63]

Thanks for doing that (graph).  I will have to tinker, because I suppose the Dunlop HotPot is really a W curve anyway (S).

I just need to find a sweet spot to use, I still think I may be able to get it with a trimmer or adjusting the rack....  But as it was 100K Log (or W), the resistance slope was just too high (critical).