Mods for More Bass Friendly Pedals

[rocket8810]

I figured I'm not the only bassist here, and thought it would be helpful to begin compiling information in one thread specifically about this sort of modification and those that revolve around it. After spending a lot of time searching the forum for information of on modifications to circuits to help them retain the low end, and make them more bass friendly, but have not found really any guidelines values or why to make some the the changes.

What I've found so far was
1. Changing the input cap to a higher value allows more bass into the circuit - good starting point value?
2. Changing the output cap to a higher value allows more bass out of the circuit - good starting point value?
3. Alter low and high pass filters in the circuit to change what frequencies cut off or pass in drive circuit - how to identify?
4. Alter coupling caps - have not found why (the act like input caps to the next stage of the circuit?) - good starting point value?

I'm still on my way out of beginnerhood, thanks in part to R.G. and Paul, as well as many others on here, not to mention a few good books, but when it comes to these things I still feel like its mostly trial and error. I don't want to just be like a blind squirrel, who occasionally finds a nut. There has to be a starting point and go from there. There are tons of great pedals that I would love to use on bass, but they have to be moded to work correctly, and I would rather spend more time playing and enjoying the pedal rather then wasting hours feeling like I'm just randomly changing values. BTW, I figure this should not just be about what to do, but why to do it. Not all pedals are the same, but there should be a method used to approach every one, hence the idea of guidelines.

[GibsonGM]

For 1 and 2, try doubling the value of what's there (ie, a .1 cap would become .22...)  Just as a starting point - you'd have to play around to find the right value for YOUR tone...this is a general rule of thumb when going from guitar to bass, by the way...

3.  Check out the topology of basic HPF and LPF's.  You can definitely identify them on a schematic, and with some poking around you can find them on a board.  After all - caps in the signal path only do a couple of things - coupling caps to block DC between stages as well as add/subtract bass,  or HPF/LPF.  There are a couple of other cases you may see, but generally speaking, they're either blocking DC or filtering...or both....leads into:

4. Coupling caps work with the output impedance of the stage prior and the input impedance of the following stage.  They isolate the DC bias voltages from each stage, and are useful to play with tone.  I'd use caution when increasing them, as you can end up with some odd effects (blocking distortion) in tube circuits.  Not so much with opamps etc., but there IS the possibility of making things all muddy - especially if you're then overdriving/clipping.   You can always switch them back to a lower value if things get muddy, however )   Small values are actually GOOD, in that they promote tightness and crispness and avoid flabby, flubby crud.   

Sort of a balancing act is going on, as you can see. You have to experiment to find the best place in the circuit to increase the amount of bass. That may or may NOT be at the input...could be after the third gain stage...or the output..or some combination of them.   But this should get you in the neighborhood!

What really is in play with most of this is the RC rolloff frequency...do some reading and you'll see why 'double the cap' makes sense.

https://en.wikipedia.org/wiki/Roll-off

[Gus]

First try the stock guitar values
Question what you read on the web.  Making the input and output caps values larger can cause issues.
The bass strings can have more string metal moving over the pickups
Lower strings are larger in diameter
So a bass has more output voltage with the lower notes
Sometimes I reduce the input cap value for bass this is to keep the lower bass notes "tighter"

A passive bass can have a more output than a guitar
A larger output at lower frequency can make thing "muddy"
A larger input cap value can make things "muddier"

Sometimes a stock TS can sound good live on stage with bass

[rocket8810]

Ahh GibsonGM, it always goes back to the dreaded RC filter. R.G. suggested I get a specific book on filters and I'm almost done with it. I'm glad to finally hear more about the reasoning behind certain things, as opposed to just hearing "do this, do this, etc."

Gus, by having a small input cap doesn't that prevent some of the bass frequencies from entering the circuit?

Now if you keep an input cap relatively small, as i understand it limits frequencies entering the circuit, wouldn't changing the output cap not do much since specific frequencies have already been limited from entering the circuit? Or am I misunderstanding something?

[amptramp]

Here is the quick way to do frequency calculations once you learn the basics:

http://www.muzique.com/schem/filter.htm

[GibsonGM]

Ahh GibsonGM, it always goes back to the dreaded RC filter. R.G. suggested I get a specific book on filters and I'm almost done with it. I'm glad to finally hear more about the reasoning behind certain things, as opposed to just hearing "do this, do this, etc."

Gus, by having a small input cap doesn't that prevent some of the bass frequencies from entering the circuit?

Now if you keep an input cap relatively small, as i understand it limits frequencies entering the circuit, wouldn't changing the output cap not do much since specific frequencies have already been limited from entering the circuit? Or am I misunderstanding something?

Good questions, and good input happening here, too!

First-order RC filters aren't THAT tight, so to speak...that's why I suggested doubling the input cap value.  In reality, you'll probably find "1.5x the original value" or something might do the trick better.   Gus raises a large specter, that of the much larger (and variable) input levels a bass can put out.   I think it you tried to mod an existing guitar pedal for bass, you'd either get really lucky on a simple one, or have a very tough time, since you'd be changing caps more than you might think!   One thing is always true: you HAVE to keep a handle on the bass or you'll end up with a muddy sound, and those frequencies can do a lot to mess up the normal operation of gain circuits.     Not as much low end is needed as one may think!

So yes, a smaller input cap = less bass frequencies (less of what that bass CAN put out), but also MORE low end in the 'guitar range', so to speak.  You will still have a perceived increase of bass, in other words.    You may adjust the cap value to suit, or not, depending on what it sounds like.   This isn't much of a science, sorry!  You have to experiment and find what works for YOUR sound, with YOUR pedals, amp, and bass...


Output cap:  depending on its value, it may be trimming some bass at the output, also.   I personally don't often calculate the rolloffs very often unless I'm building from scratch; I just try some spread out values to see what it sounds like.    Mess around with that AMZ calculator, or even better, sim up a gain stage and play with in/out cap values while doing a sweep to see the frequency response.....a pattern starts to emerge.

The 'art', if you will, is in finding the RIGHT complementary filters for what you want to do.

Increased bass an LOW MIDS in a clean boost may be fine for you.  If the circuit is to get dirty, the added bass may sound like junk.   

[Gus]

A larger output at lower frequency can make thing "muddy"
should have been written as
A larger bass output at lower frequency can make thing "muddy"

RC filters have a slope, sometimes they are first order high pass at the input
some effects that are built by someone thinking about input levels are designed for a certain gain range and input level

Also what sounds good in your house can or will not sound good on stage
and what effect or amp sounds good on stage or in a studio mix might not sound good at home at lower volumes
you need to think about how it will be used.
it is about EQ and how the instruments "share" the frequency range.  If a bass is made very "muddy" it can mask the other instruments but you might want that.

[rocket8810]

Here is the quick way to do frequency calculations once you learn the basics:

http://www.muzique.com/schem/filter.htm

Oh I know all about the calculator, and have used it and it helps a lot. But, I got torn into by R.G. for just using it and not being able to calculate it, so I don't use it as much. But, at the risk of getting torn into by him, I will admit its a great tool and extremely helpful.

Gus, in the big picture you're totally right about how effects sound at home, in a band situation, and live at a venue, i think one of the best example is the big muff. Has totally different personas bast on amp volume and wattage of the amp.

Remember, the goal I had in mind for this thread is to create a resource for modifications specifically to retain the bass for bass players in one source, so keep that in mind. I think using words like "muddy," "warm," "clarity," etc don't let people understand what's happening to the sound, as we all have our own idea of what those sounds mean, so we should be more descriptive in the way we describe what is happening to the frequencies. As R.G. explained to me:

Muddy" and "clarity" are -forgive me- muddy issues. "Muddy" is usually used when there is so much bass that modest distortion causes the effect of covering up much of the high frequency detail. "Clarity" is often taken to be either loss of treble, or having it covered up. However, "warm" is used when people LIKE some loss of treble or boosting of bass. A good designer will avoid thinking of sound in these terms until they go to do their advertising, and instead think of what the frequencies are where the frequency response changes in the audio band, then relate those to the vernacular terms for "bass" "mid" "low-mid" "high-mid" "presence" "treble" "edge" "sheen" "grit" "grak"...

BTW, I do agree GibsonGM, there is a lot of good input going on and I think we should keep it going.