MXR M104 more bass please.

[Isak]

Hi guys.

just build it and i very happy with his sound, very crunchy punchy
i was wandering what i need to change to have more bass?
now its just cutting the bass a bit.




thanks in advance,
Isak E.

[Earthscum]

increase the value of C2 to something larger, say .1uF.

[Isak]

thank you for your reply.

i'll try that.
beside C2, do i need to change a resistor or another cap on the way?

[Mark Hammer]

Nope.  C4 is the one.  At max gain, it rolls off at roughly 720 hz. Make it .22uf and you'll get your bass back.

[Earthscum]

Doh... take Mark's advice. It's been awhile, forgot about that, even staring right at it. 

[Isak]

Thanks for the input.

720hz you say...mmm...
I was more thinking about getting the lowers, say from 200Hz to 500Hz.
I'm using it for my x0xb0x (tb303 clone), not for guitar.

Cheers.

[Mark Hammer]

There IS full bass available, but when gain of a non-inverting op-amp is set in the manner used by this circuit, bandwidth at the low end shrinks as gain is increased.  Where gain is set by the feedback resistance, the opposite happens.  That is bandwidth is reduced at the top end as gain is increased (assuming a feedback cap).

With the stock ground-leg cap of .047uf, and min resistance of 4k7 (which is all the resistance seen when the gain pot is set to min resistance for max gain), that works out to 1 / [2 * pi * R * C] = 720.48hz.

The point where the bass rolls off can be altered via either the cap or resistance value.  So, when the gain is set to min (pot resistance = 500k), the same formula works out to a rolloff starting around 6.7hz, which I suspect is low enough for you.  The problem ocurrs as that resistance is reduced.  Though I have absolutely no empricial evidence to support me, my sense is that this arrangement was used because non-humbucker pickups were fairly common when the pedal was originally produced, and this arrangement has the by-product of applying less than full gain to 60hz hum when cranked - something most performers would appreciate.  No point in boosting annoying hum and feeding that to your amp.

Increasing the value of C4 will shift the range of rolloff points produced as the gain control is increased.  Doubling the cap value (a hypothetical .094uf) will move everything down an octave with the rolloff commencing at 360hz at max gain, and 3.35hz at min gain.  Quadrupling C4 (.188uf) shifts things down by yet another octave, to 180hz at max gain and 1.68hz at min gain.  Personally, I don't bother using more than a 100k pot for the gain on that circuit since nothing particularly interesting happens until you start to hit gains of 10x or greater, and with a 100k pot, the lowest rolloff with a .22uf cap will be 7hz.

Of course, the challenge of applying a max gain of 213x to 60hz content still remains.  If that presents a problem, one can simply employ a fixed highpass filter between the clipping diodes and the output.

[Isak]

Thank you for explaining this to me.
I'll replace C4 and see how it sounds, hope I'll like it.

BTW, do you maybe know a pedal that sound similar but dont kill the bass?

[Mark Hammer]

At a certain point they ALL sound the same to these ears, so I'm not the guy to ask.

All the pedals I've seen that use the same approach tend to also sacrifice bass at higher gains.  As I mentioned in passing above, if you set the gain via the feedback resistance (the way that the Tube Screamer, Timmy, and many many others do), you don't lose any bass as gain is increased, but lose treble instead.

[aron]

> At a certain point they ALL sound the same to these ears

Hahahaha. Closer to the truth than you might think!

[Isak]

So what your saying is to change R6 as well?

[Mark Hammer]

Well yes and no.  You are correct that the bass rolloff point is a joint function of C4 and the total resistance of R6 plus the gain pot.  But the more total resistance there is along that "leg", the lower the gain.

In some respects, the ideal way of adjusting gain in a non-inverting op-amp is to adjust BOTH the ground-leg resistance and the feedback resistance simultaneously, so as to produce the least possible change in both bass and treble rolloff points.

Consider....

Lets say that the various components are rearranged in the following manner:
- make the gain pot 1M
- the wiper of the gain pot goes to pin 2 of the op-amp,
- one of the outside lugs is placed in series with a 100k resistor, such that the resistor and that portion of the pot now replace the existing 1M feedback resistor,
- the other leg of the pot goes to a fixed resistor, which goes to the cap and then ground
- a 220k resistor is placed in parallel with the portion of the pot between pin 2 and R6,
- make the feedback cap 100pf and the C4 equivalent .1uf

What we have is an arrangement whereby when you add resistance to the feedback path, you are simultaneously taking it away from the ground leg, and vice versa.   When all the pot resistance is shifted to the ground leg, we'll have 100k in the feedback loop and 180.4k on the ground leg, giving us a gain of a little over 1.5x, a bass rolloff so low you can't hear it (about 9hz), and a treble rolloff just under 16khz.  So, nice and flat.

As we rotate the gain pot in the other direction, we start adding less and less pot resistance in parallel with the 220k, until we have zero ohms in parallel with the 220k and all 1M in series with the 100k.  That gives us a max gain of 197x (not appreciably different from stock value of 213x) with a treble rolloff starting around 1.45khz, and a bass rolloff around 280hz.  So, we've lost a little audible bass (but nothing close to the traditional shift from 6hz to 723hz), and we've lost a fair chunk of treble.  The interesting thing is that we've kept a little of that playing-down-the-bass-to-keep-the-hum-in-check, while at the same time trimming some of the upper harmonic content so that the diode clipping produces more of a smoother sound.  In other words, we've gotten the best of what the Dist+ has to offer, AND what the TS-9 has to offer.  It's not going to sound exactly like a TS-9, of course, but will capture some of the soothng/rounding that occurs as you turn up the gain.  And note that since more bass content is included, the diodes will clip a little more easily, even though  the max gain is a little less.

Individual players may or may not appreciate this sort of tonal change accompanying gain change.  But it's something to ponder.  The nice thing is that you can play with the value of all resistances - parallel and series - to achieve a variety of gain-change function with desired impacts on the bass and treble rolloff points.  The same range of gains could be obtained with less treble rolloff, or less bass rolloff.

[Isak]

Mark,
thank you very much for the explanation, thats help a bit but still a bit confused 
if its not to much to ask....maybe you can drew a scheme of everything youve just said, i was trying to assemble it in my head and by drew but got lost in the mid way 
i thank you in advance for your kind help, i need to learn some more and this is a great way to learn also for the synths matter, VCA's, VCF's etc..
im still trying to do it tho..

cheers.

   

[Electron Tornado]

Great info, Mark.

Isak, you can also try changing C6. The MXR Dist + uses 1uf, the DOD OD250 (which is nearly identical) uses 10uf. I thought 1uf was a little too bright, and 10uf was a little too dark, so when I built a pedal around this circuit I used 4.7uf for C6 (which was juuust right  ).

I know it's tight inside the Dist + enclosure, but if you can find a small enough switch and small enough caps, you can have two different cap values for C4 that are selectable. I did that to add a "fat" switch to my build.

[Isak]

thank

Oohhh ...man forgot to say that for the gain I'm using 100k Lin pot, not 500k rev log.
Does it suppose to change something if I replace it to 500k?

[Electron Tornado]

thank

Oohhh ...man forgot to say that for the gain I'm using 100k Lin pot, not 500k rev log.
Does it suppose to change something if I replace it to 500k?

If R8 is still 1M ohm then, yes, big difference, or you can change R8 to 200k. With a linear pot, you might not like the taper you get. Here are some pages that discuss op amp gain, and might help you with understanding that section of the circuit. After you read those, and just for fun, compare that part of the MXR circuit with the same section of the Proco Rat.

http://www.electronics-tutorials.ws/opamp/opamp_3.html

http://www.wisc-online.com/Objects/ViewObject.aspx?ID=SSE7906

http://www.technologystudent.com/elec1/opamp2.htm

http://www.eecs.tufts.edu/~dsculley/tutorial/opamps/opamps2.html

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar.html#c3

http://www.radio-electronics.com/info/circuits/opamp_basics/operational-amplifier-gain.php

[Bill Mountain]

I have 2 ways I like to increase the bass in this circuit.

1. Since this is in a non-inverting configuration there will always be a unity gain clean signal mixed in with the boosted signal before the clipping diodes.  The gain control doesn't cut bass as much as it boosts mids and highs.  So...if you lower the gain in the D+ but and increase the gain going into it (maybe with a booster) then you'll have more of your full range signal being clipped.  I like this because boosting too much bass makes it farty sounding.

2. My other method for increasing bass is to put a series capacitor to ground with the diodes.  This creates a low pass filter with the 10k and the cap (I like a 1 uf) and any lows over the diode threshold will not get clipped by the diodes.  I did this as a mod on my D+ clone (among a few other things) and my friends really liked it for use with bass.  For guitar I would try lower value series caps.

Good luck!

[Isak]

thanks for the inputs guys

ok, now for the results..
replacing C4 to 220nF was to good, to much bass for my ears, so i tried 100nF which was great sounding bass for me.
after that i replaced the 2 red leds to germanium diodes, that helped to have smoother overdrive, the Red leds was to harsh sounding overdrive.
i replaced R8 to 200k cause of the 100k lin pot as mention above but the gain was to weak for me, so i tried few values which ended up using the same 1M resistor as the original scheme says, that sound the best for my ears.

thank you for your help guys, it was very interesting for me.
the world of overdrive is very versatile and fun.

keep up rocking and pumping 
cheers,
Isak E.

[Gus]

here is a idea for circuits like this


You can increase the cap value in the - leg and then use the 100k potentiometer to adjust the high pass frequency.  It has an input buffer and also has a X2 gain stage(the gain can be adjusted).  The X2 stage is to help if you use Ge diodes and want more volume.

You can also place other distortion fuzz etc circuits in the "middle"

There should be a 100 ohm or more series resistor after the input buffer The schematic is more for ideas however it might work well for a dist + like circuit

[Isak]

Hi guys.

sorry to brought this old thread again, i need your advice please.
i use this overdrive for sometime now and not for guitar, for synthesizers.
ive notice that if the source signal of the synth have very fat low frequencies the result sound at the output is very muddy, very chocked.
the moment i take those low frequencies off its sounds amazing.
till now i used it only with x0xb0x/TB303, that was really really good cause when i turn the reso pot all the way up the bass goes low and sounds amazing.
if i want to use it say....with old moog synths, old SH101 etc it get this chock.
i will really love to know how get the M104+ amazing overdrive sound and get the low freq of my synths.
my thought was maybe to make a high pass filter at the input and make more bass internally, what do you think guys?
any help will be appreciated.

cheers,
Isak