R-C filters in the DOD250

[philipneri]

Hello, I'm new here.  I don't actually play the guitar nor do I build stompboxes.  I just have a general interest in electronics.

http://www.rabbathrecordings.com/DOD250.htm

According to this website, raising the Input Cap C1 would make the signal bassier.  Is this because the Input Cap C1 and the Bias Resistor R4 form a high pass filter?  So the stock values of .01u and 470k roll off at 33.9 Hz.  And if we raise the Input Cap to .022u, we would get a roll off frequency at 15.4 Hz.  Or am I all wrong about the R-C filter? 

Similarly, do C3, R6, and R7 form a variable high pass filter?

Lastly, do R8 and C5 form a low pass filter?  If so, how do the clipping diodes affect the cutoff frequency of the filter?

I'm sorry if I'm totally off base on all of this.  All my knowledge concerning R-C filters come from WikiPedia.

[Mark Hammer]

Kudos to you for recognizing the limits of your knowledge.   That's a good first step in any endeavour.

While in many circumstances, input capacitors DO play a role in limiting how much bass content can "enter" a circuit (ultimately constraining how much can "leave" the circuit), a great many effects pedals are generally "overdesigned" to conserve bass, at their inputs and outputs, simply because you never have any idea how many pedals will be placed in series.  So, they generally have a bass rolloff that is much lower than needed at their input, just so that the final bass rolloff at the end of 8-10 pedals is not too severe or tone-robbing.

In this particular circumstance, this pedal has decent bandwidth at the low end when the gain is set low, but rolls off (cuts out) a great deal of bass when the gain is turned up.  The culprit here is C3.  Note that this design uses a non-inverting configuration of the op-amp chip.  In such a configuration, the gain of the chip is set by how much larger the feedback resistance (R5 in this instance) is in comparison to the resistance between the inverting pin and ground.  When the Gain pot is up to maximum resistance (500k), we have 1M in the feedback path, and 504.7 going to ground.  The gain is = (1M+504.7k)/504.7k or roughly 3x.  As the Gain pot resistance is decreased, you can see that both the numerator and the denominator in our fraction gets smaller, but since the 1M stays the same, it's the denominator that changes more.

So, making the pot resistance smaller creates more gain.  BUT.....

Is that gain applied equally across the entire audio spectrum?  No.  C3 determines where that gain gets applied more and where less.  If we calculate 1 / [2 * pi * R * C], we find out where in the audio spectrum that gain gets applied.  If the gain pot is set to maximum resistance (i.e., minimum gain), then our gain is applied equally to all content above 1/[2*pi*.5047M*.047uf], which works out to 6.8hz, or as they refer to it in the textbooks, pretty frickin' low.  As R7 is made lower in resistance, the gain is increased, but the resistance in the formula gets smaller, and as the denominator gets smaller, the result gets larger.  So, with R7 = 100k, our bass rolloff begins around  32hz, still not too bad.  At R7 = 50k (gain of 28x), the rolloff moves up to 62hz.  At R7 = 10k (gain = 69x), the rolloff starts around 230hz.  Finally, at R7 = 0 ohms, the rolloff starts around 720hz.

When I say "rolloff starts", I mean that whatever the gain is set to, it is applied equally and fairly to all content above that corner frequency (or thereabouts).  Below that frequency, there is 6db less gain applied for every octave below.  So, at max gain, content at 360hz has 6db less gain applied, and at 180hz, there is 12db less gain applied.  the net effect, even though gain IS applied, even to the bass content, is that it sounds like you have amplified everything, and then stuck a bass cut filter on the end of it.

So, in this context, neither the input nor the output capacitor do nearly as much bass chopping as C3 can do, and does do, when the gain is increased.

[Joe Hart]

Hey! That's my website! Yay!!

I hope it was helpful!!

-Joe Hart

[Steve Mavronis]

Hey! That's my website! Yay!! I hope it was helpful!!

Excellent page. It helped me out a lot too. My knowledge is also limited but this stuff is growing on me. Thanks Joe (and Mark).

[philipneri]

Thank you for the responses.

@Mark Hammer - I've read several of your posts on this board and they are always so enlightening.  Thank you.  I actually learned the f=1/2*(pi*R*C ) formula yesterday from one of your pedal designs (http://hammer.ampage.org/files/The_Crank.gif).  I really like that schematic for The Crank.  It's a very clever design, and you explain what's going in the GIF very well.

@Joe Hart - Your website is wonderful.  I've read it several times.  It taught me what a voltage divider was and how to calculate the gain of an op amp.  And probably a ton of other things too.

[PRR]

> According to this website, raising the Input Cap C1 would make the signal bassier.

The point he pushes is that a -smaller- C1 cuts bass. This is very often desirable. Big distortion in big amps, you drown in muddy boom. Hence the suggestion for 0.001uFd and ~~300Hz bass-droop. All the "melody", "voice", and most harmonic structure is midrange, not bass.

That's a lot of why the C3 R6 R7 network is, as you say, a variable boost-versus-bass control. At low gain you get extended bass, at high gain the bass is not boosted and the midrange sings out clearer.

Saying that a bigger C1 is "bassier" would be true for bassier instruments than guitar. But standard guitar only goes to 82Hz. Using the 0.01uFd 480K 34hz network, the loss at 82Hz is less than 1dB, and essentially negligible (notwithstanding Mark's point that a long chain of negligible cuts can add-up to a big cut). Even using drop-tuning, the 34Hz is not significantly nicking your bottom (not near as much as your speakers). Going to 0.1u for 3Hz response is not going to be "lots bassier".

For bass, tuba, pipe-organ, other low-octave instruments, you might go 0.02u+470K to be sure you don't shave your bottom note.

In recording systems (or outsized pedalboards), you might aim lower so a long-chain of slight bass-cuts doesn't shave your bottom out. Smaller consoles use a dozen or so ~~2Hz coupling networks, and large systems often aim well below 1Hz, sometimes with a very exact steep 50Hz at the end to keep useless bass-garbage off the disk or broadcast.