Sensible input-output caps values

Started by AM, September 28, 2008, 08:23:15 PM

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AM

I'm working on an overdrive/fuzz circuit which should also provide some full frequency boost when needed. I've been studying various designs and I see various choices for input and output caps.
I want something to keep all the guitar freequencies intact without accentuating the bass frequencies. I've read somewhere that 0.1 uF should do that and everything above that is a bit of an overload. One thing I observe though is that a lot of pedals
(fuzzes)  have way higher values at the input and then cut bass at the output. I understand that. On the other hand a lot of overdrives seem to do the opposite. Tubescreamers, Anderton Fuzz/Llama, etc use 10 uF as output caps. Isn't that a bit too much? If there is some compensation for previous bass cut I understand the need for going above the 0.1 uF value but souldn't a 1uF output cap be enough on those circuits?

R.G.

You're missing an important concept. It is not possible to define the input or output frequency rolloff point by just picking a capacitor. You must also know the impedance driven by the capacitor to some degree at least.

The frequency rolloff of a capacitor driving a resistor is easy to calculate. It's just
F = 1/ (2*pi*R*C)
or, the frequency is one divided by the product of two, Pi, the resistance and the capacitance. If you don't know what the input resistance of your effect is (for example) you cannot correctly set a capacitance to give a specific frequency.

I suggest you read "The Technology of the Tube Screamer" for some looks at how capacitor/resistor rolloffs affect frequency response. It's at GEO,
http://www.geofex.com

With that as background:
Quote from: AM on September 28, 2008, 08:23:15 PM
I'm working on an overdrive/fuzz circuit which should also provide some full frequency boost when needed. I've been studying various designs and I see various choices for input and output caps.
With all due respect, you really need to be putting some work into understanding what you're doing first. Not knowing how to set R/C rolloffs means that you can at best be easter-egging in parts in hopes of it sounding good. It also implies that you don't understand the design process for the rest of it. 
Quote
I want something to keep all the guitar freequencies intact without accentuating the bass frequencies.
No problem - standard tuning has low E at 82.4Hz. Drop D goes down to 73.4Hz. Low E on a bass is 41.2Hz. However, it is not possible to keep guitar frequencies intact while accentuating bass. R-C filters are passive and can only introduce a loss. So if you keep all guitar frequencies, the best you can ever do is keep the bass.

Or did you mean attenuate bass, not accentuate??

QuoteI've read somewhere that 0.1 uF should do that and everything above that is a bit of an overload.
Either the person who wrote that knows nothing about RC filters either, or there was a bigger context that you missed. As I said, it takes a capacitance AND a resistance to define a rolloff frequency. Saying that a 0.1uF capacitor is big enough and everything above that is an overload is like saying that one foot (304.8mm) is big enough for a box to put things in and anything bigger is a waste. It means nothing unless you know what things you're talking about and how big the things are.

QuoteOne thing I observe though is that a lot of pedals (fuzzes)  have way higher values at the input and then cut bass at the output. I understand that.
Again, the capacitor value by itself means nothing. You have to know what it drives.

QuoteOn the other hand a lot of overdrives seem to do the opposite. Tubescreamers, Anderton Fuzz/Llama, etc use 10 uF as output caps. Isn't that a bit too much?
No way to tell. And then there are other things that enter into it. The function of an output cap is to pass all the signal from the circuit without letting any DC through. An electrolytic cap of 10uF may be cheaper than a 0.1uF film cap. The designer may have put it in there to be inexpensive and smaller. It cannnot pass any more bass than the input feeds it, so the exact value may not matter. Or it may be deliberately big so it can drive low input impedances on whatever it drives. Without knowing the context, it is impossible to say that 10uf is too big, too little, or just enough.

QuoteIf there is some compensation for previous bass cut
That is not possible. Once bass is cut, it can't be put back in, at least not the way you're describing.

QuoteI understand the need for going above the 0.1 uF value but souldn't a 1uF output cap be enough on those circuits?
There is no way to tell unless you know that is being driven on the output.

Or you could play it safe and put a big cap on, figuring that 10uF will drive an input resistance of
R = 1/(2*pi*F*C) = 1/(2*pi*82.4*10E-6) = 193 ohms down to 82.4Hz, so it's good for anything you hook onto it, including a 600 ohm PA input.

A 1uF cap would only offer a rolloff at 1930 ohms, and would show bass loss on guitar into a 600 ohm input.

A 0.1uF cap would give you response down to R = 1/(2*pi*82.4*0.1E-6) = 19.3k ohms.

The resistor to be driven matters.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

AM

#2
Thanks for the reply. It made things a lot clearer. I'm reading your T:S article right now and I hope it will help further.

Quote from: R.G. on September 28, 2008, 09:59:50 PM
With all due respect, you really need to be putting some work into understanding what you're doing first. Not knowing how to set R/C rolloffs means that you can at best be easter-egging in parts in hopes of it sounding good. It also implies that you don't understand the design process for the rest of it. 

No offence taken...That's why I asked about the caps at the first place. I have always managed to make my overdrives/fuzzes sounding the way I want at the end but it  involved a lot of trial and error. I feel I could save some of this time by starting to understand some concepts so I can predict better.

Quote from: R.G. on September 28, 2008, 09:59:50 PM
However, it is not possible to keep guitar frequencies intact while accentuating bass. R-C filters are passive and can only introduce a loss. So if you keep all guitar frequencies, the best you can ever do is keep the bass.
Or did you mean attenuate bass, not accentuate??

I said keeping the frequencies intact without accentuating the bass. What I was refering to was that output cap. I was thinking that it might add some extra bass at the end. You answered that later in your reply for me when you said that it can't add anything more to what it has been fed to it by the input.

Thanks for all the info R.G. You have actually helped me understanding a few more things apart from my initial question (like the importance of the output cap on driving low input impedances etc). I'll continue my reading on the T.S. topic and see how it goes.

R.G.

OK, I understand.

No, a passive filter, one which does not have amplifiers in it, can't ever add more signal power of one frequency or another. It can only subtract. The classic guitar amp tone stack is an example. It appears to boost treble or bass depending on where the knobs are set. In reality, boosting treble (for example) is actually cutting everything but treble, then later amplifying the result back up. It's the combination of passive filter plus amplifier that is a selective boost.

One situation which looks like a passive filter is amplifying is an L-C filter that is ringing. An LC filter will store signal energy in the inductor and cap and there will be a bigger signal at the resonant frequency. But that is actually the stored energy of all the signal which has already passed through the filter being kept and bled out slowly. If you look at it with a high impedance (non-loading) probe, you see the stored energy wiggling around in there. As the LC-filter is loaded, the resonance starts disappearing.

Don't hesitate to yell if you run into questions.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

Mark Hammer

It is also worth considering that many beginning builders' conception of how these filters act can be a little unrealistic.  Note that the single-pole filtering action of any input and output caps provides 6db/octave rolloff.  It is NOT any kind of "brick wall".  There is still plenty of bass coming in or out.  Same goes for treble rolloffs.  One needs to approach such filtering action in terms of the whole circuit (including what it connects to), and not just specific points.

The MXR Distortion+ provides an excellent example.  At low gain settings, the circuit lets through a reasonable amount of bass.  As you turn the gain up, though, the nature of the circuit results in the bass rolloff moving up and up until such time as it reaches 723hz at max gain (although also at a 6db/octave rolloff).  Clearly, whatever the input or output caps do or don't do is entirely overridden by what the gain control and its associated cap do.

It becomes a bit like tracking the cost of a vehicle.  Owning and operating a car is much more complex than the mere sticker price.  There are taxes, gas, maintenance, insurance, resale value, etc.  There may be many aspects of the whole circuit that affect the overall bandwidth at the output jack.  So, for instance, there may be plenty of treble filtering distributed throughout, but if the circuit is intended to increase the harmonic content, then the resulting audible output may well have more treble than the input even though it seems like steps have been taken to reduce it.


R.G.

Oh, sure, Mark - consider ALL of the factors involved!  :icon_biggrin:

You're absolutely correct - as always. A lot can happen inside the pedal that changes the auditory result.

Another you didn't mention is even subtler - Fletcher-Munson. Just the change in loudness from bedroom to gig levels will change the tone we hear.

I was doing my normal divide-and-conquer thing - know all the bits and then add them up. That "adding them up" is a daunting task sometimes; a look at it from the top is a good thing to do as well.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

newfish

I may well be barking up the wrong postman here,

...surely you can predict (calculate) the rolloff frequency more accurately if you use a pull down resistor to effectively 'set' the input impedance?

If you know that your pull down resistor is 1M, this *should* then help with the maths...

This bit of 'calculating stuff' is all a bit shiny and new to be - I've been lucky so far, but can feel the need to learn more just around the corner.
Happiness is a warm etchant bath.

R.G.

Quote from: newfish on September 30, 2008, 08:57:47 AM
I may well be barking up the wrong postman here,
...surely you can predict (calculate) the rolloff frequency more accurately if you use a pull down resistor to effectively 'set' the input impedance?
If you know that your pull down resistor is 1M, this *should* then help with the maths...
That's a good intuitive grasp of electronic process - if you can't figure out what it is, stick in something that forces it to be more predictable. It's a very, very useful dodge in many situations.

Just not this one. What you stick in has to make what's already there not matter. Let's take a few cases. An older effect may not have paid attention to high impedance and could have an input impedance as low as 10K - 100K, both enough to cause big treble sucking. If you add 1M in parallel with such low impedances, it has almost no effect. If the input impedance is already 1M, then a 1M pulldown lowers it by a factor of two. Only if the input impedance is in the 10M range does the 1M pulldown dominate the input impedance and make the math easy. That's one reason 1M is a nominal value for pulldowns. It almost never hurts by making treble sucking worse.

Don't forget your approach though. If we had to worry about series impedance, putting a resistor in series, not parallel, a 1M would usually swamp any value of 100K or lower.

Quote
This bit of 'calculating stuff' is all a bit shiny and new to be - I've been lucky so far, but can feel the need to learn more just around the corner.
Yes - it's coming quickly. It was not apparent to me in school, but at some point you can't understand what is really happening without knowing the math. Math isn't just "can I get the right number?", it's often a way to visualize what is happening.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

Gus

http://www.aronnelson.com/gallery/main.php/v/gus/plus.GIF.html?g2_imageViewsIndex=1

Is an idea for more high pass input control of a circuit like D+.  The schematic opens big for some reason.  Someone redrew it here I did not find the redraw yet.

Like R.G. and Mark posted it is all about the details. Volume level and where the high pass and low pass or bandpass or peaking/cutting filters are placed. 

What the above link is about is allowing a controlled input high pass to the distortion stage it also has a make up gain stage if one used ge diodes.

Circuits like the FF one needs to adjust input caps because of how it interfaces with a passive guitar/bass etc.  Other effects like say a TS the buffers remove the interaction and filters buffered so you have greater control.

The boss(yellow SD?) sounds good stock at louder stage type volumes so if someone compares it to a TS at bedroom volume it might be overlooked.  Don't forget the volume you want to use the effect at that changes the filtering you might want.

IMO if you can control the filtering using buffers before and after the distortion etc. you can make the buffers wider bandwidth than you need and then control the circuit inside the buffers.



AM

Quote from: Gus on September 30, 2008, 10:49:01 AM

IMO if you can control the filtering using buffers before and after the distortion etc. you can make the buffers wider bandwidth than you need and then control the circuit inside the buffers.


Question: Isn't the buffer gonna limit the control one has over the distortion by using the guitar's volume knob?
My fuzz faces sound different when there is a buffer in front of them. The circuit I'm working is based on the 4049 chip. Since I always liked my Red Llamas I'm building something new based on that circuit. One of the things I like the most about Fuzzface-type and Llama-like circuits is how they clean up with guitar's volume knob. I literally always live them on and just get everything between little crunch to fuzz by just rolling my guitar's volume between 6 and 10. OK, I have tailored the circuits to fit my rig but I've always managed so far to arrive at that point even with my very limited knowledge of electronics(just tweaking by ear).
Unfortunately I've given my previous three Llamas to friends and I can't do the same test with the buffer in front of them as I did with the FF. But if the buffer in front of it diminishes that property it will be a problem...

Gus

AM

I posted

"Circuits like the FF one needs to adjust input caps because of how it interfaces with a passive guitar/bass etc.  Other effects like say a TS the buffers remove the interaction and filters buffered so you have greater control."

In the FF case the clean up is due to how the guitar is part of the feedback network.  In other causes it is how the well the gain stage is setup.




AM

Oooops...sorry, I got into trying to analyse the schem and overlooked that sentence. The greater control you're talking about in the case of pedals which have a buffer before the distortion stages comes from adjusting the gain through the gain pot of the pedal. The whole clean up thing comes from lowering the pedal's gain control than just rolling back your guitar's volume knob. So in other words, once you put a buffer between guitar's volume  control and pedal's gain stages the gain knob of the pedal takes over the volume control of the guitar to a great extend, right?