polarized cap for output

[jeff]

 I've been getting into pedals and noticed that some pedals use polarized caps for the output.
Why polarized caps? Why not nonpolar?
 Isn't the signal at the output going positive and negative with respect to ground and therefore a polarized cap would be reversed(wrong) every half cycle?

Seperate question:
Do two polarized caps wired back to back make a nonpolarized cap?
If yes do two 10uF polarized caps make one nonpolar 5uF or one nonpolar 10uF?
Do you wire + to + or - to -?

[HotBluePlates]

I've been getting into pedals and noticed that some pedals use polarized caps for the output.
Why polarized caps? Why not nonpolar?

Most of the time, polarized caps are used because polarized electrolytics pack the most capacitance into the smallest package. Look sometime at the size of a 22uF polypropylene Solen cap next to a 22uF Nichicon electrolytic.

Big capacitance values are needed to get a given frequency response when circuit resistance is small.

Guitar amps using tubes have large circuit resistances, because it suits the tubes and the supply voltages. Transistor/opamp pedals have a 9vdc supply instead of 400v+, and use small circuit resistances. So circuit caps become large values compared to tube circuits. 4700-16000uF filter caps are not unusual.

So a 20-220uF output cap would be too big to fit in the pedal if you used a film cap.

So why not non-polar electrolytics? Regular electrolytics have flaws that are the tradeoff for packing big uF's in a small package. Non-polar electrolytics are only 2 regular e-lytics back-to-back. They have all the flaws of non-polars, x2.

Isn't the signal at the output going positive and negative with respect to ground and therefore a polarized cap would be reversed(wrong) every half cycle?

Your pedal isn't blowing up caps is it?

The output is going positive and negative relative to some median voltage. But that middle-voltage isn't ground. The output end of that output cap is effectively connected to ground through a resistor, so it's at 0v. Since the cap is polarized, the other end must be at some more-positive voltage.

This is usually provided by the natural d.c. level present at the output transistor/FET/opamp. The designers know the cap will never be able to be driven with a signal able to reverse the polarity of the voltage applied to the cap. So, the varying signal voltage just makes the cap see a larger or smaller net-positive voltage at its + lead, as compared to its - lead.

Seperate question:
Do two polarized caps wired back to back make a nonpolarized cap?

Yes. Effectively, you take two polarized caps, connect the + leads together, and wrap up that package. Now, you're left with the two - leads accessible. These leads are now the leads of your non-polar cap.

If yes do two 10uF polarized caps make one nonpolar 5uF or one nonpolar 10uF?

Caps in parallel add their individual capacitances. What I described was caps in series; these have a lower total capacitance than the caps you started with, after the same manner as resistors in parallel. The same formula is used to find the value, but C in place of R.

So two 10uF caps placed back-to-back to make a non-polar cap yield a 5uF non-polar. I have seen a rule for the voltage rating of such a cap, but given the anti-phase nature of the arrangement, I'd give the total package the same voltage rating as the lower of the individual caps.

Example:
You use two 10uF 100v caps to make a non-polar cap. You know this will give you a 5uF composite cap; the voltage rating should be 100v, because the maximum voltage the composite can handle in either direction/polarity is the 100v given by 1 of the caps.

Example 2:
You use a 10uF 100v cap and a 10uF 25v cap to make a non-polar cap. You now get a 5uF 25v non-polar cap. The 10uF 25v cap is the weak link, so you can't exceed its rating or it will pop.

[jeff]

Thanks HBP, as always food for thought.
I'm getting into pedals(trying to make a tube screamer). I think I get you but need to think more about it.

Thanks for the help
   Jeff

[jeff]

OK I got a tube screamer breadboarded and it sounds OK.
If I remove the clipping diodes should I just get a clean boost?
I tried it and am still getting distortion, Am I just overdriving the OP amp? is this OK or am I hurting something by just removing them?(actually doesn't sound that bad)

Also I was thinking of messing around with soft/hard clipping. For hard clip would you connect the diodes from pin 1 to ground or to the +4.5V supply?

Thanks again for all the help
  Jeff

[HotBluePlates]

OK I got a tube screamer breadboarded and it sounds OK.
If I remove the clipping diodes should I just get a clean boost?
I tried it and am still getting distortion, Am I just overdriving the OP amp? is this OK or am I hurting something by just removing them?(actually doesn't sound that bad)

Also I was thinking of messing around with soft/hard clipping. For hard clip would you connect the diodes from pin 1 to ground or to the +4.5V supply?

Thanks again for all the help
  Jeff

I'm not really a pedal genius...

But did you notice the cap across the anti-phase diodes? That rounds the edges of the clipping. Remove that and you get squarewave clipping. Increase the cap enough, and you filter out more and more of the harmonics generated by clipping, though you'll probably not get "completely clean".

[jeff]

Thanks. Any thought on the hard clip doides? Ground or 4.5V.

I don't know much about transistors/opamps etc.
I think this is how it works but not sure.
 Normally you use +4.5V, 0V, -4.5V but we're "fooling" it by using +9V, +4.5V, 0V and referencing the input to 4.5V so it "sees" 9V as 4.5V, 4.5V as 0V(ground), and 0V as -4.5V. Or is this completely wrong?

 So is it correct to ground the hard clip diodes to what it thinks is ground(4.5V, what the input is refernced to) or actually ground them?

[thelonious]

So is it correct to ground the hard clip diodes to what it thinks is ground(4.5V, what the input is refernced to) or actually ground them?

If HBP says he is "not a pedal genius," then my pedal IQ must be lower than my age. ;) But I will give this a shot.

In a pedal where there is virtual ground at 4.5v (Vref? Vbias? whatever is the more appropriate term), I have often seen hard clipping diodes connected to that, rather than ground. It depends on where they are in the circuit and whether DC is present at that point. For instance, see the RAT schem I attached below. I think the reason the RAT clipping diodes can be connected to ground there is because the 4.7u cap after the op amp blocks DC. So if the diodes are in a location where there is no DC present, connecting them to ground is probably ok. Otherwise, connect them to Vbias, OR add a coupling capacitor in series with the diodes so that they are not conducting DC. BTW, changing the value of that coupling capacitor can yield interesting tonal results.

The other thing to consider is that the signal voltage has to be high enough to clip the diodes at the point where they are connected, and you have to choose diodes with the appropriate forward voltage. A germanium might get slammed. An LED might not clip. You'll have to experiment with what kind of diodes to use, and/or with the value of the resistor in series with the diodes (if any).

BTW, have you read http://www.geofex.com/Article_Folders/TStech/tsxtech.htm? It's a really interesting explanation of what is going on in the tubescreamer circuit. The mods section near the bottom has some good info about the clipping diodes, especially the idea about substituting a different type of diode for only one of the tubescreamer's feedback loop diodes so that it introduces some asymmetry.

Hope that is helpful rather than confusing! I am definitely early in my learning process with this stuff, too.

[thelonious]

Also: there is a super helpful explanation by JKowalski here: http://www.diystompboxes.com/smfforum/index.php?topic=76621.0 . It's a little over halfway down the page.

[jeff]

Thanks for the info and the help.
 
 I tried it to the 4.5V and it sounds pretty good.
Going to ground caused the LEDs to stay lit because, well, they had 4.5V on 'em. Maybe like you said, after a cap, where there's no DC wouldn't be a problem, but they seem to be good where they are now.
It's kinda cool using LEDs because they light in rhythm while you play, sound good too. Not super distortion, but not bad for a couple bucks at radio shack.

          Thanks
Now to build a DR Quack!
           Jeff

[jeff]

I not sure about this but is that even how it works?
To my understanding the diodes just clip the part of the signal that is high enough to "turn on" the diodes. So if you use one pedal to clip then send it to another pedal is seem to me either the first pedal is doing the clipping or the second pedal is.
Maybe multiple pedals work because the first pedals output level is greater than the level of a straight guitar, and with that higher output plugged into the second pedal it can clip a higher % of the wave, weither or not the input signal is clipped to begin with.

Know what I mean, is there a point of having more that two clipping diodes? What's the difference if the first pedal clips the top 1/4 of the wave if the second is clipping the top 1/2 anyway?

So maybe I don't even have to build two whole pedal circuits but just a boost in front of the distortion pedal so the signal is bigger and more of it gets clipped.

[thelonious]

Know what I mean, is there a point of having more that two clipping diodes? What's the difference if the first pedal clips the top 1/4 of the wave if the second is clipping the top 1/2 anyway?

If you're talking purely about clipping, I think that's right. But part of the magic (or disaster, depending on the result!) of having more than one fuzz together would be about how the input and output impedances interact, too, I think. That's something that just a boost/buff wouldn't do, unless you planned it that way.

[jeff]

But part of the magic (or disaster, depending on the result!) of having more than one fuzz together would be about how the input and output impedances interact, too, I think.

Could you explain what you mean more?

[thelonious]

Could you explain what you mean more?

Aw man. Trying to understand impedance issues makes me feel like a dummy. I'm not going to be able to explain the technical details because I don't get them myself yet.

Smackoj was talking about stacking fuzz pedals. They're infamously finicky about what goes before and after them in the signal chain. One of the things people love about them is how interactive they are with the guitar's volume control, type of pickups, etc., but they are also really interactive with other pedals. They have a low input impedance that heavily loads pickups and other pedals. Put a fuzz in one part of the signal chain and it sounds amazing, ala Hendrix, but put it in another part and you have a muddy, attenuated, indistinct mess. I tried one fuzz with my G&L ASAT with single coils, and it sounded mush because it loaded it too heavily. I put a buffer in front of it to see what would happen, and it instantly got harsh, because it was designed to interact with the output impedance of a guitar pickup, not the buffer. I built a second fuzz, and that one fit the guitar perfectly, but as soon as I kicked in my bluesbreaker clone after it, it sound wretched because of some impedance mismatch.

With other types of overdrive or distortion, it seems to be far less of an issue because of the high input impedance of the op amps.

If you want to learn more about it, check out http://www.diystompboxes.com/smfforum/index.php?topic=14019 . I'm still trying to digest that thread...

[jojokeo]

Overdrives and many distortion pedals get along together, some better than others for various reasons. Fuzz pedals don't get along well w/ others for a reason - hard square wave clipping leading to intermodulation distortion.
Intermodulation distortion is the bane of fuzzbox technology. If you take any two pure sine waves and intermodulate them, you get not only the two sine waves, but two other frequencies - the sum of the two originals, and the difference. If you had A-440 and A-880, and intermodulated them, you'd get 440, 880, 440 again from the difference, and 1320, which is three times A-440. This would actually sound pretty good.
The bad part about this is that it's the best possible example. If you take A-440 and the note an octave and a third up, 1467 Hz, you get the original notes plus 1026 Hz, and 1907 Hz. These frequencies are NOT related to the original note and sound harsh or out of tune. We're also talking about sine waves. Real world signals always have some harmonics, and you get the sums and difference of all the harmonics, too. Very unmusical.
So the name of the game is to produce harmonic distortion, which makes musically good sounding notes and to minimize intermodulation distortion, which makes un-musical squawks and harsh buzzes. Unfortunately no matter how hard we try, we can never get all harmonic distortion and no intermodulation distortion but we can pick between distortion mechanisms to use the better ones and avoid the worse ones. One way to make mostly harmonic distortion is to not have sharp "square wave" corners. This is much like the tube-type of distortion we all know and love.

[six_eight]

Thanks. Any thought on the hard clip doides? Ground or 4.5V.

I don't know much about transistors/opamps etc.
I think this is how it works but not sure.
 Normally you use +4.5V, 0V, -4.5V but we're "fooling" it by using +9V, +4.5V, 0V and referencing the input to 4.5V so it "sees" 9V as 4.5V, 4.5V as 0V(ground), and 0V as -4.5V. Or is this completely wrong?

 So is it correct to ground the hard clip diodes to what it thinks is ground(4.5V, what the input is refernced to) or actually ground them?

It really doesn't make any difference in sound.  Hard clip diodes that go directly to ground are usually found in circuits that have no other semi-conductors that follow so that the voltage does not need to be re-biased.  Circuits that "hard clip" to virtual ground are usually circuits that have another semi-conductor that follows the clipping.  Take a look at the schematics for the Marshall Shredmaster and Marshall Guv'nor.  Up to the clipping diodes, they are essentially the exact same circuit.  The Guv'nor clips to ground.  After that, the signal goes through a tone stack, volume knob, and out.  The Shredmaster still needs to go through two more op amps after clipping.

Of course there are exceptions to this, so this is not a hard and fast rule.

[six_eight]

OK I got a tube screamer breadboarded and it sounds OK.
If I remove the clipping diodes should I just get a clean boost?
I tried it and am still getting distortion, Am I just overdriving the OP amp? is this OK or am I hurting something by just removing them?(actually doesn't sound that bad)

Also I was thinking of messing around with soft/hard clipping. For hard clip would you connect the diodes from pin 1 to ground or to the +4.5V supply?

Thanks again for all the help
  Jeff

Using hard clipping on the tubescreamer circuit won't do much.  You need a diode with a very small forward voltage, like a 1N34A or 1N60 or some other germanium diode.  You'll hardly hear a difference with 1N914 or 1N4148.  That's why Ibanez/Maxon used 1N60's in the OD855.  It's basically a Tubescreamer with hard clipping.  But notice that they put the active tone control first, and the gain stage second. 

AND!  The diodes are connected to ground even though there is still another semiconductor that follows, but notice that they re-bias it.