Using a polarized capacitor for C10 in Kingslayer 2

[solderfumes]

I've got a Kingslayer 2 on my breadboard right now (sounds great!) and I have a question about C10 in the schematic.  It calls for a 1uF tantalum (the notes in the original Kingslayer document say an electrolytic works fine here, which is what I have), but the positive side of the capacitor is referenced to ground.  Why use a polarized capacitor here?

That said, it does seem to work.  I tried replacing it with a 1uF film capacitor and it sounded more or less the same (well actually the film capacitor does seem to give it a little beefier tone, but I'm not sure I could really put my finger on it nor could I be sure my ears aren't just playing tricks on me).

[copachino]

you got it wrong, positive side its not ground, its connected to R14 and R13,negative side its grounded, i cant tell whats going on there, but looks like a filter

[solderfumes]

Sorry, let me clarify.  The negative side is grounded, yes, but what I mean is that the positive side also has a "resting" (bias?  quiescent?  I don't know the right word here) voltage of 0V DC -- when signal comes in it swings from negative to positive.

It functions as a filter, as you said -- lifting the capacitor darkens the tone greatly.

[Vallhagen]

In many cases (in this too, i guess), the choice of electrolytic or tantalum caps is based on available surface. They can be made physical small for larger capacitance.  More convenient for the design.

You can always replace a tantalum with an electrolytic, and you can also always replace it with a film cap (but NOT always the other way around!!). Just check the voltages for the caps, but its rearely an issue in 9V pedals. There are some discussions however changing cap type will affect the sound, i will not go into that discussion here.

Cheerio

[solderfumes]

Right, but my concern is that the positive lead on the capacitor is not strictly at a higher voltage than the negative.  I don't understand why any polarized capacitor should go here.  In the Sunking it's OK because the bias voltage of the circuit is 4.5v and the waveforms can never go below 0v.  But in the Kingslayer, that's not the case: the DC level of the signal is 0v, and waves travelling through them can have peaks going below 0v.

Re: voltages, in fact the Kingslayer and Sunking gave me my first gotcha moments: I almost started building before I realized my electrolytics were only 16v and I needed >34v for some of the caps in the Sunking

[RobA]

Right, but my concern is that the positive lead on the capacitor is not strictly at a higher voltage than the negative.  I don't understand why any polarized capacitor should go here.  In the Sunking it's OK because the bias voltage of the circuit is 4.5v and the waveforms can never go below 0v.  But in the Kingslayer, that's not the case: the DC level of the signal is 0v, and waves travelling through them can have peaks going below 0v.

Re: voltages, in fact the Kingslayer and Sunking gave me my first gotcha moments: I almost started building before I realized my electrolytics were only 16v and I needed >34v for some of the caps in the Sunking

I'd say that you are correct. I guess that it was done to stay more true to the original. I'd go with a non-polarized  cap in that spot and preferably a film cap if you can get it to fit. I wouldn't put a tantalum in that spot at all, but I guess that's for holding more true to the original too. But, tantalum caps are even more prone to failure from reverse bias.

The difference in the sound you hear with the film cap could just be due to the actual values of the caps altering the frequency response of the filter a bit or it could be due to the nature of the caps and the point that film is a better choice for audio. Unless you are trying to stay true to the original, go with the film. 

[midwayfair]

If space is a concern and you want to use a film cap, use a 100nF and swap R13 and R14, but you should be able to get a 1uF box cap in there no problem.

Tantalum might be more prone to failure from reverse bias, but it has lower ESR so it makes a better filter cap at least in theory, though the resistances we're talking about are probably no worse than the 1% variance of a metal film resistor.

[RobA]

Tantalum might be more prone to failure from reverse bias, but it has lower ESR so it makes a better filter cap at least in theory, though the resistances we're talking about are probably no worse than the 1% variance of a metal film resistor.

That is true, although some of the newer low ESR electrolytics do rival the tantalums in ESR to the point where they are even recommended in some power supply related circuits where the ESR is critical. Still, you'd have to be careful which electro you did use because some of them can have pretty high ESR and could cause a problem in a filter.

If space is a concern and you want to use a film cap, use a 100nF and swap R13 and R14, but you should be able to get a 1uF box cap in there no problem.

That's a good idea too. WIMA has a series of film caps with 2.5mm spacing that get at least up to 100n.

[solderfumes]

OK cool.  I'll actually be building it on perfboard (plus), so space isn't an issue.  I might just go with the film cap, but I'm also going to try tying the negative leg of the electro to -9v, which I think is more in the spirit of the original.

[bordonbert]

C10 is indeed a filter but not the sort it looks like most people are imagining, it is not there to alter the signal in any way.  There are a number of feedback paths around IC1D and this is only one of them.  R14 and C10 make a low pass filter but it's corner frequency is 10Hz!  In the other direction R13 and C10 would give 100Hz.  If you swap R13 and R14 the feedback corner frequency becomes 100Hz, NOT a good thing!  (Doesn't anyone bother to work anything out before advising?)  I think it's basically removing all the signal and only passing the DC level back from IC1B to IC1D to stabilise the setup and perhaps passing some signal forward past the distorting mechanism.  If anyone has a different slant on its task I would be interested to hear it, I may of course be missing something.

Changing the value of this capacitor will have NO good impact on the sound at all unless you go for wildly different values which will affect its intended role.  The lowest note on a guitar is 82Hz so you would need to drop it's value to about 100nF before there is any real change (10Hz -> 100Hz).  Increasing it will only drop the corner frequency lower than 10Hz so it again will make no difference to sound.  Don't even play with it, it's there for a different purpose!

In this position, if you keep things as they stand, you would be best advised to use a non-polarised type if you have it.  You will get away with a tantalum or an electrolytic but running them without a reasonable bias voltage is not best for them.  They get noisy and don't maintain their insulation layer as they should.  (Running them reverse biased is disastrous!)

By all means try to connect C10 -ve to -9V but don't be surprised if it introduces noise or sensitivity to pops and clicks.  The -9V line is only a synthesised line which will carry elements of IC2 switching frequency, (around 45kHz with pin 1 to +9V).  It has limited current capability, (higher impedance than normal), so may also carry relatively high levels of signal related noise too.

One way you could get around this is to create a 'false ground' with 2 unequal resistors across the +9V/-9V lines and a large electrolytic cap from their junction to true ground, (off the top of my head 22k/10k/100uF would be fine).  Let's say you use these values, then the voltage between them will sit at about -3.4V and 10k/100uF has a corner frequency of 0.16Hz so it will be a clean DC voltage.  You can now connect your C10 -ve to this point and it will have a 3.4V DC bias across it but still be effectively attached to ground as far as the signals are concerned by the 100uF capacitor.

The choice of capacitor type is only to do with the DC conditions, you will NOT hear any difference between types at audio frequencies in this application.  You are trying to introduce huge levels of distortion to the signal then saying you can hear an additional <0.1% introduced by a capacitor?  It's a myth!  For the same reason, as long as you take care to not introduce IC clipping you will hear no difference in your choice of IC1, as long as the pinout is the same, (virtually guaranteed), opamps like a TL074 would do exactly the same job and be indistinguishable.  Remember, the opamp is only giving clean gain which is introduced to the distorting mechanism.  Differences between any good ICs are to do with other parameters than audio frequency distortion in this application.  Blind listening tests show that the whole issue of component types is largely nonsense.

[RobA]

Jon didn't mean exchange the positions of the two resistors, at least that's not the way I took it. He meant change the two values in correspondence with the cap value change so that the frequency response of the filter would stay the same.

[RobA]

To be clear about this, my interpretation of that section of the circuit is different. IC1D is a summing amplifier. Pin 13 sits at virtual ground. The side of C2 where this path rejoins will also be at or very close to ground. R13 and C10 form a low pass filter that bypasses some lows around the clipping section to the summing amplifier. R14 sets the level of contribution of this signal in balance with R15 and the more complex network set with R17, etc. So, you can, with some care, change the values of C10, R13, and R14 and get very close to the original sound. The only difference might be a small change in the rolloff slope of the lowpass filter.

[bordonbert]

Hi Rob.  I see what you mean about the swapping of R13/R14, it was the word swap which confused me.  'Adjust' was no doubt what was intended.

And more importantly, of course you are right about the bypassing effect too.  There is no feedback via this path with it being attached to the virtual earth of IC1B.  It would be unable to feed DC back as I stated anyway with series caps in the output path, Duhhh!  I take your point there, it is bypassing the distortion area for low frequencies and feeding clean low end signal forward to IC1B.  The 1k5/1uF is still setting the break point to 106Hz which is about right for just the very bottom end. I wouldn't have thought you would want too much more in this area, it could get muddy very quickly.  But I should point out that the only thing you can change by adjusting these components is the rolloff point, without introducing additional components to increase the order of the filter the slope will remain at 6dB/oct.

Thanks for the corrections, it's difficult to be totally accurate with just a quick glance in a circuit you are unfamiliar with.  I should also apologise for starting on the other aspect, component types.  I noticed Vallhagen was very tactful with his comments regarding this and I've no doubt he has seen this topic flair up before.  Sound engineering principles and practice should be what is offered as advice, not 'common knowledge' and rumour, and that means what can be proven to be true.  The trouble is it isn't really what most amateur pedal builders want to hear.  'Nuff said!

[midwayfair]

Edit: Nevermind, it's not worth it.

[madbean]

A non/bi polar cap is a better choice there and I should have noticed/mentioned that in the doc. I think I was just so used to seeing it a certain way that my brain shut off there. In any case, obviously an tant works but for the reasons mentioned above it is not a preferred choice from a stability perspective. I don't even think it should be a preferred choice in the Klon because the amount of signal feeding through that line is very low compared to the parallel gain stage.

I'll modify the doc to mention the change. Also, I have spent quite a bit of time with different IC's and I am also going to change the recommended one from TLC274 to TLC2264. The 274 has caused a few people some issues and the 2264 sounds great as a sub.