#1
Hello,

I'm planning to make a DOD 250 Overdrive and according to the schematics I need a 25pF capacitor, however I cannot find one online. What could I use instead? And what effect would it have?


Yeah, now you're gonna die wearing that stupid little hat. How does it feel?

Help me to live.


I make custom guitar wiring harnesses and I'm pretty damn good at it!
Last edited by flyingmarlin at Aug 16, 2011,
#3
Quote by Mathamology
If you can find a number of capacitors that would equate to the same value (for example, a 20pF and a 5pF) you can achieve 25pF by running them in parallel.



This is what I was originally thinking, I'm a noob when it comes to electronics haha! I'll try this out. Thanks a lot for you help, very much appreciated!
Yeah, now you're gonna die wearing that stupid little hat. How does it feel?

Help me to live.


I make custom guitar wiring harnesses and I'm pretty damn good at it!
#4
Mouser and Digikey not have them? They should have everything =/
Then there's this band called Slice The Cake...

Bunch of faggots putting random riffs together and calling it "progressive" deathcore.
Stupid name.
Probably picked "for teh lulz"

Mod in UG's Official Gain Whores
#5
From what I can see cap C5 (25pf) in combination with the 1M resistor in parallel across the op-amp provides a low pass response. The cutoff frequency is approx 6.4Khz. This is by f(low pass) = 1 / (2 x pi x R x C)

Changing the cap will change the frequency at which the low pass reponse will begin to roll off using the above formula. So this will affect what you hear mainly in the tone of the guitar.

If you use a larger value cap the cutoff freq will be less than 6.4KHz (more 'bassy'). If you use a smaller value cap the low pass rolloff freq will be higher than 6.4KHz (more treble).

The op amp as a whole is set up as a bandpass filter with a max gain of about 200 when the 500k pot is set to minimum position which effectively produces the distortion.

That's how I read it.
#7
22pF is a more common value and will work just as well. pretty much anything from 10-47pF will work as well. that cap is mostly to stop the circuit from oscillating at high gains.

the thing with caps in fx circuits is that rarely is a cap value critical. just get as close as you can. like with this instance, you arent even going to notice the 3pF difference. plus most caps are 20% tolerance parts, so 22p and 25p have a wide range of overlapping values. low-value caps in an p-amp feedback loop are particularly non-critical.
Last edited by Invader Jim at Aug 16, 2011,
#8
Quote by Invader Jim
22pF is a more common value and will work just as well. pretty much anything from 10-47pF will work as well. that cap is mostly to stop the circuit from oscillating at high gains.


Yep 22pF will work fine.

A 47pF will change the sound significantly out of the DOD though.

The cap forms part of the RC feedback filter for the amp
Last edited by Phoenix V at Aug 16, 2011,
#9
It's a single pole filter, the low value of the cap is exactly what Jim said. If the value were much higher we'd have something to talk about, but as-is it's there to stop oscillations and reduce hiss. To determine the gain at a frequency, you use the 10k and compare that to the paralleled impedance of the 1M resistor and the cap with your op-amp gain formulas. Ignore the 1M for a second, set R=Xc,
10000=1/[2pifC]
f=1/[2piRC]=1/[2pi10000C]
f=1/[20000piC] C=47*10^-12
F=1/[2951600*10^-12]= 1/[2.951600*10^-6] = 338799.30Hz, or 338.8kHz

Even if I am off by a factor of 10 due to using the computer calculator, the -3dB is not of any concern to us. The gain at 338.8kHz will actually be slightly less due to the 1M resistor, but not by any appreciable amount.

If you (or anyone else) is interested in more than the simple single pole filter, check out Don Lancaster's Active Filter Cookbook, absolutely fantastic.
#10
Quote by blandguitar
It's a single pole filter, the low value of the cap is exactly what Jim said. If the value were much higher we'd have something to talk about, but as-is it's there to stop oscillations and reduce hiss. To determine the gain at a frequency, you use the 10k and compare that to the paralleled impedance of the 1M resistor and the cap with your op-amp gain formulas. Ignore the 1M for a second, set R=Xc,
10000=1/[2pifC]
f=1/[2piRC]=1/[2pi10000C]
f=1/[20000piC] C=47*10^-12
F=1/[2951600*10^-12]= 1/[2.951600*10^-6] = 338799.30Hz, or 338.8kHz

Even if I am off by a factor of 10 due to using the computer calculator, the -3dB is not of any concern to us. The gain at 338.8kHz will actually be slightly less due to the 1M resistor, but not by any appreciable amount.

If you (or anyone else) is interested in more than the simple single pole filter, check out Don Lancaster's Active Filter Cookbook, absolutely fantastic.


That doesnt sound correct.

The gain is the ratio between the 1M resistor and the series combination 4.7k and 500k pot. When the pot is at max value the opamp gain at centre freq is roughly 3x. When the pot is set at 0 ohm then the op amp gain at centre freq is roughly 200x, but the output will clip long before that which is what gets you that overdriven distortion.

Additionally the RC component that matters is the one delivered via the 1M/25pF resistor/cap feedback combination.

The rolloff freq for 1M/25pF is 6.4KHz which is certainly audible
Changing the cap to 47pF the rolloff freq becomes 3.4KHz which certainly makes an audible difference and will cut out the higher harmonics of the distortion.

I would consider this change of tone a concern.
Last edited by Phoenix V at Aug 17, 2011,
#12
Quote by AcousticMirror



I found a few American shops, but I'm in the UK. I've Ebayed them now anyway

So yeaaah got about 98 more capacitors than what I need though


Quote by bigbitefx


I found that, but they wanted minimum orders of like £20 if my memory serves me correct.

I'm going to use a 15pF and a 10pF in parallel.
Yeah, now you're gonna die wearing that stupid little hat. How does it feel?

Help me to live.


I make custom guitar wiring harnesses and I'm pretty damn good at it!
Last edited by flyingmarlin at Aug 17, 2011,
#13
Quote by flyingmarlin
I found a few American shops, but I'm in the UK. I've Ebayed them now anyway

So yeaaah got about 98 more capacitors than what I need though


I found that, but they wanted minimum orders of like £20 if my memory serves me correct.

I'm going to use a 15pF and a 10pF in parallel.


ahh ok, I tend to buy quite a bit at a time