I know i could research this and read my textbook but i will ask ug instead. I want to filter out the grainy static fizz noise that my high gain cheapo amp makes through my home made speaker cabinet. I want to put an inline cap as a low pass filter. What cap should i use and where does it go? On positive or negative? If i remember right my cab is a series of two 4 ohm 6 inch speakers. I just want to filter out the fizz.
Guess that's not something anyone is willing to help with
Last edited by geo-rage at May 30, 2017,
I've been avoiding posting anything because I don't think a simple low pass filter is going to solve your problem.  If you are filtering out high end buzz you are also filtering out all the the good tones above the frequency of that high end buzz and there is a lot of important tone in that region.  You would need to make some sort of notch filter to get rid of the specific tones being created by the buzz and that is a much more complicated circuit.  Also the values of caps and resistors would change depending on the specific frequency you are trying to eliminate.  Building something like this would require parts that can handle large current and those aren't cheap so it's probably not worth doing.

You might find it more beneficial to get a cheap 7 band eq and zero in on the noise on the front end rather than trying to eliminate it on the back end..
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A simple low pass filter may not solve anything but its worth trying. All im worried about is damaging my amps and speakers. I will find the parts. Thats not a thing. Unless someone helps me i will juat stay ignorant or have to reinvent the wheel.
Last edited by geo-rage at Jun 1, 2017,
We can't tell you what cap to use because we don't know exactly what you want to eliminate.  Buzz can potentially happen at any frequencies.   I believe this is a bad idea and if done wrong it can damage your amp.  Please don't do it.
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Last edited by CorduroyEW at Jun 2, 2017,
If the cap fails it can cause a short which can fry your amp or even worse start a fire.  There are other things that could go wrong too but the 1st reason should be good enough. 
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That would be if the cap bridged the positive and negative of the speaker. If it was in series that wouldn't happen. Besides caps are in everything. Why would anyone use them if they could fail? I appreciate the wisdom though.
Last edited by geo-rage at Jun 2, 2017,
"Why use them when they can fail" is a good question and shows exactly why your solution is not common or advisable; you use parts when it is smart or necessary, accepting the risk of failure but limiting it when possible. Why use them? Because we have to. Why use them when they can fail? Well, again, because we have to, but we can be smart about where and why we use them, so that their failure does minimal harm. This is the basis of quality product design, and is exactly what engineers think about when trying to make products that last longer and are easier to service when they do stop working. 

The issue here is using an unnecessary part in a vulnerable location for something it isn't really meant to do. You have to have capacitors in (for example) the power supply. Can't make one without them. So of course you use them there, because it's what the circuit calls for. When not to use them is exactly the case you suggest: in a spot where failure could be catastrophic, and where the issue could almost certainly be fixed by less drastic methods. As mentioned above, a notch filter might help a lot more, and sound better; it would use a cap, but in the preamp, meaning that if it fails, the worst that might happen is that the amp goes quiet until you fix it. That's a lot better than shorting out and smoking your output transformer (which is what would happen with it in series). This is why you're getting the suggestions you're getting. Your solution seems a lot like trying to open your mail with a chainsaw. Maybe you can do it, but there's probably a better way. 

This is a lot like the famous "death cap" in Fender amps. They didn't fail often, but the price of failure is severe enough that every tech knows to remove the cap when servicing old amps, and why no amp company makes a design like that any more. You can make a different decision if you want, but don't expect people to jump at the opportunity to help you with it. 
Why not build the circuit in a pedal enclosure, and run it through your effects loop? Can easily build a passive one with much smaller and cheaper components, and no risk to your gear.
Quote by Roc8995

That's a lot better than shorting out and smoking your output transformer (which is what would happen with it in series).

Why would crowbarring a series capacitor in this situation cause a smoking OT?


Sure, bang a cap in series and see what happens. You might learn something. The result might not be what you expect.

I assume you know the peak voltage, to select the correct voltage rating.

The possibility of what you are proposing to cause any damage is very unlikely, but no guarantee.

I can tell you it won't work, but doing it will raise many questions for you, and the only way those will be answered is by study and understanding.

I have no way of knowing what you do or don't know about electronics, but without belittling you, I assume very little, otherwise you wouldn't be asking a question like this or putting forward your cap in series as a low pass filter idea. Do a Google on passive speaker crossovers. Ask yourself what they do and how do they do it. Why don't the caps fail? What is a 6db per 8ve roll off ? What is a first order filter? What is the fundamental frequency of the "fizz" you are trying to eliminate? What is its bandwidth? What is its amplitude? What is an RC time constant? What does the term Xc mean? Likewise xl, and how do you calculate and use this information?

That's just a few basic questions, barely scratching the surface. As others have said, it's not that easy. There is no "bang this value cap in there, and it will cure all your problems" answer to this.
Quote by Vreid
Why would crowbarring a series capacitor in this situation cause a smoking OT?

Sorry, I should have said it'll do that if it fails open, not shorts. Going from full load to open circuit could blow the OT in a hurry.
Just had a thought.
As Liaztraht  said, the effects loop would be a good place to experiment. Shorting out would be unlikely to cause any damage. (unless a tuned tank circuit was accidentally designed ) You could use parts from junked equipment for no cost to see what happens. Experiment. Can learn a lot, and raise more questions that study and understanding will answer. Pretty much everything you need to know is available on line. 


Have a look at the tone circuit in a Guitar. How is that wired? How does it work? 
Last edited by Vreid at Jun 4, 2017,
Thank you! Now i get it. The cap in series would stop the bass right? So bridging the positive and negative at the speaker would let the highs pass to ground. That does sound dangerous, but i see that from time to time in radio and car speakers.
Last edited by geo-rage at Jun 4, 2017,
One important difference is that solid state amps can tolerate zero load without any trouble, so if this happened to a car radio it wouldn't do anything besides mute that speaker. Tube amps under zero load are in deep trouble.
It is admittedly rare and unlikely that this would happen, but with a tube amp I just don't think it's worth the risk. 
That makes sense now. I am using all solid state amps. Still if the cap fails and shorts. That is no problem for a tube amp either in a series situation. Anyway this is a common thing to do. Im sure i will figure something out to take the static out. But you all are so afraid that maybe i will start with a homemade 9v amp and some i home speakers before moving up to the frontman 15.

But what happens when you run your tube amp into a woofer+tweeter cab with a band pass filter and the cap fails? Are you not supposed to use those types of cabs with tube amps?
a crossover network doesn't work in the same way. if a series cap blows it fails open.
Prs se Holcomb is the answer
Quote by AcousticMirror
a crossover network doesn't work in the same way. if a series cap blows it fails open.

Doesn't it?

How does a crossover network work then?

Why does a "series cap" fail open?
Quote by Vreid
Doesn't it?

How does a crossover network work then?

Why does a "series cap" fail open?

I don't know what you are going on about since I agree with you
A three speaker hifi system crossover has multiple part working in parallel.

All throw speakers would have to fail for their to be catastrophic failure.

Guitar speakers aren't wired like that. He op is looking to fix a problem that requires a really complex solution to make safe.
Prs se Holcomb is the answer
What I'm saying is I don't agree that a single series cap doesn't work the same way as a crossover network.

A cap always does what it does. An inductor always does what it does. No matter what the circuit design is.
Capacitive and inductive reactance are related to frequency.

What the OP was proposing was a first order high pass filter, which would give a 3db Fc, and a 6db per octave roll off.

This picture above shows a first order high and low pass circuit. Eliminating the high pass section just leaves a low pass with the inductor in series.

That pic shows a first order network with a bandpass for the mid.

The pic you posted is the same principle with more poles or orders added for a tighter knee Fc, and greater dB rolloff.
Crossover design and calculation can get fairly complex.
But, a first order is simple and no different to a tone circuit in a preamp.

It doesn't really matter, because none of this will help the OP's problem. We don't even know the center frequency, bandwidth, amplitude of the noise he wants to eliminate. I think he said he has 6 inch speakers in series as well. In a guitar amp, we don't want any power robbing circuits, especially in the frequencies we want to keep.

Capacitors can fail in different modes, and can be either open, closed or partial. A common failure in valve amps is the coupling cap between stages starting to conduct. This presents DC offset on the next stage causing scratchy pots and grid blocking saturation. They very rarely fail open unless mechanically damaged. It's not uncommon for an electrolytic to go open or near enough especially if it's vented and blown the electrolyte out. But if a standard non electro cap is not mechanically damaged, and not subject to HV, they are more likely to fail shorted. This can be due to moisture ingress or puncture, breakdown of the dialectric, and then bridges can form between the plates causing a short.

AcousticMirror Not having a go at you, , just didn't quite agree with what you said.
Last edited by Vreid at Jun 11, 2017,
You want a two way cross-over with a high wattage resistor instead of the tweeter. Suggest you try one at 6KHz as anything less with be too dull.

Do not worry about failure, if you choose the correct voltage rating for the caps and correct current rating for the inductors - modern caps are very reliable.

But actually where is this fizz comming from - not the power amp surely? So maybe another solution would be a noise gate?
Last edited by PSimonR at Jun 10, 2017,
A noise gate would probably help a lot if the amp had an fx loop. The amp is a very old frontman 15. It has a good tone through my custom 2-6 cab. The internal speaker cant handle the distortion channel but my cab can. Only, it also picks up some static which i called fizz. Its cool that some people posted some schematics. I will read those and see what they do. 
You know, i wouldn't worry about a cap failing. The speaker it's self is just as likely to fail. When the voice coil burns in half your  amps are all doomed. You shouldnt use speakers.

Guitar speakers aren't wired like that. He op is looking to fix a problem that requires a really complex solution to make safe.

Yeah that seems like the correct ....accept we can safely through safety out the window.
Last edited by geo-rage at Jun 13, 2017,