Newbie ?, inductors for pedal power filtering. is this ever done?

[jimmybjj]

i just got reading this article.

http://www.muzique.com/news/dc-power-filter-box/

and seems like it would be something easily put in a pedal, but since i have seen this done before I'm guessing there is a reason.

[newfish]

A simple cap network (as in the Beavis Huminator) is equally simple and effective.  Caps are maybe what most folks have to-hand when they're building.

Am now wondering about the inductors.  Do they 'leak' any induced current out into the box?
If so, would this exclude them from the higher gain builds (Dr.Boogey, etc), in terms of oscillation?

My guess is that a box like this (with higher-rated inductors) would be handy in-line with a number of FX boxes.

A simple electro cap (47uF, for example), is usually adequate filtering in most builds.
Doesn't hurt to have a cap to filter your vRef (used with Op-Amps) either (if your build needs one).

Personal choice may also play into why this isn't commonplace.

[familyortiz]

It's very common to use the inductors as chokes for the low frequency noise, and this type of circuit is commonly found in ac power inlet modules. I think the reason why it's not common for pedals is that filtering usually takes place in the regulated power supply. This is a very effective way to achieve a quiet supply but a bit overkill given the performance of using filter caps only.

[Paul Marossy]

Doesn't the amount of current involved come into play when talking about chokes in a power supply? Tube amps sometimes use them, but we are talking about much more current than a stompbox - like amps vs. maybe 20-30mA typically.

I know I have seen small inductors in various electronic things like fax machines and stuff like that. I wonder what the rule is on when to use them and when not to use them. I'm sure that in most cases, it comes down to money and not wanting to spend any more than just the bare minimum that will get the job done 95% of the time.

[R.G.]

i just got reading this article... and seems like it would be something easily put in a pedal, but since i have seen this done before I'm guessing there is a reason.

Yes, there is a reason. L-C filters are subject to some restrictions that mean they're more costly for the amount of noise rejection they give you than R-C filters or active regulation.

Inductors are more non-ideal components than either resistors or capacitors; that is, they have more quirks about using them that are not obvious from they being thought of only as inductors. And it takes some thought and application to do L-C filters to ensure that you're getting what you want. Except in special circumstances, those largely being high-audio or above, inductors are impractically large and expensive to do the same filtering as a capacitor.

I dumped the filter in question into a simulator. It has no rejection at all up to about 30Hz, and only 6db (that is, half power, and almost imperceptible loudness) at  60Hz. That means that it doesn't reject 60Hz hum to any useful degree. Up to 1kHz, the filtering is essentially no different from a 470uF cap itself. The inductors don't help you at all up to midrange audio. It's only above midrange audio that you get advantages from the inductors. The inductors are quite effective at the highest octave of audio (10kHz to 20KHz) and above. As noted, but not obviously, it's a buzz and RF filter. What it will do is reduce apparent "hum" which may be caused by the 60Hz or 120Hz repetition rate of fluorescent lights, computer power supply rectifiers, or the rectifiers in amps. Other than that, it's not a hum reducer at all.

You need *big* inductors to reduce actual power line frequency hum. That's why the filter chokes in tube amps look like transformers.

So it's OK to reduce RF and "buzz". Notice that any noise on the "ground" wire itself is not attenuated at all. Nor is any common mode noise where both wires wiggle around. This is a serious issue with the simple amplifiers in some effects having dead-zero common mode noise rejection. So don't think of this filter as a panacea for noise. It helps in an RF-noisy studio with fluorescent lights.

A simple cap network (as in the Beavis Huminator) is equally simple and effective.

For hum and audio band noise, yep, you're right. 95% of the low frequency value of this thing is the 470uF cap.

Am now wondering about the inductors.  Do they 'leak' any induced current out into the box?
If so, would this exclude them from the higher gain builds (Dr.Boogey, etc), in terms of oscillation?

I wouldn't think so. They're not good transmitters from the way they're set up. They may pick up hum if you set the filter minus the bottom plate on an amp near the power transformer.

My guess is that a box like this (with higher-rated inductors) would be handy in-line with a number of FX boxes.

With a redesign, it might be. But it does take some filter design. It might be good for buzzy-hum as noted above. And frankly, it makes a good power-amulet to keep around and put in line with the DC to exorcise buzz demons when you want things to be quiet. 

A simple electro cap (47uF, for example), is usually adequate filtering in most builds.

Yep. Actually, you can go fairly wild on the value of that cap.

It's very common to use the inductors as chokes for the low frequency noise, and this type of circuit is commonly found in ac power inlet modules.

Actually, those chokes are there to keep high frequency noise from a switching power supply either in or out. They're ineffective at power line frequencies.

I think the reason why it's not common for pedals is that filtering usually takes place in the regulated power supply. This is a very effective way to achieve a quiet supply but a bit overkill given the performance of using filter caps only.

Active regulators and caps are much cheaper for the filtering effect you get, especially at audio frequencies. This is an ineffective way to filter hum and audio noise, but good for RF issues. It doesn't hurt at audio, and might help if you have RF issues.

I note that there is a practical problem. It would drive me nuts that the jack on both sides is female. There are very, very few male-to-male DC power cords. it would be more effective to have one of those jacks be male, or a male pigtail. Then it could be inserted in any 9V power line you're likely to find.

Doesn't the amount of current involved come into play when talking about chokes in a power supply? Tube amps sometimes use them, but we are talking about much more current than a stompbox - like amps vs. maybe 20-30mA typically.

I know I have seen small inductors in various electronic things like fax machines and stuff like that. I wonder what the rule is on when to use them and when not to use them. I'm sure that in most cases, it comes down to money and not wanting to spend any more than just the bare minimum that will get the job done 95% of the time.

It comes down to money, yes, and objectives. You do only need 20-30ma. for most effects, and the 100ma of the inductors shown is not a bad current rating for such a box. But the object of most of the inductors you've seen in such machines is as a power converter in a switching power supply, or noise suppression from the switching power supply.

[Paul Marossy]

It comes down to money, yes, and objectives. You do only need 20-30ma. for most effects, and the 100ma of the inductors shown is not a bad current rating for such a box. But the object of most of the inductors you've seen in such machines is as a power converter in a switching power supply, or noise suppression from the switching power supply.

Thanks for explaining that. I've always wondered why I see inductors in that type of equipment. I figured it was either for noise reduction/rejection or for a more stable power supply.

[MetalGuy]

Actually, those chokes are there to keep high frequency noise from a switching power supply either in or out. They're ineffective at power line frequencies.

Did you mean to use a common mode choke for filtering the DC line in case you're using switching power supply?

I've seen such chokes and some 3 terminal caps in the input of some hybrid pedals:



Also inductors at the output:



I guess they are for RF suppression.

[R.G.]

Did you mean to use a common mode choke for filtering the DC line in case you're using switching power supply?
I've seen such chokes and some 3 terminal caps in the input of some hybrid pedals:
Also inductors at the output:
I guess they are for RF suppression.

Good examples. Yep, those are there for RF, and they're there to keep the RF interference generated inside the switching supply in the pedal from getting back out and using the power supply lines as antennas to broadcast their grak.The cap on the inside of the common mode choke and on the outside is intended as a short circuit at very high frequencies, and the inductors as open circuits. What gets odd in LC filters is that you have to be very careful of what source, load, and internal filter resistance is, because the wrong combinations of L and C will form a resonant circuit and make RF worse, not better, or tune in radio stations. You have to have the right amount of resistance at the LC resonance(s) to damp the response peaks and keep from making it worse. And lead length, all by itself, is an inductor. That's why all capacitors have a maximum frequency where they seem to be capacitors. Above that, they're inductors, and they resonate at the turnover frequency. And a foot of wire can be a good antenna.

RF is another world.

[MetalGuy]

I've seen this type of RF protection only in hybrid and SS devices maybe because opamps have a wider frequency range. I wonder if it's going to be beneficial in all tube amps/preamps or only the first grid resistor will take care of RF?

[Paul Marossy]

OK, so here's a dumb question: If you come up with a pedal design, put it out on the market and it causes RFI which causes interference on other people's electronic equipment, can the FCC bust you for it?

[R.G.]

OK, so here's a dumb question: If you come up with a pedal design, put it out on the market and it causes RFI which causes interference on other people's electronic equipment, can the FCC bust you for it?

Yes. I don't know the current penalties, but back when I first learned about it, the fine was up to $10,000.

Per day.

Per item causing interference.   

And that's why "type approval" and agency certification are important. Then you at least have one other company swearing that at least one of your widgets didn't cause too much interference, at least on the days that you tested it. 

This is watered down somewhat by the fact that the  FCC really only comes looking for you if someone complains.  But if you have any competitors... hmmm... 

[Paul Marossy]

Ha ha. I'm sure that 99% of the boutique pedal companies don't get their products tested so that they are kosher with the FCC. I'm not even sure about the more well known companies...

[MetalGuy]

That's why you basically have two options:

1/ Have your equipment tested by certified labs/authorities which is quite expensive and is possible only for big companies.
2/ Make sure your stuff complies to regulations even if it's not officially tested by observing and implementing the same techniques used by big companies which are supposed to have their products tested/certified for RF compliance. It will cost you only couple of more bucks.
AFAIK in EU the CE mark and RoHS is compulsory for all electronic products but in most cases you don't have to go through official certification. If however it's tested and doesn't comply you may have a problem.

[Paul Marossy]

That's why you basically have two options:

1/ Have your equipment tested by certified labs/authorities which is quite expensive and is possible only for big companies.
2/ Make sure your stuff complies to regulations even if it's not officially tested by observing and implementing the same techniques used by big companies which are supposed to have their products tested/certified for RF compliance. It will cost you only couple of more bucks.
AFAIK in EU the CE mark and RoHS is compulsory for all electronic products but in most cases you don't have to go through official certification. If however it's tested and doesn't comply you may have a problem.

You know, it was a hypothetical question. 

I was just thinking that probably 95% of these boutique pedal guys wouldn't know if their distortion pedals were oscillating at RF ir not. Although I don't know that the typical stompbox has enough power to really cause any problems within 10 ft of them, being in a shielded enclosure and all. But this is a realm that I really don't know much about... 

[R.G.]

Most of the time people get away with just selling something without knowing anything about the safety/EMI regulations. That amounts to setting a trap for yourself unless your stuff is (1) entirely powered by low voltage DC and (2) does not do any electronic action at any frequency significantly above audio.  A fuzz face or tube screamer clone, for examples, are pretty sure not to run afoul of either safety or EMI laws, and only need to be made with all RoHS parts to be legal pretty much everywhere. Put any clocked digital logic in the box - like, say, a microcontroller - and you need to either be lucky or have it certified.

I see handmade boutique amps that are highly thought of with no safety certification. If someone ever gets electrocuted or a fire starts and the amp is involved in any way, it's fairly likely that a summons will appear at the maker's door - whether they're actually at fault or not.