Capacitor filtering question

[ranchak]

I recently built the Regulated Power Supply from GGG. I noticed that I have hum on the + side of the output. I used an audio probe and you can hear the hum. I thought that the purpose of the xxuF cap was to filter out unwanted hum. I have replaced it with a 2200uF cap and still have hum. I have been obsessed lately with trying to build a clean filtered power supply as well as a filter AC power supply. I have tried many different ideas and have not seen much success. I am not using any sophisticated measuring techniques, just listening and seeing if anything has changed. I have tried these ideas that I found on the web:

http://www.geocities.com/jonrisch/index.htm
http://www.10audio.com/diy_power_conditioner.htm

Needless to say I am still trying to filter out unwanted noise and hum. How large can a cap be to filter out hum? Is there a point of no return? Any thoughts or ideas to try? Is there a better way to filter AC power and/or DC hum?

[Paul Perry (Frostwave)]

If a regulated power supply has hum on the output, it's usually a sign that the raw input voltage isn't high enough, or (unlikely) there isn't a large enough cap after the rectifier.

[ranchak]

I have 120+ input voltage and the power supply hums at my house as well at a friend's house. I have a 2200uF cap after the diodes. This did quiet some of the hum. How big of a cap can be installed before it is not effective? Will there always be some hum on the line?

[the_random_hero]

It all comes down to what pedals you're running and how the wiring in your house is. By itself, there should be very little hum from the power supply. I think Paul was asking you more about the input voltage to the regulator (I'd have at least 3v headroom to account for the fluctuating power grid coming into your home).
I was under the impression that the law of diminishing returns applies to choosing the filtering caps - anything much more than 2200uf/amp of current drawn won't give you a lot of improvement.

[ranchak]

I'm not really sure what the voltage is before the regulator. I'm using a flatpack transformer, I'll measure the voltage and see what I have. I was really surprised to hear the hum on the output as well. Is there any way to determine what frequency the hum is and then be able to eliminate it with......? I just thought of something else, I have install voltage regulators inside of my pedals for protection in case I would power a pedal with 18v instead of 9v. Would this contribute to added noise if the input DC was 12v and there was a 12v regulator inside of the pedal?

[George Giblet]

I can't get to the ggg site at the moment so I can't see the circuit.

It would help a lot if you measured the voltage across the filter cap whilst the pedals are connected.  If you can measure or estimate the current drawn from the supply in this configuration.

Given you have had numerous problems I have a feeling you transformer voltage is too low.

There's no real limit to the cap size.  You might be able to salvage the situation with a larger cap, at least try 4700uF If need more than 10000uF then I'd be seriously consider changing the transformer.

If the transformer voltage is too low you might not be able to salvage the situation at all.  You could do some trading by dropping the output of your regulator.

I don't quite understand the regulator configuration.  You you have a ggg regulator followed by additional 12V regulators?

(Ed: oh another trick to gain voltage is to use larger diodes like 1N5404's, or Schottky's (check they are low drop).)

[the_random_hero]

I'm not really sure what the voltage is before the regulator. I'm using a flatpack transformer, I'll measure the voltage and see what I have. I was really surprised to hear the hum on the output as well. Is there any way to determine what frequency the hum is and then be able to eliminate it with......? I just thought of something else, I have install voltage regulators inside of my pedals for protection in case I would power a pedal with 18v instead of 9v. Would this contribute to added noise if the input DC was 12v and there was a 12v regulator inside of the pedal?

That could definitely be causing the problems - your internal regulators don't have enough headroom to operate correctly. I was under the impression that you used Zener diodes on the ground connection to lift the output voltage?

[ranchak]

It would help a lot if you measured the voltage across the filter cap whilst the pedals are connected.  If you can measure or estimate the current drawn from the supply in this configuration.

I will measure the votage with the pedals hooked up, but I have hum with no load applied.

I don't quite understand the regulator configuration.  You you have a ggg regulator followed by additional 12V regulators?

I have a regulators installed inside the pedals at the power inlet. I had installed these in my pedals because at first I was building a power supply that was going to have multiple voltage outputs, ie 9v, 12v and 18v. The thought was in case 18v was applied to a Boss pedal it would be protected from the higher voltage. Sort of a "safety net".

I can try larger diodes. I will also measure the voltage across the cap and post them.

[the_random_hero]

Do you mind posting the schematic? I'm building what I think is going to be a pretty similar power supply in the next few days and I can compare my hum levels to yours.

[sushyoshi]

Hey
I have the same problem you have. Ive built the clean 9V power supply and its not that clean at all. I get some hum just like i got with a cheap wallwart. The only difference is that my transformer intead of delivering an output of 25 VAC is doing an output of 16 VAC.
As i read before maybe you guys said that maybe this is the problem. When i get a higher output transformer ill test it and ill tell you what  i got.
By the way i got a 1000uF cap before the regulator and another 1000uF on the output voltage.

[George Giblet]

> I have a regulators installed inside the pedals at the power inlet.

The GGG site (I finally got the PDF) specifies a small 200mA 12V transformer.   I don't have any information about the voltage that transformer produces under no load and fully loaded conditions.  Even with the large cap and low drop rectifiers I'm doubtful you will be able to get that transformer to work with the 12V regulators.  Using 9V regulators, which is what the project specfied, it should work OK with normal diodes and a reasonable sized cap, say 1000uF.

Using your audio probe you will always get a fair amount of humm at the filter cap but the output of the regulators should be relatively quite.  If you crank enough gain after the audio probe you will even be able to hear the regulator humm - in other words there is no quantative measure of the humm.   You can use your sound card to measure the humm but you will need to couple the supply to the sound card with a cap (and perhaps some protection diodes) - if you don't know what you are doing here you can blow your sound card.

One final issue is if you have large capacitors at the input of the regulators and run multiple effects the charge current pulses can get into the audio.  You need a single big cap at the rectifier then quite small caps the input of the regulators (like 10uF).