low pass and high pass filtering question

[pinkjimiphoton]

well, it was a brand new huge log cabin... more like a mansion... when it was built years ago.

i know the problem is the power there. i think it has something to do with the solar panels on his roof.. and probably running cables randomly overhead in parts of his "studio" under i mean over the drop ceiling.

but he is unlikely to ever change that. i realize its just at his place. i have to run noise gates there when i play with him. the power is stable voltage wise but noisy as hell.

so i just gotta find a way outta his problem. i appreciate all the advice thoughts and suggestions.

i try and explain to him it could be cuz the melody maker isn't shielded correctly, and his eyes glaze over kinda thing... which i suspect is a big part of the problem, as he's never played with any kind of dirt before. ayyyyyyyy .... lol

[diegocw]

Hi there. Filtering the power supply with a low pass filter will help with the noises that might come from the power line. But if the cables around the house create big magnetic fields then that magnetic field will get in the circuit after the power supply (That is: in the signal path).  To avoid letting those magnetic fields inside your signal path you should run your cables as close together as possible (so you leave no space between the cables) and the magnetic field can't "get inside".
You have to pay special attention to the signal path before the amplification stage (before transistors). If you let the magnetic field get inside your signal path before the amplification stage you are going to amplify that induced voltage (noise & hum). Try to keep the cables from the input jack (ground & signal) as close as possible. If you twist them you make it even more difficult for the magnetic field to get inside. You can google twisted pair, and see that it is a technique used in many applications (like modem cables).
Another option would be using differential or balanced signal path, but it might be an overkill.
I hope that helps. Good luck!


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[diegocw]

* by twisting the cables you don't only make it more difficult for the magnetic field to get in, but you actually cancel the induced voltage.


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[pinkjimiphoton]

yeah, diego, ya literally "humbuck" it!

great tips, brother, thank you!

[ElectricDruid]

but the question remains... better to go the nearest standard value? in  my case r for both filters should be 56k, so a 50 k pot... or double it, to get more range?

The usual policy is to have whatever the "preferred" or "expected" value is in the middle of the pot for maximum trimmability. That implies a pot twice the expected value, so in your case, I'd go for the 100K.

If that gives a range which is far wider than useful, drop down to the 50K pot, but put 25K in series to give you a 25K-75K variable resistance.

As with all rules, you use and abuse as you see fit.

Tom

[amz-fx]

Passive Filters, like any other circuit, do not exist in isolation in the real world. They are driven by something and loaded by something. Ideally, the impedance of the driver is infinitely low and the impedance of the load is infinitely high. A Big Muff style tone control will sound different when driven by a transistor gain stage with a 10k output impedance than it will when driven by an opamp with only a few ohms output Z.

For power supplies, as has been said previously, you want a low pass filter with a low cutoff frequency - at least 3 octaves below the 60 Hz line frequency, i.e. 7.5 Hz or lower. (just a rough guideline though) It is not a good idea to make the resistance too high in the RC filter of a power supply, so increased capacitance is most often used. Increasing the R causes a voltage drop across the resistance and that is not what you want.  Use 5.1 ohms and 4700uF for a cutoff under 7Hz. Also, remember that all of the current from the power supply is going through the resistor so it must be capable of handling the power. P = I^2 * R, so at 1 amp and 5.1 ohms, the resistor must rated higher than 5w! This is why you see big beefy components in power supplies.

Best regards, Jack

[pinkjimiphoton]

thanks Tom and Jack

sage advice as always

i hear ya on the power supply resistor, i have smoked MANY of them