Reasonable value for RC filter for AM radio

[aron]

I'm trying to figure out what would be a reasonable combo for eliminating radio station frequencies. Assuming this:
http://hyperphysics.phy-astr.gsu.edu/Hbase/audio/radio.html

I'm thinking a 1K+470pf cap might be a decent start. (338799Hz) (not counting output impedance of previous pedal)

I'm trying to reference this article:
http://www.geofex.com/circuits/what_are_all_those_parts_for.htm

Rrf and Crf.
My Morley JD-10 is picking up radio stations quite a lot and I'd like to use it more. Any suggestions?

[Paul Marossy]

I thought the "standard" way of getting rid of radio stations was to place a 47pF cap to ground after the input cap...

[dukie]

Try 10pf at the input to ground, eleminate all radio frequency bleed and small enough value so will not do any harm to guitar tone. Its now my standard method of building any stompboxes.


Cheers!

[R.G.]

I thought the "standard" way of getting rid of radio stations was to place a 47pF cap to ground after the input cap...

Try 10pf at the input to ground, eleminate all radio frequency bleed and small enough value so will not do any harm to guitar tone. Its now my standard method of building any stompboxes.

Then there's Rrf. That, and Crf form a radio frequency (rf!) lowpass filter to dissipate any radio-station pickup that might find its way to the input. You'll sometimes see Crf used without Rrf. That's bad practice. It relies on something further to the left/input side of the circuit to impede the radio waves. A real, honest-to-Pete resistor does a much more reliable job. Sometimes Crf by itself makes radio interference worse by tuning the radio waves and actually presenting more to the input.

Although a capacitor by itself works many times, it will occasionally tune the wire inductance at the input so it actually tunes in the radio station you're hoping to kill. A resistor in series adds enough dissipation to the path to kill the Q of any tuned circuit the cap happens to form and will eliminate RF on a broadband basis.

[liquids]

In practice, it does seem that most people just stick a cap right at the input (or around the input) to ground with no series resistance.

But if I'm not mistaken, R.G. has mentioned that in some cases this works, in some cases it can actually induce Radio Frequencies and even when it doesn't, it is considered bad design in general.

Simple fix - all my builds  hit an ohm-value resistor as soon as they hit 'the board input' (680R on down to 100R), with a pf value capacitor (100pf or less usually) to ground.  Helps me sleep at night.

I forgot to do this on a buffer, and got RF from it, which I only realized when I was recording in a building a stones throw from a college radio station.     I eventually hack modded it by soldering a 47pf cap (what I first grabbed) at the input capacitor's lead on one end and ground on the other end...easy fix, if it's the only option, or bad design doesn't make you loose sleep.  

Edit: You beat me to it yourself, R.G.!  Thanks.  

[Rob Strand]

I'd err on 2k2 and 220pF, since 220pF, 220pF reduces any chance of pickup loading and the R's not high enough to add noise.

Look at what amps use.

[Paul Marossy]

Although a capacitor by itself works many times, it will occasionally tune the wire inductance at the input so it actually tunes in the radio station you're hoping to kill. A resistor in series adds enough dissipation to the path to kill the Q of any tuned circuit the cap happens to form and will eliminate RF on a broadband basis.

Hmm... good to know. I've never had a problem with picking up radio stations on anything I have built thus far, but I guess there's a first time for everything.

[Rob Strand]

> I've never had a problem with picking up radio stations on anything

I depends where you live.

I remember one guy had a (brand name commercial) amp with a 1k/39pF filter and he was getting frustrating RF problems.
I recommended a 4k7 and 100pF and the problem was solved (went for 4k7 because the input impedance was higher than usual.)

[aron]

Thanks Rob! It's my JD10. It's pulling in radio stations! I'm going to try 2.2K and 220pf.
I will let you know.

THANKS!

Aron

[Paul Marossy]

> I've never had a problem with picking up radio stations on anything

I depends where you live.

I remember one guy had a (brand name commercial) amp with a 1k/39pF filter and he was getting frustrating RF problems.
I recommended a 4k7 and 100pF and the problem was solved (went for 4k7 because the input impedance was higher than usual.)

Yeah, I figured that is the case. Geographical location would definitely be a factor. Anyway, all good information to store in the memory banks!

[Rob Strand]

I don't know why but I I pulled out the schematic and something caught my eye, the input cap on that unit is *very* small indeed.

You should put the filter between the input socket and the input cap (C1).

If you put it between the input cap and the opamp you will get some unwanted signal attenuation and it will affect the cut-off frequencies and tone.

(You never know there may be some other fault, it looks like the unit is grounded to the metal case through one pot, the one with the nut - buzz the case out to the circuit ground.)

[aron]

That is where I was intending to put the resistor/cap. I was going to lift the cap and do it, but in the back of my mind, I never remembered this thing having that problem until I took it apart. It may be the case is not grounded well since it seems everything else is isolated.

[aron]

Well the case is grounded so I guess I will try the RC combo.
Thanks!

[PRR]

What guitar-amps historically used was 34K and 100pFd.

The 100pFd is the input of a 12AX7.

Fender 2-jack inputs had two 68K but for either input alone the effective resistance is 34K.

This is a 49KHz low-pass, giving >20dB attenuation in the AM band, yet negligible loss in the audio band.

Higher than 34K or so (maybe 100K) will add hiss even on tube or FET inputs.

Lower than 34K won't give much loss for high-power AM broadcasts, unless you raise the capacitance enough to start loading-down the pickup's treble.

It will help a LOT to use a high-overload first device. BJTs overload at 60mV input. We use NFB so they take larger audio cleanly, but this NFB is liable to be ineffective at radio frequency. This is why JFETs persist in radio tuner first stages: they (and 12AX7) have ~~1V overload. While you can get that much RF under an AM tower or when a boosted CB radio is in the truck behind the tavern, mostly a 1V radio signal is unlikely.

Round-about grounding is another troublemaker. Grounding gets into a religious debate, but when RF is hammering at your input, you really need to get the RF off your cable shield and onto your case ASAP. One simple proven way is to use metal input jacks, and work out the rest of your signal grounds relative to that (but 3rd-pin power ground ALWAYS goes to case where the linecord come in).

[aron]

Thank you for the info. I appreciate it!

[R.G.]

The 100pFd is the input of a 12AX7.

Sylvania's 1975 databook says the grid-to-plate for a 12AX7 is 1.7pF. The g-to-p cap is multiplied by the voltage gain by the miller effect, but not all 12AX7s run at gains of 58 or more to get to 100pF. And since Miller cap is dependent on gain, it's wildly variable.

I always try to swamp out variable capacitances with fixed real caps. But Miller capacitance is real, just not obvious.

[PRR]

> grid-to-plate for a 12AX7 is 1.7pF

The tube maker only tells about the tube.

Stray wiring capacitance is significant, though hard to guess.

The actual figure for Fender-like tube-amp gain and construction is often more like 110-130pFd total at grid. This suggests 0.5pFd-1.0pFd stray C grid-plate.

> Miller cap is dependent on gain, ...variable.

Yes, though they almost all go for ALL the gain they can get. Once guitar amps settled on 12AX7, gain is usually 40-55. If you low-gain and tight-wire you might get 80pFd, if you max-gain and run long leads you may get past 130pFd. This variation is only 4dB different in the AM band.

There's other ways to skin this cat. I'm just saying that 33K+100pFd is similar to a "reasonable" plan that has worked well for 50+ years.

[artsinbloodshed]

Hey guys!
I modded a buzz box from home-wrecker and as it's a very high gain thing, I can't get rid off the RF.
I put an RCfilter with different values but either the tone changes dramatically or the RF are still here (or even worse).
as i don't have all the spare parts of the world or all the good ideas (duh!) maybe you could have some brilliant tips!

here is the mod:


anyone?
Thanks!

[amptramp]

Artsinbloodshed, it looks like you are trying to make a radio.  The signal goes directly to a rectifying junction with logarithmic response.  A series R-C filter would help as stated above.

Another suggestion I have not seen yet is the use of ferrite beads as suppression of RF.  If you choose the right material, you get resistive inductance that dissipates RF without adding series resistance that adds to the noise level.  This can be used in conjunction with a small series resistance by slipping it on the resistor lead.  There are high-loss beads optimized for EMI suppression (electromagnetic interference).

[liquids]

What are you feeding and or following this pedal with?  

Buffers do help with this kind of consistency and troubleshooting, as you may be getting the interactions of variable high and low impedances, just by turning this pedal on/off.

If your guitar is going straight into it (with passive pickups), a very small resistor and a small cap (22pf) would eliminate the RF variable...to the point of, as you said, affecting the high frequencies/tone.    By the way, where is the volume control?    This is all assuming the circuit is truly as drawn.

But it may actually be that the fairly low output impedance of the pedal itself, when on, is causing RF in some other pedal (or the amp) that follows it.  Other pedals or amps you have may be improperly designed and don't have an RF filter, but are 'assuming' that you'll be plugging a high output impedance signal into them (which alone can eliminate RF).  So sending them a low output impedance signal can excite their poor design at eliminating RF properly.

In either case, a small (1k or less) resistor and a smallish cap (22pf on up, depending on your chose resistor value) after the 10uF cap out may solve this.  Or even just a 22k type resistor at the end by itself with no cap, if that doesn't bother you in terms of buffering, tone, bad design, and higher output impedance.

Likewise, if the output impedance of this circuit interacting with other gear is the issue, a buffer instead of this pedal could similarly provoke RF in whatever gear follows it (though it will induce quieter RF than such a high gain circuit as you posted).  So, if you can, get this pedal out of the equation, and breadboard a simple buffer with Rf filter at the input only, and put it first in line in your chain....turn up the volume, and see if that provokes similar RF from the other pedals or gear, eliminating the variables down to just the affects of low output impedance.