Silent Circuit question (Beej?)

[candid_x]

The Silent Circuit has no discernible effect on a set of Fralin blues specials, installed into a Silo Special, at the original factory setting. However, when I turn the small adjustment screw of the SC pot almost all the way to one side, the 60 cycle hum does filter out quite significantly.

My question is, would increasing the SC's filtering capacity also cut into the full output range of the pickup? I'm hearing that it does, but would like to confirm this from a technical point of view.

 

[whitestrat]

I hear you too... That's why I usually don't use the SC unless I absolutely have to. Considering none of my pups on the Silo Specs are stock, the SC in some ways has become a "last resort" for me. Thankfully, I've rarely needed it.

But I'm curious. Why is it that the SC mainly works most effectively with the DiMarzios?

 

[beej]

Ok, two questions. First, the trimpot on the SC adjusts the output level of the SC. It has to be perfectly out of phase with noise from your pickups to work. So as you change the strength of your pickups, you're going to have to adjust the output of the SC. So yes, you should set it where you hear the least amount of hum coming through- that's exactly what it's there for. When I change pickups I do exactly this. It would also change depending on where you put the SC ... if you move it, you'll notice the hum also changes. Point being, don't be afraid to experiment and do what sounds good.

Second question- would dialing up the SC "cut into the full output range of the pickup?" Do you mean, will it affect the output signal, or tone of the pickup? Short answer, no. (Apologies if I read that wrong.)

All it's doing is subtracting 60 Hz hum. If you want to prove this, unsolder the SC's blue or violet wire from the pickup ground and stick a pair of alligator clips from the actual ground (black) to the ground of your instrument cable, and the blue or violet to the hot of your cable. Basically, plug the SC right into your amp and hear what it produces. That is what's going to be added or subtracted from your guitar's signal, nothing else.

It's important to note that the SC is buffered. It's not going to load your pickups like a resistor, capacitor, effect, cable, etc. would. So there won't be a tonal change on your pickups.

All the SC is going to do is subtract noise, the amount of which is varied by the little trimpot.

 

[whitestrat]

First, the trimpot on the SC adjusts the output level of the SC. It has to be perfectly out of phase with noise from your pickups to work. So as you change the strength of your pickups, you're going to have to adjust the output of the SC.

This explains everything!!! Thanks Beej!

 

[candid_x]

So the trim pot doesn't turn it (or anything) up, per se, it simply adjusts out of phase with noise. Excellent, and very cool.

Thanks, Jay!

 

[candid_x]

Oh heck, might as well ask..

Jay, would you explain how the SC (or an fx pedal etc) is "buffered"? Buffered means to protect or absorb, correct?

 

[Marc Jennings]

Ok, two questions. First, the trimpot on the SC adjusts the output level of the SC. It has to be perfectly out of phase with noise from your pickups to work. So as you change the strength of your pickups, you're going to have to adjust the output of the SC. So yes, you should set it where you hear the least amount of hum coming through- that's exactly what it's there for. When I change pickups I do exactly this. It would also change depending on where you put the SC ... if you move it, you'll notice the hum also changes. Point being, don't be afraid to experiment and do what sounds good.
.

Does this mean that if you adjust the height of any pickups so you incease/decrease the signal output you should adjust the SC also?

Marc

 

[candid_x]

Does this mean that if you adjust the height of any pickups so you incease/decrease the signal output you should adjust the SC also?

Marc

Good question, Marc.

I didn't think the Fralins bs's are especially hotter than the stock silo special pickups: 6.2K neck, 6.2K middle, 7.2K bridge.

 

[beej]

So the trim pot doesn't turn it (or anything) up, per se, it simply adjusts out of phase with noise. Excellent, and very cool.

No- turning the screw actually turns up the volume of the hum. Different pickups are going to have different levels of hum, so you need noise that's equally loud but out of phase to cancel it out.

But again, it just produces hum. That won't mess with your signal other than adding or subtracting hum to it.

Does this mean that if you adjust the height of any pickups so you incease/decrease the signal output you should adjust the SC also?

No. The hum isn't coming from the strings- it's being induced in the pickup windings by noise in your environment (the windings are just like an antenna, picking up stray signals).

When you raise the height, you're going to pick up more vibration from the strings, but the amount of noise they pick up won't change. So no need to adjust the SC.

 

[beej]

Jay, would you explain how the SC (or an fx pedal etc) is "buffered"? Buffered means to protect or absorb, correct?

Buffered means to isolate your signal from loading due to the impedance of devices further down in the signal chain.

Background

A guitar pickup is an "LCR" circuit. It has inductance (L), capacitance (C) and resistance (R). All three of these electrical properties help define it's sound. Change one property and you'll affect the output frequencies. For example, adding more resistance will shift the loudest frequencies of the pickup upward, causing the output to be more trebly, etc. Increasing the inductance will shrink the window of frequencies passed by the pickup, causing more of a midrangey sound, etc. Using a longer cable will add capacitance, decreasing highs in your signal.

The signals in passive guitar signals are what we call "high impedance".

Impedance

It's tough to explain impedance without getting too technical. Basically, it represents the relationship between voltage and current which a device is capable of accepting or delivering. For our purposes, it's just math. Every component adds together to affect the outcome, but not exactly in the way you'd think. For example, resistors in parallel add together like so:

1/Rtotal = 1/R1 + 1/R2 ... etc.

(Note that capacitance and inductance affect your signal too, it's just easier to talk about resistance for the purpose of discussion.)

Practical example- if you have one pot on your guitar (500k) and you wire up another one (500k), that formula tells you that the total resistance is 250k. By the same principle, if you wire up a small resistance in parallel it's going to drastically reduce your overall resistance (we'd call this "signal loading", it would suck highs from your signal), and if you wire up a huge resistor- 1 Meg (1,000,000) or so, it won't affect the total resistance much.

That's why the input impedance of guitar amplifiers is really high (you can think of it mostly as resistance). That way, they don't load your signal and affect your tone.

Loading your signal can be a useful tool. For example, Jimi Hendrix would use different length cords when recording, depending on the sound he wanted to get. Using a long cord with a lot of capacitance helps tame the brightness of single coils and give them more mids, exactly what you'd want for distorted guitar sounds. But you want those highs for clean playing, so a shorter cord would have less of an effect. Nowadays we can buy low capacitance cable if we're concerned about long cable runs. Or we can use a buffer.

Buffering

To prevent devices further down the signal chain from affecting your signal, as described above, you can "buffer" the signal. A buffer is basically a small amplifier that serves as a "repeater" for your signal. It has a very high input impedance (usually on the order of 1 Meg) so it won't affect your signal. And the output impedance is low (usually on the order of 10k). From the math, lower impedance signals are less susceptible to the effects of capacitance from long signal runs, etc. Active pickups are buffered in exactly the same way- there's an onboard preamp that buffers the signal, right at the guitar.

A buffer is generally "unity gain", meaning the signal coming out is just as loud as the one coming in. If it amplified the signal more, you'd probably refer to it as a preamp rather than a buffer. But it's the same thing.

Point worth mentioning here- since a buffer is, in fact, an amplifier, it will affect your tone. Some buffer circuits are more musical than others, and this explains why some guys like them and some don't.

Hope that helps explain things!

 

[candid_x]

I almost said, so a buffer is like an op-amp. My reverb pedal (Mr Springy) is buffered (not true bypass), but it does seem to sweeten the sound musically even when bypassed. Awhile ago I was driving myself nuts A/B-ing a silo special with and without a battery, and to my surprise I actually liked the sound better with the battery in. But you know with things like that it can be hard, if not impossible, to be truly objective. My next string change, I'll go in again and readjust the trim. I was afraid of turning it too far to one side, thinking I'm diminishing the signal. Now I know it's fine to do that.

Jay, thanks so much for such a thorough explanation! It helps a lot.

 

[robelinda2]

I like a bit of hum.

 

[GuitarHack]

Hah-hah...I have a degree in electrical engineering (22 years ago), but my brain was starting to hurt from this! Rob's comment was good to me

 

[whitestrat]

Hah-hah...I have a degree in electrical engineering (22 years ago), but my brain was starting to hurt from this! Rob's comment was good to me

Hahahaha... same here... Except mine was from Electronics... It just hit me how much Iv'e forgotten reading Beej's post!

 

[beej]

Happy to be of service, fellas. Bruce- glad that helped. Happy tweaking

 

[candid_x]

I've always had the subliminally conscious impression that touching the tiny slotted trim-pot with a screwdriver would implode the universe, or forever banish me and the guitar to hell. ('least we'd be there together)

Thanks for demystifying that magic little cylinder thingy, Jay.