Effect of series resistance

[Marco Pancaldi]

I know that putting a resistor in series with a pickup I will have the following effects:

1 - The resistor reduces the peak resonance amplitude
2 - Put before the volume pot, the res. have a pad effect
3 - The res. increase the output impedance, so his interaction with following pot and cap give more "darkening" effect (for example, when lowering the volume pot)

So the final results are: less volume, less resonance (and little lower in frequency due to load interaction). Please correct me if necessary.

In your experience: is it possible that inserting a resistance can attenuate low frequencies a little, ad how?

Thanks
m.p.

[Stealth]

Given that no resistor is actually just pure resistance (and the fact some are made wire-wound, thus acting like a coil - a inductor) it could attenuate the lows a bit. Not sure by which amount, though.

[Albert Kreuzer]

At audio frequencies and in this application, a resistor is just a resistor.
You could attenuate the lows with a capacitor in series. Try .002 to .01µF and work from there.

[Rick Turner]

A proper pad is a voltage divider, not accomplished with mere series resistance. There are various pad designs from simple to more complex circuits that can keep source and/or load resistance more or less constant.

[Marco Pancaldi]

A proper pad is a voltage divider, not accomplished with mere series resistance. There are various pad designs from simple to more complex circuits that can keep source and/or load resistance more or less constant.

I was thinking that the voltage divider is the series resistance AND the pot resistance to ground. Is it wrong?

m.

[Rick Turner]

What exactly are you trying to accomplish here?

A series resistor will just raise your source impedance and make the effect of cable and tone control capacitance worse (or better if that's the sound you like!).

Are you putting the resistor in series before volume and tone controls? After? The real question is #1 there...what are you after?

Generally we try to avoid upping the series resistance because it does play havoc with the LCR aspects.

That said, there can be interesting effects using a shunt resistor in parallel to deliberately load the pickup. I find it best immediately followed by a buffer so you've got total control over that "fine tuning" of the voicing of the pickup.

[David Schwab]

In your experience: is it possible that inserting a resistance can attenuate low frequencies a little, ad how?

No, but you can you can use a cap with a parallel resistor for that, like on treble bleed circuits.

[Marco Pancaldi]

Hello,

sorry: long story.

I'm making some experiment with Music Man Silent Circuit. I'm sure you already know that it is a little dummy coil with a low impedance output. The circuit output goes to pickup ground wire and works "injecting" the noise from the dummy coil into the pickup. Due to the low impedance (an opamp is probably used), the circuit is reported being very transparent.
I know, from Music Man forum, that a simple way to bypass the Silent Circuit is to remove the battery from the guitar.

I noted two things:
- I mesured the "to ground" resistence on the Silent Circuit output and it is 20k
- when i remove the power, the pickup tone is the same but the low end is "very slightly" reduced
The final test will be to remove the power AND bypass pickup wire to ground (or make a complete true bypass switch as in a stompbox), but I haven't had time yet.

So, the original (now more explained) question is: can 20k series resistance (20k to ground, when the circuit is unpowered) affect the bass response?

Thanks
m.

[CRU JONES]

The dummy coil has a buffer that is capacitively coupled to your pickup (to inject the hum). That capacitor is in series with your pickup. Increasing impedance at bass frequencies.

Cru

[David Schwab]

If you connect the ground from the pickup to ground, the circuit will be bypassed. But why would you want to do that? You like hum? Why are so many guitarists luddites? Batteries wont hurt you.

I have one of those circuits here:



In the circuit schematic, the first coil to the left (202D) is the dummy.

The guy removed it because he likes those dual rail DiMarzio pickups.

[Marco Pancaldi]

Cru:
your advice is intresting, but I don't understand how the cap influences the tone when the battery is off...in this case the only path to ground is via the 20k resistor (366 resistor in the scheme from David).

David:
I completely agree with you about guitarists (I am a guitarist)...
I have nothing against a 9v battery, only I'm searching for a versatile circuit: in lot of cases guitar pickups offer good humbucking combinations (for example using the classic "trick" with RWRP strat middle PU), so I'm searching for the less obtrusive design and best bypassing scheme, to use the Silent Circuit only when strictly necessary.
In any case: have you tried the circuit and what's your opinion?

Thanks all
m.

[David Schwab]

I haven't tried it yet. One of the forum members, Mike Sulzer, came up with a similar buffered dummy coil. I haven't tried that either.

You can use the Silent Circuit with more than one pickup. They show that arrangement in their patent.

Anyway, to bypass it for doing RWRP, just switch the pickup's common to ground. On the guitar I removed that from, that crazy looking 5 way switch bypassed the circuit on a few positions.

Either way, you have no need to run the pickup through a resistor. What your reading in the circuit has little to do with when it operating.

[Mike Sulzer]

... a similar buffered dummy coil.

I think the reason for the slightly reduced bass is this:

Consider very low frequencies where the pickup impedance is almost just the series resistance. The 20K resistor and the series resistance of the pickup form the upper part of a voltage divider where the bottom part is formed by the circuit that follows. Adding the 20K significantly increases the impedance of the upper part. Now increase the frequency, but stay below the resonance. The inductance of the pickup rises and its impedance rises above that of the resistances. Now the effect of adding a 20K resistor in series is reduced because the impedance of the pickup is already that large or larger. This is not a big effect, because it is swamped out by the broad resonance as the frequency rises, but I suspect that this is what you are hearing.

You should be able to make that 20K resistor much smaller, like 10 times smaller. The voltage output of the op amp is mv at most, and so the additional current through the smaller resistor is very small.

My circuit is somewhat different. It uses a pickup size dummy wound with about 10% more turns than the pickups so that is has somewhat higher sensitivity. The buffer is a single JFET source follower run at well under 1 ma for long battery life. The resonance of the dummy with the JFET input capacitance and its own capacitance is much higher than that of the pickups with their capacitance and the dominating cable capacitance, and so you just get the voltage of the dummy canceling the hum voltage of the pickups by subtraction in series. The JFET source follower feeds two low resistance pots, one for a level adjustment with each pickup, with the slider of the pot connected to the ground lead of the pickup. In this way you get good cancellation of hum harmonics out into the KHz range without the need for frequency adjustment as some cancellation schemes require. The resistance you add in series with each pickup is no higher than the pot value, a few hundred ohms.

[Marco Pancaldi]

Hi Mike,

thanks for the detailed answer.
So, if I understand right, your advice is to put a resistor in parallel to Silent Circuit output, to obtain a lower resistance to ground (about 200 ohm) path for the negative wire from the pickup....?

The normal current consumption from the circuit is 0.5 mA: maybe I can try a progressive lowering of the output load resistance monitoring variations on the circuit current absorption.

What is the humbucking quality you obtained? The Silent Circuit I'm testing is mounted on a P90 equipped guitar: the result is good, but the hum rejection from the RWRP combination from both pickups in parallel is largely better.

Thanks

m.

[Mike Sulzer]

Hi Mike,

thanks for the detailed answer.
So, if I understand right, your advice is to put a resistor in parallel to Silent Circuit output, to obtain a lower resistance to ground (about 200 ohm) path for the negative wire from the pickup....?

The normal current consumption from the circuit is 0.5 mA: maybe I can try a progressive lowering of the output load resistance monitoring variations on the circuit current absorption.

What is the humbucking quality you obtained? The Silent Circuit I'm testing is mounted on a P90 equipped guitar: the result is good, but the hum rejection from the RWRP combination from both pickups in parallel is largely better.

Thanks

m.

I was thinking more like 2,000; that is 10 times smaller than what is there now if I remember right. 200 would probably be OK.

Th rejection I get is about as good as an ordinary humbucker; not as good as I can get with my multiple coil pickups.