Series resistor order?

[Lizardmn]

I've been seeing some different things with the series resistors used to protect say an op amp from too high current, and was hoping someone could educate me about it.
Sometimes they're after the input cap + bias resistor like on the left, sometimes before like on the right.


Does this order affect the sound in any way? Am I right in thinking if it's before the bias resistor there's some signal attenuation because it forms a voltage divider?
When are these protection resistors even necessary? I don't see one at all on bluesbreaker schematics.
Thanks guys

[r080]

This might answer a couple of your questions:

http://www.geofex.com/circuits/what_are_all_those_parts_for.htm

[Lizardmn]

A very informative page, though not quite, as what I'm seeing is Rfr without the capacitor. I recognise a LPF when I see the resistor paired with the capacitor to ground, but I'm unclear on what it means when I see the series resistor seemingly on its own.
Basically


Does A = B (I think it does but obvs not sure)? Also what does R3 do in C?

[antonis]

what I'm seeing is Rfr without the capacitor.

There always some stray capacitance hanging around the neighbourhood..

Yes, A = B

R3 is an old-time "residual" for input bias current compensation 

[R.G.]

If you have any two components in series, and you care only about the voltages at the ends of the two part string, and the current flowing through both of them in series, then the order does not matter. For a circuit fragment that is, for example --R-C--, the same voltage will be at the ends and the same current will flow through them if you hook it up as --C-R--.  In general, if there is no connection to the middle points in a series string, the order inside the string does not matter.
As mentioned, there are tiny stray capacitances everywhere, so there is always a tiny capacitor leading out from the middle of any series string. A test of reason applies: stray capacitances are usually a few picofarads. If the R and the C have much lower impedances, you can generally ignore the stray capacitances unless you're working with RF circuits.

[PRR]

...Does A = B..?

Shave and a haircut, two bits.
Haircut and a shave, two bits.

[Lizardmn]

The stray capacitance is really interesting. Invisible tiny capacitors is a good way of thinking about it. Thanks 

I guess my confusion arose when looking at electrosmash analyses where they don't mention the series resistor's effect on the high pass filter after it. I can understand why not, if the series resistor is too small to affect the calculation in a meaningful way.
But then I see this config in a circuit I've been looking at



where the 12K (since we can swap the order with the capacitor) I guess not only has a big effect on the cutoff frequency of the filter, but also attenuates a lot of signal because it divides the voltage together with the 1K5 to ground.

[antonis]

The voltage divider works under Vout/Vin = R3 / (R12 + R3 + 1/2π*f*C2) voltage transfer function..
(R12 & C2 are in series with output signal where R3 is in parallel..)

The above equation can be rearranged into k*[2π*f*(R12+R3)*C2 / 1 + 2π*f*(R12+R3)*C2], where k = a constant and (R12+R3)*C2 is the time constant..
Hence, cut-off frequency point is set by f = 1/[2π*(R12+R3)*C2] which is at 25Hz for your circuit..

P.S.
Does your example concern an audio circuit..??

[Lizardmn]

P.S.
Does your example concern an audio circuit..??

It is an audio circuit, yeah. It's just a bit of the CE1 preamp. Even with the chorus effect bypassed it goes through this op amp boost stage. Then the dry signal hits a bunch of passive attenuation, I guess because it's to be mixed with the effected signal again later.

[antonis]

It's just a bit of the CE1 preamp.

Definately NOT with R12 set there...

[Lizardmn]

It's up there



Idk why I called it R12, it's R47 on here but definitely a 12K.
And definitely needed, cause if it's just 470nf and 1.5K I make the cutoff freq of that to be 225Hz, above the fundamental of the G string so I figure that's incorrect. 

[antonis]

 

That's a whole 'nother story..
(it involves pre & post switching signal dividing for reasons beyond the scope of current thread..)

To stay tuned, the order of series elements makes no difference as far as we only consider for their equivalent configuration..
e.g. both a 10k+5k and 5k+10k result into 15k..
(same of course stands for parallel combination - we don't mind which resistor is placed upwards or to the left and which is placed downwards or to the right..)

[Lizardmn]

That's a whole 'nother story..
(it involves pre & post switching signal dividing for reasons beyond the scope of current thread..)

Oh no, so because its after the signal being split into multiple paths there's a whole other rule that I don't know about? Just when I thought I had it lol.
Aight, guess I'll go look that up now 

[anotherjim]

Sometimes a series resistor after a cap isn't there as an input safety current limiter, but to form a low-pass filter with the device's Miller Capacitance and that can be more significant than the stray capacitance...
http://www.aikenamps.com/index.php/what-is-miller-capacitance
It might be done into a solid-state version of a tube circuit that had a resistor even if they may not exhibit as much Miller capacitance as some tubes but that doesn't stop someone from copying the tube originals input Grid resistor value. Nevertheless, it can still do some good as an RF filter but a low-value cap as well can be added.
Some designs are made from building blocks, so you sometimes see an input arrangement used further down the path where it isn't really needed.

Be careful when the circuit is for bipolar supplies. Coupling caps may not be needed at all in the original but are added when the circuit is adapted to a single supply.

Having too much gain in an input stage and then having to attenuate can be a tactic (hoping) to improve the SNR since the attenuation will reduce the added device noise of the preamp compared to the signal.

[POTL]

Hi, it seems to me that it makes no difference where the ESD resistor is located before or after the bias resistor. Located before the bias resistor, there will be a voltage divider and a volume drop, if placed after the bias resistor there will be no volume drop, but the resistor will still protect against static. if i am wrong correct me

[POTL]

Since we started talking about resistors, I have long wanted to ask. Look at resistor 19 at the output of the circuit. usually this resistor goes up to resistor 21 and forms a voltage divider at the output of the circuit. In this configuration, this resistor comes after r21, there is no voltage divider and the output of the circuit is not tied to ground (via r21), but is simply in the "air", is this method correct or is it better to set r19 before r21?

[r080]

I have wondered about that one too, but never asked or tried to figure it out. My understanding is the 470 Ohm resistor is there for output protection, so what's the difference?

1. 470 floats: If the output is shorted to ground, you have the 4.7k and 470 in parallel. The 470 dominates, giving 427 Ohms (~10% lower), which is not going to cause a problem in most cases. As you noted, POTL, there is no divider.

2. 470 first: If the output is shorted, the 4.7k is shorted, and it goes through the 470 Ohm resistor. But, this gives you a ~10% lower signal output. For an overdrive pedal with loads of gain, this probably doesn't matter.

In this example, I would have no problem doing it either way, but I could be missing something.

[PRR]

...is simply in the "air"...

If so, then why do we care? The sound isn't going anywhere.

Draw the WHOLE circuit.

In real life it may go to a short, a ton of cable capacitance, the output of another preamp, the output of a power amp......

If it goes where we expect, an audio input with >47k impedance, then 470Ω is 1% loss and inaudible. If it goes to a 10V source it can pass 20mA which does no good but rarely smokes.

[POTL]

Yes, this is an audio output I'm wondering is this the wrong design? I've always heard that the input and output of a circuit should always have a resistor between audio and ground, and here after the ground resistor there is a resistor hanging in the air, it doesn't look right.

[duck_arse]

the last in line resistor is only there, when it is there, to pull-down a hanging capacitor plate, such that there are no/reduced pops. your circuit shows the caps with pull-downs fitted, what happens past that point matters not. it's not the input and output of circuits that need resistors to ground, it's floating capacitors that need resistors to ground.