Series resistors at the input -- good and bad points?

[midwayfair]

Some designs have a resistor in series with the input -- e.g. a 100K in some overdrives or really small like a 22R-100R. I always assumed that the really tiny ones were just there to lop off a very small amount of RF (similar to using a 10pF to ground or some such), and that the larger ones were a general tone shaping thing.

Then someone in a PM to me recently mentioned that "too small and it loads the guitar pickups, especially with a long cable." This seems backwards to me, but it does seem to make intuitive sense that a super small resistor at the input means lower input impedance. However, I have trouble understanding plenty of stuff about impedance (I'm sure I'm not the only one ...), and it's maybe a counter intuitive thing.

So is there something here I'm missing? Is a tiny resistor at the input a "bad" thing?

[PRR]

Insufficient data.

What *else* is in the circuit? Surely this resistor goes-to something?

[chptunes]

I'm intrigued.. ..in some of Jack Orman's project notes (like the Mini-Booster), he suggests a 10k series resistor at the input to cut RF interference.. indicating too that the pull-down resistor can then be omitted.

I'd like to hear more about when to use such a mod, and when to refrain.

..staying tuned....

[Gurner]

Then someone in a PM to me recently mentioned that "too small and it loads the guitar pickups, especially with a long cable." This seems backwards to me, but it does seem to make intuitive sense that a super small resistor at the input means lower input impedance.

you were right in the first part of that last sentence (before you changed your mind!)

Adding in a series resistor *adds* to the input impedance the guitar pickups sees. For example If you have 1,000,000 ohms of circuit input impedance before adding a series resistor, then, if you add a super low 1 Ohm resistor in series with the input signal, then now you've got 1,000,001 Ohms of input impedance that the guitar pickup 'sees'..

[Johan]

I'm intrigued.. ..in some of Jack Orman's project notes (like the Mini-Booster), he suggests a 10k series resistor at the input to cut RF interference.. indicating too that the pull-down resistor can then be omitted.

I'd like to hear more about when to use such a mod, and when to refrain.

..staying tuned....

the pulldown resistor biases the FET(needed). the series resistor works together with the FETs internal (gate to source) capacitance to form an LF-filter, cutting RF out( optional, but a good idea).
you see the same in many many tube circuits and the result is refered to as the Miller-effect..it does not apply to BJT's
J

[darron]

I've always thought of them being a part of a low pass network. They can also help to protect the semiconductor that follows incase somebody feeds it a stupid signal that it can't handle. I've been using a 220r (seems like a MINIMUM) and 220p at the front of my builds.

I'll watch this topic too

[midwayfair]

Insufficient data.

Surely this resistor goes-to something?

Any generic amplification or buffering stage, I guess.

Here's a few examples off the top of my head:
http://www.madbeanpedals.com/projects/Cosmopolitan/docs/Cosmopolitan.pdf
100R following the input coupling capacitor, BS170

circuitsaladdotcom.files.wordpress.com/2012/11/mosfet-compressor.gif
100K following the input coupling capacitor (this is the circuit I was messing with), 2N7000

http://2.bp.blogspot.com/-oBz_XMAZSd8/T-jvPvJdWTI/AAAAAAAABk0/pF9SOGcl8PI/s1600/ROTH.png
200R before the input coupling capacitor, TL072

^ I guess they're all FET stages of some sort. Does the TL072 have G-S capacitance like what Gurner says is going on with the MOSFET stages?

And if Gurner's correct, then lowering the size of this resistor should lower the input impedance. The sound usually gets *brighter* with higher impedance. But here it's both part of a low-pass filter, too. And is there a meaningful difference in a stompbox between using the series resistor into a FET method vs. just putting a tiny cap to ground to kill RF?

Tea time. Maybe it'll make more sense with caffeine in me.

[puretube]

..it does not apply to BJT's
J

are u sure?

[midwayfair]

Hrm:



"In this case the emitter resistor is 10K, and the typical gain of the 2SC1815 is 300, for an input impedance of 3M at the transistor base. Therefore, the 510K biasing resistor accounts for almost all of the signal loading at the input.
This input stage is always connected to the input signal, not switched at all. Since complaints of "tone sucking" do not follow the TS9 and 808 around, apparently this is a high enough impedance to avoid loading guitar pickups too much like the common wah pedals do." --R.G. Keen

That's a BJT stage ... 1K resistor in series and the load without the biasing resistor would essentially that of the transistor ...

More from RG:


"The input voltage is divided down by the voltage divider composed of the 68K input resistor and the effective transistor input impedance."

I feel like I need a "guitar pickup impedance and loading for dummies" here. Anyone got a good link for required reading?

EDIT: wait ... the resistance is in series with the input, so it's just +R to whatever the pickup's natural R already is. So ... it's just a little extra impedance?

[Johan]

..it does not apply to BJT's
J

are u sure?

No, but that s how I understand it.at least for the type of circuits and frequencys we typically see in stompboxes. But please educate me..

[Gurner]

And if Gurner's correct, then lowering the size of this resistor should lower the input impedance.

I should just clarify...my previous comment was earlier on in the thread when no specific schematic had been pasted in to talk about....since posting there are now schems appearing which have caps *after* the series resistor ...my earlier comments were just a reply to a more simple line of questioning  "does adding a series resistor increase or decrease input impedance" :-)

[ashcat_lt]

A guitar pickup is essentially a fairly large inductor in series with a middle-largish resistance in parallel with a smallish capacitor.  It is not a pure resistive load, but a reactive one with different "R" at different frequencies.  There are generally also pots parallel to this, and there's the middle-ish capacitance of the cable there also.

The resistor in series with the input does tend to add to the impedance at the end of this chain, and if there was no capacitance to ground afterward it would add some brightness.  That added brightness comes on a logarithmic scale, though.  Once the bias resistor gets big enough, it needs to get a whole lot bigger for a smaller amount of change.  1K is well within 1% tolerance for a 1M resistor, and would barely make a measurable - let alone perceptible - difference.

I think it was PRR who recently said "everything useful is a voltage divider".  I'd started to catch on to that not too long ago myself.  Once you start seeing the "legs" of the voltage divider in amongst all those components things start to make a bunch more sense!

[PRR]

> to lop off a very small amount of RF

And how? Again, what else? If a 1Meg resistor and an infinite-impedance device (TL072 and small JFETs come close), 100K and 1Meg is a 10:11 or 0.90909 voltage divider to *infinite* frequency.

Sometimes there's important "parts" not seen. A 12AX7 tube input is >100Meg plus 100pFd capacitance. 100K against 100pFd is flat to 16KHz then rolls-off above. So 10K-100K series with a reasonable capacitance (a cap, or a parastic of some other part) is a good RF protector.

Again: a JFET input is a diode. If you apply 28 Volts (say, a speaker signal), it tries to conduct huge current and blows-up. However if there is 100K in series, it will only conduct 28V/100K= 0.3mA. Most little diodes will stand 1mA all day. Many JFETs will stand 10mA for a while, so even 10K will protect to 100V which is more than you should find laying around a stage.

And again you find these little resistors in series. 100r-1K into a MOSFET gate may make sense: these things can go-crazy at high radio frequency just connected to a little wire. Sometimes as little as 100r spoils their fun.

But RF supression with 100 ohms? Even if you hardly dim the low end of the AM radio band (500KHz) you need 3,000pFd against 100 ohms. But that's similar to a 100 foot guitar cord working against the rather high impedances of a guitar pickup. It really loads-away all your treble. Some fuzzes may do that; mostly it's too brutal.

Sometimes I think a designer threw-in a few hundred ohms for no specific reason. It does give a place to use 10K-100K if that becomes necessary.

The WAH pedal (and some others) are a special case, Inverting Amplifier.

[pinkjimiphoton]

guys, dumb newb question, so hate to hijack the thread, but doesn't putting a resistor in series with the input change the output of the guitar from a voltage to a current as far as the input of the circuit is concerned?

i seem to remember from the tube amp days that you could use 1k resistors in the bias supply of some amps so you could read the bias current across this resistor..
instead of reading the bias voltage, you could read the tubes bias draw in milliamps, instead..

hope that makes sense.. like i said, dumb newb question.

[midwayfair]

guys, dumb newb question, so hate to hijack the thread, but doesn't putting a resistor in series with the input change the output of the guitar from a voltage to a current as far as the input of the circuit is concerned?

i seem to remember from the tube amp days that you could use 1k resistors in the bias supply of some amps so you could read the bias current across this resistor..
instead of reading the bias voltage, you could read the tubes bias draw in milliamps, instead..

hope that makes sense.. like i said, dumb newb question.

Hijack away ... this is really important for the Circuit Salad compressor that prompted this question, since I believe that it works on current-control instead of voltage-control.

[darron]

Geofex: What are all those parts for?

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

shows one example (Rrf), but only with the capacitor to complete the rolloff network. In his version mr R.G. reminds us that we shouldn't reply on just a capacitor without a resistor for rolloff. you guys are telling the other story, don't rely on just a resistor without a capacitor.

it's not mentioned there that the resistor helps to protect the circuit, but then there are those diodes (Dp) so even better.

[alexradium]

sometimes 1k is used to protect the opamp from static surge