I want to finish a build but am short a 27r resistor. Is there a way to stack a larger size with a capacitor to come up with the same value?
You can put two or more resistors in series, in which case their sum is the total resistance (16 + 11 = 27 for example.) Or you can put two or more in parallel, in which case the sum of their reciprocals is the reciprocal of the total resistance (62 and 47 in parallel are 26.7, which is close enough, for example.) This calculator will let you put in your target resistance (27 ohms) and will give you several combinations of larger resistors in parallel that will give you that. http://www.sengpielaudio.com/calculator-parallel.htm
Rad. Thanks. I knew about adding in series but I didn't know I could go smaller with parallel.
Quote from: SprinkleSpraycan on July 03, 2022, 08:31:30 PM
Is there a way to stack a larger size with a capacitor to come up with the same value?
Of course you can but for a specific frequency only...
Just wire the capacitor across the resistor..
(in parallel, like MikeA told you..)
Quote from: antonis on July 04, 2022, 05:19:46 AM
Quote from: SprinkleSpraycan on July 03, 2022, 08:31:30 PM
Is there a way to stack a larger size with a capacitor to come up with the same value?
Of course you can but for a specific frequency only...
Just wire the capacitor across the resistor..
(in parallel, like MikeA told you..)
So if I'm trying to reach 27r, how to I calculate which resistor and capacitor to combine? Will a capacitor create a lag time? I doubt I would hear it but I'm curious.
If it is a cap and R in parallel, if you add 2 R's to get the value and bridge them with the cap, the response of the network will be identical to the original design.
If you post a pic of exactly what you're doing, we could give you a lot more precise info :)
Quote from: SprinkleSpraycan on July 04, 2022, 09:41:42 AM
So if I'm trying to reach 27r, how to I calculate which resistor and capacitor to combine?
It depends on resistor values availiability..
For resistors of value bigger than 27R, you'll have to wire the capacitor in parallel where for resistors of value smaller that 27R you'll have to wire the cap in series..
Capacitors exhibit a "resistance" (Xc) calculated via Xc = 1/2πfC, where f = frequency of interest and π = 3.14..
33R // 3.3nF (at 318Hz) -> 27R
22R + 100μF (at 318Hz) -> 27R
WHY are we talking capacitors at ALL???
This misses the problem of "smaller resistor".
> Capacitors exhibit a "resistance" (Xc) calculated via Xc = 1/2πfC
And since "audio" covers 100:1 of frequency, this is not useful for a "resistance" problem.
Quote from: PRR on July 04, 2022, 04:16:11 PM
WHY are we talking capacitors at ALL???
Because that was OP's itch.. :icon_smile:
>Is there a way to stack a larger size with a capacitor to come up with the same value?<
And we still don't know what fragment of what sort of circuit this resistance and/or capacitance value is applicable to.... :icon_confused:
Quote from: GibsonGM on July 04, 2022, 11:40:07 AM
If it is a cap and R in parallel, if you add 2 R's to get the value and bridge them with the cap, the response of the network will be identical to the original design.
If you post a pic of exactly what you're doing, we could give you a lot more precise info :)
Forget "capacitor". Folks are going off on a tangent because you unwittingly mentioned capacitor in your opening post. Mike's first reply is what you need.
Quote from: bluebunny on July 05, 2022, 05:02:55 AM
Forget "capacitor". Folks are going off on a tangent because you unwittingly mentioned capacitor in your opening post. Mike's first reply is what you need.
:-[ :(
We still love you, Antonis! ;D
(and also, perhaps the 27Ω value isn't hyper-critical and you could sub in something that's close that you already have, but we don't know because we haven't seen what circuit this applies to...)
^^ This is also a good point. I'm guessing a value that low is part of some basic power filtering? In which case a piece of wire would suffice for the time being. Or any other low-value resistor you have to hand (<100R). Show us the schematic and we'll know for sure.
Wow! Such enthusiasm. It's for an OCD clone. The adhd from pedalpcb. It is the first resistor after the diode at the incoming 9v. I used a 51r in parallel with 47r as those where the values I had in hand that was closer to the calculator. Pedal sounds great and thanks to everyone. I said capacitor not knowing I could use resistors in parallel. I've only been doing this since November so every project is a new learning experience.
Looks good too. I went a more aggressive direction than the fulltone art.
(https://i.postimg.cc/LnBcFnLq/IMG-20220704-115149-060.webp) (https://postimg.cc/LnBcFnLq)
Quote from: SprinkleSpraycan on July 05, 2022, 08:02:02 PM
It is the first resistor after the diode at the incoming 9v.
Yep, part of the power conditioning. Your solution fits the bill (the value isn't critical). Good that you've got it all working to your liking. Nice build. :icon_cool:
Quote from: bluebunny on July 06, 2022, 04:04:15 AM
Your solution fits the bill (the value isn't critical).
I suppose OP would like to know WHY particular resistor value isn't critical.. :icon_wink:
(after all, he declared that: "every project is a new learning experience."..)
(https://i.imgur.com/iyFp2Ih.png)
So Marc, could you plz give us some info about power supply LPF (R20/C11) and Vcc voltage (compared to (+)) regarding circuit's current consumption..?? :icon_mrgreen:
MY farmer's "analysis" is that 27R/220u LPF gives a cutoff of ~27Hz. This is well below normal ripple voltage for a common pedal power supply.
The divider is 20k total; so you have 4.5mA going thru it. The 27R allows ~300mA "on tap", so that is more than fine (greater than 10x), and the opamp certainly can't be within that range of current requirement!
If you changed 27R to 47R, your cutoff would be ~15Hz, with almost 200mA available...better filtering still, tho less mA for the opamp (did not check how many mA the opamp needs). A lesser value R would raise the cutoff frequency and also have to dissipate more power.
Doesn't the in-line Schottky D1 also contribute significantly to the LPF with C11?
Based on the data sheet, D1's capacitance is only 110 pF....only a concern at RF...
I probably don't know what I'm doing, but when I simulate an RC LPF in LTSpice, like a 27R in series and 220uF to ground, I get your corner frequency figure of ~ 27 Hz. If I then replace the 27R with a 1N5817 in the forward direction, I get a lower corner frequency and a faster roll-off. Can't say I understand the mechanism, though...
@Sir Mike: I think that 4.5mA isn't the sole current consumption.. :icon_wink:
@MikeA (w/o address): In case you implement an AC signal in LTspice, 1N5817 acts as half-way rectifier..
Of course not...I can only 'estimate' what the opamp will consume at this time, however. :icon_lol: I'm a potato farmer / house painter.
Do tell, Antonis!
Mike & antonis, I breadboarded it this morning to ensure LTSpice wasn't lying to me. The diode + cap do rectify AC and establish a small DC offset. If we're wanting to filter AC noise on a DC bus, diode + cap work pretty well. With a diode present, adding a resistor in series (as in the OP's circuit) would do more to reduce the supply voltage than to reduce any AC signals.
Quote from: MikeA on July 07, 2022, 07:47:55 AM
The diode + cap do rectify AC and establish a small DC offset. If we're wanting to filter AC noise on a DC bus, diode + cap work pretty well. With a diode present, adding a resistor in series (as in the OP's circuit) would do more to reduce the supply voltage than to reduce any AC signals.
I'm not sure if I follow you right, Mike..
It should more enlightening to make a discrimination between
AC waveform (+/- fluctuation around x-axis "zero" ordinate) and
DC ripple (only + fluctuation much higher than X-axis)..
The former is semi-rectified by series diode (with a diode forward voltage drop "gap") where the later simply maintains the ripple unaffected..
(it just lowers the ripple waveform by a forward voltage drop..)
(https://i.imgur.com/Glo3qBf.jpg)
Quote from: SprinkleSpraycan on July 05, 2022, 08:02:02 PM
Wow! Such enthusiasm.
Yup. Thats the norm around here. Ask a question, get accurate answers then stand back as the discussion deepens and wonderful snippets of information emerge for the willing to pick up. On subjects only vaguely associated with pedals sometimes. Rare precious and beautiful this place.
Glad you got it working