Something I've never understood about voltage dividers

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Mbira

Well-known member
Joined
Jun 4, 2004
Messages
2,422
Location
Austin, TX
In a voltage divider, say you are doing a 10/1 division.

Is there a difference between using a 1 ohm and a 10 ohm network vs a 10k and 100k network, etc.

Do people just grab what is closest to them in their bins, or do things like impedance come into play? 

 
depends what your driving afterwards. (i.e. what the load will be).

Also, depends on the current being driven through the resistors, (power dissipation).

Finally, Doug Self has a good chapter on this in his latest book, the potential johnson noise cause by the resistors, introducing noise to the signal.
 
Rochey said:
depends what your driving afterwards. (i.e. what the load will be).

In my case, I'm working with avery high impedance piezo pickup into a tube guitar amp, so I know those can handle high input impedance, right?

Also, depends on the current being driven through the resistors, (power dissipation).
Would this affect resistance choices, or just resistor wattage?

 
Piezo pickups don't just work with high-impedance loads, they need high-impedance loads. They sound even more like dog-crap when loaded with lower impedances.

So if you make a pad that drops 1/10, the series resistor needs to be something like 9 megs and the shunt resistor needs to be 1 meg. Which will generate a lot of Johnson noise. And unless the cable to the input tube is very short (like, say, 3 inches) you'll lose highs as the high output impedance of the pad forms a voltage divider with the cable capacitance.

Besides, why should you need a pad when driving a tube from a piezo pickup? They don't put out that much signal.

Please tell us more about what you're building!

Peace,
Paul
 
> say you are doing a 10/1 division.

Say you are doing a 2/1 division to get 120VAC to power a USA Fender Twin from a 240VAC line.

I have 1Meg resistors handy. Two of these, my Perfect Voltmeter shows 120V.

The Fender Twin probably draws 120 Watts from the wall. At 120V, that is 1 Ampere. We may approximate this as 120V/1A= 120 ohms.

Stick that on your 1Meg+1Meg divider. You now have a 1Meg+119.99r divider, and get less than 0.03V from your 240V supply.

DRAW THE WHOLE PLAN!!

If there is a load (and there always is), draw it and figure it.

> avery high impedance piezo pickup

Oh?

IDK, but a 1,000pFd piezo is 10Meg at the bottom of the audio band and 10K at the top of the audio band. You think it is high impedance because low loads suck bass, but it is fairly moderate further up.

The Fender is 1Meg+100pFd, so 1Meg to 2KHz declining to 100K at 20Khz.

Not an easy network.

Because the piezo won't be harmed by a dead short, if it is "too hot" the quickest way to get 1/10th the voltage is to put 10X the capacitance directly across it. Try 10,000pFd (0.1uFd). This will also make it less fussy about bass-loading.
 
Hi guys,
Here is a better description of exactly what I'm dealing with:
http://www.groupdiy.com/index.php?topic=39632.0

And thanks PRR!  I'll slap on a cap in the morning and see how that does.  The weird thing is people talk about "bass loss" with these things, and i generally have more bass than I know what to do with...
 
That's a superb explanation PRR.

I often wondered the same thing, but in regards to using two resistors of the same value to create a virtual ground for the non-inverting input when using opamps on single rail supplies.
 
Back
Top