# Modifying Amp Highpass (resistors)



## letzleta (Jan 11, 2007)

I have been looking into modifying a highpass on a sub plate amp by changing the resistors.

I recently contacted the manufacturer of my amp to get the correct resistor values since they weren't listed on the chart. However I cannot find resistors to match the values I was given.

I was told: "In order to reach 15Hz highpass you can try R17 = 26k1 and R18= 90k9"

I think it is weird he used the word "try", like these may not be the correct values. Anyway, I cannot find any 26100 Ohm or 90900 Ohm resistors. Are resistors meant to be combined to achieve the correct values? For example, will 3 30k resistors equal 1 90k resistor?

If not, can someone tell me where to find 26k1 and 90k9 resistors? Thanks


----------



## brucek (Apr 11, 2006)

> Anyway, I cannot find any 26100 Ohm or 90900 Ohm resistors. Are resistors meant to be combined to achieve the correct values?


Those value are exact 1% resistor values. Resistors come in 1%, 5% and 10% variety.
For 26100ohms and 90900 ohms the closest values for the three types and the errors involved are:
1%(26100 @0% error), 5%(27000 @3.4% error), 10%(27000 @3.4% error)
1%(90900 @0% error), 5%(91000 @.11% error), 10%(100000 @10% error).



> For example, will 3 30k resistors equal 1 90k resistor?


Yes.

brucek


----------



## letzleta (Jan 11, 2007)

I don't see any 1% resistors on Radio Shacks or PE's sites. Does anyone know where to get 1% resistors? Or should I just use 5%?

I would like to be as precise as possible, while not spending an arm and leg to ship these little things.


----------



## brucek (Apr 11, 2006)

> Or should I just use 5%?


Just use 5%. They're readily available.
Use standard [5% 91K] and [5% 24K in series with (2.2K or 2.0K)]

bruck


----------



## letzleta (Jan 11, 2007)

brucek said:


> Just use 5%. They're readily available.
> Use standard [5% 91K] and [5% 24K in series with (2.2K or 2.0K)]
> 
> bruck


Boy am I glad you said in series, because I didn't even think about that. How would you reccomend I attach the EDIT:resistors to each other?


----------



## brucek (Apr 11, 2006)

capacitors?


----------



## letzleta (Jan 11, 2007)

Sorry, I mistyped... Meant resistors.


----------



## brucek (Apr 11, 2006)

> How would you reccomend I attach the EDIT:resistors to each other?


Ahhh, OK.
Well, if the single resistor is on a printed circuit board, remove it and just solder one end of each new series resistor into the spot where the single resistor was located and solder the tails together by bringing them together in a teepee shape...

brucek


----------



## letzleta (Jan 11, 2007)

Sounds good.

Thanks for all your help Bruce.


----------



## letzleta (Jan 11, 2007)

Does using 2 resistors in series cause any issues?

If I am correct in assuming 5% tolerance means a 100k resistor is within 5% of 100k (95k-105k), then 2 5% resistors in series would create a 10% tolerance series, right?

The reason I ask is I cannot find 91k resistors or 24k, the closest I have found is 2x47k=94k and 22k+4.7k=26.7k. I found 1/2 watt 22k+3.9k=25.9k and 47k.

So, my options right now are:
1/4 watt 5%:
2x47k=94k and 22k+4.7k=26.7k
or
1/2 watt 5%:
2x47k=94k22k and 22k+3.9k=25.9k

What would you do Bruce?


----------



## Otto (May 18, 2006)

letzleta said:


> Does using 2 resistors in series cause any issues?
> 
> If I am correct in assuming 5% tolerance means a 100k resistor is within 5% of 100k (95k-105k), then 2 5% resistors in series would create a 10% tolerance series, right?


Yeah, worst case you could now have that big of a swing. Can you buy a handful of each and measure them till you get what you want? How about a potentiometer?


----------



## letzleta (Jan 11, 2007)

Potentiometer??

Not sure what you are talking about there.


----------



## Otto (May 18, 2006)

Oops. A potentiometer is a type of resistor that you can set to any value within its range by turning a screw -- aka "varistor" (variable resistor) Here's an example of one available at RadioShack.

There are different shapes and sizes, and diffent resistance ranges. If you can fit them into your application, then you can easily adjust them to get the target values you are looking for. Then, if you want to tweak later to a different cutoff frequency, all you have to do it turn the knob! Of course, you'll need a tool to measure the resistance. If you don't already have one, there are lots of DMMs out there for reasonable prices. I have a nice one that a friend gave me, and I find it to be a very useful tool for lots of stuff.


----------



## brucek (Apr 11, 2006)

> The reason I ask is I cannot find 91k resistors or 24k, the closest I have found is 2x47k=94k and 22k+4.7k=26.7k.


Fine, but the tolerance overall doesn't change. A 47K with a 5% tolerance can swing 2.35K, and a 91K 5% can swing 4.55K. The two 47K equal the 5% tolerance of the 91K. Right.. 

The tolerance is the guaranteed spec of resistance. Measure the resistors with a meter and see what you got.

Use the same or greater wattage that was there before the change.

brucek


----------



## letzleta (Jan 11, 2007)

Very good point bruce...

2 47k 5% tolerance resistors in series will have a 10% tolerance, but it is 10% of 47k which roughly equals 5% of 91k.

Thanks for the specifics, it was like a lightbulb going on after I read it.


----------



## letzleta (Jan 11, 2007)

Does anyone know if this mod has been done successfully? If so, I would like to be able to talk to anyone who has completed it. The resistors are located in a tough to get to location and I would like to hear how people have gotten to them.


----------

