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A resistor question for CAT SL-1...help needed

Hello. I have a question about resistor values and what I should be looking for to perform this modification. I don't have the rca phono loading plugs that came with the unit so I'm attempting to make my own.

I need to add a couple of resistors to my preamp phono section (CAT SL-1) to match my cartridge output of 100 ohms (Shelter 501 Mk II, low-output mc). I'm confused about what value I should get. My preamp is currently set for 47K ohms which is not the recommended setting for my cartridge.

I'm looking to experiment with values of 100 ohms to 1000 ohms but I don't know what resistor watt value to buy...ie, 1/4 watt, 1/2 watt, etc. Also, what tolerance should I get...ie, 1%, 2%, 5%, etc.

Your help or suggestions is greatly appreciated. Thanks much!!
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Showing 1 response by marakanetz

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marakanetz
5,473 posts
 
A 1/4Watt would definitely suffice for input resistor for the cartridge.

The simple rule is that the total resitance if you connect a resistor R and 47k input resistance in parallel will be smaller than R. Since 47K is much larger than 100Ohm you may
a)replace 47K resistor with 100Ohms
b)Connect ~101Ohm resistor in-parallel with 47K Ohm resistor.

The general formula for your application goes from the following formula of the resistors connected in-parallel:

Rtotal = R1*R2/(R1+R2)

where Rtotal is the resistance you desire, R1 is 47K resistor and R2 is the actual resistor you need to match your experimented values

Rearranging the formula for R2 as follows:

R2 = [Rtotal(R1+R2)]/R1

Using these relationships you can build your own resistor bank that will step thorugh as many values as you desire until you find the best load resistance.

You may get a RadioShack project breadboard for such xperiment.
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