SUT loading calculations.

[bob orbell]

One for the moderators which I am sure will help all that don't know how to do the maths to arrive at the value for the above, I was going to put something on, but I am sure it needs to go in the FAQ's, it is very simple with the help of on line parallel resistor calculators, but if one of the mods can lay down the procedure ie 125 ohm load squared = 15625 so 47K with 23.2K gives the nearest figure for the added resistor, but explain in more detail than I can it would be very helpfull. Bob

[John Caswell]

Hi Bob et al,
So you start with the load required by the pickup cartridge - say 100R
Then step -up ratio of the SUT - say 1:10, primary:secondary (usually written as 10:1
The impedance ratio is the step-up ratio squared -100:1
So for a reflected cartridge load of 100R you will need a secondary load of 10K ie 100R x 100:1 = 10000 = 10K.
As you already have a 47K grid leak resistor in Pre2/3 all you need to find is a resistor in parallel with the 47K that gives you a secondary load of 10K. 13K does the trick.
You do not have to be pedantic about it as long as the reflected load is near 100R.
Hope that helps

John

[bob orbell]

Thank you John, but I was not asking the question of how to calculate, I know how, I thought it a good idea to put the equation and wording in the FAQ for all to see, my reasoning is that I have in the past been asked by no fewer than four members, "how do I calculate the value of the extra resistor" I know there is a section in the build sheet with some loading figures, but no explanation of how to arrive at these figures and some may wish to try different values of loading, for example, some Ortofon cartridges recommend a load of 50 - 500 ohms and the build sheets only go as far as 100 ohms. This can and I think is very confusing for those that don't quite understand maths. Bob

[John Caswell]

Well the thread heading said "SUT loading calculations" so that is what you got.
Judging by your response what you require is the resistors/capacitors in series/parallel formulae, which are some of the basic electronics building bricks.
So....
Resistors in series - Rx = R1+R2+Rn
Resistors in parallel - 1/Rx = 1/R1+1/R2+1/Rn. For 2 resistors this is simplified as
(R1xR2)/(R1+R2) =mnemonic - "product over sum"
As an additional point 8 identical value resistors in parallel will give you 1/8th of the resistor's value eg 8 x 8R2 in parallel = 1R close enough

For capacitors the exact opposite applies

Capacitors in parallel - Cx = C1+C2+Cn
Capacitors in series - 1/Cx = 1/C1+1/C2+1/Cn Once again for 2 capacitors this is simplified as (C1xC2)/(C1+C2)
Once again as an additional point if you have 8 identical, capacitors in series the the total capacitance is 1/8th of the capacitor's value. Eg 8 x47µF in series will give you 6µF approx, and before the pedants among you say my maths is poor electrolytic capacitors are often +50% -20%.
It is reasonably rare that you use more than 2 capacitors in series generally they are in parallel to make upside odd value.

The simplest way nowadays is just to "Google" for a suitable table or tables on the web.

John