recalculating resistor values


2003-11-20 6:25 pm
howdy ;)
I'm about to build Rod Eliot's DOZpreamp from his site. He states that transformer should be 16 V AC, yet the one I have is 20V AC so I need to recalculate values of R1 and R2 (100R). I thought of putting R1 120R and leave R2 100, am I right or someone please correct me...

Heres Url:

and pic of PS:


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It will work with whatever value you pick for R1/R2, with higher entrance voltage there will be more current flowing through the BD139 to make voltage drop to 30v over R1+R2.
By choosing higher values for R1 and R2 you make life easier for the transistor, above 1 watt dissipation it will need a heatsink.

Instead of a bit over 40Vdc on entrance, dropping to 30Vdc, with 20vac secondaries entrance voltage will be above 50 volts.
The voltage drop over R1 and R2 will be around twice as high.
If you wish nominal current level to remain at 50mA, R1+R2 need to be 400 ohms instead of 200.

If you go for lower values you will need much higher power rating for R1 and R2.
To drop 20 volts over 120+100 ohms it takes 91mA.
Means R1 at 120 ohms needs a power rating of at least 1 watt.
If you choose R1=200, R2 = 200 ohms, the minimum is 0.5 watt, and half the thermal load on the BD139.
And choosing R1 as 200 ohms changes the corner frequency of the R1-C3 low pass filter to 0.36 Hz from 0.72Hz, even lower ripple.