[justhavingfun]

Tangent's recommended resistor values for my STEPS:

R1 1/4w 1M ohm
R2 1/4w 100 ohm
R3 1/4w 120 ohm
R4 1/2w 2k ohm
R5 1/4w 2.2k ohm

But I can't seem to find proper 2k ohm resistor for R4 and closest resistor value that I can locate is 2.2k ohm. Can I use the 2.2k ohm instead of 2k ohm? Since all above resistors are 5% tolerance value, I figure 2.2k ohm can be used instead of 2k ohm resistor on R4.

 

[ruZZ.il]

This really depends on your desired output voltage. R4, in conjunction with R3, set the output voltage range (adjustable with the VSET trimpot). with R4=2.2kohm and R3=100ohm you get an adjustable output of 29-35V. if you're using this solely with amps that can take this voltage, thats fine (check your opamps datasheets or amp specs).

The formula he gives for Vout is:

Vout=1.25*((R4+VSET)/R3+1)+0.0001*R4

so with R3=120 you'd get 24.4-29.6V output range. (with a 500ohm trimpot)

you shouldnt go any higher than 120ohm for R3 cause it it sets the current the regulator uses and this shouldn't be below bout 10ma

Tangent talks about this in his STEPS Parts Selection Guide.

now, you should set your voltage wisely, too. I set mine to be just above what my opamps can take. a little voltage is lost mainly through diodes on the way, in the amp, so my opamps get just below their max voltage.

what I didnt think of when building my steps was the Regulator voltage drop, this is the voltage difference between the unregulated DC and the regulators output. I used a 30V trafo(unloaded) which is about 37V unloaded.. (effective AC voltage, peak is about 1.4*Veff and the unregulated DC is pretty close to the peak..) minus a few volts lost on the rectifier bridge so, partly loaded I got something like (35-1.4)*1.4=Vunregulated=47V and I regulated it down to around 26.5 which is a 20V voltage drop.. which causes my regulator to heat up quiet a bit (not sure if I'm right here.. but with half an amp.. 20V drop.. its dissipating 10W in heat). I'm waiting to get a 24v transformer(unloaded), giving me about (27-1.4)*1.4=36, 36-26.5 is just under 10V to regulate, so its regulating half the voltage.. so I hope it'll generate half the heat.
(a bit of babbling about this, with much help from others getting me set straight, is here)

you should take a look at the Power Supply Parameter Estimator too.

hope this helps a bit

-ruZZ

 

[justhavingfun]

Thanks for the helpful info ruZZ. I am building my STEPS to be used with LaRocco PPA amp that I recently bought. I wanted to do some comparison test using different quality power sources, if that makes any sound quality differences to my ears. I also have several different opamp combo that I can try with my PPA. So far, I have a generic PS (it looks like it was used for powering laptop) that came with the PPA, Elpac wallwart, a recently built TREAD, and thus now in the process of building STEPS. I wanted to have my STEPS to give out about 24V in the middle range. And after reading Tangent's helpful tips, I came up with above resistor values to give me 24V middle range. I guess I need to hunt down proper valued resistor. Again thanks for the help, ruZZ.