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Old 31st January 2011, 02:59 PM   #21
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Download PSU Designer 2 from Duncan Amps and play with different values of R in the CRC to see the effect. Higher values of R will result in lower ripple, but lower rail voltage and higher dissipation in the R. Engineering tradeoffs...

The Mini A will sound good whether you use a CRC or just a bunch of C.
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Old 11th January 2012, 10:51 PM   #22
slomatt is offline slomatt  United States
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Quote:
Originally Posted by jleaman View Post
in the psu board, just stuff ALL the holes for the crc with 0.47 P0.47W-3BK-ND's it will work perfect for the mini a's.

Jase
Quote:
Originally Posted by Beftus View Post
The other resistors form a first order RC filter with the final PSU cap. Example: With a 1 ohm resistor and a 10000uF cap in C3 you get a cut-off frequency of 15.92 Hz using this formula: 1 / (2*Pi*R*C)

The five resistors you can place are in parallel, choose a combined value for R, then calculate which values you have to place in parallel to get your value for R. You will want to use four or five 3Watt resistors in parallel so they share the load. Mount these resistors at least 0.25" off the board, they will get hot.
I'm in the process of ordering parts for a Mini Aleph build and just need to finalize the list for the power supply.

The "Power Supply Parts List" spreadsheet comments that for R3-R12 "for the standard CRC configuration, no R is needed". This would mean that the standard CRC configuration is in fact just a CC and does not act as a low-pass filter to remove 60Hz line noise.

In other posts people recommend setting the filter at 20-30Hz, which can be calculated using the equation above.

jleaman recommends using 0.47ohm resistors in "ALL the holes", which would result in 5 resistors in parallel (0.094ohm) for each side of the power supply board. Assuming C3 is 10000uF this results in a cutoff frequency of ~170hz, which is well above the target 60hz frequency.

It would seem that the target resistance for a 20Hz filter would be 0.795ohm which is 5 3.98ohm resistors in parallel.

Can anybody see any issues with my calculations?


To calculate power dissipation of the resistor array we'll need to know the voltage drop across the array or the current through the array. I read somewhere that on the mini aleph you can assume 1A draw per channel, but this seems low. Using P=I^2R this would put total power dissipation for the 0.795ohm array at 0.795W which is only 0.16W per resistor and seems very low. Am I missing something?

As always any input in much appreciated. Some of the parts on the original chipamp.com build sheet are no longer available, so when I'm finished I plan to post my substitutions in case they are useful to someone else.

- Matt
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