some question about soz


diyAudio Moderator Emeritus
2002-01-21 2:42 pm
near Atlanta, GA
Speaking of heatsinks, would two of my previously mentioned heatsinks:

8" x 9" x 2.6"
baseplate - 1/4" thick
19 fins - 8" long x 1/8" thick each


What wattage SOZ with one of these for each channel? Is two of these enough for a stereo 5W SOZ?

Also, would it be possible to use IRFP044 units for the 5W SOZ? The specs are:

VDS=55 RDS=0.024 ID=49 IDM=180 PD=100


Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
IRFP044's would be just fine.

The sinks in the picture could probably handle 100
watts, and so would certainly be able to do a 5 watt

One of the tricks to sinking the SOZ is to isolate the
resistors from the transistors. The resistors can run at
higher temperatures.

Also, consider using light bulbs as in Zen-lightenment,
which solves the problem of sinking the resistors, and leaves
you with lots of margin for the transistors.
I would consider building a 10 watt SOZ as the minimum. At 10 watts on demanding music I found at times the amps ran out of steam. At 15 to 20 watts the proplem was gone. A 15 to 20 watt SOZ is a pretty big thing to build. You are pretty much forced to make a pair of mono blocks if you want to maintain your sanity. Going from 5 to 10 watts is not much more work. And you could squeeze that into one chassis. SOZ is a major chassis and heat construction problem. While, the basic electronic circuit is really nothing to complete in comparision.

If you have speakers that are a very easy load then 5 watts could be a lot of power. But, you may be restricted from using other types of speakers in the future.


diyAudio Moderator Emeritus
2002-01-21 2:42 pm
near Atlanta, GA
If I built the soz using light bulbs as in Zen-lightenment as Nelson suggested, this should alleviate quite a bit of heat dissipation on the heatsinks, and if I was to use two of the above heatsinks, I could probably get 10W or so. Is it true that the resistors are the source of most of the heat? What is your opinion on using lightbulbs for an soz?

[email protected]


2001-08-17 6:00 pm
The major obstacle in a SOZ will be the heatsinks to bring the operating temp. of the resistors to a comfortable level. And this would be a really big chunk of metal at higher power. Bulbs runs white hot, operating resistors at this temp. doesn't sound reliable.... Bulbs are way cheap compared to the power resistors and the associated hardware.
The SOZ light bulb idea is very clever. I do not know how long the bulbs would last. But over the long term it could cost more. Looking in the Mouser electronics catalog I see Vishay 50 watt 16 ohm 1% wire wound power resisters at $4.80 for one and $3.60 for ten or more. Using two of these in parallel will give you the 8 ohm resistror value. You need 8 per channel or 16 total for a cost of $62.40. You can use the Vishay 25 watt 1 % wire wounds for the 1 ohm resistors. 6 at $2.70 costs $16.20. You now have all the power resistors you need for a 10 watt SOZ with no bulbs to replace. The 1 ohm resistor R7 in the PassLabs's circuit drawing has a low load and does not have to be mounted on a heat sink. This gives you more room on your heat sink. Mount resistors on separate heat sinks from the MosFets. I would find another pair of heat sinks and build it this way. Also, this is very subjective. The light bulbs just look bad. I am a DIYer but I still like my stuff to look good and last.
I think your missing Nelson's point.

The heat dissipated by the resistors is huge ..... and they need their own, quite generous heatsinking:

The heatsinks on top of this beast are for the resistors only and run at 270F (this is not a mis-print) if I don't use a fan.

This problem was the inspiration to the light-bulb solution ...... as bizarre as it first seemed :)

cheers, mark
SOZ bias-SOZ loads?

I have some amateaur questions....

-How can I calculate the bias for the SOZ? ( I know that I=U/R, but with which resistors need I calculate? )
-Should I use 4 or 16 ohm speakers for the SOZ? ( Iknow the 8 Ohm is the best...)
-What would happen if I would use for the load 16 Ohm resistors instead of the original 8ohm?