SOZ Temperature

Question about SOZ operation temperature:

I built SOZ with the following:

-230V~25-0-25V 600VA toroidal transformer;
-2 x 150000uF Capacitors;
-there is 30.0V+30.0V DC on the capacitor terminals after the circuit is on

I used a 375 x 200 mm Aluminium heatsink from alutronic (PR170) per channel,
and greased all the components including the resistors that are rated at 50W

The temperature at the heatsink after 30mins of operation is about 100ºC!!!

What do you guys think of this?

Will it burn up in a couple of hours? days?


Pedro Oliveira
[email protected]
If you are using only one of these heatsinks you need about three more. Realisticaly you should be able to touch your heatsinks. At 100 deg. C you should be able to cook food on them, forget about touching them. Are they turning red when their hot? I built the same rating that you've got and with the heatsinking that I used my amps weighed in at 45 lbs. apiece. At the temperature yours are running I don't think they are going to last very long. The problem with that, when it happens, unless you have some sort of speaker protection circuit hooked up you might lose a good set of speakers. Be Careful.:eek:
Pedro I'm not sure about the size in mm, but if they are a little bit taller than a coke can I used 4 per channel. The extrusion # was 6209 ahamator. They have a web site but I don't know the address. It's under Ahamator or Wakefield if you do a search. My amps ran at about 50 deg.C or a little cooler. Good Luck;)
when one of those mosfets fails because of the heat, one of two things can happen. If the Mosfet fails open, you will get DC through the speakers, and you will likely burn out a speaker. If the mosfet fails shorted, you can also hurt a speaker, as well as overload the power supply. Make sure you have the amp is properly fused.
turning red


I'm not sure if you were kidding or not when you asked if Pedro's amp was turning red, but that wouldn't happen until it's in the vicinity of 750 degrees C. Aluminum heatsinks would melt before they started to turn red.

I spent a long time in the ceramics kiln business, and we considered that temperature as "starting to get warm." :)
Re: turning red

vpharris said:

I'm not sure if you were kidding or not when you asked if Pedro's amp was turning red, but that wouldn't happen until it's in the vicinity of 750 degrees C. Aluminum heatsinks would melt before they started to turn red.

I spent a long time in the ceramics kiln business, and we considered that temperature as "starting to get warm." :)

so i take from that that you are more a white/blue = hot kinda person :)
...yes i know....


I am building a SOZ to about 50 Watts per channel. An i mis-calculated the Heat-sinks, too.

i`m going to use one Big Heat-sink for each resistor, about 46 pieces 330*165*60 mm.
I have 46 Pieces of 50watts resistors from Dale.
And a big one for the Mos Fets.

This amp will be verry big and heavy !!!- and expensive.

I thing to be finished with it in the middle of this year and will do a dokumentation in the pass sites.



Most of my experience with metal has been steel, but thanks for the info.

You can measure your own wattage at the output on the speaker terminals. Use the formula: V squared/resistance. This will tell you but look fast at your voltmeter because its going to move like a jack rabbit. Look for peak and then average. has sold heatsinks before as well as matched transistors for some of Nelsons designs. He is in Germany which should be closer to you. He is also a very nice and helpful guy. Good Luck;)


2000-12-31 9:31 am
the point that no one seem to have addressed in the replies is that 100 deg C maybe hot for carbon based life forms like you and I but for wire wound resistor rated at 250 C is not too bad. For metal film resistors and semiconductors it's a different story. Sure it would be better if everything was operating at 40 C but lets face it, unless you want to put 100 Kg of aluminum in your livingroom that's just not gonna happen.
My advise to you is buy more heatsinks, re-distribute your components on them making sure that the mosfet don't get hotter than 50-60 C and everything will be ok.
SOZ Power Rating and Heat Sinks

I Have built a 50 watt version of SOZ. The best thing to do is to mount the mosfets on different heat sinks then the power resistors. The power resistors produce far more heat than the mosfets. If you can mount each mosfet on a separate heat sink and mount them inside your chassis using nylon screws and washer to isolate the heat sink from the chassis. Then you do not need the mica insulators. Getting rid of those mica waffers will allow far better heat transfer to your heat sink. The best insulator is no insulator. When using thermal grease. Apply the thinest layer to cover the part. Most of the diy projects I have seen have thick globs of thermal grease around the mosfet. To much thermal grease makes for poor heat transfer to you heat sink.

I would not make a 50 watt SOZ. I had a variable power supply and I found that a 15 to 20 watt version worked just as good. The speakers I used are not amp friendly. They are a large modern three way with a manufacturers stated 4 ohm impedance and 87 dB efficiency. I am sure they dropped below 4 ohms at times. This is what most speakers do. The impedance swings above and below what manufacturers claim. So, what happened when I turned the power up to 50 watts? The sound got louder. But, not by a large enough amount that I would consider worth the size and expense of going to 50 watts. I also thought at 20 watts it sounded a little better. At 50 watts the power supply, resistors and mosfets are being stressed which generates more noise. Another thing to note is even at 15 to 20 watts you think your listening to a much more powerfull amp. And I found that the low dampening factor was not a problem either. The very over built power supply I used may cause these to be less of an issue. I have been listen to tube equipment for years and I have to admit this is an amp worth building because it knows how to play music too.

People and their 50 watt SoZ amps!

I just find it funny how big of a challenge for DIYers wanting a full out 50 watt / channel SoZ amp. You guys must have deep pockets for buying large heatsinks.

For my SOZ amp

I find that it's the mosfets that get the hottest the most. The power resistors I used don't seem to give nearly has half as much heat as the fets - but maybe it's due to where the mosfets are mounted. I found that isolating the mosfet on a separate heatsink wouldn't make much difference or a waste as it's better to utilize the aluminum chassis as a heatsink also.

My guess, the temperature i've measured off my heatsinks (where it's hottest) is no more than 40°C at room temp. I can keep my hand for several minutes so i'm quite happy with the amount of heatsinks used. I might have enough heatsinks to get squeeze 40 watts per channel but with efficient speakers.. there's really no need.
you`re right

After getting more experienced about SOZ i will build my SOZ at about 35 Watts, thats more than enough.

Each Watt above 10 Watts kosts a lot of Money.
And I wish to have my SOZ for a long time and don`t want to change MosFet`s and Resistors one time a Year.

I thank Mr. Pass for such a schematic like the Soz and the Bosoz.

I have tested it against very good leak tube Mono`s and other Amps: I can tell: it`s better. But the sound depens on the quality parts you use: the better parts the better the sound.

When my SOZ, BoSoz is finished i will test it against the best Commercial Amps.