SOZ virgin needs help!

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OK, I've given in. After a month of reading the Pass Labs forum (I don't know why, but I'd ignored it for months previously), I've decided to build SOZ.

I'm determined to build something that will not fall foul of the houshold authorities (my wife), nor permanently scar my inquisitive children. I've sketched out internal layouts for the amp and rekon I'm looking at the following (stereo) specification:

1.2KVA transformer
300,000u resevoir capacitance
4x 0.12 deg C/W heatsinks

Before I start commiting myself to a SOZ project far too low on one of these 3 parameters, I'd really appreciate some advice from you guys on just how far out I am - maybe with some rough and ready rules of thumb for me to bear in mind.

Thanks in advance!
Hi , Lenin

in pass sites you can see with how much Voltage of your transformer you get how much on Power.

With your Heatsinks i guess you can drive the SOZ about 20watts.

Nevertheless, i`m in Soz/Bosoz Project actally, myself and they work in eyaluatio since some month.
The Combination Soz/Bosoz is outstanding good in sound, depending on the parts -quality you use.


Your right, I didn't mention what output power I'm going for. From what I've picked up from threads on the forum, the "I've built a 50wpc SOZ" boys have got it all wrong, as 15 - 20w sounds fine (or possibly better - !) and provides enough power for real world speakers.

With this in mind, I'm aiming for about 20w.

As far as guidance from pass sites is concerned, I've not been able to find any! The Pass DIY site has info on Zen-lightenment only (at least it did yesterday, maybe the site's SOZ info is being updated or something - I don't know). With occasional references to power supply formulas popping up all over the place (but no links posted as to where!) you can imagine how fustrated I am!

Any danger of posting some links here for me guys? (I'm particularly worried that the transformer may just be too small)

Oh, one final newbie question (thanks for your patience). Isn't the rail voltage for SOZ a touch low? Wouldn't cranking up the rail voltage help to increase power output? (Excuse my ignorance!)

Finally, OK, SOZ is hardly a good choice for someone aiming to maintain domestic harmony, but there lies the challenge! Let me explain. I've looked at galleries featuring completed SOZ projects and I have to say, I really feel that people have generally been lazy with their implementaions, relying on the fact that the design needs to dissapate huge amounts of heat to justify amplifiers the size of small fridges in their living rooms. If my wife really were that accomodating (or just plain mad) I suppose a 1 and a 1/2 foot square anodized radiator would be fine. Unfortunately, she isn't.

So what I'm building is a pair of 'half width' 20w SOZ monoblocks. Dimensions: 5 in x 8in x 14 in (h x w x d) (Yes, you read that right). Sat together, the monoblocks should take up about the same space as an average CD player - about one third the size of typical SOZ implementations I've seen.

Now guys I can almost hear some of you scoffing as I type this info, finding it hard to believe that some idiot honestly believes it is possible to buils a SOZ the size of a CD player, but I'm happy to provide more info on just how I'm doing it.
Retired diyAudio Moderator
Joined 2002
That seems like too small of a form factor to put the monoblock 20W soz in. Reading over the article on PassDIY, it appears that the total dissipitation of the 20W zen would be 500W. That seems like it would require a lot of heatsinking, definately more then you are considering.

How were you planning on setting up the heatsinks? One for the resistors and another for the transistors?

I think that the best idea would be to get a variac and at what wattage you can obtain reasonable temperatures. I would imagine that you would have to make it 10W, from seeing others SOZ amplifiers.

I guess it just depends on how much of the 4"x8"x14" is heatsink.

500w dissapation - No problem!

Surely I'm ok on heatsinking?

Even at 500w total disappation for a 20wpc SOZ (I'm presuming that includes the power resitors dissapation requirements). Four 0.12 c/w heatsinks should be more than adequate.

Splitting the 500w dissapation load across the 4 heatsinks to give 125w per heatsink, were looking at a measly 15 degree temperature rise on each heatsink. (0.12 deg c/w x 125w).
Have I got this right?

OK, I've not considered the allowances that need to be made for thermal interface issues, the type of thermal interface material etc, etc... but hey, let me say it again, we are looking at just a 15 degree temperature rise on each heatsink! This will be one cool running 20w SOZ!

I'm far more worried about the power supply. If I'm to stick to my 'dinky monoblock minatures' idea, I'm really stuck on 600VA and 300,000u per channel. Is this on the wimpy side?

My SOZ output devices and resistors will SHARE heatsinks

Oh Brian, setting up the heatsinks.

No, I'm not using one heatsink for output devices and one for resistors in each monoblock. I've picked up bits and pieces in the forum on this issue (and the often repeated advice to mount them seperately for best sound quality). I don't doubt that there is an audible effect, but I suspect that what people are hearing is the resistors 'cooking' the output devices.

Paying proper attention to the arrangement of devices and resistors on the heatsink should minimise this problem for those running everything (heatsinks - and the devices junction temperature) pretty high. As I say, I'm aiming to have a 'cool' running (I know this is a relative term!) 20w SOZ, so this problem shouldn't arise.
are you sure you want to pop your cherry with the SOZ? Oh, by the way I think you will soon realize how off you are on your temperature rise estimates.
I would wait a couple of weeks for the new and hopefully better SOZ with CS. You maybe able to squeeze a few extra watts out of your trafo or save some $ on the electric bill.
Cherry popping now on hold!

A new SOZ with CS?!!! (Candle Sticks? Custom Spraypaint?)

You mean to say that Mr NP has been developing CS for the SOZ on the QT!

I didn't know anything about this. I guess it explains the absence of any detailed reference to SOZ on the Pass DIY site. Of course I'll wait. Thanks for the tip. And seriously, what is CS, by the way?

I don't suppose you could tell me whether 600VA/300,000u SOZ monos are just too wimpy.

I don't know if you thoght about it but if it's going to be a miniature SOZ with dimentions you specified are you sure you will fit 600W transformer and 300,000uF of caps inside the chasiss?
If you covered that side than my best bet is to use cooling fan and possibly variac if you have access to one and then you might even like your cake.
My original post starting this thread explained that the (stereo) spec is: 1.2KVA and 300,000u.

Somewhere in the twists and turns of the thread I decsribed the monoblock as being 600VA and 300,000u. Of course, that should have read 600VA and 150,000u. My silly mistake.

Yep, there is space (just) for 600VA - it's actually arranged as 2 300VA transformers. The intention is to independently rectify and feed each half of a balanced configuration arranged on 2 signal boards (hence 2 heatsinks per monoblock).

And finally, I'll come clean. Yes, it is a fan cooled arrangement. I've simply not mentioned it up till now because of the derision which seems to meet the idea of fan cooling whenever it is raised on the forums.

Personally, I feel fan cooling has had a bad press from the audio community and implemented correctly, is the answer to most class-A heat dissapation headaches.

Yes, fans are often noisy (Purchase a low noise fan)

Yes, a poor implementation can leave you injecting noise into the system (imlement some decent isolation for the power supply - incorporate it into the regulator even!)

Run the fan at a lower voltage. (Noise tends to increase logarithmicly with fan motors, running them at half voltage works a treat)

Finally, isolate the fans from the heatsinks and the chassin with an appropriately large guage of sorbothane or similar, to finally kill any remnants of noise.

I tried this out of desperation about 7 years ago, just accepting that the end result would probably be noisy, but that it was the only way to get my high bias power amp projet at the time on the road.

The end result (complete with sorbothane mounts, etc) astounded me. For about 5 years now, I've been using some truly incredible 12db fans from Papst (sorry, I don't have the web address, but they won't be hard to find). These really are the icing on the cake and surely it's just a matter of time before Class-A afficionados release that as long as class-A amplification is associated with scalding hot mini power stations dominating peoples homes, it will never reach mass acceptance. Fan cooling is the missing link, in my view. Unfortunately, it will only shake off it's unfair reputation as unthinkable for high end audio, when others on this forum are far-sighted enough to point out that it has generally been poor implementation that has led to the audiophile 'truth' that fans are too noisy for high end audio.

Now that should put the fan-cooled cat amongst the scorched pidgeons...

While I'm at it, why doesnt someone (with far more technical ability that me) come up with a nice little schematic that solves most peoples electronic objections? I'll certainly post mine as soon as I've got to grips with just how to do it!
CS is an abbreviation for current source.
A current source can lower the heat dissipation by allowing you to lower the rail(s). I did a thread on this a while back.
Whether you like the sound of current sources (current mirrors, et. al.) is another ball of wax...
(Jam will show up shortly to defend current sources.)
Don't judge a power supply on the number of microfarads, judge it by Joules of storage/output Watt. Low voltage rails require far, far more capacitance to equal the J/W ratio of an amp with higher rail voltages--something tube folks have known for years.


J= Joules
C= capacitance in Farads
V= Voltage

Note that the voltage is squared. Assuming that the capacitance remains the same, going from 40V to 400V makes a <i>BIG</i> difference. Conversely, dropping the voltage to 20 or 30V requires a fairly hefty investment in caps to retain the same number of Joules of storage.
I've used fans...and not, depending on the project, the phase of the moon, and how much money I had on hand at the time. I generally prefer not to. If you really want to get serious about dissipating a lot of heat in a small area, go water-cooled.

Thanks Grey. Now what about rail voltage?


I've got one final question. I'm a bit perplexed at Nelsons advice to add approx 20% onto the transormer secondary voltage in order to get the rail voltage figure. Shouldn't we be multiplying by 1.414 (and swallowing rectifier and other losses?)

I suppose one of the reasons I ask this is that I'll be using shcottky rectifiers in my SoZ project (with thier lower voltage drop, absence of switching spikes, etc). With this in mind, what do you guys reckon I need to be specifying as secondary voltages for the transformer? I'm guesstimating I'll need 30V secondaries, for a theoretical, lossless 42V, or a real world 40V.

(I'm now after a 40V rail, having found a way to fit a 1.5KVA toroid into each chassis).

Or should I just stick to Nelson's recommended 32V?

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