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Old 13th February 2004, 05:44 PM   #1
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Default Penultimate Zen Transformer

Dear all,

I am currently building my first zen project, a penultimate Zen using the PCB from Pass Labs. The Pdf specifies a 300VA Transformer per channel, however this seems a little high to me. I think i need 2 Amps at 50 Volts, which should be 100VA. I understand the need for some headroom, but 300VA seems a little excessive (expensive!) I am thinking of using 160VA, but would appreciate some advice from those in the know first!

Thanks!
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Old 13th February 2004, 08:09 PM   #2
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I wish it was that simple.......This is a Class A amplifier that wastes alot of power in heat. The efficiency is somewhere about 15%. This means you will deliver about 25 watts of power to your load and the rest of the power, will for the most part be dissapated as heat. Unfortunely this is the price we pay for a simple Class A amplifier. Even though there is a little safety margin built in on the 300 VA toroid, it will be a squeeze to get 25 watts out with a 160 VA toroid.
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Old 13th February 2004, 08:35 PM   #3
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The move up to Class A also required a move up in my concepts of "whats right".
I got my amp up and running with a free (non toroid) xfmr I had, but soon found out that the xfmr was humming and not happy. I then realised to get the max out of it, it would require a serious power supply and then went for the spec'd Plitron.
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Old 13th February 2004, 08:39 PM   #4
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Unhappy You've gotta think big !

At idle the amp is using something like 2 amps at 50 volts per channel, that equates to 100 watts per channel as heat, I just built a penultimate zen a few months ago and I built it on the cheap. I used an Avel 625VA transformer for both channels, so around 310VA per channel or so.
The 625VA transformer was cheaper than two 300VA transformers.

Its worth it in the end, because the pen. zen is a great amp, I love mine!

Chris
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Old 13th February 2004, 08:56 PM   #5
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I have a Zen V2, without a closed enclsoure. It works with 160VA toroid transformers, one for each channel. I did not experience much difference to my Zen V4 with 330VA for each channel or a dowsized 1,5W Zen lite with 500VA for both channels. The toroids do all make mechanical noise, but no audible hum in the speakers. The mechanical noise seems to be more temerature dependend than % load dependend. So the way of cooling of the transformer may have more effect than the difference between oversizeing 2 or 3 times.
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Old 13th February 2004, 11:49 PM   #6
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Thanks guys, I think i will go for the 2*300VA option to avoid any suprises!
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Old 14th February 2004, 02:44 AM   #7
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well, i guess you need an overspeced trans for 2 reasons.


1) trafo makers tend to inflate the current rating buy 10%-30%, i`ve got thai made onez that would only do 50% of the rated.

2) even considering the trafo is of good quaility, when you draw 2a at 50 volts on idle, nelson pass recomends of doubling the current rating especially during Class A operation. he does this to cover the circuit`s power factor of the AC to DC conversion. i guess this too is to prevent a trafo from over heating.
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Old 14th February 2004, 02:29 PM   #8
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I donít think the real reason has been identified. The real reason is the peak currents in the transformer are much larger than you may think.

I have posted some graphs to identify this phenomenon:

http://www.diyaudio.com/forums/showt...536#post318536

Here is a summary: if you use a diode bridge and some capacitors for a power supply that sources 2 Amperes DC, then the currents in the transformer are over 10 Amperes peak. Since heat (and the magnetic field) is a squared term of current (P = R * i^2), the peak currents generate tremendous amounts of heat in the transformer. As the transformer windings heat up, their resistance increases with temperature. Therefore, the transformer heats up even more Ė positive feedback.

There is a technical term to describe this effect: Power Factor (PF). If the peak currents are 10A and the power supply delivers 2A, then the PF is very poor. Low power factors also tell you that the filter capacitors have to handle the ripple currents (hum), too.

High power factors mean the peak currents are lower (proportionally to the delivered current). The efficiency inside the transformer is better, the diodes donít have to conduct the peak currents, and the filter capacitors will have less ripple currents.

The only way to solve this problem is use a massively over-rated transformer that can handle the peak currents, or to use a more complex rectifier. Choke-loading the rectifier can increase the power factor. Another solution is using a tube rectifier, but thatís something Iíd rather not mess with, and it also requires a choke. Beware, using a choke will reduce your transformerís voltage output by a couple volts.

For a 2 Ampere DC power supply for a Class-A amplifier, only a small choke is required. Digikey sells high-current chokes in the 1-5mH range for about $5 from JW Milller. Radio Shack also sells a high-current choke for mobile CB radios for about $5. Iíve successfully used the Radio Shack choke in my Zen power supply. Iím using a 120VA transformer, and with the choke installed, I donít get any transformer heating.
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Old 8th June 2009, 01:34 PM   #9
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Is there any issues of using one 300VA trafo with two secondaries instead of 2x160VA ? I'm running zen4 at 1.5A.
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Old 8th June 2009, 03:42 PM   #10
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No issues, that's how I do it.

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