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Old 2nd December 2002, 03:33 PM   #1
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Join Date: Aug 2002
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Default zen4 adjustement


I want to build a Zen4, and in the adjustment chap. a Variac is used.
Where must this device be connected and what caracteristics do it have.

Of course I'm a newbee !

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Old 2nd December 2002, 03:49 PM   #2
Taco is offline Taco  Netherlands
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Join Date: Nov 2002
Location: Chicago
Instead of your powersupply you use a variac, so you can power things up very slowly. And measure if things are right. It's a saver way to test your work. A variac is a variable powersupply.
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Old 2nd December 2002, 04:19 PM   #3
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Join Date: Mar 2001
I would like to offer up a more refined definition. A Variac (tm)
is a variable AC power source. You plug your amplifier's
power cord into it and turn up the AC from 0 volts to as high
as 20% over the voltage from the wall.

If you turn it up slowly, you can catch amplifier faults often
before the smoke and flames.

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Old 2nd December 2002, 05:21 PM   #4
miguel2 is offline miguel2  Portugal
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Join Date: Jun 2002
Location: Portugal

Having seen smoke and flames and not having a variac I came across the idea of using the zeners on the Zen PSU to increase the voltage up. So if you use a single zener the voltage in the regulated rail will be around 4-5 V. Puting another one will increase it a bit to around 14 V. Can this be done? Any change in the resistors of the PSU (or caps)?
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Old 2nd December 2002, 05:57 PM   #5
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Join Date: Dec 2001
Location: Philadelphia
Yes, but....

You are assuming that the regulator portion of the circuit is wired correctly and all its devices are working. Correct?

Also, by doing so, you are making the pass transistor in the regulator stage (Q5) do all the work of reducing the voltage. So if you amplifier is biased with 2 amps of current, the power dissipated across this transistor will be (50-5V) x 2A = 90 watts for your first case, 70W for your second. [I'm not checking your 5V number]. That's a lot! Probably too much for more than a couple of seconds. And.... if something else is wired incorrectly so that it draws more than 2 amps, you could really be in trouble.

So; nice idea. But it has some real limitations.

A variac or similar adjustable transformer is a great investment if you are interested in doing further DIY projects.
Vince Harris
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Old 2nd December 2002, 06:11 PM   #6
miguel2 is offline miguel2  Portugal
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Join Date: Jun 2002
Location: Portugal
Hi Vince,

I always check the PSU first and then the rest of the circuit.

The Zen amp has a resistor that controls the bias current, so we can adjust it so to have no problems on the pass transistor. Of course the current cannot be very high but maybe it is sufficient to see if everything looks well.
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