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.
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.
Hi
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)?
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)?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.
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.
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.
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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