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Old 13th January 2005, 12:12 PM   #1
Fuling is offline Fuling  Sweden
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Default Half a SoZ?

Hi

Has anyone ever built a non-balanced Son of Zen?
I´m asking because I want something extreeeemly simple to feed my BL Fostex horns with a bit of output impedance and I would like to cut the need for a preamp with balanced outputs.

I´m thinking something like a 60V PSU, a 22 ohms drain resistor, 1 ohm (or so) source resistor, input and output caps and some kink of simple voltage divider for the bias.
About 1,5-2 A bias current.
No feedback except for the source resistor.
Ultimate simplicity, huh?

I have all the parts at home, just want some feedback (hmm...) before I start drilling my big heatsinks...

Cheers/
Daniel
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Old 13th January 2005, 01:09 PM   #2
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I can't help you, but I'm very interested in your results. Why not copy the biasing of BoZ(http://www.passdiy.com/pdf/brideofzen.pdf) and make it dead simple.

/Niclas
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Old 13th January 2005, 02:52 PM   #3
Fuling is offline Fuling  Sweden
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Yes, something like that.
Pretty much the same circuit but with lower resistor values and bigger caps
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Old 13th January 2005, 03:19 PM   #4
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I did a simulation of the circuit and with your resistor values you would have a gain of about 4.5 (with irfp044), which means you need a serious preamplifier. Or maybe not if you have a sensitive fullrange horn.
Bias was about 1.5A.

/Niclas
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Old 13th January 2005, 03:28 PM   #5
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60V PSU seems far too high, with a gian of about 4,5 and 2 A bias you're never gonna use that... 20V Dc might be more appropriate allthough linearity will suffer..
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Old 13th January 2005, 03:59 PM   #6
azira is offline azira  United States
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In theory, it's a power implementation of a standard diff pair. You should be able to cut it right down the middle and one side at 1/2 the gain. It's how you would begin analysis of a circuit like this.

I believe you would change the 1-ohm R7 to a 0.5-ohm R to ground but it's been a little while since I last analyzed a diff pair.. guess I should do it again.
--
Danny
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Old 13th January 2005, 04:41 PM   #7
Fuling is offline Fuling  Sweden
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Maybe 1 ohm source resistance is a bit too much, I guess some practical experiments will be necessary.
The preamp will probably be something with tubes.

I think 20VDC is way too low, this is more like a Zen Light thingie than a SoZ
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Old 14th January 2005, 08:03 AM   #8
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If you only use a 0.5 ohm source resistance you will only get about 0.5 volt across the drain/source of the MOSFET. To make it work you would have to lower the drain resistance from 22 to 11 ohms. The new circuit will give you a bias of 2.6A and gain of 6.5 (again with an IRFP044).

/Niclas

EDIT: Forgot to mention... You will dissipate about 75watts in the MOSFET with the 2.6A bias and 60 volt rails!!!
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Old 14th January 2005, 10:08 AM   #9
Fuling is offline Fuling  Sweden
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Quote:
If you only use a 0.5 ohm source resistance you will only get about 0.5 volt across the drain/source of the MOSFET. To make it work you would have to lower the drain resistance from 22 to 11 ohms. The new circuit will give you a bias of 2.6A and gain of 6.5 (again with an IRFP044).
I don´t quite get it, since the gate bias voltage is adjustable the source resistor should only affect the AC parameters.
I might be wrong though...

I´ll have too take a closer look a this before I start building this thing.

Thanks for the input BTW
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Old 14th January 2005, 10:58 AM   #10
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Bias with 22 Ohm drain and 1 Ohm source resistor is about 1.5A
=> 1.5*22 = 33V across the drain resistor
Bias with 22 Ohm drain and 0.5 Ohm source is about 2.6A
=> 2.6*22 = 57V across the drain resistor
In the latter case you will only have about half a volt across the MOSFET (rest is over the source resistor).
The gate biasing will only put the MOSFET in conducting or nonconducting mode of operation.

The above was confirmed by simulations, for whatever it's worth.
Am I missing something?

/Niclas

EDIT: I appologize for the error (or not whole truth) of the above statements. The above results were make with a BoZ like biasing of the gate voltage, when I changed it to a Aleph-L type biasing of the gate voltage it is as you say Fuling: The bias current through the MOSFET can be changed at least to some practical extent. There is still the problem of too much voltage drop over the 22 Ohm drain resistor at high biasing.
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