Zen jfet v.8 problems, help! - diyAudio
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Old 14th November 2008, 01:20 PM   #1
liberty is offline liberty  Italy
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Default Zen jfet v.8 problems, help!

Hi all,
I tryed it with
q1=IRFP250 and q2=IXTH50N20.
P1=22k and R5=47k, since I have these in junkbox.

P1 with pin1 and pin2 soldered to GND and pin 3 to R5/R4/C2, so that turning completely clockwise it become a 0ohm resistance, and turning completely counterclockwise it become a 22kohm resistance.
(I think...I hate pot pinout )

So, first of all, which is the correct starting position?
the pot completely clockwise or ounterclockwise?

I tryed to turn on the circuit (with the pot turned clockwise, 0ohm), but I have 0V on Q1 Source, 0,43V on drain, and 45V on Q2 drain (the same in power supply).

I Turn off the circuit and the 45V on Q2 drain go down very slowly (as it is in power supply), and in this situation (using only the charge of power supply caps 25000uF), turning counterclockwise P1 does not change anything (but the Q1 drain voltage, which increase proportionally), so I still have 0V on the source of Q1, and 45V on the drain of Q2 (going down with same slow effect).

There is anyone so kind to explain me where I am wrong?

Second question:
If I will decide to try transistors (TIP141, BUW11A and so on) for Q1 and Q2, I can simply put them in the circuit (base for gate, collector for drain and emitter for source), or I have to make some little changes in the circuit?

Thanks.
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Old 14th November 2008, 05:06 PM   #2
liberty is offline liberty  Italy
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Nobody?
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Old 14th November 2008, 10:28 PM   #3
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The P1 is there to form a voltage divider in cooperation with R5. The purpose of it is for us to get about 9V at the junction of R4/R5/C2/P1 and to get about 1.1V at the source pin of Q1, by adjusting P1. If P1 is adjusted down to 0 ohm, it means that P1 is shorted to the ground. Then, we will read 45V (your rail voltage or supply voltage) at the drain of Q2 and 0V at the junction and the source pin of Q1. And, all the 45V will be dropped through R5.

If the P1 is soldered correctly, and if P1 is adjusted to the value greater than 0 ohm slowly, we would meet the target voltages at the junction and the source pin. Otherwise, I would be first suspicous of the correct soldering of P1.

Best Luck!

Cheers,

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Old 14th November 2008, 10:58 PM   #4
Fuling is offline Fuling  Sweden
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As I understand it you have substituted the power Jfet with a mosfet, is that right? (I donīt have the ZV8 schematic in front of me).
If thatīs correct you have to arrange a bias voltage to the lower Mosfet, Jfets donīt need it but a mosfet needs ~4V on the gate to start conducting.
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Old 15th November 2008, 03:13 AM   #5
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Hi,

You have to adjust P1 to the minimum before you first turn on. Then with your meter attached to the source of jfet and ground, you turn on the power. Gradually turn P1 until you get the reading of around 1V. With the source resister of Jfet at 0.5 ohm, it means that you have biased the amp to run at 2 amps.

Funnily, I can't get good souding when I bias at 2 amps. Instead, when I run using 1.2 ohm resister here, the sound is so much sweeter.

Wachara C.
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Old 15th November 2008, 01:08 PM   #6
liberty is offline liberty  Italy
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First of all thank you to all for answers.


Yes, I know that P1 form a voltage divider with R5, this is the reason I started with P1 at 0 ohm.


BTW the circuit doesn't work.
Yes, I am using a mosfet as Q1 (since I haven't JFTS...).

Maybe this is the reason of failure?
I have to add a bias circuit for mosfet gate?

Can I try to use transistors (I have TIP141 and BUW11A)
(connecting tranistor base for gate, collector for drain and emitter for source) for Q1 and Q2?

It need changes?

Thanks again!
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Old 16th November 2008, 09:52 PM   #7
liberty is offline liberty  Italy
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nobody?
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Old 16th November 2008, 11:39 PM   #8
Fuling is offline Fuling  Sweden
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Quote:
Yes, I am using a mosfet as Q1 (since I haven't JFTS...).

I have to add a bias circuit for mosfet gate?
Yes. Jfets and mosfets have fundamentally different bias requirements. The best advice I can give you right now is to study all the Zen variations on passdiy.com and pay attention to how bias is arranged for Mosfets compared to for Jfets.
Mosfets require a positive voltage of about 4V measured from soucre to gate in order to pass any current.
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Old 17th November 2008, 08:57 AM   #9
h_a is offline h_a  Europe
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Hi liberty,

try to get the Lovoltech jfet. Using a mosfet in this position is really not a replacement.

www.audioprojekte.de still has them I think.

Have fun, Hannes
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Old 17th November 2008, 10:33 AM   #10
liberty is offline liberty  Italy
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Thanks to all.

To be honest I had some doubts, but I tryed the circuit with mosfets, after I read this discussion:

Pass JFET Power Amplifier

I read that every N channel device will work (for Q1 and Q2), also a TIP141 or a mosfet (IRFP044 for example) will do the job.

Trying the circuit, now I know that a mosfet (IRFP250) doesn't work as Q1 (it will work only as Q2)...maybe a TIP141 will work as Q2 also...but for Q1 I am afraid I need only a JFET (not mosfet and not transistors)...

I am right?
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