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Old 3rd January 2001, 09:00 PM   #1
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I just finished my Zen amp. I plugged it in and let it idle for about 4 hours. The output from the power supply as specified is more like 44VDC not 34VDC. I wasn't all that worried about the extra voltage because the transistors were rated at 50 volts.

However, at least the output transistor blew after 4 hours. And took at least one high power resistor with it. I know because I found half of the resistor 3 feet away. I could still touch the heat sinks after 3 hours. However, an hour later, they were radiating heat.

So... Any ideas?

Do you think it was the increased voltage, not enough heat sinks, or just something else wrong with the circuit?

I was not happy with the measurements when I hooked it up. I was measuring 15mV AC and 15mV DC at the output with no input. What's going on here?

Thanks,

pixie
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Old 3rd January 2001, 10:08 PM   #2
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Unhappy POP!

I tried to tell you that a month or more ago. I wanted to email you to see if it worked because I wanted to do the same mods. I almost did email you yesterday.

The second version seems to have a schematic mis-print for the power transformer output after the rectifer.

30v x 1.414= 42.42v, close to what you got. The original is 25 x 1.414= 35.35v. Can anyone else verify this math??

Were the FETs at least 10 amps? Did you look for cold solders, if you hard wired it?

Did you look for 17 volts at the DRAIN pin of the mosfets. Adjusting it by the pot? I had to ask Pass himself what he meant by "symetrical clipping".



I can tell you this...my ZEN amp's heat sinks starts to get HOT after 10 minutes or less.

I had a simular problem with it when I reversed the small transistor in the constant currrent source. Ran cool then the resistor just popped!! But that only took one minute to happen.
3 times mine caught fire!! I almost gave up.

But you hard wired it right? I think that's that you said when I told you about this the first time.
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Old 4th January 2001, 09:32 AM   #3
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Check one thing - the Zen amp is a single stage amp so it is inverting. If you look into the schematic, you can see that the speaker output + phase is the Ground, the MOSFET output is the - phase. So if you follow normal practice to hook the - phase to Ground, you are basically short circuiting the amp. Try to check this one. I heard that many people burn their Zen with this mistake.

I build the Zen and got it work one first fire up, absolutely no issue. It is a stable amp.
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Old 4th January 2001, 04:00 PM   #4
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Oh.

I'm not one to hide my supidity when it's obvious. You'll learn this.

Was the 17 Volts at the Drain in the original document?

I think I did a good job of soldering. I've been doing it for 15 years. I'm not being snotty, just establishing a confidence level...

I only used the 20,000uF because I couldn't find a 2mH inductor. Another detail I forgot.

I'll check the transistors' specs. I think they were rated at 5A. I'll have to go back to Digi-key today anyway.

So, 10A 50V is what I need, right?

Thanks for the tips.
I just realized, this is my first analog circuit since the simple circuits in college.
Digital is much easier...

Thanks,
pixie
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Old 4th January 2001, 04:10 PM   #5
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No, I was wrong. I just went to Digi-key.

I have the following transistors:

IRFP244 250V 15A
IRFP9140 200V 12A

So, that's not the problem.

At those ratings I'm not even sure the transistor blew. It may have just been the resistor. I'll try more wattage on that resistor.

pixie
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Old 4th January 2001, 06:06 PM   #6
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Both Zen and Zen revisited show a 34 volt supply even though the diagrams show different transformer voltages. You may want to contact Pass via email. I am currently building the A-40 and he did reply to my power supply question. As well it would help to know which resistor went POP! was it R1. Also at the Pass site others have posted their finished project some have provided email addresses Who better to communicte with?
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Old 4th January 2001, 08:29 PM   #7
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17 volts at the drain was from the original artical, but since your voltage is higher, that probably changes too. I don't know what it would be with a higher secondary.

You can use 1 to 2 mH coils. I just got 2- 1mH from Parts Express for 9 bucks. Rated at 300 watts.

Just a tip if yours are fried...use lower voltage mosfets. That helps lower distortion. Say 100v since your sec voltage is almost 45v. I used 50v for a 34v secondary. Are you biasing at 2 or 3 amps w/ R1?

I think what's happening is the higher voltage and ampage is too high for some of the component values in the second version of the design. I would take the hit and get a 25v + 25v @6 amps secondary trans. 10 watts output in class A isn't so bad. You'll be surprised at how nice it sounds.
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Old 5th January 2001, 02:13 AM   #8
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If the wiring is correct, then check the power rating of the resistor as you said.

Another possibility is that your MOSFET run into oscillation. This is harder to check unless you have a high freq CRO.

With the CRO, you can also check symmetrical clipping. That is the most accurate rate to adjust the so call 17V, which is an approximation. By CRO, you can turn up the input until the amp clip, and when it do so, it should clip for the same amount in the positive and negative side of the waveform.
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Old 5th January 2001, 04:33 PM   #9
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I went to parts express. I got about 200 inductors in the list. Which one would you suggest? They have everything from 1mH to 2mH in three or four different types. It looks like the air core donut type are the best. They are more like $11-14 each though. What would you suggest closer to 1 or 2mH?

It was R1 that blew. The schematic calls for 2W. I think that's what I got. So, I'll try 4-5W. If I can find it.

Would anyone suggest a fuse after the power supply? Say 10A fast-blow. Maybe that would save some parts until I get this thing completed.

Digi-key has a limited selection of MOSFETS in the 10-15A range. Where would you suggest I get them?

I don't have a CRO. Anyone have a used one they don't want anymore?

Thanks,
pixie
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Old 5th January 2001, 06:04 PM   #10
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Use either 1 or 2 mh your choice, get the 14 AWG if you can.
Do not buy Bobbin or iron core, GET THE DONUT ONES!

It wouldn't hurt too much, but it will intorduce some resistance, but if you feel safer doing it go ahead. It just extra protection.

Try mouser electronics for the mosfets.
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