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Old 5th February 2002, 12:30 PM   #1
Bakmeel is offline Bakmeel  Netherlands
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Smile ZEN Downsized!

So here I am, with my first DIY home-brew Amp (at least, in design)...

I want to use the parts from my former amp, which was a Velleman K4010 Mosfet Power Amp. It performed well, but due to a horribly bad constructed bias-cirquitry, it performed below my standards.

A DIY amp would be a good option, and the ideas of mr. Pass appealed well to me. I would take the knowledge, and use it to create my own design. But amps are expensive to build, mainly because its power supply and heatsinks. As a student, I am not that wealthy you know

But look! there is sitting in front of me, an unused power amp by Velleman containing: a 2 x 30V/ 225VA transformer, 40.000uF caps, and IRFP140 / IRFP9140 power mosfets Add som new resistors, a transistor here and there and VOILA! there you have a ZEN pure class A amp, for less than a cheese sandwich

There is only one problem (here is my actual question). The heatsink used in the Velleman is capable of dissipating only 30W continuously at a temperature rise of 25 . A ZEN would require at least 70W... So, then I should use less power, and it should work, (I hope) or I would have to settle with an amp running at 75 C. Also, my rail power would be 42V in stead of 34V

If I use a 1 ohm resistor for R1, the bias current should settle at 0.76A, which leaves me with little less than 5W output power... Is that correct? Or will it do nothing at all?

The project is actually for me to see if I can make a home-brew amp work, so the amount of power is not the problem for me...


Regards,
Bakmeel
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Old 5th February 2002, 02:24 PM   #2
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It would be beneficial to find bigger heat sinks. The Zen struggles as it is at 10w. If you are going to reduce the power, make sure your speakers stay at or above 8ohms and are very effecient. In the order of 96db or higher per watt. In fact that is what is recommended for the 10 watt Zen.

In the Everything Else forum I posted a link for some nice sized heat sinks. Run a search.

You'll go from a cheese sandwich to steak!

Vince
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Old 5th February 2002, 08:50 PM   #3
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I have to agree with vdi_nenna. Find some bigger sinks and you will be OK. If you like, I can provide you with some. Drop me an email and we will discuss it further.

From what I can see, lowering R1 will indeed lower the bias to aprox. 0,7 amps. This will yield a little over 4 watts in 8ohm class A

I have build some ZEN's and found more bias yields better performance. So I am afraid that lowering the bias current will get you a working circuit but not much music.

Hope to hear from you.
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Old 5th February 2002, 09:22 PM   #4
paulb is offline paulb  Canada
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Is there any possibility of putting a fan in the case somehow? Or even external, blowing across the heatsinks? I'm a big proponent of reuse of parts, they're expensive!
If you somehow allowed for adjustable bias, you could just set it to as high as you're willing to let the transistor temperature be.
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Old 5th February 2002, 09:47 PM   #5
Bakmeel is offline Bakmeel  Netherlands
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Point was to make it just work... Performance was not the issue I will work for better performance and more power when I have the money to buy bigger sinks....
I have some resistors to spare, so I think I will experiment with some values... thanx for all the help

Bakmeel
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Old 5th February 2002, 10:48 PM   #6
paulb is offline paulb  Canada
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Good plan. Have fun!
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Old 6th February 2002, 02:34 AM   #7
Bakmeel is offline Bakmeel  Netherlands
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And after 3 hours of breathing solder fumes, I am proud to report that it is working just fine! ... There was a problem with the bias current at first, I believe because of a half-broken mosfet, and some differences in resistor values.... It ran about 1.5A bias, where only .76A was allowed... No problem there, it worked just as well...

For the sound? well, because of the hasty soldering and the amp beeing just a test object, it was just OK.... I think there is a lot of room for improvement left

To be continued.....

Bakmeel

PS: It was a tedious test, since I had no fuses at hand... Luckily nothing exploded....
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