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Old 25th May 2009, 11:51 PM   #21
njepitt is offline njepitt  United Kingdom
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Default How to lower bias

Hi all. I have a working version of the F4 up, following the ideas mentioned above. I am using two pairs of mosfets per channel, in this case IRF630 and IRF9630, since they were very much cheaper to buy in sufficient numbers to match and I gathered from the big thread that other people used them successfully. I had three pairs per channel, but I'm afraid I let the magic smoke out of one of them... Not quite sure how...

Just as well, perhaps, because with P1 at minimum I have about 400mV across the source resistors (.47R), which according to the manual is about twice what is suggested. I should add that I'm using a cap multiplier supply I have on hand, which gives about 20-21V unloaded, but which is sagging a fair bit under the strain of running this F4. It all works fine (and first impressions are that it sounds great!) but it's a bit too hot... Been on for about 40 mins, and the heatsinks near the mosfets are at 54 degrees. (ambient is about 27 this evening.)

So it seems to me that an adjustment of something is in order, to bring the bias down a little... so my question is how to do this. My naive idea is to put another resistor parallel to R9 (the 10K resistor connecting to P1) to lower the overall resistance between the two pins of the TL431, but I am unsure if I need to do something in the symmetrical position (by which I mean R8). Any ideas?

I'll post photos and other build-experiences when I can, and as always v. grateful for any help.

Cheers

Nigel
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Old 26th May 2009, 12:33 AM   #22
Formerly "jh6you". R.I.P.
 
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Default Re: How to lower bias

Quote:
Originally posted by njepitt


...

So it seems to me that an adjustment of something is in order, to bring the bias down a little... so my question is how to do this. My naive idea is to put another resistor parallel to R9 (the 10K resistor connecting to P1) to lower the overall resistance between the two pins of the TL431, but I am unsure if I need to do something in the symmetrical position (by which I mean R8). Any ideas?


The fixed reference voltage of TL431 (about 2.5V) is between ref-pin and anode-pin (i.e. across (R9+P1)), I would slightly lower R8 to adjust the bias at lower zone.


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Old 26th May 2009, 12:58 AM   #23
njepitt is offline njepitt  United Kingdom
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Default Re: Re: How to lower bias

Quote:
Originally posted by Babowana



The fixed reference voltage of TL431 (about 2.5V) is between ref-pin and anode-pin (i.e. across (R9+P1)), I would slightly lower R8 to adjust the bias at lower zone.



Hi Babowana,

Do you mean in addition to lowering R9, or instead of?

Cheers

Nigel
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Old 26th May 2009, 01:05 AM   #24
njepitt is offline njepitt  United Kingdom
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And how much do you consider would be "slightly" lowering?
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Old 26th May 2009, 02:03 AM   #25
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Default Re: Re: Re: How to lower bias

Quote:
Originally posted by njepitt


Do you mean in addition to lowering R9, or instead of?
And how much do you consider would be "slightly" lowering?


Instead of . . . because lowering R9 inceases the bias.

Try R8 of 20K . . .
Still high? then, 18K . . .

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Old 26th May 2009, 02:55 AM   #26
njepitt is offline njepitt  United Kingdom
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Default Re: Re: Re: Re: How to lower bias

Quote:
Originally posted by Babowana



Instead of . . . because lowering R9 inceases the bias.

Try R8 of 20K . . .
Still high? then, 18K . . .

OK, I'll try this... Actually I just tried lowering R9 to see what would happen... (this is diy, after all, right?) the answer is a fuse blew in the power supply

I'll let you know how things work out.

Thanks

Nigel
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Old 26th May 2009, 03:58 AM   #27
njepitt is offline njepitt  United Kingdom
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Well, it turns out I'm a bonehead... (but then, you all probably guessed that anyway, right?) RTFM, as they say... I was turning P1 the wrong way, since all the other amps I've done so far worked that way. Oddly enough I *did* check that "raising resistance raised bias", but obviouslky did it wrong. (I presume I checked different mosfets befre and after adjusting P1...) In any event, I got the bias down without more difficulty... I now have between 240 and about 270 mV on the various mosfets, and temp touching the heatsink very close to the hottest mosfet is 51 Celsius after an hour, which seems to be reasonable. I can probably get this down a bit farther with a few tweaks...

Thanks again

Nigel
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Old 26th May 2009, 05:30 AM   #28
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If I count my mistakes I did at the company,
I should have been fired long time ago . . .

If I count my mistakes I did to my wife,
I should have been already killed by her . . .


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Old 26th May 2009, 09:52 PM   #29
njepitt is offline njepitt  United Kingdom
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Default Photos

Here are a couple of photos, as promised. First shows the F4 together with the F5 monoblocks I made a few months ago.
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Old 26th May 2009, 09:54 PM   #30
njepitt is offline njepitt  United Kingdom
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The next shows a little more of the internal workings. The idea was to put all twelve of the mosfets on little veroboard pieces together with source and gate resistors, and then to spread these around the chassis, for better heat management. I only have eight connected up now, as explained above, but same idea applies. When I have the inside ready I'll fix some sort of grille on top and bottom to makethings look nicer without affecting airflow (for cooling) too much.

Thanks again for all the help

Nigel
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