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Old 13th November 2001, 12:29 PM   #11
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Hello John,

my experience with the Hitachi Fets is that you can get big gains changing the bias from 100 to about 200-225 mA per Fet. After that the changes are quite subtle. Do you really need 100V supply rails? This will give over 500W/8 ohms with no power gain into 4 Ohms cause max current of 2 paralled Fets is only 16 amps. My first amps had about 75V supply rails but Im down to 42V now and therefore able to use better (noisewise)transistors/capacitors in the driver stage. The amp is still perfectly capable of driving large speakers (Thiel CS6)without clipping (in a normal living room).
In my experience 2 paralled fets (Hitachi that is) are not enough when youre using "difficult" speakers, with a 100V supply I would use at least 4.

william
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Old 13th November 2001, 08:31 PM   #12
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Default Clarification

Hi Guys

I have been reading through your responses again and was wondering if I have caused a little confusionon my initial posting. I quoted "I have built myself a Power Amplifier using Hitachi mosfets, 2SK176 and 2SJ56, two per channel...." To clarify this matter what I should have stated was that there are two of each per channel ie: four transitors per channel. I don't know whether this will make a difference to your initial comments but my apologies to all.

John
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Old 14th November 2001, 11:49 AM   #13
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Hello John,

your first message was clear (2x 2sk176/2SJ56 per channel)

william
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Old 16th November 2001, 09:20 PM   #14
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Default Well Done !!!!!

Hi All

I said I would get back to you after I had played around with the bias current. Well I decided not to pfaff around with 25mA increments but went staight for 250mA. I set this up and let it settle for approx. an hour for each channel, no load connected. It took about that time for the heatsinks to reach what seemed to be the stable temperature with no load. This was scientifically done by placing ones hand rather gingerly on said heatsink and leaving it in that position until you figure that you haven't actually turned your hand into a major burns injury. Yep they were hot but not too hot to hold on to. what puzzled me was when I built this amp some four years ago I set the quiescent current to 155mA as recommended but when I connected my meter up to set the 250mA it was only reading 85/87mA I don't know why it had dropped so much.
Now for the best part a) my amp is still working, the heatsinks get mighty hot when driving my Magneplaner MG11c speakers. b) The performance of the amp sonically is a revelation. The bass is much deeper and much better defined, The sound stage is deeper front to back, there is more space around the individual musicians, There is lots of low level detail that I have never heared before. It's JUST GREAT!!!
Thank you to everybody for your fine advice it has been very much appreciated. I still have the drifting image problem but I don't think I am capable of sorting that so I must find someone who knows what he is doing.

Best Regards

John
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