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Old 14th November 2005, 11:05 PM   #11
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Hi Reactance,

As has been said the lateral MOSFETs need a tempco tracking bias chain to reduce Vgs accurately as they warm up.

When I designed my Simple Killer Amp using IRFP240/9240 pairs I characterised the devices thoroughly over 100pcs before designing the amp. The tempco of these at 50mA bias is -5mV/deg C. I then designed a cct to track these at that rate and the result is very stable bias with temperature. Having done that I didn't need source resistors and was able to take full advantage of the rugged good performance of these devices.

Hope that helps.

Cheers,
Greg
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Old 14th November 2005, 11:35 PM   #12
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Hi, Reactance,

Thanks for swearing a schematic that comes from my website. If you buy the real kit (which the pcb is carefully planned and the components are carefully matched), you will not swearing on my schematic.

Quote:
The hell with that mosfet amp it going to give me grey hairs if i pay futher attention to it , why does ppl put diagrams like those on the internet in the 1st place,,
The schematic is put on internet is not for you to clone, or it is put there to give you grey hair, but for explaining what is inside the kit. I do not expect people to hijack the schematic and then swear on it.
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Old 15th November 2005, 06:06 AM   #13
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Hi Lumanauw,
this connection have one basic mistake : no voltage limiter. By shorting of output will be broken gate, 'cos driving voltage rise over 20 V and power fets will be destroy.
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Old 15th November 2005, 07:43 AM   #14
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Quote:
Thanks for swearing a schematic that comes from my website.
Try building an amp and find out after 2 and a half hrs of checking and rechecking ur work, it does not work only bad dc that melts ur money quiker than a blink of an eye u feel humiliated and stupid as well as robbed in a sense that u wasted your time money for an audio amplifier circuit that looked promising.At least have the decency to tell people it does not function correctly.

Quote:
The schematic is put on internet is not for you to clone
NO WHERE ON YOUR WEBSITE DO U SPECIFY THIS !!!!!!!!


From now any diagram that comes from u will remain questionable
In terms of readability and performance.

Learned a lot from this none the less.
Never build "easy looking amplifers" with no support from others.

Upupa Epops said
Quote:
It is original schematic designed with old Hitachi SK/SJ , which some nitwit " redesign " for vertical IRF... Andrew is correct, without thermal compensation it will goes into " heaven "....



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Old 15th November 2005, 08:09 AM   #15
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Hi AmpGuru,

"The tempco of these at 50mA bias is -5mV/deg C. I then designed a cct to track "

Would that be similar to Doug Self's variable tempco arrangement in his thermal "compensation" article in EW?

Brian.
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Old 15th November 2005, 10:13 AM   #16
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Hi, Reactance,

Thanks again for being so nice to me.

In your simulator it works fine, then why is it don't work in your clone? Do you have a slight thinking that you must have missed something, before you write something that shows everyone about how you are being so nice to me?

If you just try different combination of VBE's of BC556 AND D667, you will find a pair combination(s) of both that works fine.

The key is this : this is a "dual differential" design WITH both legs of differential is used. You HAVE to match the performance of the first differential (BC556) AND the second differential (D667). Matching only the BC556 is not enough, because both legs of the differential is used in the first and the second stage.

I admit that this topology is not easy to implement, but once you have done it, it sounds very nice. This topology is not so forgiving like ordinary 3 stages amp (where the VAS is single loaded). This topology requires matching like the AlephX (I mean in the difficulty of the matching transistor). Also the same difficulty as the Hiraga types of CCT (But I don't recall anyone being so nice to Grey Rollins or Hiraga like Reactance do to me)

This topology is sensitive to VBE mismatch, because both legs of the differential is used, both in the first differential, and the second differential (balanced VAS to some).

Imagine this. If the transistor's gain is 50X. 1mV mismatch in the first (or second differential, where the other pair is perfectly match) will result in 50mV mismatch in output stage. This topology is not forgiving like the single loaded VAS, where the DC offset is determined ONLY by the matching of the differential. VAS only follows in that configuration, DC offset is dictates alone by the differential pair, but not with this type of circuit. Can you imagine what happens if the first and second differential transistors are not carefully matched? You will end up in something like 2-3V DC offset.

If you don't like matching the transistor, there is one easy way to make it properly. I don't know what configuration in your clone. The voltage rail is sensitive too.
Remove the 12K resistor (or use smaller value, say 4k7). This way you will not have to match anything.

I really hope I can help you implementing this cct. Or you will behave more and more nice to me?

Hi Upupa Epops,

You are right, this schematic is lack of VBE multiplier. It will work fine with lateral FETs.
The first intention of this amp is to make an amp that is not too expensive. Look at the price of the PCB and full kit. But I slipped in thinking, it turns out difficult in matching the components, so even a nice people like Reactance is behaving so nice to me.

If he just BUY the kit, he will turn into bad man, not the so nice man like this time.
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Old 15th November 2005, 02:05 PM   #17
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Quote:
I really hope I can help you implementing this cct. Or you will behave more and more nice to me?
Ok, i admit i was a bit rude to u sorry man.
I apologize for saying to hell with your diagram.

Ok then so what should i do to make this amplifier work ??
Any help u gots the diagram what now ??

Also the circuit i used was simulated using ideal components, spice is very tricky, sitting on the work bench is is always different from spice.
but its close enough though i dont have extentive knowlege in this area of circuit simulation.

Cool man no hard feelings ok ,,,,


I still hate the smell of burned output transistors,,,
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Old 15th November 2005, 02:31 PM   #18
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Hi Luman,
just admit the circuit as published does not work and either say so on the schematic or remove the schematic from your site.
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Old 15th November 2005, 02:42 PM   #19
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Quote:
Hi Luman,
just admit the circuit as published does not work and either say so on the schematic or remove the schematic from your site.
I've think about it. It's gone now.
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Old 15th November 2005, 02:52 PM   #20
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Hi Lumanauw,

Is the schematic representative of your kit? I'm genuinely interested in how the quiescent current remains stable with just a pot for bias. Does the tail current of the balanced VAS take care of the change by reducing when warm? Or have you just found in practice that it's not a problem and bias just drifts up by a small amount?
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