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Old 14th May 2007, 07:17 AM   #11
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Default SIMPLY AMAIZING

after all this time and all these e mails so many people still willing to help me .....

mr nelson pass ...fotios ..... thank you so much people

let us not forget please that amp as we speak is working very well in rails no more than 65 volts but from the original drawing which also i can confirm that is correct ...though there are couple of changes

A) vas ccs is created now with 2 resistors of 6K8 instead originall 4K7 to maintain vas curent
B) current resistors are fited to the out of all mosfets
and C) botom feedback resistor is 270R instead of original 470R to make sensitivity less than 2 volts ......
[B] in this setup all worked just fine but one mistake connecting input lead with gain potensiometer fully on will send all mosfets to hell ( actually happend to me in a setup just yesterday )DO YOU THING THAT PROTECTION DIODES WILL SOLVE THAT ???[B]

THANK YOU SO MUCH !!!!!
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Old 14th May 2007, 11:15 AM   #12
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Looking at that circuit, its very prone to serious bias variation due to dynamic rail variation during loading, Also the circuit is suited for BJT output only, driving the mosfets these way could bring lot of troubles.What is the purpose of D4, D5 when the Mosfet already has inbuilt body diode.
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Old 14th May 2007, 11:52 AM   #13
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Looking at that circuit, its very prone to serious bias variation due to dynamic rail variation during loading, Also the circuit is suited for BJT output only, driving the mosfets these way could bring lot of troubles.What is the purpose of D4, D5 when the Mosfet already has inbuilt body diode.

FIRST OF ALL .... the circuit IT IS !!! working in rails of 40-42volts ...... my approach is to operate it up to 60-65 volts .....

second this is either the approach of NELSON PASS made in the early 80's as a mosfet version of hardman kardon ciatation 12
or the approach of dr Bora Jagodic as legend 4 and it is working .....

and as far as i know the irfp 240 which i use DO NOT include diode at least in the gate section ( just looked it up in IR web page )

so the question remains is the diodes going to protect against the particular problem mentiond above considering the abuse of connecting input leads while gain potensiometers are fully on resulting burned mosfets ?????????
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Old 14th May 2007, 12:11 PM   #14
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Sakis,
Donot shout

Listen carefully what i mean to say that this circuit is not a reliable one to drive mosfets, if you want to put zeners than put them across gate to source terminals, but again they would not protect your amp when overdriven, because the overdrive would eventually destroy the zeners also. Regarding IRFP240, there is no gate diode in it but all the Hexfets exhibit inbuilt body diode which is antiparallel in termination.
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Old 14th May 2007, 12:13 PM   #15
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The IRFP240 does include a body drain diode, which is what Iyremenko was talking about. But your D4 & D5 is not excess IMO as the body drain diode is not very strong. However, these components are really only needed if you plan on connecting/disconnecting the loudspeaker while music is playing, or have a limiting circuit in the amp. As these situations will cause abrupt reverse current spikes.

The gate protection zeners you have used will save the amp if the fault when plugging sources in/out is due to excess Vgs. If the problem is massive current spikes blowing the MOSFETs then they will not really help with that. Personally I always use the gate protection zeners as it's one less potential problem.
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Old 14th May 2007, 12:21 PM   #16
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Quote:
Originally posted by richie00boy
But your D4 & D5 is not excess IMO as the body drain diode is not very strong.
Certainly not correct, because the current rating of body diode is same as of mosfet which in case of IRFP240 is around 30A, also the IN4007 have 3uS reverse recovery time and 1A 1000V rating, whereas mosfet body diode exhibit less than 500nS reverse recxovery time. Still much faster than IN4007, therefore its clearly evident that additional back emf clamp diodes in case of mosfet outputs are just superfluous.
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Old 14th May 2007, 03:33 PM   #17
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Default i dont mean to shout .....

i would really like mostly to take away nagatives about this amp cause for the perpous is made, the specs, the cost and the simplicity that is made of ...the amp is fine

to my standards anything that elliminates this problem i.e amplifiers reaction to abuse as mentioned above amp is working just fine ...at least for me

so the question remains the same......
do you thing that these diodes will provide or even better improove stability under these conditions ????
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Old 14th May 2007, 03:56 PM   #18
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Default Re: i dont mean to shout .....

Quote:
Originally posted by sakis
do you thing that these diodes will provide or even better improove stability under these conditions ????
The IN4007 doesnot help the mosfets in any case.
Placing zeners also donot guarantee 100% protection from overdrive, they only protect the gate from marginal spikes only.
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Old 14th May 2007, 05:54 PM   #19
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Default as i said

a litle here and a litle there will make perfect ....meaning that not every time that you connect a lead with gain potensiometer fully on you loose the mosfets .....

many times this will happen but once you gona loose the fets ....

so if a litle protection is added i thing will reach perfect .....please notice that if you overdrive with signal you have no problems amplifier will distort and speakers will scream but work for very long time ....i actually overdrive with almost 3 volts and working ....except distortion no other probs ...

SO COME ON GUYS !!!!!!! JUST A LITLE MORE WILL MAKE PERFECT
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Old 14th May 2007, 07:08 PM   #20
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Quote:
Originally posted by Nelson Pass


If you drop D1 it will swing more positive output.

With a 2N5551 in the place of D1 it is resolved the most negative going output swing (and thus the negative d.c. offset)?

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