That is not fully true! When I measure on my pcb TP1-TP2 and TP3-TP4 I have to rotate the pots in different way to get on both TP the lowest (zero) resistance. yes you are right the pots are mounted in the same direction but electrically they behave oppositional.
Correct. Adjust for minimum resistance across the TP, and mark the pots in the direction that will increase resistance.
biasing the F5T
Now I started to bias the 1st channel, up to now everything behaves as expected. I can increase the voltage drop over the source resistors and the heatsinks getting warm.
What I don't understood is: beside the temperature on the HS how do I recognize that I can't go further with the voltage drop? In other words how will I find that point short before the diodes start conducting?
Thanks in advance for your help!
Now I started to bias the 1st channel, up to now everything behaves as expected. I can increase the voltage drop over the source resistors and the heatsinks getting warm.
What I don't understood is: beside the temperature on the HS how do I recognize that I can't go further with the voltage drop? In other words how will I find that point short before the diodes start conducting?
Thanks in advance for your help!
when the diodes start conducting you will see the voltagedrop run away. you must be very carefull. and turn it back when you hit that point. if you get to that point at all before your heatsinks are too hot. or just calculate when they start conducting. and leave the voltage drop a far bit lower.
Mallard, Follow Nelson's article advice and stay far out of fearless amp builder range. His suggested safe zone of 0.3v across the Source resistors will still give you roughly 2.4 A in class A outpout with two pairs.
0.3V gives you not more then 1.2A bias With 2 pairs. 2.4A with 4 pairs🙂
given that the source resistance is 0.5ohm.
given that the source resistance is 0.5ohm.
I used 0.68Ohm as source resistors. And I'm using the big 5U chassis from hifi2000.it which is 500mm in depth (not only 400mm as the diyAudio chassis). 😀
My bias target is 2 Amp for the pair... at least!
My bias target is 2 Amp for the pair... at least!
Thanks AudioSan, when you say "far bit lower" what do you mean by that? How many mV lower...roughly?
ohh man... with open chassis, 680mOhm resistors I'm at 380mV voltage drop.
Is it enough? I've not seen the diodes start conductiong yet. Temperature is really warm but not that warm that I can't touch it for a minute...
When I calculate correctly 380mV drop gives a class A peak power of ~200W, correct?
Is it enough? I've not seen the diodes start conductiong yet. Temperature is really warm but not that warm that I can't touch it for a minute...
When I calculate correctly 380mV drop gives a class A peak power of ~200W, correct?
first off. is it dual 0.680ohm resistors?
if so. 0.38/0.34=1.1A pr device. so you got 2.2A bias.
4.4*4.4*8=154W peak class A at 8ohm
Power dissipation pr fet is about 35W(if you got +/-32V rails).
if so. 0.38/0.34=1.1A pr device. so you got 2.2A bias.
4.4*4.4*8=154W peak class A at 8ohm
Power dissipation pr fet is about 35W(if you got +/-32V rails).
as low that its no way they accidentally start conducting if the ambinent goes a little up. and/or the bias drift a bit.
Yes, but in a push pull circuit you get double the bias current before leaving class A.
bias problem?
I might have a problem on the left channel to setup the correct bias.
Up to now I was not able to get both p-channel and n-channel to similar values.
After heating up I still have a gap of ~12mV between n and p channel.
I didn't have this problem with the other channel, that one was pretty much equal.
What could cause such a discrepance? Is there anything I can do about it?
I might have a problem on the left channel to setup the correct bias.
Up to now I was not able to get both p-channel and n-channel to similar values.
After heating up I still have a gap of ~12mV between n and p channel.
I didn't have this problem with the other channel, that one was pretty much equal.
What could cause such a discrepance? Is there anything I can do about it?
that do not matter. as long as the offsett is as close to 0mV as possible. under 100mV is okei.
the cause is most likely resistor value differenece do to resistors tolerance.
the cause is most likely resistor value differenece do to resistors tolerance.
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one polarity bias (meaning Iq , measured as Urs - voltage across source resistor) and output offset is all you need to measure , for proper setting of amp
if you didn't match source resistors , you'll never see same Urs voltages
if you didn't match source resistors , you'll never see same Urs voltages
Stable bias
Just a question for my understanding:
In order to get a stable bias and DC offset is it recommended to connect a B1 buffer preamp (no connection to CD player or so)?
I tried it and from what I can say all measurements are more stable and fluctuating much less whereby the DC offset was increased in comparison to no B1 connected.
Any suggestions?
Just a question for my understanding:
In order to get a stable bias and DC offset is it recommended to connect a B1 buffer preamp (no connection to CD player or so)?
I tried it and from what I can say all measurements are more stable and fluctuating much less whereby the DC offset was increased in comparison to no B1 connected.
Any suggestions?