....as a method to lower the bias from almost 6A!
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Thanks Wuffwaff,
Do you think it possible to lower bias that drastically with just r19 and not transplanting all my resistors? I wonder why it is so high in the first place. Maybe you can shed some light..
Both channels measure the same:
-Irfp450 OP's
-mpsa18's
-rails +-21v
-stock resistor values
-Q5~5v
-Q4~3.5v (low)
-Q1 and 2 source~4v
-ref zener~9v
-offset~ -.5v!!
I originally had a matched front end but tried several others with no real change. The batch of 9610's I bought were real close.
I think it's possible that one or both mpsa18 MAY be in backwards. They are from an unknown source. I have neglected to switch then as the amp does function and sound fine, although I havent really been able to 'get on it'. Could the amp function with them in backwards?
I know, too many questions. Thank you.
Hi,
if you suspect them to be in backwards I would check them. Offset is way too high and bias too. Even without R19 it shouldn´t be that high.
Did you set ac-current-gain? Did the current source work at all?
Look at the schematic and try to find out what will happen if you connect them backwards. I don´t find the physical layout at the moment (it would be quicker to just look at them and see if you put them in the right way round)
I think Q4 is limiting the bias at the moment so maybe this is in the right way around. Something must be wrong with the active current source.......
William
if you suspect them to be in backwards I would check them. Offset is way too high and bias too. Even without R19 it shouldn´t be that high.
Did you set ac-current-gain? Did the current source work at all?
Look at the schematic and try to find out what will happen if you connect them backwards. I don´t find the physical layout at the moment (it would be quicker to just look at them and see if you put them in the right way round)
I think Q4 is limiting the bias at the moment so maybe this is in the right way around. Something must be wrong with the active current source.......
William
OK, now I'm pretty confused!
As I said my mpsa18's are of unknown origin. I have used the ON datasheet as reference. I have been thinking I may have another brand w/reversed leads.
Why is it that I can reverse orientation of Q4 with no change in operation? When I reverse Q5 it throws everything off and no current flows?
dag nabbitt!
As I said my mpsa18's are of unknown origin. I have used the ON datasheet as reference. I have been thinking I may have another brand w/reversed leads.
Why is it that I can reverse orientation of Q4 with no change in operation? When I reverse Q5 it throws everything off and no current flows?
dag nabbitt!
mpmarino,
To check orientation for an NPN transistor...take it out of circuit. If your meter has diode function, with the +probe on one lead stick the -probe to the 2 other leads one at a time. Try the other 2 leads with your +probe until you get about 0.6V. You have B(+probe) and E(-probe) if you read 0.5-0.7V. For PNP use -probe to look for B.
Using Resistance function same procedure but look for lowest R reading (not 0) which means you are forward biasing the B-E.
I think if you reverse the reference NPN trans on the CCS you'll get more than 1V across Source resistors.
To check orientation for an NPN transistor...take it out of circuit. If your meter has diode function, with the +probe on one lead stick the -probe to the 2 other leads one at a time. Try the other 2 leads with your +probe until you get about 0.6V. You have B(+probe) and E(-probe) if you read 0.5-0.7V. For PNP use -probe to look for B.
Using Resistance function same procedure but look for lowest R reading (not 0) which means you are forward biasing the B-E.
I think if you reverse the reference NPN trans on the CCS you'll get more than 1V across Source resistors.
mpmarino said:
Q4 is for current limiting. You could leave it out and wouldn´t hear a difference. It is only there to protect the amp in case of a shorted output.
If you´ve got mpsa´s then the layout will be as stated in the datasheet. I would look again at the current source if everything is connected the right way.
William
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