PPI PC2350 schematic or service manual??

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TO-3

TO-3

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Joined 2004
Perry Babin said:
Are you sure the bias was 0.7v instead of 0.007v or 0.0007v?

1mv across the resistors should produce clean output across the spectrum.

If I said the bias could be set without having the amp clamped to the sink, I was wrong.
Click to expand...

Oh man, good catch. I meant 7mV. And I guess I did mis-state you. Sorry, Perry.
I used to set the bias on those without the sink, but obviously you can't keep them running like that for long. With the bias-jumper in place, no problem.

For some reason this is not letting me edit my last post.
 
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soft4gsm

Member
Joined 2005
So, if this is correct, we have:
I=Ure/Re=0.007/0.165=42.4mA bias current per transistor.
We have 20 transistors, supplied with 60v.
Itot=20*0.0424=0.848A - Total standby current drained from PSU (+/-60v)
Ptot=Ucc*Icc=60*0.848=50.9W - total standby power only from ouput transistors.
Lets transfer this power into the other side of PSU (12v), if we have 80% efficiency:
Pin=Pout/0.8=50.9/0.8=63.5W - power drained from 12V accu.
Iin=Pin/Uin=63.5/12=5.3A - Current drained from 12v accu only for Output transistors.
In my amp, now, with NO bias current to power transistors, I have 2A cirrent drain from accu. This means, so we have 2A additional current from other part of the amp. (Drivers stage, preamps, and so.).
Iintotal=5.3+2=7.3A Total Current in standby mode from 12V accu.
But this is almost 2.5 times more than specification in user manual, where is written - standby current 3A.
In this way, If I want to have 3A standby current, I need 8mA bias current or about 1.3mV on emiter resistors (2x0.33ohm in paralel)...
 
S

soft4gsm

Member
Joined 2005
Yes, 3a idle current is not a precise specification of course... It depends from many things... Input voltage, temperature, ... I checked the distortion on oscilloscope, and even now, in class B, they are vary small... specialy in low freq. and high power, where this one will be used :D . Probably Negative Feedback and transistors are fast enough.
But I agree with about 1mV across emiter resistors. It should be enough.
BTW, I'll try to measure this from one other amp. - the same model...
Then I'll post the results. I hope this other amp. to be original one (means not repaired, tuned, and so...).
 
1

1moreamp

Member
Joined 2005
TO-3 said:
Oh man, good catch. I meant 7mV. And I guess I did mis-state you. Sorry, Perry.
I used to set the bias on those without the sink, but obviously you can't keep them running like that for long. With the bias-jumper in place, no problem.

For some reason this is not letting me edit my last post.
Click to expand...


Hey Jon, THX for jumping in with some spec numbers.
My worry is the lower rail supply regulator devices cooking off as they instantly have a rather large drop across them regardless of the idle bias settings... Thx for your input as always.
I also re-check the amp for stability over time so clamped to the sink is the only way i do any testing and setting on these big boys from the day... And I do try to stay near factory idle spec's as rule unless i find other issues limiting the amp I would rather not high bias any car amp that will sit someones trunk at 140+ degrees. Turned on outputs that play to spec 20 to 20K are just fine for me IMO...Any more is just asking for heat related troubles finding their way back to my bench....

Good catch Perry as always your on the mark.....:)
 
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