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Old 20th June 2010, 05:53 PM   #1
tadiam1 is offline tadiam1  Argentina
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Default lm1875 problem??

I build this amp...but the bias are much.
(r5/r5 ajjust this?)
More value ..more bias less value (100ma)the output transistors are cold.
In both cares the lm are hot.....?
No oscilation

The sund is clear an the voicers are more presents and best low frecuencyes.
But have this problem any sugestion??
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Old 20th June 2010, 07:52 PM   #2
Bone is offline Bone  United Kingdom
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Have you put VT1 &2 and LM1875 on a Heatsink? The LM1875 even with no output will get hot as it will dissipate about 10 watts with +/- 25v supplies. This is normal. See the data sheet. VT1 & 2 will not turn on until the LM1875 draws enough current through the 1 ohm resistors to turn them on. Under quiescent conditions they will be turned off and cold. They will only turn on under high (Loud) signal conditions.
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Old 20th June 2010, 10:05 PM   #3
tadiam1 is offline tadiam1  Argentina
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Default re

Ok...all is on Heatsink.but tthe output transistors are cold?? the lm in more hot than normal use (single) conditions.
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Old 20th June 2010, 11:07 PM   #4
AndrewT is offline AndrewT  Scotland
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Bone is telling you that if the chipamp draws 50mA through the 1r0 resistors that the outputs will only see the 50mV that will drop across the same resistors.
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Old 21st June 2010, 03:40 AM   #5
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Quote:
Originally Posted by Bone View Post
The LM1875 even with no output will get hot as it will dissipate about 10 watts with +/- 25v supplies. This is normal. See the data sheet.
Your 10W figure sounded a bit extreme so I went and checked the datasheet Typical quiescent is quoted as 70mA, max 100mA. So the no output dissipation will typically be 0.07 * 50 = 3.5W. Worst case 5W.

You're right about the output transistors, they'll only have 70mV of bias in the quiescent condition.
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