LME49810 biasing procedure temp comp and simple resistor
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 Chip Amps Amplifiers based on integrated circuits

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rhythmsandy
diyAudio Member

Join Date: Aug 2012
Quote:
 Originally Posted by multisync +/- 35 volts will give you about 50-60 watts @8ohm. 1.5 amps of bias is about 18 watts at 8 ohms class A and then the amplifier will go into class AB. This will be quite loud with most speakers. There are lots of DIY members who are happy listening to amplifiers in the 1-5 watt range Ti/national make a special version for fets LME49830, the bias voltage can go higher for the fets yes, just adjust the pot, measure and wait to see how hot the heatsink gets remeasure and adjust again. There is a bit of space between the bias transistor that senses the heat and the output transistors, therefore there will be a significant delay between the furthest transistor getting hot and the bias transistor detecting that heat and lowering the bias. The final case you put the amplifier in, other heat sources, sunlight, summer temperature where you live will all affect the final design. Before I was happy I let my amplifier sit for days and I would check and adjust and check and adjust till I was satisfied things would not get too hot to blow up. The amplifier is stable if the transistors are not matched, some transistors will get hotter than others, that is why you must measure each transistor bias for worst case. You did not read all of the link I gave you.
here the amp bias is 1.5Amps peak so the final output in class A to be

P RMS = [ (Ipeak x 2)^2 * 8ohms ] / sqrt(2)
P RMS = Sqrt ( 3^2 * 8 ) /sqrt(2)
P RMS = 72/1.414
P RMS = 50.91

yeah it takes time to stabilise the bias.

I am looking forward for working on two pole compensation improving the HF distortion.

so I have a doubt that the way you recommended to measure the Bias current is continuous not peak is it?

the above calculation is nelsons method of calculating class A.. he first calculates everything in peak and the divides it by 1.414 to get the final power...

Last edited by rhythmsandy; 30th January 2013 at 04:49 PM.

 30th January 2013, 05:10 PM #12 multisync   diyAudio Member   Join Date: Jan 2008 Location: Carp When you measure bias current you measure DC current flowing through the output transistors with the input shorted. There is no peak current, just DC. The current into your speakers will vary with frequency as nearly all speakers are a reactive load with varying phase angles between voltage and current and varying impeadeance with frequency. A good capacitor will nearly dissapate 0 watts of heat while having tens of amps of current flowing through it and having hundreds of volts across it. Same with a good inductor. Think about where the currents flow in a class A output stage, with no load, with no load and signal, with a load and no signal, with a load and signal.
 30th January 2013, 05:17 PM #13 rhythmsandy diyAudio Member   Join Date: Aug 2012 according to your way it seems correct as its all about the I^2 x R which is right but how come nelson is measuring peak but why is he going that way rather than direct I^2 x R... strange... does anybody has an explanation? why is nelson considering peak first? but how is he calculating peak when one can measure the DC directly..
 31st January 2013, 04:36 PM #14 rhythmsandy diyAudio Member   Join Date: Aug 2012 what will be the Vce for the above amp is it some value less than the psu like if my psu is 78V so Vce could be 70V? what about Ic? Im just calculating the max transient current that can be drawn for a duration of 100ms and 10ms so that the psu can be optimised... so how to calculate the Vce?
 1st February 2013, 02:38 AM #15 rhythmsandy diyAudio Member   Join Date: Aug 2012 this is the equation but we dont have the values of Rb and Re values are not given in the datasheet. Ib = Ic * beta. Vcc - IbRb - Vbe - (beta + 1)Re = 0
 1st February 2013, 01:51 PM #16 rhythmsandy diyAudio Member   Join Date: Aug 2012 Vcc - IcRc - Vce - IcRe = 0 how to calculate the Rc and Re values? its not in datasheet
 4th February 2013, 01:25 PM #18 rhythmsandy diyAudio Member   Join Date: Aug 2012 i had a strange HF rolloff after 14khz so i suspected that its becoz of the RL network on the output so I just connected the output straight before the RL netwoked and all worked super good and even the transparency increased alot. what RL values are recommended at the output. what problems one would face if its completely removed?
 4th February 2013, 04:04 PM #19 multisync   diyAudio Member   Join Date: Jan 2008 Location: Carp the output R and L should not cause any HF roll off
panson_hk
diyAudio Member

Join Date: Aug 2005
Quote:
 Originally Posted by rhythmsandy i had a strange HF rolloff after 14khz so i suspected that its becoz of the RL network on the output so I just connected the output straight before the RL netwoked and all worked super good and even the transparency increased alot. what RL values are recommended at the output. what problems one would face if its completely removed?
Did you measure it? What R and L were used?

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