LME49810 biasing procedure temp comp and simple resistor - diyAudio
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Old 29th January 2013, 03:22 AM   #1
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Default LME49810 biasing procedure temp comp and simple resistor

I am looking to bias the lme49810 to 1.5Amps with temp compensation circuit given in the application note. The formula is as follows..
VBIAS = VCE = RB1 * I + Vbe(V)

its said that Vbe is limited to 6V in the circuit with 2sc5171 as biasing transistor.

what is the general value of vbe observed or does it changes when biasing?

what should be RB1 for a bias of 1.5amps? considering
I = 1.5A
Vbias = 10V
Vbe = 5v

so the result is RB1 being 3.3 ohms? what is the general procedure to bias in this case..
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Old 29th January 2013, 03:39 AM   #2
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you do not bias the LME chip, you bias the output transistors. It is difficult to do this mathamatically, as the bias current depends on the hfe of the output transistors, the number of output stages, the resistors used, the configuration etc. You may notice nearly every bias circuit uses an adjustable resistor. Some people after adjusting the bias with an adjustable resistor replace the adjustable resistor with one of a fixed value.
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Old 29th January 2013, 03:46 AM   #3
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yeah i agree that we are not biasing the chip but the output stage..
here is the schematic.
Imageshack - lme49810.jpg
please guide me in biasing this in 1.5Amps
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Old 29th January 2013, 12:31 PM   #4
AndrewT is offline AndrewT  Scotland
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have you built it yet?
have you bought the bits?
__________________
regards Andrew T.
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Old 29th January 2013, 12:32 PM   #5
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its ready with me...
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Old 29th January 2013, 05:14 PM   #6
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I have something similar at home. The boards were bought assembled.

see this thred
Anyone recognise this Design..? based on LME49810

None of the 5 pairs of output transistors were matched. I set the bias to an average of 30ma each pair. Some pairs were at 20ma some at 40ma. The total for all 5 was 150ma. I used +/-65volt rails. The power dissapated was 130volts x 0.150 amps or 19.5 watts per channel. In your case you can set the bias to an average of 300ma per pair or 1.5 amp per pair. Depending on what you do, your heat sink will vary in size from very large to gigantic or large and force air cooled. Higher bias currents will not necessarly give you a lower distortion with BJT transistors. You have to find the sweet spot. Measure the voltage across one 0.22R resistor. Current will be the voltage across the resistor divided by the resistor value. 0.220 volts DC across one resistor will be about 1 amp of bias current, 0.022 volts will be about 100ma of bias current
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Old 29th January 2013, 09:43 PM   #7
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Quote:
Originally Posted by multisync View Post
I have something similar at home. The boards were bought assembled.

see this thred
Anyone recognise this Design..? based on LME49810

None of the 5 pairs of output transistors were matched.
but how is it stable when they are not matched?
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Old 29th January 2013, 09:59 PM   #8
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Quote:
Originally Posted by multisync View Post
I have something similar at home. The boards were bought assembled.

None of the 5 pairs of output transistors were matched. I set the bias to an average of 30ma each pair. Some pairs were at 20ma some at 40ma. The total for all 5 was 150ma. I used +/-65volt rails. The power dissapated was 130volts x 0.150 amps or 19.5 watts per channel. In your case you can set the bias to an average of 300ma per pair or 1.5 amp per pair. Depending on what you do, your heat sink will vary in size from very large to gigantic or large and force air cooled. Higher bias currents will not necessarly give you a lower distortion with BJT transistors. You have to find the sweet spot. Measure the voltage across one 0.22R resistor. Current will be the voltage across the resistor divided by the resistor value. 0.220 volts DC across one resistor will be about 1 amp of bias current, 0.022 volts will be about 100ma of bias current
I got this from the source http://connexelectronic.com/product_...products_id/78
I think this guy is a forum member and seemed a bit reliable than one on ebay. Like you said the design seems interesting.

I also bought lm4702 with a pair of 5200 1943 transistors and they sound pretty good no complains here..

300ma per pair makes sense but 1.5 amps per pair seems impractical I think you wanted to tell on the total its 1.5 amps.

Like you said there wouldnt be much difference in THD after certain level. Its there in the National app note that after 500ma there is no perceived difference but I think its for one pair.

regarding Heat issues once "andrew said that he tried at 500ma and cant put fingers on the heatsink imagine how how it would be at the junction"

I will be using this heatsink two of them if required each about 6 inches in length..
Imageshack - photo0306t.jpg

so the max per pair I think we can go is 500ma or ill be rather stay at 400ma max per pair..

right now i have this in BJT soon will be with Toshiba mosfet or Hexfets.

but all this is done varying the pot on it?

thank you very much for the kind help...

Last edited by rhythmsandy; 29th January 2013 at 10:11 PM.
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Old 29th January 2013, 11:07 PM   #9
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when driving this in class A what should be the supply voltage ratings? im guessing +/-35v should be decent enough but what is actually recommended. How do I calculate this?
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Old 30th January 2013, 03:13 PM   #10
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+/- 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.
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