Class A with LME49811...

Dxvideo · 2009-02-18 3:35 pm

Hi all,
Do you think is it possible to design a really good 20-25W Class A amplifier with LME49810?
Why I am asking that? Because this opamp has really good THD figure and I guess it has a class A biased driver stage (consider that it has just 9mA output capability while 49810 and 49830 has ~50mA).
So with gate drivers (AN1645 pg:14) two pairs of HEXFETs like IRFP240/9240 can handle 20W or more class A output with 2.5-3A bias..
It would be an easy and good Class A amplifier...
What do you say?

EWorkshop1708 · 2009-02-19 12:34 am

Sounds like a plan.

Are you planning a CCS or Push-Pull Class A output stage?

I'm pretty sure the chip will run on the lower voltage rails, most chips do.

Keep us posted.

Eworkshop1708

Dxvideo · 2009-02-19 6:49 am

Yes.. A plan for now!
I will use P-P output stage.. And will feed the LME with higher voltage than the outputs.
Will keep you informed.

Dxvideo · 2009-02-19 9:28 am

What about this one;

An externally hosted image should be here but it was not working when we last tested it.

AndrewT · 2009-02-19 3:35 pm

Hi,
what is the Vgs of your output FETs when they pass your intended bias current?

Now apply that Vgs across the 100r emitter resistors of the driver pair.
What current are you passing through the drivers?

Dxvideo · 2009-02-19 4:03 pm

In fact I expect to adjust the Vgs (and of course the bias current) via the trimpot..
This output design is from National AN1645 pg:14.. I did not make a simulation yet. But with this way I expect to have 2.5A at least... What do you think?

bobodioulasso · 2009-02-19 5:26 pm

one pair of mosfets at 1.3A bias would do the job? Look at Mr Pass F5.

AndrewT · 2009-02-19 6:33 pm

Dxvideo said:
In fact I expect to adjust the Vgs (and of course the bias current) via the trimpot..
This output design is from National AN1645 pg:14.. I did not make a simulation yet. But with this way I expect to have 2.5A at least... What do you think?

you have not thought yet.
What is the Vgs of your output FETs when they pass the intended bias current.
Go and measure them, then do the rest.

Dxvideo · 2009-02-20 7:09 am

bobodioulasso said:
one pair of mosfets at 1.3A bias would do the job? Look at Mr Pass F5.

But my amplifier will give 25W RMS into 8R.. So;
25/8 = 3,125, SQRT(3,125) = 1,767 A RMS into load
That makes 1,767 * SQRT(2) = 2,489 A peak into load
And if you take 20% more for bias that makes ~3A...
Do you still think that 1,3A will enough?

AndrewT said:
you have not thought yet.
What is the Vgs of your output FETs when they pass the intended bias current.
Go and measure them, then do the rest.

Ohh.. Sorry for the misunderstanding..
As I read from the datasheet of IRFP240 then the Vgs for 3A is just ~5v @ >10v Vds...
Ok I'll make the circuit, but I'd like to make a simulation first.. And have no model for LME49811 !! Do you know any solution for this?
Thanks..

AndrewT · 2009-02-20 9:46 am

Dxvideo said:
But my amplifier will give 25W RMS into 8R.. So;
25/8 = 3,125, SQRT(3,125) = 1,767 A RMS into load
That makes 1,767 * SQRT(2) = 2,489 A peak into load
And if you take 20% more for bias that makes ~3A...
Do you still think that 1,3A will enough?

Ohh.. Sorry for the misunderstanding..
As I read from the datasheet of IRFP240 then the Vgs for 3A is just ~5v @ >10v Vds...
Ok I'll make the circuit, but I'd like to make a simulation first.. And have no model for LME49811 !! Do you know any solution for this?
Thanks..

do the calculation in your head, or if you must use a calculator/slide rule or whatever.

A single ended amplifier current limits at the bias current level.
A push pull amplifier reaches the ClassA limit at twice the bias current. Most will then go further operating in ClassAB. Read the F5 to see how it's done.

bobodioulasso · 2009-03-13 9:01 am

need advice: Is this possible?

minhtriet · 2009-03-14 5:30 pm

Dxvideo said:
Hi all,
Do you think is it possible to design a really good 20-25W Class A amplifier with LME49810?
Why I am asking that? Because this opamp has really good THD figure and I guess it has a class A biased driver stage (consider that it has just 9mA output capability while 49810 and 49830 has ~50mA).
So with gate drivers (AN1645 pg:14) two pairs of HEXFETs like IRFP240/9240 can handle 20W or more class A output with 2.5-3A bias..
It would be an easy and good Class A amplifier...
What do you say?

Hi Dxvideo.
I see you've installed LM4702 with lateral mosfet output, now you continue with the later version is LM49810. I have a plan with LM4702. Can you give me several review for sound quality?
Thanks.

Dxvideo · 2009-03-16 8:27 am

Dear Minhtriet,

I'm listening LM4702+Renesans MOSFET amplifier for over two years.. It was biased at 105mA and working at +-55v rails. And the component selection was not good.. So firstly I've decided to make somethings on it. Then this weekend I exchanged all the suppy capacitors with low ESR ones (Panasonic FM series), NFB capacitors with nichicon muse series and for the main filter capacitors I've installed one 22,000uF-63v Rifa PEH169 instead of two Jamicon 10,000uF/63v in parallel per rail.. And the rails reduced to 40v (30v AC) without load...
And a final action; bias increased to 650mA, thus makes ~3,4W class A condition..
Sonic result was incredible! I did not measure anything but as my ears saying; my amplifier upgraded to another league..
I dont know which modification makes this result exactly but I am sure if I will make an amplifier with LME49811+hexfets @2,5A bias(like above) it will be a really class a amplifier..

minhtriet · 2009-03-16 7:05 pm

Hi Dxvideo
Thanks for your reply
I have read the National technical document for LM4702
www.national.com/an/AN/AN-1645.pdf
In this document, thì good bias current for lateral mosfet (2sk1058, 2sj162) is 150ma. Now you set to the 650ma and sonic is good. I was very suprise, but I will try your way.
Thanks for your answer.

SpittinLLama · 2009-03-16 7:48 pm

The bias was set in AN-1645 to be a stable bias with temperature with the most simple bias scheme possible (a resistor + cap). It is noted that if a higher bias is desired with these FETs then something like a Vbe multiplier is needed for stability across temperature.

-TH

Dxvideo · 2009-03-17 7:31 am

It was just a pre listening experiment for my next LME49811 project.. I have tested it just for half an hour.. And again: I havent measure anything in this test! May be the bias increased to very high levels or a huge DC offset appeared on the outputs.. Who knows! But the heatsink was really hot after 30 minutes...
As you will see in my new design a Vbe multiplier and gate drivers added to system.. But my old LM4702 circuit have no precautions like these..
So do not try bare 4702 (or 498XX) with MOSFETs or darlingtons with this bias level!

juma · 2009-03-17 1:19 pm

Once again: you don't need 2.5 A bias for 25W/8R.
1.3A is enough because:

as AndrewT wrote already in post #10 and bobodioulasso in post #7:
"A push pull amplifier reaches the ClassA limit at twice the bias current.
Most will then go further operating in ClassAB. Read the F5 article to see how it's done."

Dxvideo · 2009-03-17 1:26 pm

I will...
OK.. I am confessing; I did not consider "the half bias" idea after your (or bobodioulasso's) first posting. But you look serious.
I will examine the F5 topic and try this 1.3A bias idea with my current (LM4702 with laterals) amplifier.. 44 x 1.3 = 57.2W of dissipating! Not too much...
I will inform you about the results.
Thanks.

minhtriet · 2009-03-17 5:27 pm

Hi Dxvideo!
If you have time, Please review the different sound of LM4702 and LM49811.
Thank you!

Dxvideo · 2009-03-18 6:55 am

I did not make a LME49811 amp yet! I am just trying something on my current LM4702... But I will do soon. And believe me, my first task will be to make a comparison between them (and with my other amps).

Class A with LME49811...

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