LME49811 + 2SK1058/2SJ162

[Buslik]

I would like to build 50W 8Ohm amplifier based on LME49811 + 2SK1058/2SJ162 pair.
Does it worth to add BD140/BD139 stage?
Or it will be even better to use LME49830 and connect FETs directly?

 

[AndrewT]

National released the 830 specifically for mosFET output devices.
Why choose an 811 when it is intended for BJT output devices?

If you or other Members come up with a list of good reasons for choosing the 811 then go ahead.

 

[Buslik]

THD of 811 is smaller. So I think that 811+BD140/139 will result in better slew rate and smaller THD than just 830.

 

[Arius]

So let me get this right - 811 with external driver will have better performance than 830 with internal matched drivers?

Just asking....not being sarcastic.

You can probably get away with the 811 driving a single output pair. It can do up to 7mA. I have a similar app and I'm using the 830.

 

[tomchr]

Dude. Just build them according to the application notes. You won't be disappointed.
My LME49811 + STD03 amp sounds better than the Parasound A23 it was replacing. And the Parasound is a damn good amp.

Yeah, there might be a mathematical difference in THD between the LME49811 and '830, but the difference in slew rate is much more likely to be audible than the difference in THD. If you want to use FETs in the output, use the driver designed for FETs.

~Tom

 

[homemodder]

So let me get this right - 811 with external driver will have better performance than 830 with internal matched drivers?

Just asking....not being sarcastic.

You can probably get away with the 811 driving a single output pair. It can do up to 7mA. I have a similar app and I'm using the 830.

Yes, the external drivers can be biased to higher current to overcome the gate capacitances of the fets. Not only will THD be better but the speed too.

Im not advocating one opamp for the other as in both cases I would probably use external drivers on both versions. If I remember correctly in one of the application notes National shows speed and THD tests done with external driver and no driver, its no surprise the driver type is superior.

 

[AndrewT]

But there is a voltage issue.
BJTs need low base to base voltage.
Both Vertical FETs and Lateral FETs need higher voltage gate to gate.

830 is designed for FETs

 

[Buslik]

Ok, got it. So LME49811 + STD03 seems to be better couple.

Next step is to find amp case and transformer.

As for transformer I am thinking on this one MCTA100/18 - MULTICOMP - TRANSFORMER, 100VA, 2 X 18V | Farnell België. But it will be very close to LME49811 minimum voltage. Is it an issue?

Any ideas for nice case around Europe? With 50-60mm height?

 

[AndrewT]

A 100VA transformer would normally be paired up with a 50W to 100W amplifier.
Or you could pair it up with a stereo amplifier of 25W+25W to 50W+50W.

 

[homemodder]

But there is a voltage issue.
BJTs need low base to base voltage.
Both Vertical FETs and Lateral FETs need higher voltage gate to gate.

830 is designed for FETs

True. The bias voltage parameter for this chip is not given ....??

 

[pieter t]

True. The bias voltage parameter for this chip is not given ....??

It's all in the LME49811 datasheet

 

[homemodder]

Another quick look and Vbias not stated on datasheet. For 49830 it is. Offcourse it can be calculated but Im lazy.

 

[Buslik]

Any case advices?

 

[homemodder]

For mosfets rather use the lme49830. For higher performance with the mosfets use small drivers. National application note 1645 gives some valuable tips.

 

[pieter t]

Another quick look and Vbias not stated on datasheet. For 49830 it is. Offcourse it can be calculated but Im lazy.

Maybe a bit slower look?
It's really there, even with different + and - supply voltages.
But as you are lazy: output swing is 33 VRMS with +/- 50 volt rails, and 68 VRMS with +/- 100 volt rails.

 

[AndrewT]

The base to base, or gate to gate, bias voltage is not the same as the output swing.

 

[jackinnj]

Where's SpittinLama when you need him? There's a chart of THD% vs bias current in the LME49830 data sheet. Best advice I can give is that the lateral MOSFETs don't need thermal compensation which is a big design simplification, secondly -- if you tweak the values of the gate stoppers on the MOSFETs you can reduce the THD% (which is already quite good.) This is illustrated in AN-1645. There was going to be another paper/application note on the compensation techniques for this series of drivers, but National decided to channel their resources toward solar power/alternative energy chips before that bubble blew up, and they got swallowed by Texas Instruments in the interim.

 

[pieter t]

The base to base, or gate to gate, bias voltage is not the same as the output swing.

I know Andrew; I guess that homemodder should look at bias voltages required for the output transistors. That has not so much to do with voltage swing of the driver. Important though is output current of the driver (is the driver capable of driving the input capacitance of the output device, where fets generally show higher input capacitance than BJT's - hence the LME49830 with it's much higher output current meant to drive fets).

 

[homemodder]

Interesting, TMC perhaps ??

 

[AndrewT]

FETs are reputed to have similar capacitance drive requirement to BJTs.

The BJTs also need current drive for the base.
Generally this makes the drive requirement for BJTs more onerous.