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mantabernd 16th December 2012 03:23 PM

Problem with Bob Cordell's LME49830 and LT1166
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I built the lme49830 with bias IC LT1166 but it doesn't work very well.

The quiescence current at about +/-30V is 10mA.

The first issue was a oscillation at the gates of the fets. I changed C6 from 100n to 180n and the oscillation was gone.

Sine wave looks fine at the oscilloscope, but if I connect a speaker and a music source the sound is very distorted. It sounds terrible, like if the bias voltage is not high enough.

So I measured the gate voltages, the irfp240 gate voltage is about +1.7V and the IRFP9240 gate voltage was -0.7V. Is it ok?? I think not… ?!

So the next step was chancing C4 and C5 from 10pF to 27pF but nothing happened. It still sounds distorted…

Maybe it's because the red marked current source? Do I have to connect PIN1 of the LT1166 to V+ rail? I think my LME49830 will fire up if I connect Bias + with V+. So what does I1 mean?

I hope you can help me :) Thanks in advance!


Headbanger 16th December 2012 04:10 PM

You have to use 10mA current source there, as drawn.
Cheers, Vic.

mantabernd 16th December 2012 04:54 PM

Ah ok thanks for reply.

Can I simply use a transistor and three resistors like shown in the image?

Re would be Pin 1 of the LT1166.

It's a bit dangerous I think, if the current source blows up the hole rail voltage (60V) is at the lme49830 and will kill it :(

AndrewT 16th December 2012 05:30 PM

You have the CCS labels back to front.
The resistor R1 is the constant current sink.
I is fed from a +ve supply via a limiting resistor.
Re is set to zero ohms.

Now you need to turn the whole thing upside down to create a current source and use a PNP for the inverted voltage.

mantabernd 16th December 2012 09:13 PM

1 Attachment(s)
I simply added a current source with a lm317 and it worked... not perfect but much better than before.

I figured out that this circuit is very, very sensible to the current source. So if it isn't exacly 10mA it doesn't work. So now I know what the problem is and can fix it.

@Andrew: why do I have to build the current source upside down? I added a circuit for the current source, can I do it this way?


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