LME49710 into higher class A - diyAudio
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Old 14th January 2013, 02:14 AM   #1
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Default LME49710 into higher class A

I have an LME49710 as a low pass after the I/V stage, it runs on +-15v rails, I love the sound of it from the mids down to the bass and low bass. But I find it a little cold sounding from the mids up. I prefer the OPA627 or OPA134 in this area. But they do not do the bass and attack the LME49710 can.
So I would like to give the LME4949710 a bit more sweetness by trying to run it into more Class A with the simple resistor from output (pin 6) to negative rail, what value of resistor do you guys think I should use given the 15v rails?

Thanks George
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Old 14th January 2013, 03:51 AM   #2
jcx is offline jcx  United States
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Class A bias current level depends on the load - heavier load, higher Vswing requires higher bias to remain in Class A

I've measured some op amp's distortion rising quickly at even 1/2 of the rated output current - so thats another side of the box

and people with distortion analyzers claim you can't predict which rail the current bias for the output stage should be drawn form for greatest benefit - you have to measure
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Old 14th January 2013, 03:59 AM   #3
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THX JCX, The load it sees is a dual fet input single ended to balanced output LME49720.

Cheers George
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Old 14th January 2013, 04:21 AM   #4
jcx is offline jcx  United States
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any feedback componet's load on the output stage has to be included too, often Class A bias is set to >~2x the actual current demand - but measurement of distortion min vs bias may change that

the last link in http://www.diyaudio.com/forums/analo...ml#post3321441 actually does a Class A output stage bias measurement (towards the end) - with resolution way below noise floor
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Old 14th January 2013, 05:11 AM   #5
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I don't have the circuit of this cdp and it's multi layered board is too hard to reverse plot what the componets are around the LME4972.
Could I just try starting with a certain value resistor to give a small increase in class A temp to the LME49710 as it is a tin can package so I should be able to feel if it's getting a little warmer than it is, which is cool at the moment?.

Cheers George
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Old 14th January 2013, 09:03 AM   #6
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Well I managed to do it the stupid idiots way, measure the case temp at normal +-15v no bias resistor it was 40c. Then starting with 51k from output to -ve I monitored the case temp and started to reduce the resistance till I got a rise kept going down in resistance till I got to 50c. Final resistance value 3k. Done deal the stupid idiot way. Funny I expected to see a rise in dc offset, but no.

If your interested in the difference, there was big time, the upper mids and highs sweetened up, but I lost detail and snap, it sounded much like the OPA627, and then the reason why I liked the LME49710 the magic fast punchy bass that goes down to subsonics, all gone too, now it sounds just like the OPA627 in the bass as well, you just can't win this sh*t!!!!!

Cheers George
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Old 14th January 2013, 10:25 AM   #7
qusp is offline qusp  Australia
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personally I think class A bias for most modern opamps is a waste of time (as you seem to have found), the good ones are already there for the loads they are expected to drive normally and forcing a static bias could easily make performance worse. the older opamps used to benefit a bit more than the ones today IMO. its so trendy to do, but unlike you most seem to not consider the load, not think about it at all, whack a resistor on there and more is 'more betterer' for these guys.

if after something like that, better to just add a simple discrete follower and bias that hard IMO
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Old 14th January 2013, 10:43 AM   #8
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I will say that cooking the LME49710 at 50c with the extra Class A for the half day of listening and then going back to standard has actually sweetened it up a little and it still has it's magic bass and attack, I just think it needed a bit of cooking to bed all those microscopic parts in.

Cheers George
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