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Old 5th August 2007, 01:24 AM   #1
sam9 is offline sam9  United States
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Default Multi-stage amplification

In the latest edition of Audio Express, Denis Colin presents data to show that overall THD can be lowered by chaining several op amps end on end rather than achieving all the gain in one stage. He is thinking mostly of phono preamps. The idea is that three opamps with a gain of 5 each (5x5x=125) will result in lower overall THD than a single stage with a gain of 125.

He used AD797's to test this and presented measurements (nor sims) to arrive at an optimized gain per stage, about 4.8.

This got me to thinking about the LM3886 and similar IC amps. A complaint I read on the site is that the ap notes always show these devices configured with a gain of 20 which some feel is lower than they desire. The suspicion is that at gains of 25 or 30 the THD figures deteriorate. It occurs to be that an opamp on the front end (perhaps part aof a "super gain clone" scheme) with just a little gain might be away to have one's cake and eat it too. Opamp gain = 1.5 x GC gain 20 = gain of 30. or 1.25x 20 = 25. Or 2 x 15 = 30. etc etc.

Anybody tried this? It seems to me that the "catch" might be noise, I noticed Colin always mentioned THD, never THD+N. But if you are using AD797s that might not be much of an issue. More cynically if you are just thinking phono amp, perhaps the LP surface noise covers iy all up anyway.
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Old 5th August 2007, 05:23 AM   #2
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Sometimes ago I asked a question here, why make 2 stages with each gain =3x when we can make a single stage with gain=9x. Someone answers that these 2 approach has difference in bandwith (where gain x bandwith = constant, so less gain gives more bandwith). Cascading several low gain stages will work in higher bandwith. Don't know about the noise, though.
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Old 5th August 2007, 04:49 PM   #3
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Multiple stages can work fine and has often been done.
One can even combine the 2 amps to create one, high gain amp, thus providing more feedback and lowering distortion.
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