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Old 26th August 2011, 04:45 PM   #11
jcx is offline jcx  United States
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I just wanted to point out where there are limits and where there is real improvement – the “207 dB loop gain” isn’t a “real” improvement – emphasizing it may confuse/mislead the many here who haven’t mastered feedback theory

I certainly do think high loop gain feedback and composite amplifiers are great techniques, have built and verified some with distortion below –160 dB with an indirect IMD test and a Juli@ soundcard – a measurement that did take averaging to see into the noise may have a cool factor for feedback geeks but has little audio relevance beyond establishing that linearity for audio signals is not an issue

High loop Gain Composite Op Amp Circuits
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Old 27th August 2011, 10:00 AM   #12
1001 is offline 1001  Italy
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hi JCX, I saw the circuit you had simulated, very interesting. I would like to ask you which are the pro and cons of these high feedback amp in real audio world, are these not useful of usable, sound worse, don't offer real benefit?
thanks a lot for your posting
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Old 27th August 2011, 08:58 PM   #13
jcx is offline jcx  United States
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Default do headphone amps count as the "real world" even in audio?

Bode linear stability of high loop gain amps is the initial challenge - very fast output devices help with this - DSL driver op amps seem perfect for many dynamic headphones and audio signal line driving with extended GBW that makes the CFA DSL drivers usable inside many "audio" op amp's feedback loop without difficulty

if you want to go further than Bode's "Maximal Feedback" ~ 30 dB/decade loop gain you encounter "conditional stability" - reduced gain from clipping, slew rate limiting can cause higher order roll off loop gain amps to have nonlinear oscillations - clamping diodes for internal nodes, limiting "wind up", other measures can help but the clipping recovery will often look worse than "simpler", "low gain" amps

for headphone amps it is easy to just have the headroom for driving your headphones to >120 dB SPL before clipping

B.J. Lurie's papers, books are a real help understanding this particular conrer of very high loop gain feedback - and they aren't perfect - "buggy" texts and sometimes hard to understand - but offering useful insights, perspectives I've never seen spelled out elsewhere
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Old 27th August 2011, 09:31 PM   #14
jcx is offline jcx  United States
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wthin the restricted setting of DSL op amp output V swing and current I would say these high loop gain/composite op amp techniques pretty much "solve" for any practical purpose the audio amplification “gain block” "problem"

but if you "dissect" "feedback amplifiers" by the block diagram you see that gain is only 1 of the 3 parts: gain, feedback and difference/sumer

feedback part selection is as open as your belief in part's "sound" but bulk metal foil R and polystyrene Caps have few measurable nonlinearities

the input sumer that differences the input and feedback signal to get the "error" that the gain block amplifies has limits

at low Z, low signal like MC phono cart some discrete designs can beat monolithic op amps on noise

there are common mode V nonliearities that are "outside" of the feedback loop - bootstrapped cascode diff pair like AD627 can help

RF rectification in input device nonlinearity is another concern - higher bias, bigger junctions can give discrete an advantage, fets supposedly are superior - but I've seen little on GHz input BJT with local degeneration, or comparisons with AD's new BJT input for AD8099, ADA4898

Last edited by jcx; 27th August 2011 at 09:42 PM.
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Old 28th August 2011, 11:05 PM   #15
1001 is offline 1001  Italy
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well, I will try to stay away from high feedback composite amp
but now I have doubt, if these topology are so difficult to implement and not useful for audio circuits why Halcro electronics use this technology?
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Old 29th August 2011, 12:28 AM   #16
jcx is offline jcx  United States
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the basic "multiloop" with a high current fast unity gain buffer in the op amp loop is quite common, even in hobbyist diy headphone amps with discrete or integrated buffers

adding loop gain by making the "buffer" a CFA op amp with local "flat" gain, adding some loop gain, is described in Walt Jung's "Op Amp Applications" book and other articles so it is a practical engineering technique if a little more difficult than cookbook hobbyist level

the potential nonlinear oscillation/clipping recovery is something you can encounter in discrete circuits too but it will be new to most users used to single monolithic op amp circuits with dominant pole compensation - unity gain stable dominant pole op amps are designed that way because it makes them easy to use but does give up a little potential performance

it is the higher order loop gain shaping/rolloff that allows >100 dB loop gains to high audio frequencies where the complexity really steps up

Halcro's patents don't really "teach" the added nonlinear stability/clipping recovery details need to be successful at that level of complexity - but higher order loop gain composite/multiloop amplifier topology is a tool that can be used if "parts per billion" distortion numbers from the active components interests you



the "simple" "formula" of good "audio" op amp with a fast unity gain buffer in its feedback loop is probably a good trade off of minimal added complexity/design difficulty for most of the potential performance improvement

you still can use sub regualted input op amp rails, the composite amp breaks the thermal feedback of output stage heating to the input diff pair, the buffer presents such a high load impedence that the input op amp is working Class A driving the buffer input and you can use Class A output stage bias

Last edited by jcx; 29th August 2011 at 12:40 AM.
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Old 9th September 2011, 01:50 AM   #17
roline is offline roline  United States
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I prefer tube input and gain stages with Mosfet output in class A operation. The Mosfet takes care of the output drive for various impedance head-phones and the Mosfet has the same transconductance as a tube, Voltage in / current out....
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