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Old 25th November 2011, 03:05 AM   #11
gootee is offline gootee  United States
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Quote:
Originally Posted by jcx View Post
paralleling 20-30 mA output current op amps seems lame when you can get op amp/buffers with much higher current ratings, up to .4-1 A

the other probelm is that its much easier if the output op amp/buffer is lots faster than the input op amp for multiloop feedback stability

and if your'e really working from your junk box then small signal Q are likely output buffer parts fro headphone amp power levels
Actually, paralleling opamp buffers is said to reduce output noise, due to random cancellations. Noise with two parallel opamp buffers is reportedly about 3 dB lower than one opamp and four in parallel decreases noise further but is about the practical limit.
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Old 25th November 2011, 06:05 PM   #12
jcx is offline jcx  United States
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output device noise is irrelevant inside another feedback loop - it is the noise of the input/global loop op amp that matters

just do the input referred noise calc with the loop gain of the input amp at audio frequencies

and the TPA6120, or discrete Q buffers are really input low noise compared to 5534
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Old 26th November 2011, 01:30 PM   #13
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The OP's concept could run the output buffers both inside and outside global feedback, so their noise would be relevant in the latter case (similar to O2).

There is one problem with paralleling output buffers, each one sees a source impedance that multiplies accordingly, which ultimately worsens distortion. The importance of voltage noise vs. current noise also shifts IMO.

The TPA6120A2 has one issue, it has to be used with a series resistor of at least 10 ohms to avoid oscillation with capacitive loading. For some types of phones, that still is too much. These chips are like racecars, fast but fussy. There always is a tradeoff between stability and distortion, looks like things are leaning more towards the low distortion side in this case.

As far as effective input noise is concerned, with the output noise values given in the datasheet this computes to about 3.5 nV/sqrt(Hz) for the Av = 40 dB case (which is fairly but not unusually low), up to a more modest 18 nV/sqrt(Hz) for Av = 6 dB. That assumes a 20 kHz measurement bandwidth, which the DS does not bother to mention.
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Old 26th November 2011, 10:41 PM   #14
jcx is offline jcx  United States
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output isolating Z can be a inductor, or R bypassed by inductance at audio frequencies to give low signal loss, high damping/frequency response flatness

fast loops are actually easier to isolate from C load - I've discussed TPA6120 load isolation series Z parts choices before:

Quote:
Originally Posted by jcx View Post
in audio power amps the load isolation series impedance uses an inductor to keep the audio frequency efficiency/damping ratio high while "disconnecting" the amp output from the load (mostly cable C causes stability problems) at high frequency near the loop gain intercept/corner frequency

the TPA6120 is so fast that the load isolation impedance can be lower than you normally see in audio power amps which may have only ~ 1 MHz loop gain intercept and at best 30-50 MHz output Q - requiring a few uH inductance to get load C isolation near 1 MHz and beyond

the TPA can have >50 MHz corner frequency and uses GHz Q so the ~10 Ohm series load isolation impedance doesn't have to fully develop until beyond ~10 MHz so < ~1uH inductance can be OK

lossy ferrite bead can be a good technical fit to the load isolation requirement - you may even find some that physically fit the QRV09 10 Ohm output series R smt pad

to avoid the often claimed audio distortion of output load isolating ferrites I spec beads with Isat ratings ~ 10x of the op amp short circuit current Ferrite Products | Laird Technologies

those really concerned by audiophile tweaker's unverified claims about ferrite "ruining the sound" can always use air core inductor and parallel R - old carbon comp with a winding over the body is one option


even John Curl seems to agree that < ~ 1uH series load isolation L doesn't harm "the sound" with dynamic speaker loads


except for the multi-armature high end iem all other dynamic headphones have much higher Z than loudspeakers, and usually only a single bass impedance peak followed by a slow high frequency Z rise due to coil inductance - this makes their response very insensitive to small series load isolation L

I can't see worrying about the special impedance requirements of multi-armature iem when drive level mismatch, amplified input noise caused hiss from a general purpose amp will be audible - these special iem require a specialized amp design
Quote:
Originally Posted by jcx View Post
the audio frequency concern is nonlinearity from the inductance modulation - probably only easily visible as IMD - for these parts audio is "DC bias" so a few 100 mA peak as you might see at bass frequency in a inefficient orthodynamic headphone could give very low level IMD with higher audio frequency signals
but since the inductive impedance is so low at even high audio frequencies I expect the numbers would be well below -60 dB @ 20 KHz even from inductors that show substantial L drop with real current levels - and the distortion products would fall with frequency for typical largely resistive impedance headphones

to minimize this possible distortion I could even suggest looking at cable bead cores - I think the principle is the physically largest core that just gives enough impedance will have the better DC bias performance in the same core material

long cylinders give better high frequency performance from less C between wire in/out than ring/toroid shapes

I'd guess larger center hole would mean more uniform magnetic path length - delaying onset of DC saturation - better for low level linearity but more abrupt saturation characteristic at large signal levels

they would just be slipped over the wires to the output connector

Steward 28B0375-100 has > 10x the material volume compared to the leaded 28L0138-10R


I also think you could go as low as 200-300 nH, as long as |Z| > ~10-20 Ohms above ~10 MHz

the lower values make air core single layer coil dimensions reasonable for putting in the wire between the pcb and the output connectors
but you should allow for lots of air space to any ferromagnetic material with the extended leakage field of the coil

Last edited by jcx; 26th November 2011 at 10:46 PM.
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