LM6171 input stage design

[rpell]

I don't know of any audio products with the LM6172.

The LM6172 was used for i/v conversion in Sony's SCD-C333ES SACD player.

The LM6171 is also used in the CORDA headphone amplifier from Meier Audio ( http://home.t-online.de/home/meier-audio/ ).

[Chucko]

Anyone tried using the Linear Technology LT1358 or its kin (LT1354-LT1365) in an audio application? It has a topology that sounds similar to the LM6171; I haven't looked at the '6171 data sheet so I can't say for sure. The LT application engineer described it as a CFB amp in VFB drag, modest GBW but very good slew rates at modest quiescent currents.

[rpell]

Anyone tried using the Linear Technology LT1358 or its kin (LT1354-LT1365) in an audio application?

I have a Modwright-modified Sony 333ES that uses the LT1364 in its output stages. The 333ES (which retained the LM6172 in the i/v conversion stage) sounds very clean. Tonally, it sounds very close in A/B comparisons with an SDS Labs tube DAC, which of course uses no op amps at all. (This is in contrast to my 9000ES DVD player, which was modified with OPA627s throughout. It seems to have a "warmer" less open sound than the other units.)

[fmak]

Much is made about decoupling of power supplies but if board decoupling is sound and power supply of high order, surely it is valid to directly compare op-amps of high bandwidth ie 6171 versus 825 for sound quality. This is especially so if there is no measurable difference in distortion or noise at the output.

What might happen at chip level, I don't know; it takes a lot of care and equipment to measure.

[Chucko]

You can't always do a drop-in substitution. Ironically for what was once touted as an "universal analog component", op amps have become quite specialized, and you have to use some care in choosing an appropriate device for the application.

For instance, a DC coupled gain stage or an integrator designed around a FET input device's picoamp bias current will not take kindly to the 1 uA bias current of an LT1364. Some devices are stable into any capacitive load and can be used as line drivers with impunity; others oscillate at the drop of a hat. Some can deal with large differential voltages, others will fry in the same circuit. Some will drive headphones directly without breaking a sweat, others will current limit and clip, still others will simply smoke when presented with a low-Z load.

And, to directly address your comment, some of the very fastest devices will oscillate with the wrong combination of decoupling caps! As much as geeks like me try to reduce it to a science, audio electronics is still an art.

[capslock]

The socket has adequate decoupling and the LM6172 has an extra capacitor soldered across the supply pins. The output is isolated with a 400 R resistor. The amp being used in a multiple feedback, second order low pass configuration, there is a 220 pF capacitor from output to inverting input, but this should be fine as the LM6172 is unity gain stable. I am going to solder a 20 R insulation resistor directly to the noninverting link when I evaluate some even faster VFB op amps.

Coming back to the topology question: Once the output has settled, both inputs are at the same voltage, no current flows in R_E and there should be no distortion in the input buffers. The question is how long the settling takes. CFB amps have been notorious for having long settling times to 0.01%, I haven't yet figured out why. There are many cases where a real VFB op amp of slightly slower bandwidth/slew rate than a comparable CFB will have a longer settling time to 0.1% but faster to 0.01%. Note that this feature is not advertised for the 6171.

I have found the AD link again, take a look at page 11:
http://www.analog.com/technology/amp...ards/pdf/1.pdf

I wonder what the advantages of coupling the input buffers through current mirrors and using voltage-controlled current sources in the voltage gain section are. Anybody "auditioned" a quad-core amp?


Take care,

Eric

[fmak]

I agree; but manufacturers claim otherwise. Also no two or even the same manufacturer produces data in a form suitable for direct comparison.

Surely the user can only base assessment of suitability on the literature provided and not be expected to measure each type of chip.

[capslock]

Yupp, the LT1358 seems to use the same topology as the LM6171. LT claims it is a true voltage feedback amp...