Why does this schematic have an extra IC with the LM3886?

[MotoMan_Yz400]

I've been poking around these forms for a few months now, listening, learning, and understanding chip amps the best I can. I came across this design for a bridge/parallel design, apparently tested by National Semiconductor and has promising results BPA-200
I have a PCB design mocked up in ExpressPCB from one of the schematics show in that paper (last schematic in the paper, one of the last pages). I've been comparing them with designs people here have, and I noticed a difference. Most people here (more or less) run their input signal directly into the LM3886, while this design does the same, but it has an extra op amp in the feed back. I read the reason for this, makes sense, however, why isnt everyone using this? Does it's function provide so little, people opt not to incorporate it? Should I have it in my design? Is it not needed for single chip designs, but necessary for multi chip designs? Can anyone shine some light onto this design?

[hitsware]

It's not in the feedback, it's driving the whole array. It is needed because the input impedance of the inverting amps is 1 k Ohm, making for 500 Ohms for the 2. This is lower than many sources are comfortable with.

[ionomolo]

Originally posted by hitsware
It's not in the feedback, it's driving the whole array. It is needed because the input impedance of the inverting amps is 1 k Ohm, making for 500 Ohms for the 2. This is lower than many sources are comfortable with.

AN-1192 BPA-200 has two opamps, one is the buffer you mentioned and the other is the dc servo and it is in the feedback loop. I assume MotoMan_Yz400 to be asking about the one in the feedback loop.

This circuit is a dc servo. It integrates the output signal (calculates the mean voltage) and then it substracts it to the input signal (not as easy but that's what it does).

This is a way to reduce dc offset that has some advantages:

· Does not use caps in the signal path

· Offers better precision

But it has many problems:

· It complicates the layout and forces (almost always) to add a second power supply to feed that little opamp.

· In fact it is a magnified capacitor and it may also have some of it's defects, severely attenuated by the fact that it operates at low frequency and it does not *theoretically* interact with the audio signal.

· It requires high precision parts to do it's job well. That does mean close tolerance, not audiophile grade.

I havn't experimented with it already so I can't give any clue about if its sound improovement is worth the time and money

[MotoMan_Yz400]

Well one would think, if it's helping regulating the DC offset by eliminating it the best it can, then couldnt you conclude that the LM3886 would then run cooler? If theres always extra current running through the IC, then it's not aiding to the audio output, just heating things up. That sound right?

[pjp]

Your speakers would run cooler too ...

[ionomolo]

The difference would be 8 mV best case (gainclones usually have offsets arround 2 mV and this means 30V*(2mV/8Ohm) = 7.5 mW.

This isn't worth the extra price, and a DC servo circuit is more prone to fail that a single cap. This is for peple who fiercely belive that there shouldn't be any caps in the signal path.

[leadbelly]

Originally posted by ionomolo
This is for peple who fiercely belive that there shouldn't be any caps in the signal path.

Adding a servo NECESSITATES adding a cap to the signal path. With a servo, you need to AC couple the input so that the servo can do its thing. What the servo actually does is allow you to avoid the use of a large cap on the OUTPUT.

[ionomolo]

Symmetrical-powered amplifiers don't need a cap in the output either with servo or without it!

AN-898 encourages it's use precisely for removing the cap at the input. Anyways, the servo removes the offset regardless of it's origin.

[leadbelly]

Originally posted by ionomolo
AN-898 encourages it's use precisely for removing the cap at the input.

Where? I can't find anything at all about servos in AN-898.

[ionomolo]

You are absolutely right, I was refering to AN-1490 where it states:

In listening tests at National’s sound room evaluating different circuit components used in the LM4702 demo amplifier,
there was one part whose negative effect on audible signal
quality was undeniable. A DC blocking capacitor on the input
of the LM4702 degraded sound quality. In multiple listening
tests, with different participants and at various locations
around the country, the negative effects of even the best film
and foil polystyrene DC blocking input capacitors in the
audio signal path was confirmed. It is therefore recommended
that DC blocking capacitors not be used in the
signal path for mid to high-end audio equipment. Where DC
offset from another signal source may be a problem then the
use of a DC servo circuit that keeps DC offset from appearing
at the output of the amplifier is recommended.

I have a whole folder full of application notes, sorry for the mistake.

I actually belive that the DC servo must also degrade the sound and probably in a much less benefical way than the coupling cap since it (partially) referenciates the feedback loop to an active voltage source whose low impedance is reached trough incredible ammounts of NFB and will vary severely with frequency.

I would be very pleased if anybody has done capacitor versus servo A/B comparison and can give a clue.