High performance OPA1602 + TPA6120A2 Composite Headphone Amplifier

[jasonhanjk]

Originally Posted by BamboszeK

Feedback in composite amplifiers needs precise tuning, otherwise even small changes can make them unstable.

That's only partly true.
I can make 5532 or 2068 stable with 4556, after a lot of trial and error. Composite amp schematic online need to have the low noise op amp and amplifier bandwidth to be the same for stability. That's the main reason not to anyhow change ic.

L30 already put a lot of good engineering, so better leave it as is. They add something to make it stable.😅

My current design Tpa6120 use 0 ohm output resistor.

[kn0ppers]

BamboszeK is absolutely correct in my opinion. You throw around the term stability and claim to have a design with 0 Ohm output resistor, but how exactly did you verify stability and what kinds of phase margins are we talking into typical headphone loads?

[jasonhanjk]

Great question.

I won't tell you the answer directly, however I will ask you just 1 question.
What's the reason(s) for the TPA6120 needing resistance at it's output?

[BamboszeK]

It isn't TPA6120 specific. To maintain loop stability, output of the amplifier need to be isolated from capacitive load. Resistor is cheapest way of doing it.
Some opamps are stable up to certain load capatiance without additional isolation, up to couple hundreds of pF usually. That's why you often won't see any resistor at output of headphone amplifiers.
Inductor is the better way of doing it, but it's considerably larger and more expensive. Doesn't limit power and output impedance however. Standard practice in speaker amplifiers.

I really simplified things. More in-depth explanation could be found in most books about analog design or this article Op Amps Driving Capacitive Loads | Analog Devices

[chris719]

Originally Posted by jasonhanjk

Great question.

I won't tell you the answer directly, however I will ask you just 1 question.
What's the reason(s) for the TPA6120 needing resistance at it's output?

Capacitive load isolation isn’t some trade secret.

[Mark Johnson]

Someplace here on diyAudio is a posting containing several photos of a nosebleed high end manufacturer (Jeff Rowland Design Group)'s high power power amp, which contains eight or twelve LM3886 chips inside. You can see on the PCB that each of the LM3886 chips has an isolation resistor in series between the chip and the loudspeaker output, but none of the LM3886s has a series inductor.

[jasonhanjk]

Originally Posted by chris719

Capacitive load isolation isn’t some trade secret.

Yes.
A few more questions and get it answer will eventually lead to 0 ohm output resistor.

It's also known that inductor cannot isolate capacitance, a ferrite bead should work better. Or better, both in series.

[RussellKinder]

Originally Posted by MASamplifer

Good morning all,
I was not actually aware all this composite amp work using the TPA6120A2 was going on. In parallel, talking with Jan Didden, I had embarked a similar trek that now show up on page 42 of the March 2021 AudioXpress. In the early Comlinear days as apps manager, we were doing a lot of composite amplifiers as the only source for high voltage CFA (1985 to 1990 about). Yes, the ADSL (or more generally differential wireline drivers) are intrinsically very low distortion of necessity. That was my main area for many years - all of those standard DMT (discrete multi-tone) - think of them as little radios in 4kHz slices (for xDSL). The key test for linearity is called and MTPR test - multi tone power ratio - the lineal descendent of the old microwave NPR test (noise power ratio). Essentially, you fill up the test spectrum with carriers with a notch of no power and look at how much those carriers fold into the notch (a comprehensive intermodulation test).

Very true about the ADSL line drivers. I was hired into TI in 2002 to work on what became the TNETD7122, the analog front-end for the AC7 chipset. It drives and receives ADSL signals at the telephone company end of the phone line. I remarked one day that the TX amp would make an amazing headphone amp (-126 dB THD+N into 40 ohms at far beyond audio BW). That's when they told me the audio guys had repackaged the previous ADSL line driver as the TPA6120, and that it was, in fact, very good.