Is the output impedance of Topping's L70 and D90 1.65 ohms?

[mike7877]

So I was looking at some pictures:

On the very right is the A90. I marked red lines for the traces for illustration purposes. The E70, the PCB between the two heatsinks carries four traces with the output. The red line connects the output to a 1001 ohm resistor used for feedback to the same pin that gets the signal fed to it from a 1612 with 1001 ohms on its output.

It looks like the L50's output impedance is two parallel 3.3 ohms, in series with 4.7 ohms,
the L70 looks like two 3.3 ohms in parallel,
and the A90 looks like two 3.3 ohms in parallel.

Now, 1.65 ohms is pretty low, 6.35 ohms is low enough for 100 ohms, maybe a little less. But neither is <0.1 ohms.


From the layout, it does appear the 6120A2 is the final stage (and I doubt they chose the 1612/1656 over them lol).

The 6120A2 datasheet suggests a 40 39.2 ohm resistor on its output, and I believe "allows" down to 10 ohms. I don't think there is, specifically, any super important reason this resistor is required. The datasheet does mention it's for limiting the current with low impedance headphones and maximizing SNR. Since Topping implements over-current protection, going below 10 ohms shouldn't be a problem.

I've driven speakers with my L70, and their sonic signature was notably different than when connected to proper speaker amplifiers with actual <0.1 ohm output impedance. I also have an A70 Pro and the change in tone is similar (more tweeter output is the most obvious difference, a tonal change not present when driving actual headphones {with no crossovers obv}).

Does anyone dispute?

https://www.ti.com/lit/ds/symlink/tpa6120a2.pdf

(also, I have no idea what NFCA is after looking at this... looks like standard implementation to me - at least from this angle!)
I'm not a Topping hater, I actually really like my D90 III and A70 Pro.
And my L30 II and my E50 and my E70 Velvet and L70.
And my E30 II and DX1 and HSO2.
Oh, and my (now cooked but it was my fault I think) G5!

Topping, I still would've bought my headphone amps if you advertised them as ~1 ohm!!

And if they are 0.1 ohm / 0.2 ohm balanced, I'm sorry for being stoopid guise!

 

[NTK]

The Topping headphone amplifiers, AFAIK, utilize composite designs. The feedback signal is taken after the output resistor of the TPA6120A2. Therefore the values of the output resistors for the TPA6120A2 do not directly relate to the output impedance of the amplifier. See page 34 of this thread for discussions on the Topping L30 topology.

Topping L30 Headphone Amplifier Review

I am pretty sure that someone posted 2 images of the inside of L30 and I went through the entire thread and can't find it. Anybody recalls the page number or can upload the images, please?

www.audiosciencereview.com

Member @Bamboszek also has a thread over at DIYAudio on his version of a composite TPA6120A2 headphone amp.

High performance OPA1602 + TPA6120A2 Composite Headphone Amplifier

Hello guys, About half a year ago I decided to design my own high performance headphone amplifier. The idea came more from some kind of engineering challenge rather than serious need - my O2 was probably more than enough. My designs considerations were pretty simple: Challenge myself and...

www.diyaudio.com

 

[Bamboszek]

Since Topping implements over-current protection, going below 10 ohms shouldn't be a problem.

Output resistors in this case are not for current limiting only, but more importantly for isolating capacitive load from TPA6120A2 and maintaining feedback loop stability.
To achieve this and preserve low output impedance one could use output inductor (air core usually, much like in speaker amplifiers) or include output resistors in feedback loop like Topping did here.

There is topic on DIYaudio like @NTK mentioned, and here on ASR if you are interested in more details.
Definitely I wasn't first to use TPA6120A2 in nested feedback / composite amplifier configuration for driving headphones, but very similar Topping L30 appeared shortly after I published my findings there.

 

[mike7877]

Output resistors in this case are not for current limiting only, but more importantly for isolating capacitive load from TPA6120A2 and maintaining feedback loop stability.
To achieve this and preserve low output impedance one could use output inductor (air core usually, much like in speaker amplifiers) or include output resistors in feedback loop like Topping did here.

There is topic on DIYaudio like @NTK mentioned, and here on ASR if you are interested in more details.
Definitely I wasn't first to use TPA6120A2 in nested feedback / composite amplifier configuration for driving headphones, but very similar Topping L30 appeared shortly after I published my findings there.

Ah, yes - I remember reading that in the datasheet as well.

Is extra feedback used to essentially make the 3.3 ohm resistor look transparent (by providing the amount of extra power required to drive the load as if it wasn't there??



Unrelated, these inductors on the LA90 - good example of what you're talking about?

Is the resistor ess

 

[solderdude]

So I was looking at some pictures:

View attachment 400387
On the very right is the A90. I marked red lines for the traces for illustration purposes. The E70, the PCB between the two heatsinks carries four traces with the output. The red line connects the output to a 1001 ohm resistor used for feedback to the same pin that gets the signal fed to it from a 1612 with 1001 ohms on its output.

It looks like the L50's output impedance is two parallel 3.3 ohms, in series with 4.7 ohms,
the L70 looks like two 3.3 ohms in parallel,
and the A90 looks like two 3.3 ohms in parallel.

Now, 1.65 ohms is pretty low, 6.35 ohms is low enough for 100 ohms, maybe a little less. But neither is <0.1 ohms.


From the layout, it does appear the 6120A2 is the final stage (and I doubt they chose the 1612/1656 over them lol).

The 6120A2 datasheet suggests a 40 39.2 ohm resistor on its output, and I believe "allows" down to 10 ohms. I don't think there is, specifically, any super important reason this resistor is required. The datasheet does mention it's for limiting the current with low impedance headphones and maximizing SNR. Since Topping implements over-current protection, going below 10 ohms shouldn't be a problem.

I've driven speakers with my L70, and their sonic signature was notably different than when connected to proper speaker amplifiers with actual <0.1 ohm output impedance. I also have an A70 Pro and the change in tone is similar (more tweeter output is the most obvious difference, a tonal change not present when driving actual headphones {with no crossovers obv}).

Does anyone dispute?

https://www.ti.com/lit/ds/symlink/tpa6120a2.pdf

(also, I have no idea what NFCA is after looking at this... looks like standard implementation to me - at least from this angle!)
I'm not a Topping hater, I actually really like my D90 III and A70 Pro.
And my L30 II and my E50 and my E70 Velvet and L70.
And my E30 II and DX1 and HSO2.
Oh, and my (now cooked but it was my fault I think) G5!

Topping, I still would've bought my headphone amps if you advertised them as ~1 ohm!!

And if they are 0.1 ohm / 0.2 ohm balanced, I'm sorry for being stoopid guise!

The output resistors on the outputs of the TPA can be lower than 10ohm as suggested by the datasheet.
There are 2 of them on each channel because the outputs are paralleled.
As every op-amp (which is what they are) has a small DC offset the resistors are mainly there to 'mix' the outputs of the op-amps and to ensure that the outputs (when they would be tied together hard) would be pumping currents in each other due to DC offset and small gain differences (due to resistor tolerances).
The nested feedback lowers the output resistance of the entire circuit to near 0 ohm.
The resistors could be a little higher in value but that would limit the output power and paralleling the outputs is done to increase output power (double the current a single TPA can do).

So no need to worry about those resistors increasing the output resistance of the amp. The 'losses' are compensated for,

 

[mike7877]

The output resistors on the outputs of the TPA can be lower than 10ohm as suggested by the datasheet.
There are 2 of them on each channel because the outputs are paralleled.
As every op-amp (which is what they are) has a small DC offset the resistors are mainly there to 'mix' the outputs of the op-amps and to ensure that the outputs (when they would be tied together hard) would be pumping currents in each other due to DC offset and small gain differences (due to resistor tolerances).
The nested feedback lowers the output resistance of the entire circuit to near 0 ohm.
The resistors could be a little higher in value but that would limit the output power and paralleling the outputs is done to increase output power (double the current a single TPA can do).

So no need to worry about those resistors increasing the output resistance of the amp. The 'losses' are compensated for,

So feedback essentially makes the two paralleled 3.3 ohm output resistors effectively disappear? Pretty neat! I wonder why my speakers with passive crossovers behave the way I'd expect them to with an amplifier that has a ~1 ohm output impedance though - could that be a side effect of this method? OK for unfiltered drivers, but not OK for crossovers?

Also:

 

[staticV3]

The A90's differential headphone output has 0.37Ω output impedance.
Source

 

[solderdude]

So feedback essentially makes the two paralleled 3.3 ohm output resistors effectively disappear? Pretty neat!

That's possible when you have an overall feedback after both 3.3ohm resistors.

John already mentioned several years ago (before his first amp came out) that the TPA6120A also works fine with lower output resistors and a circuit that prevents a capacitive load on the output destabilizing the circuit.
So Topping is not using the TPA6120A in a typical configuration.