Measuring and comparing two headphone amps' outputs

[Jose Hidalgo]

Hi all,

So I have received my two replacement Topping L30 headphone amps (2102xxxx). They both work, but I have noticed a big sound difference One of them outputs a sound that could be considered as "normal", but the other one has way too much treble, to a point that I have to EQ it down -6 dB in order for it to become "hearable". Let me emphasize this : the difference is not small, one of them just hurts my ears beyond any reasonable doubt.

Here's my setup : Windows 7 PC (USB) --> Topping E30 (RCA) --> Topping L30 (n°1 or n°2) --> Headphones
If L30 n°1 is connected, sound is about "normal", and if L30 n°2 is connected, sound is "not normal".
I'm using the same RCA cable alternatively, and also the same L30 power adapter alternatively.
So I think I've narrowed down the issue to one of the two L30. I don't see what else it could be.

I'd like to identify/prove what's going on precisely, so I'm guessing that measuring each L30's frequency response would be a good start.
Unfortunately I'm a measurement beginner, so I wonder how I could achieve that without having to buy any additional stuff.

So far I've just tried connecting the L30 headphone output by my PC microphone imput via a 3.5mm cable, and then recording that input via Audacity.
Then Audacity could generate a spectrum plot, so I could compare spectrum plots from both L30 amps and see what's going on.
But that doesn't work: Audacity doesn't record anything, no matter what I try. I'm guessing it's an impedance problem or audio level problem.
Audacity settings are :

• Devices > Interface > Host : Windows DirectSound
• Devices > Recording > Device : Main audio capture driver

I've also thought of using PRE mode instead of HPA mode (via a RCA to 3.5mm cable), but I haven't tried yet, because maybe the issue is limited to HPA mode, and in that case testing in PRE mode wouldn't prove anything. I don't know, I'm a bit lost.

Are there better/other ways of achieving this ? Thanks in advance.

 

[the_brunx]

If You can open them up and see if they have the exact same parts inside.

 

[Jose Hidalgo]

If You can open them up and see if they have the exact same parts inside.

That is out of the question. Too complicated for me, too risky, and most of all it would void the warranty of two new amps.
Plus it wouldn't prove anything: they could have different parts, but they supposedly have the same specs.
Only measurements would prove that indeed they don't sound the same at all.

 

[the_brunx]

Interesting, where did you get the info that opening up a device voids the warranty?

 

[Jose Hidalgo]

Interesting, where did you get the info that opening up a device voids the warranty?

Let's not get off-topic. I simply won't do it. Kindly read this topic's title again if required. Thank you.

 

[the_brunx]

All good, but just for the sake of spreading the right information. As far as I know, Opening up devices even devices with the small void warning sticker does not void their warranty.

 

[abdo123]

All good, but just for the sake of spreading the right information. As far as I know, Opening up devices even devices with the small void warning sticker does not void their warranty.

it depends on the company and their terms, it's not set in stone.

Anyway for OP's argument, you can record two tracks with Audacity, sync them with the 'find sound' function and you can hear both of them and share it with the rest of us as well.

 

[Jose Hidalgo]

Anyway for OP's argument, you can record two tracks with Audacity, sync them with the 'find sound' function and you can hear both of them and share it with the rest of us as well.

Sounds great, I'd love to do that... except that it doesn't 'sound' at all for the moment :

But that doesn't work: Audacity doesn't record anything, no matter what I try. I'm guessing it's an impedance problem or audio level problem.
Audacity settings are :

• Devices > Interface > Host : Windows DirectSound
• Devices > Recording > Device : Main audio capture driver

 

[abdo123]

Sounds great, I'd love to do that... except that it doesn't 'sound' at all for the moment :

do you have an audio interface? an analog to digital converter?

 

[the_brunx]

as far as I know, it does not depend on the company it depends on the law. US and EU it is illegal to void warranty for such reasons. in fact you can repair your device without voiding the warranty.

opening up your consumer electronic devices to have a look inside, 100%, without a doubt, does not void the warranty.

 

[Patrick1958]

Do you have an external recorder?

 

[Jose Hidalgo]

do you have an audio interface? an analog to digital converter?

Do you have an external recorder?

As said in the OP, I have a microphone input. Why wouldn't that work?

 

[abdo123]

As said in the OP, I have a microphone input. Why wouldn't that work?

it would, is it recognized in the recording tap under sounds?

 

[Jose Hidalgo]

... problem solved ! And it's all good news !

I finally managed to record both L30 output by using a second PC. First PC wouldn't work. Nevermind.
Then with Audacity it was easy to plot their spectrum.
That allowed me to see that (first good news) both L30 sounded exactly identical ! That was a huge relief.
Identical... but both VERY different from the original signal spectrum.
So something was definitely going on, and it wasn't in the L30s. So how could they sound different then ?

Going back through my audio chain, I finally discovered that the cause resided in my player.
For unknown reasons, a convolution DSP got activated at some point between my two L30 listening tests. So first test was OK, but second test wasn't.

The thing is that such activation wasn't indicated in the player's DSP drop-down menu. That's the weird part. So all this time I was persuaded that there was no active DSP and that my player was outputting pure, unmodified sound. Having explored all other possible causes, I had to go to its settings "just to be completely sure"... only to discover that the DSP was indeed activated.

The fix was as easy as kindly telling my player "Disable all goddamn DSPs once and for all and just let me be !".
Sound is perfect now on both L30 amps, as it should.