I was just playing with the idea to demonstrate the effect on a square wave of different frequency responses.

So I gave it a try.

Looking at 2 amplifiers:

**Benchmark AHB2**
Frequency response, from Amir's review

View attachment 306150
If I see correctly, that's behaving like a 1st order low pass filter at around 200kHz.

Let's say it's a Butterworth 1st order filter.

What would a 10kHz square wave look like ?

View attachment 306151
Cool.

Now, let's try with the NAD M23, or C298, or whatever amp using the Purifi module.

**NAD M23 **
Frequency response, as per Amir's measurements above

View attachment 306152
That's a steeper response.

Purifi's datasheet says -3dB at 60kHz and -6dB at 75kHz.

So let's try with a Butterworth 2nd order filter at 60kHz

Square wave at 10kHz would look like

View attachment 306153
That's different, indeed.

Does it matter ?

Now let's introduce another parameter:

**Human hearing**
What happens after our ear's own low pass filter is applied ?

That's a very steep low-pass filter, since some may hear at, say, 16kHz, but at 32kHz, for sure, we don't hear anything.

Meaning 0. Nada. -100dB or so at least.

Let's try and apply such a filter on top of the amp filter.

Here is what we'd get as a result.

**Benchmark AHB2**

View attachment 306154

NAD M23

View attachment 306155
In both cases, we get a pure 10kHz sine wave.

Of course, since 3rd harmonic and above are completely inaudible to any human.

(And the theory -and practice- tells us that a square wave may be reproduced by an infinite sum of sine waves at odd multiples of the fundamental frequency, with level decreasing as 1/order)
So are those differences in 10kHz square wave any relevant ?

Not at all.

By the way, don't say a Purifi amp can't do any square wave correctly.

Here is one (predicted) for NAD M23 :

View attachment 306156
Well, granted, that one is at 1kHz.

So at 60kHz, we already have 30 harmonics (including the fundamental), which allows a pretty accurate reproduction of the square wave.

OK, I had fun

Now let me grab a beer...