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Trevor Wilson[_3_] Trevor Wilson[_3_] is offline
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Default Introducing a New Horse to the Stable

On 14/10/2019 8:00 am, Mat Nieuwenhoven wrote:
On 12 Sep 2019 20:28:33 GMT, Trevor Wilson wrote:

snip

**Let me be very clear about several things:

* NFB is fine. In fact, NO audio amplifier can work without it.
* GLOBAL NFB is also fine. When properly applied.
* I have a personal preference for the amplifiers I use, which employ
lots of local NFB and no global NFB. Others may have a different opinion.
* As part of my education into the world of zero global NFB amplifiers,
I subjected myself to a couple of single (unfortunately) blind tests,
between two, otherwise identical, amplifiers. One employed zero GNFB and
one employed a modest amount of GNFB. I preferred the zero GNFB one.
Since that time, I subjected several (10) of my clients to the same test
(DBT). The zero GNFB models was preferred every time. Except one.
* Once mo I would posit that part of the reason why some listeners
prefer valve amplifiers, is due to the fact that global NFB levels are
very low, or non-existent.


But, again, in the real world, negative feedback, done properly, has
many more advantages than disadvantages.

**Again: No issue with NFB. In fact, no issue with GNFB, when done well.


How do modern switching amps (class D) stack up for HiFi use?


**Provided the switching frequency is high enough and the load impedance
is benign, then they should work well. I was particularly impressed with
the interesting design from Devialet. A small Class A stage is used in
much the same way that Quad did several decades ago for their Current
Dumping„˘ products. They used a Class A stage, combined with a Class C
power stage.

Aren't
most PA systems now fully digital?


**No such thing.

Do they actually use FB?

**EVERY amplifier uses NFB. Every single one. Regardless of technology
or claims from manufacturers.

If I look
at the spec sheet of the TDA7492 it doesn't look like it. Do they
sound worse than a good analog amp?


**I see a loop feedback mechanism in the block diagram. I see some
audibly significant problems with the amplifier. Max THD is cited aas
0.4% and the frequency response is poor, compared to even modest Class
A/B amplifiers. The low switching frequency ensures that low impedance
(4 Ohms) loads are not well catered for.


The class-D amps typically have a series inductance between the
switching elements and the speakers, does that influence transients?


**Of course.

Even a tweeter has already 15-20 microHenry of inductance.

Mat Nieuwenhoven








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Trevor Wilson
www.rageaudio.com.au

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