On 19/10/2019 11:03 pm, Mat Nieuwenhoven wrote:
On 13 Oct 2019 21:26:46 GMT, Trevor Wilson wrote:
snip
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 TDA7492 has a switching frequency of typically 310 kHz. How is
this related to bad handling of a 4 ohm load, and why is that
dependent on the load resistance? And this IC is specified for 4 ohm
or more.
**The output filter will affect phase and frequency response, when used
with 8 Ohm loads. A switching frequency closer to (or exceeding 1Mhz)
is desirable to ensure phase/frequency response errors are not extant
with lower impedance loads.
The low frequency fall off is deliberate (page 24 of the ST spec),
easily fixed by increasing the input size capacitor.
**Yes, but the problems are also obvious at the other end of the
frequency spectrum. The problems with this chip will be audibly obvious,
if it is attempted to be used in a high quality system.
The
THD-versus-frequency plot is indeed not impressive, THD rises in
spots to 0.2 %. You don't happen to have a link to a similar plot
from a good quality analog amp? Also, a link for a similar FFT plot ?
I am curious.
**Here's one:
http://www.ti.com/lit/ds/symlink/lm3886.pdf
Here is one of my all-time favourite Class A/B chips (sadly no longer
manufactured, but excellent performance):
http://pdf.datasheetcatalog.com/data...ps/TDA1514.pdf
Either will comfortably outperform your cheap 'n cheerful Class D chip.
At the cost of efficiency, price and heat sinking, of course.
The IC is also dirt cheap, on a board for less than 10 $. For that
price, it is superb value for money.
**No argument from me. Telephones, TV sets, portable audio equipment can
all make good use of such chips. For QUALITY audio, however, forget it.
Far too many audibly significant problems. Same as cheap tube amps - too
many audible problems to bother with.
There is indeed a feedback path from the OUTx pins to the second amp
inside.
**Of course. See my previous comment. ALL amplifiers use some kind (or
kinds) of NFB.
snip
For a professional product, see e.g.
https://icepower.dk/products/other/a-series/ . Its datasheet is at
https://icepower.dk/download/2414/ . It supports loads down to 2.7
ohm loads. Again I wonder what the switching frequency has to do with
load.
**The last ICEpower amp I had on my bench sounded horrible. It's been a
few years. Perhaps they're better now. That said, the issue with
switching frequency is pivotal the the high frequency performance when
used with low impedance loads. The output filter is almost always a
simple 6dB/octave affair. As such, it can introduce phase shift and/or
frequency response errors down into the audio band. Raising the
switching frequency to 1MHz means that almost any speaker system can be
used without causing such problems. IMO, once switching frequencies
reach (say) 1.5MHz, then Class A and Class A/B amps will no longer need
to exist. Right now, at the present state of the art, Class A and Class
A/B amps still provide superior performance, when used with low
impedance loads. Of course, this means that for subwoofers, Class D is
the only sane choice.
I wonder if the phase plot can be matched by any analog amp, or even
the output resistance of 50 mOhm .
**Of course. Achieving an output impedance of 50mOhm is easy enough. The
catch is achieving that output impedance at 20kHz.
--
Trevor Wilson
www.rageaudio.com.au
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