Re: Why Is High Feedback Considered Bad In Audio? In Simple Terms

On Tue, 27 Jan 2009 21:58:27 -0800 (PST), "miso@xxxxxxxxx"
<miso@xxxxxxxxx> wrote:

On Jan 27, 8:25 pm, JosephKK <quiettechb...@xxxxxxxxx> wrote:
On Tue, 27 Jan 2009 00:23:17 -0800 (PST), "m...@xxxxxxxxx"



<m...@xxxxxxxxx> wrote:
On Jan 26, 10:04 pm, JosephKK <quiettechb...@xxxxxxxxx> wrote:
On Mon, 26 Jan 2009 14:10:10 -0800 (PST), "m...@xxxxxxxxx"

<m...@xxxxxxxxx> wrote:
On Jan 26, 10:17 am, Eeyore <rabbitsfriendsandrelati...@xxxxxxxxxxx>
wrote:
"m...@xxxxxxxxx" wrote:
 "abby" <abbybr...@xxxxxxxxxxx> wrote:

Can someone explain, in simple terms, why high feedback is considered
inferior to low feedback in audio circuits?  To me, linear is linear
and a high feedback op-amp circuit is linear.  Apparently, that isn't
entirely true.

There were a few things in older transistor amp design that weren't
done well, and the lower feedback designs were more tolerant of these
things. Besides the Doug Self papers in Chris Jones post, dig up the
old Marshall Leach papers.
http://users.ece.gatech.edu/~mleach/
http://users.ece.gatech.edu/~mleach/lowtim/

If I were to roll an amp together, I'd be more inclined to go for
linear MOS on the output. Randy Sloan's amp book is a good reference,
even if you only want to read about such design.
http://tinyurl.com/b2ltfa

Sadly the excellent Hitachi lateral mosfets have now gone, but you can get
approx equivalents from Semelab/Magantec and Exicon.

I built an amp module using the Hitachi parts with 0.0005% THD ( yes -106dB
SINAD) @ 350W into 8 ohms btw. The final product THD was determined by
op-amp buffer stage performance @ 0.003%. You can get better op-amps now
though. It would also drive 600W into 4 ohms and around 900W into 2 ohms.

That used tons of feedack both local, overall and pole-zero. It also
sounded lovely. Quite a few made their way into recording studios as
monitor amps.

Graham

http://tinyurl.com/c8ydwm
for the Renasys parts. It is the semi spin off from Hitachi.

It must be a small market for the lateral current flow mosfets. They
are pretty simple to build, so technology isn't an issue. I didn't see
a die photo on the Renasys part, but I assume the fets are just waffle
pattern.

Mostly hexagonal now, slightly better HF corner.  Say thanx to IR.

The IR hexfet is a vertical flow device. I did a back of the envelope
sketch of a lateral current flow with hex shaped poly and it doesn't
pack as well as the waffle. Do you a particular device with a diagram
showing the layout?

No.  It is a horizontal device with a hexagonal channel pattern.  See:http://www.irf.com/technical-info/guide/device.html

I don't see a lateral current flow device on that page. You can see
the epi layer on the hexfet with drain contact labeled on the bottom.
Here is the text from that page:
"As shown in Figure 17 above, the HEXFET® power MOSFET is named for
the hexagonal shape of its individual cells. Current flows from the
source metallization down through the device, and out through the
drain contact. Vertical current flow is the reason the HEXFET is also
called a vertical MOSFET. Nearly all power MOSFETs on the market
employ this vertical structure."

It says vertical current flow, which matches the diagram.

The channel flow is horizontal across the near surface the hexagonal p
regions. It is lateral, as opposed to the earlier VMOS parts. Also
look up TMOS (TI IIRC).

.

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