Does the servo control line in some modern power amplifiers prevent
them from kicking butt like some of the older amplifiers did? Is servo
control really just a way of preventing the output transistors from
melting down, rather than some kind of sonic marvel, as the
manufacturers seem to want us to believe?

Servo control that I've seen mainly relates to controlling DC offset.

Could also refer to a motional feedback system for controlling a speaker
system, usually but not always a subwoofer.


Mark Z.


wrote in message
ups.com...
Does the servo control line in some modern power amplifiers prevent
them from kicking butt like some of the older amplifiers did? Is servo
control really just a way of preventing the output transistors from
melting down, rather than some kind of sonic marvel, as the
manufacturers seem to want us to believe?

In . com, on 03/04/05
at 09:56 AM, said:



Does the servo control line in some modern power amplifiers prevent
them from kicking butt like some of the older amplifiers did? Is
servo control really just a way of preventing the output transistors
from melting down, rather than some kind of sonic marvel, as the
manufacturers seem to want us to believe?

I'll vote for Mark's reply.

Servo control for loudspeakers has never been very popular in the
marketplace because it tends to wed the amplifier and speakers.
Customers like to chase the ultimate amplifier and the ultimate
loudspeaker. A wedded pair robs the shopper of half the fun.

"sonic marvel"? Any development is a "marvel" in audioland. I can
recall a manufacturer who came up with a new engineering technique for
their amplifier line. There was a wonderful name for it, an almost
incomprehensible description, and pretty graphs. (something that
complicated *HAD* to make their amplifiers sound better -- didn't it?)
I didn't want to bust their bubble tto much, but more than ten years
prior another manufacturer had another name and description, but
similar graphs. Unfortunately for both of them, I had already seen the
graphs in my engineering textbooks some years prior to either company's
pitch. They were decent, run-of-the-mill amplifiers.

In my teens I may have fallen victim to the hype, but now that I've
seen a zillion products, I know that a novel topology does not
guarantee superior sound or a long reliable product life.

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"Barry Mann" wrote in message
om
In . com, on 03/04/05
at 09:56 AM, said:



Does the servo control line in some modern power amplifiers prevent
them from kicking butt like some of the older amplifiers did? Is
servo control really just a way of preventing the output transistors
from melting down, rather than some kind of sonic marvel, as the
manufacturers seem to want us to believe?

I'll vote for Mark's reply.

FWIW, I'll second that. Servo-control usually refers to adding some active
components to the over-all feedback loop that is present in virtually every
modern competently-designed standard-design SS power amp, to minimize DC
offset at the output terminals. In almost every case its more-or-less
gratuitous, but it doesn't usually hurt any.


Servo control isn't really necessary in a well-designed power amp. The
evidence to support his is the large number of good audio amps that lack
this feature. I imagine that someone could **** in the soup and screw up a
good power amp with an improperly-designed servo feature. But, usually it
just ups the parts count and adds another line to some marketing guy's
features and benefits summary.

wrote:
Does the servo control line in some modern power amplifiers prevent
them from kicking butt like some of the older amplifiers did? Is servo
control really just a way of preventing the output transistors from
melting down, rather than some kind of sonic marvel, as the
manufacturers seem to want us to believe?


I've repaired an older Sansui amp with Active servo blah blah..whatever
it was.. (model,uhm.. A1130P?)
Basically offset/bias correction..
I had a hell of a time repairing it (more than once)...and it seemed
picky,and 'easy to upset'.
The amp,when working,Was a good amp though,I used it for a few years..
The sound was okay,then it went to "pretty good" once I opened it up and
saw a bunch of 4066's in the signal path,and a bunch of other funky
stuff (yikes)..Considering all that I think it sounded great!
It also took a fair amount of abuse...but the fuses and protection
circuity wasn't enough..it would let out a giant cloud of "magic smoke"
if it got into a low-ohm load,or a dead-short at a fair volume. (that
killed it instantly,as I would expect it would do to most amps!)
Some amps will take that kind of abuse..My Father,who "permanently
loaned" me his Sansui AU7900,said that he once took it to a McIntosh
clinic,and had it tested,It performed very well,and dad said he even
drove it into a dead short,and it still survives to sing about it..
It's built like a tank too.

My Dad now has a Yamaha powered sub of some sort,that uses "servo
feedback"..It's a pretty nice sub,I'm impressed by it every time I'm
over there..it can get low,and loud,considering it's size,and it sounds
great.It seems integrate well with different main speakers (he's used a
couple ofdifferent Maggies,and a pair of Small Advents with it so far.)
I think it has 2 (maybe only 1?) 8-inch subs,and a fairly beefy
amplifier (200W?)I forget the details,but it's a pretty nice sub,but as
I understand cost a pretty penny (or two.)
From what I gather,there's an accelerometer,or something on the woofer
cone,that detects how fast/far it's moving,etc. and feeds that to the
amp to 'compensate' for the speaker's response curve/box,etc.
I dunno.. it's late(early!),and I need sleep..

In , on 03/06/05
at 04:30 PM, Fran ois Yves Le Gal said:

On Sun, 06 Mar 2005 06:54:50 -0800, ptaylor
wrote:

From what I gather,there's an accelerometer,or something on the woofer=

cone,that detects how fast/far it's moving,etc. and feeds that to the
amp to 'compensate' for the speaker's response curve/box,etc.

Nope. Yamaha Active Servo Feedback or Yamaha Servo Feedback is
basically a negative impedance amplifier coupled to a tailored
bandpass filter. It is based on a similar technique, called Ace Bass,
used by Audio Pro of Sweden since 1978.

Few audio designers read the literature. "New and improved" often means
"independently rediscovered". Even if the design engineer did read and
adopt, the marking department will rename the technique and claim it
for their own.

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In article , says...

My Dad now has a Yamaha powered sub of some sort,that uses "servo
feedback"..It's a pretty nice sub,I'm impressed by it every time I'm
over there..it can get low,and loud,considering it's size,and it sounds
great.It seems integrate well with different main speakers (he's used a
couple ofdifferent Maggies,and a pair of Small Advents with it so far.)
I think it has 2 (maybe only 1?) 8-inch subs,and a fairly beefy
amplifier (200W?)I forget the details,but it's a pretty nice sub,but as
I understand cost a pretty penny (or two.)
From what I gather,there's an accelerometer,or something on the woofer
cone,that detects how fast/far it's moving,etc. and feeds that to the
amp to 'compensate' for the speaker's response curve/box,etc.
I dunno.. it's late(early!),and I need sleep..

This is how some (all ?) velodyne subs work. The accelerometer on voice
coil sends a signal back to the driving amp. The amp then compares the
signal from the accelerometer to that comeing into the amp. It then makes
the necessary corrections to keep the signals the same.
-------------
Alex

"Alex Rodriguez" wrote in message

In article ,
says...

My Dad now has a Yamaha powered sub of some sort,that uses "servo
feedback"..It's a pretty nice sub,I'm impressed by it every time I'm
over there..it can get low,and loud,considering it's size,and it
sounds great.It seems integrate well with different main speakers
(he's used a couple ofdifferent Maggies,and a pair of Small Advents
with it so far.) I think it has 2 (maybe only 1?) 8-inch subs,and a
fairly beefy amplifier (200W?)I forget the details,but it's a pretty
nice sub,but as I understand cost a pretty penny (or two.)

From what I gather,there's an accelerometer,or something on the
woofer cone,that detects how fast/far it's moving,etc. and feeds
that to the amp to 'compensate' for the speaker's response
curve/box,etc.
I dunno.. it's late(early!),and I need sleep..

This is how some (all ?) velodyne subs work. The accelerometer on the
voice
coil sends a signal back to the driving amp. The amp then compares
the signal from the accelerometer to that comeing into the amp. It
then makes the necessary corrections to keep the signals the same.

An acellerometer detects, well accelleration. Acceleration is the derivative
of the velocity, and velocity is the derivative of the position. Sucessive
stages of electronic circuits called integrators can convert accelleration
signals into signals that represent velocity and position. The desired
parameters can be electronically compared to the actual parameters and the
operation of the subwoofer corrected.

In , on 03/08/05
at 07:21 AM, "Arny Krueger" said:



[ ... ]

An acellerometer detects, well accelleration. Acceleration is the
derivative of the velocity, and velocity is the derivative of the
position. Sucessive stages of electronic circuits called integrators
can convert accelleration signals into signals that represent velocity
and position. The desired parameters can be electronically compared to
the actual parameters and the operation of the subwoofer corrected.

I think it is a good technique, but it cannot turn an otherwise poor
driver design into gold. The electronics can correct the motion at the
point of measurement, but if the cone is flexing, the measurement point
may not be a good representation of the overall piston position.

The technique does present a design challenge because the system
feedback now includes a mechanical component. I never worked through
any design details, but I would hope that for subwoofer designs, many
of the cone breakup modes would be outside of the frequency band of
interest and could be swept under the carpet with simple bandpass
filtering.

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[sorry about the puzzle, spammers are ruining my mailbox]
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Fran?ois Yves Le Gal wrote:
On Sun, 06 Mar 2005 06:54:50 -0800, ptaylor
wrote:

From what I gather,there's an accelerometer,or something on the woofer
cone,that detects how fast/far it's moving,etc. and feeds that to the
amp to 'compensate' for the speaker's response curve/box,etc.

Nope. Yamaha Active Servo Feedback or Yamaha Servo Feedback is basically a
negative impedance amplifier coupled to a tailored bandpass filter. It is
based on a similar technique, called Ace Bass, used by Audio Pro of Sweden
since 1978.

Isn't what ptaylor describes closer to what Velodyne does with its
servo-regulated subwoofers?

--

-S
It's not my business to do intelligent work. -- D. Rumsfeld, testifying
before the House Armed Services Committee

In article ,
Fran?ois Yves Le Gal wrote:

On Tue, 8 Mar 2005 20:18:43 +0000 (UTC), Steven Sullivan
wrote:

Isn't what ptaylor describes closer to what Velodyne does with its
servo-regulated subwoofers?

Yes, Velodyne is one of the few manufacturers using accelerometers in a
servo loop for some - not all - of their models.

How about something like this:


http://www.meyersound.com/products/studioseries/x-10/



Or the Bag End ELF system.

Edwin

I agree the advertising wording is embarassingly silly. The patent
number is 6,584,204. The thesis associated with the project can be
downloaded at

http://jagger.me.berkeley.edu/publications.php (about 3/4 down the
list) or directly
from http://jagger.me.berkeley.edu/papers/pack_11.pdf