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chrissugar

Member
ROMANIA

Posts:
1312

To DC servo or not to DC servo,this is the question NEW INFO

« on: January 21, 2007, 04:30:23 PM
»

to DC servo or not to DC servo, this is the
question.

In the last few years I read tons of posts about people
prefering or not DC servoed audio circuits. I remember at the old place there
was a thread dedicated to this subject and there were so many contradictory
afirmations that I thought this is something worth to be investigated. The
opinions are divided between those who think that DC servo is a bandaid because
it eliminates the coupling capacitor but has other problems (sound unnatural,
constrained, has problems with low end etc, etc) and the ones who think that DC
coupling is better because it eliminates the capacitors (low freq phase
problems, dielectic absorbtion that smears the sound, etc)

One of the
recent threads, where one member was expressing his not liking DC servos (
quote: Question is, what are remedies for that beside DC servo (which I tend not
to like for nontechnical reasons) ")  reminded me that this is an unsolved
issue, and nobody did a serious controlled test, and draw the conclusions about
this subject (or at least didn't make the results public).

So I decided
to do myself this test. In fact it is a more extended test because I'm also
interested not only about DC servo but also about the claims related to the
audibility of various coupling capacitors.

Before anything I decided to
do my tests with two different discrete opamps, with and without DC servo. The
first opamp is jfet opamp based of Fred Forssell's paper, and the second is
based on the JML Hybrid (which is a circuit similar to some used in some
expensive comercial devices). The advantage with the Hybrid is that is has a DC
servo circuit that can be activated or not depending on application. For the
Jfet opamp I had to build a separate one, so I used the DC servo circuit from
Fred : http://www.forsselltech.com/schematics/JE16%20Mike%20Preamp.PDF
The
first setup was built with four Hybrid type opamps as pairs of circuits for
stereo evaluation, one pair with DC servo and one pair without.

The only
way to make a correct evaluation is to use a coupling capacitors for each setup,
because the one without servo can't be DC coupled to the next device. To reduce
as much as possible the sound alteration induced by the coupling caps, I used
some very high quality polyprops (MITCAP) so if there is any difference between
the servoed and non servoed circuits it should be below the damage induced by
the capacitor.
Anyway, based on how radical were the opinions about the sound
of servo versus non servo, I was expecting some serious
differences.

The setup: computer playing various test files for
calibration, and 24bit 96Khz mastertape quality records, minimalist, two
microphone orchestral, choir and jazz music. The computer connected to Lavry
DA10 converter, the analog outs distributed two the two pairs of discrete opamp
based gain blocks (one with servo, the other without). The outs of each pair of
the gain blocks into the coupling capacitors and then into the inputs of the
Cranesong Avocet. The Avocet was used as an A/B test device because it has a
very transparent audio path and the switching is relay controlled.
For the
first test I made both pairs of gain circuits as 20dB noninverting circuits, all
four opamps configured in class A (LED biasing). I opted for the 20dB gain and
not unity gain to make the opamps "work harder". Also everything was calibrated
with the RME Totalyser to less than 0.1dB, to avoid the chance to draw wrong
conclusions because of any level related problems. So the resistors in the gain
blocks were perfectly matched to obtain this precission.

The audio
test. I played lots of materials to be sure that my conclusions are correct. I
forgot to mention, that my choice for using pairs of circuits and not mono
chanels, was that in mono you can make judgements related to timbral changes,
but only in stereo with coerent materials you can hear if there is any change in
space definition because of phase distortion or other issues.

After
listening for an hour and switching from servo to non servo and back for at
least 100 times, I can say that I couldn't detect any difference in sound
between the two situations. So if there is any difference in sound between a
servo and not servo circuit it should be under the distortions introduced by an
expensive polyprop capacitor. I'm a bit shocked by the results considering how
radical opinions were expressed related to this subject.

Later I did
three more tests.

The second test was with the jfet opamps in the same
circuit, and had the exact same conclusions. No difference in sound between
servo and non servo.

The third test was to use four identical mic preamps
with the same test setup. The line level signal was distributed in pairs, and
attenuated 40dB before the Lundahl input transformers. everything was calibrated
like in the first test. First pair (A) was non servo and the second pair (B) was
with servo.
At this test I finally could hear a difference in the width of
stereo image at high frequency. Interesting was that the servoed version sounded
a bit wider than the non servo but without any timbral change. Before drawing
any conclusion I swapped the gain blocks between the pairs, now pair a is
servoed and pair B is non servo. The patern of sound was exactly the same but
this time the non servo sounded a bit wider. I can say it without any doubt that
the small width difference I heard was because of the input transformers, and
there is no difference in sound between DC servo and non servo.

Later I
redid the same mic preamp test, this time with the Jfet opamps and had the exact
same conclusion. The B pair sounded a bit wider (but timbraly identical)
regardless of servo or non servo.

So my final conclusion is that
there is no audible differece between gain blocks with DC servo and no DC servo.
Also I think that direct, capacitorless coupling (with DC servo circuit) has the
cleanest audio because there are no artifacts induced by the DC servo circuit
and there is the advantage of eliminating a coupling cap.
I sugest anyone who
can do this test in controlled condition to do it to confirm my
foundings.

chrissugar

P.S. The next test I intend to do, is
to make a setup with a gain block with DC servo, and couple it's output to the
next stage directly or with various type of capacitors (polyprop, polystiren,
elco), to evaluate how much damage produce various capacitors compared to direct
coupling. But this will be in another day, I hope in the very near
future.

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Christian Mike Sugar

CMS-LAB

CJ

Member
California

Posts:
10263

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #1 on: January 21, 2007,
04:52:28 PM »

Finally a good experiment on this!
Good
work, and thats a lot of hours bolting all the combos together.
You are
probably right about the input xfmr being the difference on the high 3end, the
servo should only affect low frequencies, right?

I always wanted to try a
nickel core output with an extra winding for counter dc.
In other words, the
servo action would be done inside the transformer, not another opamp.
This is
a standard technique in the instrumentation racket.

But then whats
the point of the servo if you can not direct couple?

Logged

If I can't fix it, I can fix it
so nobody else can!
Frank's- www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar- http://tdsl.duncanamps.com/schematics.php

Guest

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #2 on: January 21, 2007,
05:06:23 PM »

It seems like the question, "What is better:
fat free cookies, or sugar free sousages?"
I always though that the topology
must reflect desired end results, and many topologies may be implemented for
similar results, some of them are more optimal, some are less optimal, some are
equal in cost and results with different technologies in hands.
Trying to
eliminate a capacitor using servo equals to moving the capacitor to another
place and adding more DC and low end feedback gain. Stabilizing working points
of some precious amplifier that can't be stabilized another way is the different
story.
However, usage of a servo to compensate errors in calculations or wide
tolerance of resistors is another story, for example in hybrid thick film ICs
where adjustment of resistor values is costly, but anyway proper technology
implementation helps to reduce the need for adjustments...
IMHO, everything
may be used if used properly, either sugar or fat.

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CJ

Member
California

Posts:
10263

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #3 on: January 21, 2007,
05:14:42 PM »

And you have to know what is being plugged
in, and what is being plugged out.
If you DIY a whole system, you can cut
corners.
If you sell to the the world, you have to anticipate unorthodox
applications and design accordingly.
So the DIY rack wins everytime, as far
as S/N and Sonics.

I have never had a Jensen circuit on the bench, but I
am willing to bet thst tempeture is a major concern.

If you put a 6.2 1/2
watt zener on the bench in series with enough resistance to current starve the
backwards p/n, and put a fluke 4/1/2  dijii on series with the supply, you
can get an idea of what is going on in a servo circuit.
The current kind of
oooozzzes up and down, like a roller coaster stuck in a dip.

Logged

If I can't fix it, I can fix it
so nobody else can!
Frank's- www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar- http://tdsl.duncanamps.com/schematics.php

JohnRoberts

Global Moderator

Member
Hickory, MS

Posts:
8911

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #4 on: January 21, 2007,
05:33:28 PM »

While this may be just a semantic
distinction,  servo's do not eliminate the blocking capacitor they instead
trade a crap big one for smaller value good ones and a hopefully neutral opamp
path, that does no harm in the process.

I used a servo in a no
compromise phono preamp. While I concede I did it partially for the design
exercise and marketing hook, I was pleased with the result. I'm not from the
camp that all blocking caps must be bad. I mainly don't like them for low
impedance applications or when you can't control immediately before or after. I
am enthusiastic about Wayne's effort to eliminate them from phantom powered mic
preamp front ends (low impedance). I've had similar thoughts for a while, but
never reduced my scratchings to practice, so good job Wayne. Now, I Just need to
do it betta (in my spare time).  :wink:

JR

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John Roberts
http://circularscience.com/
Tune it, or don't play
it...

Guest

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #5 on: January 21, 2007,
06:14:44 PM »

That 2 Hz looks like
ringing.

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Guest

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #6 on: January 21, 2007,
06:56:33 PM »

I suppose tube amps may ring on infralow
frequencies when 3 stages with interstage caps have a global feedback. Some
people like to mix micked sound and output from speaker leads. But your picture
shows too high amplitude for the single channel summed with others, most
probably it is something related to CD...

Logged

Guest

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #7 on: January 21, 2007,
07:25:25 PM »

80 Hz HPF is what doctor prescribed for
guitars. I have 20, 40, 80 Hz switch on my tube mic preamps, 12
dB/Oct.

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JohnRoberts

Global Moderator

Member
Hickory, MS

Posts:
8911

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #8 on: January 21, 2007,
07:59:50 PM »

Quote from: "mediatechnology"

Quote

So my final conclusion is that there
is no audible differece between gain blocks with DC servo and no DC servo.
Also I think that direct, capacitorless coupling (with DC servo circuit) has
the cleanest audio because there are no artifacts induced by the DC servo
circuit and there is the advantage of eliminating a coupling cap.
I
sugest anyone who can do this test in controlled condition to do it to
confirm my foundings.

Thank you for that. Very
interesting. I tend to agree. I suppose like most things there is a "right"
way to do it and a "wrong" way. I think this may be an example of the wrong
way:

Check out that
bodacious base-line shift in the right channel. 2 Hz? It's the guitar intro to
Switchfoot's "We Are One Tonight."

Don't know we can hear it, but the
asymmetry in the guitar drives this one nuts.

I would expect a properly
executed DC servo to look like a simple HPF.

OTOH guitar amps, preamps
whatever, can and do routinely clip asymmetrically for some "nice" sounding
distortion. An otherwise 0 DC content AC waveform if clipped asymmetrically will
have a transient DC component that then works it's way through any following AC
coupled stages drifting toward zero DC equilibrium again. If the LF perturbation
seems to be following signal peaks perhaps that's an alternate explanation.

JR

Logged

John Roberts
http://circularscience.com/
Tune it, or don't play
it...

bcarso

Member
San Fernando Valley, Los
Angeles

Posts:
4055

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #9 on: January 21, 2007,
08:09:01 PM »

Nice job Chris.  I confess I think about
things a lot more than I have the opportunity and time to listen to them, so I
always like to hear some real results of careful tests, particularly when I
trust the person and know they have at most a minimally skewed
preconception.

A long time ago, when I was working on a system design to
be used in conjunction with Floyd Toole's speaker mover facility at Harman,
people at Madrigal were queried as to the likely success of my designs up to
that point.  It was interesting when it came out that they had basically
abandoned servos in favor of plain old coupling caps---IIRC just Wima
polypropylenes.

But then about that same time, popping the hood on an
expensive preamp of theirs revealed they had also migrated to monolithic opamps,
which led an associate to write them off as beneath consideration as serious
high-enders from then on  :roll:

Logged

chrissugar

Member
ROMANIA

Posts:
1312

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #10 on: January 22, 2007,
04:06:12 AM »

Thank you all.
I'm happy that after many
years of reading contradictory opinions about this, I have a real world answer
to this question.  :grin:
There are two more tests I'm very interested
in, and hope I will have the time to make a setup to test them.
One is about
the sound of various coupling caps compared to direct coupling, and the other
about audibility of low frequency phase shift (at what freq and what angle it
starts to be hearable). From my experiments I have the feeling that I hear
diferences but untill I make an A/B test in controlled conditions I can't draw
any conclusion.

chrissugar

Logged

Christian Mike Sugar

CMS-LAB

Samuel
Groner

Global Moderator

Member
Zürich, Switzerland

Posts:
2841

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #11 on: January 22, 2007,
04:21:35 AM »

Thanks for this contribution! I suggest that
you could include a third test to your setup, i.e. a precisely trimmed DC
coupled amplifier without servo. For this you may use a low drift trimming
circuit as shown e.g. here: B_r1.pdf

Samuel

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chrissugar

Member
ROMANIA

Posts:
1312

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #12 on: January 22, 2007,
04:35:43 AM »

Yes Samuel, thank you, I had this in my
mind.
I just have to make a circuit that has a very low DC offset from the
beginning and add that trimming circuit. But that would probably eat more time
because of fine tuning the circuit, so I leave it as a test to be done after the
capacitor test.

chrissugar

Logged

Christian Mike Sugar

CMS-LAB

mikep

Member
Philadelphia

Posts:
450

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #13 on: January 22, 2007,
10:23:16 AM »

Quote from: "chrissugar"

Yes Samuel, thank you, I had this in my
mind.
I just have to make a circuit that has a very low DC offset from the
beginning and add that trimming circuit.

Thanks for sharing your
research.  I have been thinking about this alot.  I dont like
cap-coupled outputs because the impedance of the load is unknown, in order to
assure LF preformace you have to size for minimum load. this leads to the desire
for 1000 uF  that have all the problems that implies.

Im personally
on the fence about servos.  Ive witnessed bad sounding servos.  wierd
bouncy bass, etc.  And im not sure about the real word reliability of
direct coupled, servoed outputs.

I have been playing with
DC-trimmed, direct coupled output stages.  IMO the solution is to cap
couple into the output stage, where you can setup favourable conditions for that
cap to operate in.  Im not saying this is totally drift-free, but it
works.

(there sould be a 10k
resistor in the top leg of those biasing diodes)

the problem is, when you
DC couple into an unknown input that has low DC-Z ( like a transformer). If it
is at even slightly different potential than your output,  the current will
start to flow like crazy. same story for a servo.  This is what doesn't sit
well with me.  If you are certain that the next stage will be cap-coupled
then this is fine, but then why try to get ultra-low offset at all? ( for my
personal equipment I am perfectly happy with up to 1 volt of offset into a film
cap-coupled inputs.)  In a way a servo output stage makes alot of sense,
but in the "real" world (a much stupider world than I live in) Im just too
afraid it will blow up when joe sixpack plugs it into the "power amp in" jack on
his guitar amp, or a mic amp input with phantom on, or whatever.  I guess
it worked for the SSL 9000 though, and there are plenty of other examples out
there.

mike p

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mikep

Member
Philadelphia

Posts:
450

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #14 on: January 22, 2007,
10:31:53 AM »

Quote from: "mediatechnology"

OK John it makes sense. Could it also
be a beat frequency from doubling?

Could be alot of things.
sometimes detuned guitars do weird LF stuff, ever listen to the Melvins or
Sunn-O)) ?  the "b-wow" sound.  there is a rougly 1/2 a Hertz
modulation when loose strings are strummed hard and let to ring out.  not
saying that is what caused this, but anyway

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bcarso

Member
San Fernando Valley, Los
Angeles

Posts:
4055

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #15 on: January 22, 2007,
11:51:57 AM »

Quote from: "mikep"

Im personally on the fence about
servos.  Ive witnessed bad sounding servos.  wierd bouncy bass, etc.
And im not sure about the real word reliability of direct coupled,
servoed outputs.

mike p

Most people don't understand
feedback control theory very well, or bother to simulate, so there are doubtless
some very bad servo designs out there.

I recall Dan Siefert
telling me about Jim Bongiorno's hassles with servos.  Dan "knew that he
(Dan) knew not" but conjectured that Jim's dilemmas arose from wanting very high
gains in the loop, and got some things working better just by figuring out how
to reduce them.  But these were the days when powerful simulators were
generally unavailable, and you had to know and do the maths.

As John R
has said, and PRR has remarked (loosely paraphrasing) they are a way to get a
low-frequency cutoff based on a good film cap instead of a big not-so-good
electrolytic.  They are not  "out of the signal path" just because the
topology doesn't look the same as one with a coupling capacitor in series with
something.

However, the devil is in the details---where do you inject the
correction and what does it do to the rest of the circuit?  How much noise
comes from the servo amp that is not reduced by other circuit responses?
How much noise is associated with the higher impedances?  What
happens in overload?  What about component tolerances---for example, in the
popular differential integrator approach, what does cap mismatch
do?

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Svart

Member
Atlanta GA USA

Posts:
5137

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #16 on: January 22, 2007,
01:48:50 PM »

I abandoned my usage of servos for a project
i was working on for bipolar caps.

glad I did too.  It actually
ended up cheaper to use a single BP cap than add an opamp and a couple of poly
caps.

I did a similar test to Chris's, between the same circuit with a
servo and a bipolar coupling cap.  I didn't hear much difference either.
I'm sticking with the BP caps.

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Welcome to the GroupDIY leper
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occupation:
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Alesis X2 information repository:
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chrissugar

Member
ROMANIA

Posts:
1312

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #17 on: January 22, 2007,
03:41:12 PM »

MORE INTERESTING RESULTS!

Although I'm
busy in this period, I managed today to make two more tests.
I was afraid to
make the servo DC coupled versus non servo DC coupled, because I was sure that
it will be a big pain to make a gain circuit without DC servo to have low offset
and not to drift in time.
I installed each discrete opamp I had, one at
a time, into the gain circuit. Powered up and left them for a while to
stabilise, and measured the DC offset without the DC servo. I found an opamp
that had less than 0.5mV offset at the output and it was stable, without more
than 0.2mV variations over 15 minutes. I thought I have the perfect candidate to
make the most relevant test, the direct coupled with DC servo versus direct
coupled without DC servo.
The setup was the same like in the previous test,
everything calibrated to less than 0.1dB.

The result? I couldn't detect
any hearable difference between the two situations. No timbral change. No change
in details, space information, reverb tails.
So I can say without doubt that
in my particular test setup I couldn't detect any difference in sound between
the two situation.

I was so excited about this that I decided to take
some more time and do some direct coupled versus capacitor coupled tests.

The test was not very extensive because my time was limited and probably
will do this test again with different type of capacitors from different
manufacturers, but I managed to test three situation:
1-expensive 47micro
polyprop (biggest value I have)
2-medium price/quality 47micro
polyprop
3-electrolyt 2200 micro Panasonic FC parallel with 100nano WIMA
polyprop

1-the overall difference is very small, a very little bit the
sound is smeared. The biggest difference is at low freq where the direct coupled
has more energy and the sound goes lower (but I'm not sure that the reason for
this is not the value of the capacitor, maybe not big enough for linear
phase/freq response)
2- the same thing but a bit more smeared the
sound.
3- the biggest surprise, the Panasonic FC bypased with the WIMA poly.
This combo sounded a little bit more smeared than the expensive polyprop but
without any timbral change, and the bass was not affected like with the first
two situations. (which leads me to the idea that the bass difference at the
first two tests was determined by the capacitor value)
In my opinion this is
good news because we should not forget that the Panasonic was not polarised at
all (nonpolarised elcos have higher distortions). Also it is a big surprise
because anyone would expect that an electrolyt will sound significantly worse
than a polyprop, which was not the case. Of course the polyprops sounded a bit
closer to the direct coupled connection compared to the Panasonic, but we are
talking about nuances.

This conclusion contradicts all the radical
opinions about electrolyt capacitors. Too many times I read opinions like "ELNA
is the only way and Panasonic is crap", or "Black gate is perfect, all the
others are crap" or any other combination.
If the difference between a
Panasonic FC and direct wire is this small, then even if there are many other
capacitor better than the FC we cant talk about differnces like day and
night.

For me this test is also relevant because it will surely
affect my decissions in my future projects. For perfectly transparent sound
(mastering,orchestral recording) the DC coupled with DC servo is the way to go,
but for non critical application high quality polyprop or elco (Panasonic, ELNA,
Nichicon) can produce very good results.

chrissugar

P.S. One
more thing, the test was done only for one single capacitor against wire in the
audio path, so I can imagine that in a sitution where more caps are in the
signal chain they will make bigger differences.

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Christian Mike Sugar

CMS-LAB

Guest

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #18 on: January 22, 2007,
04:47:14 PM »

Congratulations Christian!

You've
discovered for yourself that the question, "What is better: fat free cookies, or
sugar free sousages?" has no direct answer and depends on the
situation.

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Svart

Member
Atlanta GA USA

Posts:
5137

To DC servo or not to DC servo,this is the question NEW INFO

« Reply #19 on: January 22, 2007,
06:20:29 PM »

next up: voltage biased electrolytic
caps?

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Welcome to the GroupDIY leper
colony! when something falls off, we just replace it with a tube!
occupation:
General Electron Mayhem

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http://www.theopiumdenproductions/.

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