To servo or not to Servo..

[ReSiStAnCe]

Hi guys,

I want to build a bridged parallel amp using 4 3886's to drive a sub.
I've been lurking this forum for a while and saw some nice amps.
I don't want to reinvent the wheel so i dont want to draw YAGCP! (Yet Another GainClone PCB).

I've come across 2 PCB's I really like:
1. From TLMADSEN, which uses servos.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=19157
I've send him PM, but untill now (1 month) didn't get a response.

2. From CM961, whithout servos

http://www.diyaudio.com/forums/showthread.php?s=&threadid=24155&highlight=3886

Should I be worried about DC-offset without servos or do these servos cause more harm than good (music wise)?

Can anyone get me in touch with TLMADSEN?

TIA.

 

[tecnofossil]

Should I be worried about DC-offset without servos or do these servos cause more harm than good (music wise)?



TIA.

Depends upon who you talk to. People that subscribe to the theory that the simpler the circuit the better the sound will probably say servo's polute the sound. Others still will claim that a properly designed servo system will be transparent, adding no perceivable sonic effects. There are quite a few very expensive (and not so expensive) amplifiers which utilize servos which have had absolutely glowing reviews by many different sources as well as amplifiers that use a DC blocking cap and/or a cap in the feedback network of the amp.

Now, the above applies to amplifers operated full range. If the amp is being used to power bass units I strongly believe you will notice NO sonic effects from a well designed servo. Now be aware, depending upon the time constants of the servo circuit (the correct active circuit name is: "integrator"), if the the time constants (in a nutshell the LF frequency point the circuit will act upon down to DC which is what you are trying to keep as close to zero as possible) are to high you may begin to roll off the audible bass frequencies you want your subwoofer to reproduce. If the time constants are too low it may take unusually long period for the servo to zero out your DC offset, possibly passing DC along to your woofers, so, you might need a time delay relay on the amp outputs.

Hope this helps.

-Tom

 

[tlmadsen]

Alive with servos

Hi Resistance

I am right here. Don't know how we lost communication......

Yes, you should indded worry about DC-offset if you want to parallel.
Music wise I don't belive you can hear the DC-servoes (some people might have a differnet view on servos, but please don't let us start on that once more), or at least on the my "Das Modul" implementation of BPA-200.

If you go ahead with paralleling and no DC-servos you most make sure to have as small as possible DC-offset on each of the amps.

I am working a "grey paper" about paralling/bridging of chip amps, but when it will be ready is very hard to say (hopefully sometime this year )

have fun

Thomas

 

[soundNERD]

Hi,

Sorry I can't help you. The p/b 3886 im building right now doesn't use servos.

But, what exactly is a servo. I have been told that it is a sensor that tells the amp where the cone is, and I have been told by a guy at tweeter that it is in the sub crossover. I think it is the first one, since it came from a person on this forum, but could somebody clarify for me?

Thanks!

 

[ReSiStAnCe]

Ok Guys,

Thanks very much, all of you.

I've decided to go for the servos.

Tecnofossil: thanks for the clear explanation.

Soundnerd: As tecnofossil points out, it's really an integrator.
Have a look at this for a full explanation: http://www.visionics.ee/curriculum/Experiments/Opamp integrator/Opamp Integrator1.html
What you describe is an active feedback. It's sort of like an integrator, but different ;-) (and more difficult to construct)

Ciao

 

[macboy]

I have been told that it is a sensor that tells the amp where the cone is, and I have been told by a guy at tweeter that it is in the sub crossover.

"Servo" is a general term. It means: a self-regulating feedback system or mechanism.

Some subwoofers have a servo system as you described. There is some method of feedback to tell the amplifier where the cone is, and the amplifier can adjust it's outputs so that the cone is exactly where it wants it to be (according to the input signal). This reduces distortion. This is not easy to implement, because accurately detecting where the speaker cone is at is not easy and usually involves some kind of optical sensor. This is not the kind of servo we are talking about.

The kind of servo used around here, and described in the National AN-1192 app note, is a servo designed to eliminate DC offset. It consists of a low pass filter (integrator technically) which is connected to the output, and feeds back into the input. Since DC is the ultimate low frquency ("0 Hz") it passes through the filter, and becomes the input to the servo. The servo inverts this signal and feeds it back into the input. Because it is inverted, it cancels out the DC that would otherwise be present. So there is no DC offset at all on the amplifier's output.

Oh, and the guy at tweeter doesn't have a freakin' sniff what he's talking about, which is pretty typcial of audio salespersons.

 

[soundNERD]

Yeah, I was believing you guys more than the guy at tweeter. His best example of a servo is that in the crossover it gets rid of distortion so, quote, "when you play something like acdc with a lot of bass it wont distort as much"

But they're smarter than the people at best buy. they probably dont know what the word servo is.

Thanks
-Mike

 

[soundNERD]

Another question,

I have an infinity servo sub that i asked about before and why it distorts so badly and the cone moves so oddly. It was used and I think that the speaker is bad. If the one in it is a 4-ohm, could I replace it with a better 4-ohm speaker? Or is the servo sensor designed specifically for the speaker inside it?

Thanks again, Mike

 

[ReSiStAnCe]

The servo is not designed for the speaker you use. On the other hand, the speaker enclosure is designed for the speaker, so you might want to ask this question in the 'loudpeakers' section.

 

[macboy]

I have an infinity servo sub that i asked about before and why it distorts so badly and the cone moves so oddly. It was used and I think that the speaker is bad. If the one in it is a 4-ohm, could I replace it with a better 4-ohm speaker? Or is the servo sensor designed specifically for the speaker inside it?

Do not attempt to replace the speaker. The new speaker will almost certainly not work with the servo system, and it will probably not have similar enough TS (Thiele-Small) parameters, meaning that it will not sound good at all in a box of that specific size. You usually do not get good bass by dropping a random speaker into box. The box needs to be designed for the speaker.

I would guess that the servo system is behaving badly. I would probably be possible to modify the amp so that it just operates as a normal amp and doesn't use the servo. Judging from the nature of the type of questions that you post in this forum, I do not think that you have the kind of experience/know-how to pull that off. You could surprise us though.

An easier (but more expensive) mod would be to replace the existing servo-controlled amplifier with a gainclone amp. You could probably use the existing heatsinks and the power supply, provided that the voltages are in the range needed for the GC amp. Since the new amp is conventional rahter than servo-controlled, it should behave better. Note that the sub was designed to work with the servo though, so it may not perform up to spec (frequency response may not go as deep, etc.).

 

[soundNERD]

Well, the speaker is sitting arond, so would it hurt to try and see if I am satisfied with the results?

You probably are right, i'm not going to fool with the servo nor replace the amp.

Thanks!

 

[Jamesblond]

I know this has long been an abandoned thread, but still I'd like to post, in the interest of my own education.

Hypothetically, if one wants to build their own servo/feedback circuit for an amplifier, could the second voicecoil (when present) of a woofer be used to provide the servo input signal? Due to electromagnetic induction, it should provide a signal that exactly represents the movement of the cone. The difference between that and the output of the amplifier could be used for correction of the cone movement. Is this idea in a any way correct?

I ask this, because I've seen talk about diffent kinds of controlled feedback on this forum (DIY accelerometers glued to cone, optical feedback, etc). I'm in no way experienced in building my own audio gear, but I do have a (distant) background in electronics. This idea seems very logical to me, and should not be too difficult to implement - if it is in fact correct.

The second voice coil could feed it's signal to a difference amplifier with a high imput impedance (otherwise the resulting current would generate power in the second coil, inhibiting movement), with the signal somehow being inverted. The resulting diffence-signal could be summed with the input signal, resulting in a more accurate cone movement, relative to the input signal.

The above just an idea I have, that I'd like to see either confirmed or shot to pieces, depending on the truth in it.

Or, is this impossible, because the signal fed to the first coil induces a signal in the second coil, which prevents movement detection?

 

[DF96]

Remember that the input music signal voltage represents displacement (i.e. position) of the cone, while the voice coil induced voltage represents velocity of the cone. You can't just subtract one from the other and get an error signal.

 

[richie00boy]

It can be done, but it's not trivial and more than likely above your abilities (no offence). I looked into it a number of years ago and you require to modify the response and differentiate the feedback signal.

 

[Preamp]

Or, is this impossible, because the signal fed to the first coil induces a signal in the second coil, which prevents movement detection?

So it is. At least according to several threads I've read around the web.

 

[forr]

Remember that the input music signal voltage represents displacement (i.e. position) of the cone, while the voice coil induced voltage represents velocity of the cone. You can't just subtract one from the other and get an error signal.

The input music signal voltage represents... acceleration.
Integrators and differentiators have been used in servoed drivers to pass from a kind of data to an other between acceleration, velocity and position.

 

[Jamesblond]

I'm not planning on building a servo subwoofer or - amp myself. I just asked out of interest in the subject. There have been a number of implementations of this kind of technology, I believe. One I know of is the use of a second (static) coil wired in reverse polarity relative to the voice coil. It pushes against the voice coil, supposedly improving it's efficiency, reducing distortion, enhancing transient response, etc. It's used in the Event Opal studio monitor. I think the bass end of that speaker isn't the best I've heard. May be because of the frequency dependant port loading of the speaker, may be because of the use of this second coil. I sounds a bit slow and muddy, not very defined and transparent to my ears.