John,
Give me a couple of days. I need to tidy up the schematics at home this weekend before I can post. It is a bit difficult 700km away from home.
Greetings from Holland (they won their World Cup Soccer playoff today),
Patrick
Give me a couple of days. I need to tidy up the schematics at home this weekend before I can post. It is a bit difficult 700km away from home.
Greetings from Holland (they won their World Cup Soccer playoff today),
Patrick
I would again like to talk about servos in general.
Some hi end manufacturers still avoid them. Many mid fi manufacturers don't use them, because coupling caps are cheaper to implement.
Still others have tried them, and found them audible in their circuits.
What to do?
Well, first, cap coupling is easiest. Then, slight DC offsets don't matter much, and even with differences in temperature will not make much difference. The caps can also protect adjacent stages in case one active device fails. The drawback is that the caps often have non-linear distortion at low frequencies, and they virtually all have a significant amount of dielectric absorption. This is a 'linear' distortion that doesn't show up as non-linearity, but it can change the signal path to be greater than 1% deviation from ideal, in each capacitor. Is this bad? Well, it's not hi fi.
Servos can be problematic if they behave too much like a capacitor in rolling off the bass. Sometimes, we want a low frequency cutoff in order to remove excess garbage below 20 Hz or so. I have found that using a servo to create this rolloff is not a good idea. It is actually better to use a series cap and a load resistor. With a good cap, especially polypropylene or better, this beats a servo.
However, IF a broadband response is desired, then a servo can be implemented that will only really do much with any offset below 1 Hz. This is important, so that the servo does not actively change the audio signal, especially asymmetrical components. A good, at least linear integration cap should be used, and the output of the servo amp attenuated at least 20 times, (100 is better) before it is put back into the circuit to correct the offset. This means that a 2V offset from the integrator IC will only give 0.1V of correction, for example. This keeps the servo noise and distortion out.
Using these suggestions, servos have a much better chance of being OK in a high fidelity design.
Some hi end manufacturers still avoid them. Many mid fi manufacturers don't use them, because coupling caps are cheaper to implement.
Still others have tried them, and found them audible in their circuits.
What to do?
Well, first, cap coupling is easiest. Then, slight DC offsets don't matter much, and even with differences in temperature will not make much difference. The caps can also protect adjacent stages in case one active device fails. The drawback is that the caps often have non-linear distortion at low frequencies, and they virtually all have a significant amount of dielectric absorption. This is a 'linear' distortion that doesn't show up as non-linearity, but it can change the signal path to be greater than 1% deviation from ideal, in each capacitor. Is this bad? Well, it's not hi fi.
Servos can be problematic if they behave too much like a capacitor in rolling off the bass. Sometimes, we want a low frequency cutoff in order to remove excess garbage below 20 Hz or so. I have found that using a servo to create this rolloff is not a good idea. It is actually better to use a series cap and a load resistor. With a good cap, especially polypropylene or better, this beats a servo.
However, IF a broadband response is desired, then a servo can be implemented that will only really do much with any offset below 1 Hz. This is important, so that the servo does not actively change the audio signal, especially asymmetrical components. A good, at least linear integration cap should be used, and the output of the servo amp attenuated at least 20 times, (100 is better) before it is put back into the circuit to correct the offset. This means that a 2V offset from the integrator IC will only give 0.1V of correction, for example. This keeps the servo noise and distortion out.
Using these suggestions, servos have a much better chance of being OK in a high fidelity design.
DC servo
pre-filter - integrate - attenuate.
The only piece I can see missing is a post filter.
Are there any downsides to a post filter, either on it's own or inserted into the attenuation?
pre-filter - integrate - attenuate.
The only piece I can see missing is a post filter.
Are there any downsides to a post filter, either on it's own or inserted into the attenuation?
Well, with each filter and integrator in series you increase lf phase shift. At some point you need to be sure it remains stable; a servo with lots of filter sections can easily lead to lf instability and oscillations.
jd
is this why we sometimes see a bump in the frequency response just before rolling off the higher frequencies?
Are there some rules to help avoid the stability issue?
Are there some rules to help avoid the stability issue?
It's a regular nfb-loop, so all the nfb-stability rules apply 😉
Gain margin, phase margin, etc.
jd
I promissed some information about a new measurement at Test Factory in Stuttgart.
They found that transistor amplifiers without NFB in the output stage and tube amplifiers can have an advantage when driving real loudspeakers. I found a paper ( in German, sorry) that discribes this measurement. You can download the paper under :
http://www.burosch.de/images/Display-workshop.pdf
Unfortunately i was not able to talk to Mr. Schüller, that came up with that measurement but i will try again.
They found that transistor amplifiers without NFB in the output stage and tube amplifiers can have an advantage when driving real loudspeakers. I found a paper ( in German, sorry) that discribes this measurement. You can download the paper under :
http://www.burosch.de/images/Display-workshop.pdf
Unfortunately i was not able to talk to Mr. Schüller, that came up with that measurement but i will try again.
JG, I think the subject is very interesting, but I find it impossible to interpret the presentation (... I'm just a dumb English guy). Can you please summarise the information in a few lines?
As far as i understand the test they connect a power amp to a real loudspeaker. They put in an MLS signal and measure the impulse response of the poweramp - loudspeaker combination. Then they convert the impulse respose into the frequency domain. Power amps with no NFB around the output stage and tubeamps usually do better on this test. Power amps with high NFB usually have the impedance curve of the loudspeaker imprinted. They came up with this test because many low feedback amps and tube amps did very well in subjective listening tests although they had more harmonic and intermodulation distortion and a lower damping factor.
I was not able to talk to Mr.Schüller so far but i hope he can give some more elightenment. I visited Test Factory some time ago when they made such a measurement. Unfortunately i can not remember the exact test conditions but i will try to get more information. Amplifiers that did well where Aire, Pass, Brinkmann etc. all with no or little NFB around the output stage.
I was not able to talk to Mr.Schüller so far but i hope he can give some more elightenment. I visited Test Factory some time ago when they made such a measurement. Unfortunately i can not remember the exact test conditions but i will try to get more information. Amplifiers that did well where Aire, Pass, Brinkmann etc. all with no or little NFB around the output stage.
A full-version Acrobat will allow you to extract the text into various formats (incl text), from which you can translate with Babelfish.
But you already got full service from Joachim.
Patrick
But you already got full service from Joachim.
Patrick
I've been trying to find out exactly what their results are. Is it that when Zout is larger, it is also flatter but provides less damping? Didn't we know that already?
Or is there an effect I'm missing?
Inquiring mind wants to know!
jd
Me. It seemed to sound just fine, as would any well-designed preamp being used in a suitable system.
......so Mr.Pass was right all along with STASIS.
Jam
I don't think STASIS has something to do with the fact of an output stage running outside the feedback loop. It's written somwhere in a white paper or so, the X'ed Pass poweramps (not XA...) have the feedback taken before the output stage. No STASIS there.
Just tons of bias current
Tino
What is often overemphasized is damping factor. It is overrated. Somewhat higher output impedance and with a flatter characteristic tends to show that the amp uses less feedback and has a higher open loop bandwidth. Both are important. Higher open loop bandwidth is the key.
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