DC Servos and Digitally-Controlled Microphone Preamplifiers

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mediatechnology
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DC Servos and Digitally-Controlled Microphone Preamplifiers

Post by mediatechnology »

"DC Servos and Digitally-Controlled Microphone Preamplifiers," Gary Hebert, THAT Corporation, AES 137th Convention, Paper 9198.
Microphone preamplifiers for professional audio applications require a very wide range of gain and low noise in order to provide a high-quality interface with the vast number of available microphones. In many modern systems the preamplifier gain is controlled indirectly via a digital interface in discrete steps. Often dc servo amplifiers are employed as a means of keeping the dc gain fixed to avoid large changes in output offset voltage while the audio band gain is varied. The resulting highpass filter response varies substantially as a function of the preamplifier gain. We investigate the frequency and time-domain effects of this. We also investigate several approaches to minimize these effects.
Link to DC Servos and Digitally-Controlled Microphone Preamplifiers: http://www.thatcorp.com/datashts/AES919 ... ifiers.pdf (link restored)
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JR.
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Re: DC Servos and Digitally-Controlled Microphone Preamplifi

Post by JR. »

Pretty dramatic improvement.

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ADMsystems
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Re: DC Servos and Digitally-Controlled Microphone Preamplifi

Post by ADMsystems »

Document removed due to agreement with AES. Email from Joe Lemanski with THAT Corp included the following...
I am sorry, but we were a bit premature in putting the link on our web page. It turns out that we have an agreement with the AES that prevents us from publishing the paper on our site. If the paper is not chosen for publication in the AES Journal, then we can post it six months from now.

In the meantime, you will need to go to the AES directly in order to get a copy of the paper. Here is a link:

http://www.aes.org/publications/conventions/

The convention just finished up and it is my understanding that it takes about two weeks before the AES will have the papers available for sale.

Sorry for the confusion.
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JR.
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Re: DC Servos and Digitally-Controlled Microphone Preamplifi

Post by JR. »

In a nutshel what they did was switch in a faster time constant on the servo immediately following gain changes so it could settle quickly, then switch back to the slower time constant.

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mediatechnology
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Re: DC Servos and Digitally-Controlled Microphone Preamplifi

Post by mediatechnology »

Well, now that its gone I can't remember: Did it take two switches to alter the time constant?

The Differential Deboo Integrator only requires one switch to vary the C: viewtopic.php?f=6&t=559&p=6321

Just found a copy that I saved offline. Hope the AES doesn't SWAT me for having it.
Looks like the input resistor of the "half-circuit" is switched so the differential "2C" servo would require two switches.
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Re: DC Servos and Digitally-Controlled Microphone Preamplifi

Post by mediatechnology »

The link in the original post has been restored by THAT Corporation.

http://www.thatcorp.com/datashts/AES919 ... ifiers.pdf
Craig Buckingham
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Re: DC Servos and Digitally-Controlled Microphone Preamplifi

Post by Craig Buckingham »

In my experience servos are an evil best avoided at all costs. They add a whole lot of disadvantages of their own.

What's wrong with a dual current sink on input differential LTP and gain setting resistor set between the LTP emitters or emitter degeneration resistors?

With good match and trimming could not a servo solution be avoided altogether? One disadvantage is most if not all op-amps don't have access to both emitters and don't use dual current sinks for the LTP and so a discrete front end would be required. A high-end solution most likely but not for run of the mill cost sensitive applications.

Interested in your views.
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Re: DC Servos and Digitally-Controlled Microphone Preamplifi

Post by JR. »

Nobody uses servos because they like the complexity, but they are a lesser evil than not using one. Servos are pretty mature but there are often trade-offs (well discussed).

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Craig Buckingham
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Re: DC Servos and Digitally-Controlled Microphone Preamplifi

Post by Craig Buckingham »

Servo is more complex than AC coupling for sure, but I would say maybe equally or less complex than a hybrid design that is DC stable without the need for a servo.

Putting a monolithic matched pair front end on an op-amp (curse the thought of degrading the monolithic pair by marrying it with an op-amp) adds extra complexity and much greater cost. Hence my previous comment about servos being ok for cost sensitive applications but not optimal for high-end (less cost sensitive) solutions.

Servos are mature for mainstream equipment. But for high-end equipment the previously described circuits in my opinion are not mature.
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Re: DC Servos and Digitally-Controlled Microphone Preamplifi

Post by JR. »

Craig Buckingham wrote:Servo is more complex than AC coupling for sure, but I would say maybe equally or less complex than a hybrid design that is DC stable without the need for a servo.
yup
Putting a monolithic matched pair front end on an op-amp (curse the thought of degrading the monolithic pair by marrying it with an op-amp) adds extra complexity and much greater cost. Hence my previous comment about servos being ok for cost sensitive applications but not optimal for high-end (less cost sensitive) solutions.
I can't tell if you are kidding. Dedicated digital control mic preamps use decent matching. When applying 60-70dB of DC gain even a well match monolithic will exhibit DC offset.

Servos are mature for mainstream equipment. But for high-end equipment the previously described circuits in my opinion are not mature.
I recall writing about DC servo's in my Audio Mythology column back in the 1980s. While the modern generation of digital gain control on a chip are relatively recent developments, thus the THAT paper on the subject.

I am often accused of suggesting microprocessor to solve all problems, but if money is no object it seems the tricks discussed in the THAT paper and more could be integrated into a more comprehensive solution using a cheap digital brain. A micro could learn the DC offset for various gain settings and control a DPOT to trim the DC offset for each gain setting. It would take a little time to learn the offsets and things might drift a little with temperature but I suspect this would work, without an evil servo involved... while some might consider a micro even more evil than a servo.

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