Old: A Direct-Coupled Input-Capacitorless Active Mic Preamp

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JR.
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by JR. » Wed Jul 04, 2012 11:12 am

I miss the regulars exchanges with Brad. He was one of a small handful out there who really understand stuff. It's nice on a noisy forum to have a critical mass of informed consensus. I occasionally get the random email from him but he doesn't post much anywhere I frequent these days.

I rather like this investigation into improved servos. The last servo I used back in the '80s in a phono preamp just pushed the pole(s) super LF.

I am percolating my idea for a digital servo... not completely crazy if there already is digital control content in the vicinity (like gain control). The best cap is no cap....

JR
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology » Wed Jul 04, 2012 11:24 am

I do miss Brad too and still occasionally exchange e-mail.
I sometimes see posts on the EET Times/EDN forums from him but not often.
I rather like this investigation into improved servos.
Thanks.

I did try putting a passive pole on the output but it seemed to prefer a first-order response.
I can't see any significant noise contribution by not having a passive output pole.
I am percolating my idea for a digital servo
Auto-zero of the low gain offset at turn-on would be a good start.
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by JR. » Wed Jul 04, 2012 12:07 pm

mediatechnology wrote:
I am percolating my idea for a digital servo
Auto-zero of the low gain offset at turn-on would be a good start.
Sorry for the veer...

The beauty of non-linear digital processing is we can use situational input to decide how and when to trim the DC offsets.

Not only can a "brain onboard" selectively decide when to tweak DC offset, if it is tied into and knows the gain control it can learn a DC offset curve vs, gain so it remembers history and can predict a nominal starting point for faster settling for every gain setting, and update that history on the fly. It can also turn itself off for most of the time when errors are under control.

Flash memory means it never has to start from scratch, more than one first time.

JR

PS I have semi seriously looked at this whole exercise commercially (DC coupled front end) for decades. From a marketing perspective, removing caps is IMO a merchantable high end hook, but the remaining caps in the servos, creates a dissonance with customers who do not comprehend the subtle difference between cap types and application.

EDIT_ PPS a recent thread on another forum about a new MIDAS digital console seems to be bragging on their having a soft clipper in the mic preamp stage as a useful musical effect for mix engineers to use????? frigin DJs.... soft clipping is still clipping... arghhh [/edit]
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology » Wed Jul 04, 2012 12:34 pm

...if it is tied into and knows the gain control it can learn a DC offset curve vs, gain so it remembers history and can predict a nominal starting point for faster settling for every gain setting, and update that history on the fly.
That's key.
But, I'm not sure it's worth the effort just to replace a dual op amp, a film capacitor and four resistors.
To hear clicks in this one - under the worst case of going from low to high gain - you really have to have monitor-destroying gain.
Knowing that comes from building it.

You'd never want to listen for clicky-ness in the presence of actual input.
I'm a little nervous every time I check it.
You'd never hear it unless you were looking to hear it.
creates a dissonance with customers who do not comprehend the subtle difference between cap types and application.
You can go for that segment John.
having a soft clipper in the mic preamp stage as a useful musical effect for mix engineers to use
I just did an EFX device protype using a set of mutlipliers to generate a non-level-dependent "soft clipping" third harmonic generator and it was very useful.
It wasn't actually clipping since it happened without regard to level.
But not in a preamp.
MIDAS may be trying the SSL VHD trick.
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by JR. » Wed Jul 04, 2012 12:57 pm

The first rule of tweaky audio-phoolery, if you want to claim that you sound different than everybody else, it helps to actually sound different. :o A digital console with analog soft clipper in it's front end will definitely sound different (when driven hard). 8-)

My complaint is that a modern digital console with digital gain control and comprehensive internal knowledge of gain staging can literally never be clipped. Smart management can shift gains around so front end clipping could be seamlessly adjusted out with make up gain later in the path. Of course this is counter-productive if you want to sound "different".

Distortion EFX can be liberally added as a digital plug-in if actually useful. I see the soft-clipping as a thinly veiled merchandising gimmick, presented as a feature, and not really needed with modern technology .

I bailed out of the hifi side decades ago because there was little correlation between actual fidelity and commercial success, at least in my experience.

JR
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by emrr » Wed Jul 04, 2012 1:08 pm

JR. wrote: It's probably Ok to not support the worst offenders, of good phantom power behavior.
You mean like Earthworks, who demand 10 mA current, and work with virtually zero transformer coupled preamps? Who made mics that cease working when bridged with a Jensen mic splitter transformer?

Yeah, you have to ignore the extreme cases.

This is exciting to see develop.
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology » Wed Jul 04, 2012 1:39 pm

Yeah, you have to ignore the extreme cases.
Both in terms of microphones and the people who use them. ;)
This is exciting to see develop.
Thanks. It's exciting to do. (Again, finally.)

I knew the Earthworks pulled 10 mA (5 mA/leg) and I think it clocks in at one of the all-time highest followed by Shure's large diaphragm line.

What is it about the Earthworks and transformer-coupled inputs it doesn't like? Loading?
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by emrr » Thu Jul 05, 2012 10:13 pm

I don't know that Earthworks has ever really explained it. They just warn you should use a preamp of impeccable specs and (if you must) high quality modern transformers. If I knew what the reason was, I said 'bah!' and chose to forget after a freakin' Jensen made one cease to function. The experiment in question was an Earthworks into a Sytek (transformerless modern), with a Jensen splitter added, the iso side of the Jensen feeding another Sytek channel. The Earthworks always worked with every transformerless preamp I tried, never with a transformer coupled preamp.
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology » Fri Jul 06, 2012 5:42 am

a freakin' Jensen made one cease to function.
It actually broke it?
Have to wonder if the Earhtworks doesn't like the low DC termination resistance (between tip and ring) of a transformer primary.

For the DC preamp the DC termination resistance is fairly high as it would be in an active preamp.
My target value is going to be 5K? differential.
It can never be more than 13.6K? due to the phantom source resistors.

(I got omega by typing Alt+234. Does it render in your browsers OK? Or, do you get "W?")
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by emrr » Fri Jul 06, 2012 11:25 am

It worked again once the transformer was removed from the picture. The Jensen is 50 ohm DCR.
Best,

Doug Williams
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