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

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

Post by JR. » Sat Oct 10, 2015 8:19 am

mediatechnology wrote:
I also anticipate the predictable arm chair response that this job should be dedicated to a microprocessor...
Been there, heard that, but no one has sent me code.
You called?

I haven't said that for several weeks now, :lol:

The beauty of building in a small computer is you can do all kinds of tricks. I won't irritate you with a list.
The code I could use - it would be very useful - is a rotary encoder and pushbutton/display board that has an SPI output.
That would be a very worthwhile effort similar to what Alex did at Innersonix.
I could probably do one but the missing part (IMO) is a rich, soft , display (like a smart phone).

I can imagine hybrid analog/digital products that would benefit from a soft interface.

JR

PS I recall the Hebert paper and different rate loops are used in a number of applications.

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mediatechnology
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by mediatechnology » Sat Oct 10, 2015 8:29 am

You called?

I haven't said that for several weeks now, :lol:
I know! I was thinking it was about time. LOL.
We really could use that SPI controller.
Everyone is doing Smartphone interfaces.
I don't think I would want to place the trust of a live recording in my Smartphones's battery.
People want knobs.

OK, I calculated the Q of my alleged-to-be-misbehavin' circuit using Gary's formulas and my circuit values.

I did a differential to half-circuit transformation and used the following values from my original schematic posted at the beginning of this thread.

Rsource 1K
Ccoupling 47 uF
Rinject 100K (half circuit value)
Rbias 2K5 (half circuit value of Rterm including phantom resistors)
Gain 60 dB
Servo tc 0.235 (500K*0.47uF)

With the input capacitor in place fo is 4 Hz.

The Q calculates out to be 0.704.

At lower gains the Q lowers, and with lower Rinject, Q also lowers.

It's slightly underdamped at even the highest gain with a coupling capacitor.
Not quite hitting the exact nail on the head for a Bessel response but pretty damn close for being determined empirically by a hick from Texas with only a high skool (sic) "degree."

Next?

With the real measured physical properties being subordinate to the theoretical I'm glad we determined that the AC characteristics of the servo meet the mathematical requirements for stability.
Man do I feel better now.
But we gotta divert to some other Red Herring 'cause this is the internet.

Are we going to now have to discuss the subject of clicking during gain switching again?
If so I'm just going to send links to the 5 or 6, maybe 10 other times its been discussed.

What I've learned in the last couple of days is that I need to build one of these that can travel.
Maybe after enough other people say "Hey, this thing really does work" we can move on from "it can't be done!"

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

Post by JR. » Sat Oct 10, 2015 9:24 am

mediatechnology wrote:
We really could use that SPI controller.
Everyone is doing Smartphone interfaces.
I don't think I would want to place the trust of a live recording in my Smartphones's battery.
People want knobs.
rough up a proforma definition.

I might consider a LCD graphic display for a soft menu surrounded by several rotary encoders and push buttons.

I would still like to make my uber-dynamic processor with literally tens of different adjustments.

Thinking out loud I wonder if a small cheap digital mixer could be repurposed as a controller?

It's slightly underdamped at even the highest gain with a coupling capacitor.
Not quite hitting the exact nail on the head for a Bessel response but pretty damn close for being determined empirically by a hick from Texas with only a high skool (sic) "degree."

Next?

With the real measured physical properties being subordinate to the theoretical I'm glad we determined that the AC characteristics of the servo meet the mathematical requirements for stability.
Man do I feel better now.
But we gotta divert to some other Red Herring 'cause this is the internet.

Are we going to now have to discuss the subject of clicking during gain switching again?
If so I'm just going to send links to the 5 or 6, maybe 10 other times its been discussed.

What I've learned in the last couple of days is that I need to build one of these that can travel.
Maybe after enough other people say "Hey, this thing really does work" we can move on from "it can't be done!"
The proof is in the pudding, build a few and get them out in the world. Even if it is a better mouse trap you will still need to sell it.

JR

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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by olafmatt » Sat Oct 10, 2015 10:32 am

mediatechnology wrote:"it can't be done!"
Please re-read my initial posing. I did NOT say that it can not be done. I was asking whether you came across any such interaction between the servo and the caps inside a microphone. Again, I did not say that it will not work in general. Or missbehave in general.

Olaf

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mediatechnology
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by mediatechnology » Sat Oct 10, 2015 11:19 am

I do understand that.
Thanks for making me calculate the Q.


When I first did this I didn't have the benefit of Gary's equations.
But I did know it was slightly underdamped and not prone to overshoot from watching the servo response to just about everything I threw at it.
I don't think you believed me.

I've been told that one very well-known high-end designer tried an input-capacitorless design and abandoned it due to the perceived mic "offset" issue, the need for high voltage low noise transistors, or need to have to burn off a lot of common mode DC.
I wasn't provided details.
The person telling me this chose to completely dismiss the fact that I've done this (refusing to even acknowledge it by looking at it) and that I've repeatedly tested it when they told me "it can't be done."
I know better.
It wasn't the end of the story and I didn't "just move along because there's nothing to see here."

About the variation in Q with Input C:

I think microphone manufactures using output coupling caps are constrained to values ranging from 22 to 100 uF.
Below 22 uF/leg and the LF response will suffer.
Above 100 uF there's too much stored charge, leakage and they're too big.
Maybe I should calculate delta-Q for a value range.
THAT's model doesn't factor in mic output coupling caps but that's easily done.

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

Post by JR. » Sat Oct 10, 2015 1:39 pm

mediatechnology wrote:
I've been told that one very well-known high-end designer tried an input-capacitorless design and abandoned it due to the perceived mic "offset" issue, the need for high voltage low noise transistors, or need to have to burn off a lot of common mode DC.
I wasn't provided details.
I can't find it now but I even designed a 3 transistor LTP to perform the level shifting, the 3rd transistor was fed with audio + and audio - to scrub off common mode voltage.

I know I drew it up and posted it but can't find it now... , only a different 3 input LTP I used to auto bias an amplifier output stage.


I'm in the middle of mowing leaves so later today or tomorrow I'll fire up my other computer and repost it. I even remember what I named the schemo (3-put).

Long story short even if you don't finesse a DC coupled level shift like mine (unproved BTW), you can perform the level shift at high impedance using quality film caps.
The person telling me this chose to completely dismiss the fact that I've done this (refusing to even acknowledge it by looking at it) and that I've repeatedly tested it when they told me "it can't be done."
perhaps a case of NIH?
I know better.
It wasn't the end of the story and I didn't "just move along because there's nothing to see here."

About the variation in Q with Input C:

I think microphone manufactures using output coupling caps are constrained to values ranging from 22 to 100 uF.
Below 22 uF/leg and the LF response will suffer.
Above 100 uF there's too much stored charge, leakage and they're too big.
Maybe I should calculate delta-Q for a value range.
THAT's model doesn't factor in mic output coupling caps but that's easily done.
I gave up trying to predict what other people do... you can be pretty accurate in the average, but the outliers will still be out there. As long as they are few and far between it's not likely to be a show stopper.

JR

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mediatechnology
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by mediatechnology » Sat Oct 10, 2015 2:32 pm

I can't find it now but I even designed a 3 transistor LTP to perform the level shifting, the 3rd transistor was fed with audio + and audio - to scrub off common mode voltage.

I know I drew it up and posted it but can't find it now... , only a different 3 input LTP I used to auto bias an amplifier output stage.
I'm pretty sure you posted here in the old thread as an attachment. I think I may have an off-line copy too.

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

Post by JR. » Sun Oct 11, 2015 9:40 am

3_put.jpg
3_put.jpg (61.29 KiB) Viewed 12534 times
OK here it is again...

I share this just to show the unusual topology. It has compromises i can see (like headroom at low DC phantom voltage). Since I never melted solder there may be other issues I don't see too. :oops:

Using decent film caps at high moderate impedance should be arbitrarily good performance making this complexity unnecessary, unless on a zealous marketing campaign to eliminate all caps.

JR

caveat the schematic values and devices are just whatever I could use to rough out the schematic to present the concept and not in any way final.

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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by emrr » Sun Oct 11, 2015 10:44 am

mediatechnology wrote: Everyone is doing Smartphone interfaces.
I don't think I would want to place the trust of a live recording in my Smartphones's battery.
Guaranteed best way to obsolete your product in 5 years time too.
Best,

Doug Williams
Electromagnetic Radiation Recorders

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mediatechnology
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by mediatechnology » Sun Oct 11, 2015 12:26 pm

John - Thank you for posting that schematic. I know that I have a larger copy somewhere. If you don't mind I may host a copy and embed it in a later post to avoid attachment scroll bars.

emrr - I don't like the idea either. JR's Shiney New Hammer glass cockpit with an LCD/OLED monitor with knobs, buttons and some off-screen LED warning lights could be made a Universal Controller. SPI, MIDI etc. etc.

We just recently had a phone die while it was on the charger. How do you explain that you can't finish a show to a concert hall with 50,000 people because your phone died?
I know smartphone everything as an audio controller is trendy but I've been reading about how people are returning back to desktops and shunning laptops, tablets.
When they need screen real estate or simply want to conduct e-commerce go to their desktops.
People just want knobs. How many times have we clicked on advertising links just trying to grab a handle to pull a page down on a smartphone?

And in other news...

Olafmatt has just sent me US Patent 8233643 filed in 2010 and it's just lovely: http://www.waynekirkwood.com/images/pdf ... 233643.pdf

Internet Wayback Machine capture of Prodigy-Pro September, 15, 2006. https://web.archive.org/web/20070821222 ... hp?t=18134

I will not be commenting on this.

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