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

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

Post by olafmatt » Mon Oct 07, 2019 12:57 pm

mediatechnology wrote:
Mon Oct 07, 2019 7:48 am
Olafmatt's servo that I used in the MC preamp works great at controlling the Vbe offset but I'm not sure it has enough range to also correct the larger offsets the mic presents.
I would guess it would allow for large enough a correction, but it would need less attenuation at the output of the servo opamps, thus potentially adding more unwanted stuff to the signal.

What I was trying to point out a few posts up is that even with my servo, if you make both emitters sit at the same DC offset (i.e. 0VDC across Rg) then due to the bases sitting roughly a fixed 0.65VDC away from the emitters you actually do control the DC offset at the bases. And this means you might still get interaction with the caps in the microphone (if it has some in the output). - And unfortunately with a microphone you never know how large these caps are. In a traditional preamp with phantom blocking caps you can just design around this problem because you have control over the size of the caps.

I remember a story from a few years back with a Universal Audio A/D/A box that had a common mode DC offset leaking out the input because they presumably DC coupled the input XLR to the A/D converter chip which needed a (roughly) 5Vpp swing centred around +2.5V. So the whole frontend was offset by 2.5 volts and SOME mixer outputs didn't like those 2.5V common mode being injected into their outputs. - This is one of the stories that made me aware that an input should always be an input and never "leak" anything out of it.

If the goal would be to simply get rid of phantom blocking caps I'd suggest to design your own mics to go with the preamp and simply use a dedicated power supply wire to the mics. In case we focus on phantom powered microphones, the thing should just work with ANY phantom powered microphone that exists now and might exist at some point in the future.

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

Post by mediatechnology » Mon Oct 07, 2019 2:11 pm

I would guess it would allow for large enough a correction, but it would need less attenuation at the output of the servo opamps, thus potentially adding more unwanted stuff to the signal.
I intend to find out. I really like the ZTX851 MC front end and with mods I think it will make an excellent mic preamp input-capacitorless or AC-coupled.
What I was trying to point out a few posts up is that even with my servo, if you make both emitters sit at the same DC offset (i.e. 0VDC across Rg) then due to the bases sitting roughly a fixed 0.65VDC away from the emitters you actually do control the DC offset at the bases. And this means you might still get interaction with the caps in the microphone (if it has some in the output). - And unfortunately with a microphone you never know how large these caps are. In a traditional preamp with phantom blocking caps you can just design around this problem because you have control over the size of the caps.
Yes it does reflect back into the bases and thus the output capacitors if the mic is AC-coupled. I think the size of the output caps are limited to 100 uF or less due to stored charge and physical size. At values much larger than 100 uF you can spot-weld. There is the risk of the servo becoming second-order. I think I calculated the worst-case peaking once based on sane output coupling cap values and with some minor tweaking it was manageable.
I remember a story from a few years back with a Universal Audio A/D/A box that had a common mode DC offset leaking out the input because they presumably DC coupled the input XLR to the A/D converter chip which needed a (roughly) 5Vpp swing centred around +2.5V. So the whole frontend was offset by 2.5 volts and SOME mixer outputs didn't like those 2.5V common mode being injected into their outputs. - This is one of the stories that made me aware that an input should always be an input and never "leak" anything out of it.
Must have been the same reference design as my TI A/D EVM board. Its inputs float at Vcm. It's a pain.

Well there's always some bias current leaking in or out of a direct-coupled input.

I look at it this way: From a DC perspective if the output of the mic is a few hundred millivolts out of balance and the servo forces the resistance bridge back into sub-millivolt balance how is that any worse than it being in perfect balance to begin with?
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olafmatt
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by olafmatt » Mon Oct 07, 2019 2:18 pm

mediatechnology wrote:
Mon Oct 07, 2019 2:11 pm
I look at it this way: From a DC perspective if the output of the mic is a few hundred millivolts out of balance and the servo forces the resistance bridge back into sub-millivolt balance how is that any worse than it being in perfect balance to begin with?
It can even be a good thing! If you have a mic with output transformer and the phantom voltage picked up with resistors (no center tap on the transformer) the servo would help to reduce core saturation which you'd normally get with too much DC offset being left in place.

Probably I'm just worried about potential mics with DC servos in their output.

About the CM offset, yes that TI A/D EVM board does the same. As does the ARDA AT1201 EVM board. Seems to be quite common because it lowers parts count and gives better meassurement results. And as long as it works prilliantly with lab equipment on a tidy bench, who cares? :evil:

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