THAT 1512 phantom power considerations

[JohnRoberts]

It is pretty easy to execute a very high quality HPF using film capacitors after the signal is raised to line level.

If you really want to do it without capacitors you could perform the HPF inside digital domain, but most CODEC designs use caps, so that may take some work too.

JR

 

[mshilarious]

Well I put capacitors in microphones, so . . . a capacitor-less condenser microphone can be done, except for the capsule!

 

[chrissugar]

There is one reason that speaks against doing it: you will need a filter after the micpre to filter out all the low-frequency rubbish. With the mics shock-mounted on a regular mic-stand I could see passing trains in the wave display of the DAW. And the train line is about 1km away. So unless you come up with a capacitor-free low-cut as well the whole effort is useless.

It depends.
For modern multitrack/multimic music I don't see the reason to eliminate the caps, but for two mic recording of orchestral music where absolute transparency and phase integrity is the goal, eliminating most of the caps is a good idea.
Fred's new smp2 mic preamp has two inputs for each channel, one direct capacitorless input (for externally powered mics or ribbons) and a capacitor isolated input for phantom powered mics. It is considered by many classical recordists the cleanest preamp on earth.

chrissugar

 

[olafmatt]

It depends.
For modern multitrack/multimic music I don't see the reason to eliminate the caps, but for two mic recording of orchestral music where absolute transparency and phase integrity is the goal, eliminating most of the caps is a good idea.

Yep, that's what I did it for. And less for the extended freq. response in the low end, but more because of the better phase linearity. I've settled now on a micpre with phantom blocking caps (rather large foil caps) and the rest being without caps. It's much easier to make and if you set the corner freq. low enough phase shift can be moved to below the lowest notes you record.
I also have to note that my cap-free micpre never really liked a TLM103. It worked flawlessly with all mics I tried, except the TLM which seems to draw more phantom current on one leg than the other and thus confuses the floating mechanism and servos.

Olaf

 

[JohnRoberts]

Interesting, that confirms my concern about some mics being problematic when DC coupled..

It is certainly straightforward to measure the phase response of a mic preamp for the effect of electrolytic capacitors in the path.  I suspect people underestimate the difficulty in cap coupling the typically low value gain resistor. It is at an impedance where the cap nonlinearities could be significant. I suspect effects of that cap are more likely to be audible than properly executed phantom  input blocking caps.

Ironically consumers would consider audible pot noise when adjusting gain as evidence of an inferior design, when in fact it could be audibly superior in use.

But the customer is always right... even when they're wrong...

JR

 

[olafmatt]

I suspect people underestimate the difficulty in cap coupling the typically low value gain resistor. It is at an impedance where the cap nonlinearities could be significant.

The THAT1510/1512 demo board PDF shows a circuit that uses a servo instead of a huge cap in series with the gain set resistor. But if you now imagine an input without blocking caps, one mic line sits at phantom voltage (dragged down by the current draw of the mic), the other one at the same level (with a perfect mic) plus parts of the correcting voltage the servo injects. Unequal current draw of the mic (over the two signal lines) causes another differential DC offset, making even larger correcting voltage of the servo necessary.

Olaf

 

[jdbakker]

[...] the TLM which seems to draw more phantom current on one leg than the other and thus confuses the floating mechanism and servos. [...]

I would imagine that most mic input transformers would be Not Amused by the resulting DC current.

JDB.
[typical mic in tx max pri DC current, anyone?]

 

[JohnRoberts]

Wayne has done a great deal of work with DC coupling the THAT chips.

I remain a little apprehensive since lots of devices besides microphones draw power from phantom, but whoever does it first (DC coupled) in the mass market will learn soon enough.

JR

 

[jhayjhay0918]

Its good for them that the  R-C product is smack in the middle of the audio range.Thanks for it.


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