I was wondering if there is some clever circuit that avoids using either transformers or capacitors in, say, the input of a Mic preamp and still block DC (for instance phantom)

P.S. Found this very old thread but it seems the important content is deleted https://groupdiy.com/index.php?topic=18134
« Last Edit: June 13, 2019, 11:45:28 PM by Dualflip »


gyraf

Servo?

..or upping input offset all the way up to wanted input voltage?

Jakob E.
..note to self: don't let Harman run your company..

ruffrecords

opto coupler?

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

Servo?

..or upping input offset all the way up to wanted input voltage?

Jakob E.


A servo for 48V DC?

I've used servos for the output of opamps, but never to block or offset phantom ,sounds a bit extreme to me.
« Last Edit: June 14, 2019, 08:42:48 AM by Dualflip »

opto coupler?

Cheers

Ian

Yes, that would work, but that would probably be even less linear than the other options. What I want is direct coupled, but some kind of servo or circuit to block phantom.

volker

I was wondering if there is some clever circuit that avoids using either transformers or capacitors in, say, the input of a Mic preamp and still block DC (for instance phantom)

P.S. Found this very old thread but it seems the important content is deleted https://groupdiy.com/index.php?topic=18134

Continued here.

Thank you volker!

JohnRoberts

I have been thinking about this for decades but never reduced it to a commercial product, never even melted solder, but I did buy some high voltage caps a couple decades ago (still sitting in my back lab).  :-\

These days inside a digital mixer we could float the DC coupled mic pre up to 48V and then use a digital optocoupler to get the ADC output data back down to console logic rails potentials.

Yes, Wayne's extensive work on this approach was the reason I first joined this forum, before Wayne left and started his own forum.

This could be used as an exotic hook to perhaps sell specialty preamps but the general public does not know or care much about how many caps are in their audio path, because they don't sound bad when properly executed.

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

I have been thinking about this for decades but never reduced it to a commercial product, never even melted solder, but I did buy some high voltage caps a couple decades ago (still sitting in my back lab).  :-\

These days inside a digital mixer we could float the DC coupled mic pre up to 48V and then use a digital optocoupler to get the ADC output data back down to console logic rails potentials.

Yes, Wayne's extensive work on this approach was the reason I first joined this forum, before Wayne left and started his own forum.

This could be used as an exotic hook to perhaps sell specialty preamps but the general public does not know or care much about how many caps are in their audio path, because they don't sound bad when properly executed.

JR

Yes, people dont care that much about how many caps are in the audio path, and most people claiming no caps in the signal path use transformers, which in my opinion are worse than caps!, I read in a book that DC servos can be as bad as caps in low frequency phase shift, I believe Deane Jensen did some work on this.

P.S. Who is Wayne?

JohnRoberts

Yes, people dont care that much about how many caps are in the audio path, and most people claiming no caps in the signal path use transformers, which in my opinion are worse than caps!, I read in a book that DC servos can be as bad as caps in low frequency phase shift, I believe Deane Jensen did some work on this.
DC servos generally still use capacitors in the audio path they just take advantage of op amp's high impedance and high gain to use better behaving film capacitors instead of electrolytic. With high impedance the LF cut off pole can be set so low as to not be audible. 
Quote
P.S. Who is Wayne?
Yes Wayne Kirkwood...  His website https://www.proaudiodesignforum.com/forum/php/viewforum.php?f=1 is a good resource for audio information and he has several active audio design projects.

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.


benb

Re: Do you know any other way besides transformers or caps to block DC?
« Reply #10 on: June 15, 2019, 11:52:51 AM »
There's also the argument that signal current goes through the power and ground, and thus through the filter capacitors in the power supply.

Then there are the hard-cores who run everything on batteries...

JohnRoberts

Re: Do you know any other way besides transformers or caps to block DC?
« Reply #11 on: June 15, 2019, 02:55:40 PM »
There's also the argument that signal current goes through the power and ground, and thus through the filter capacitors in the power supply.

Then there are the hard-cores who run everything on batteries...
There are many arguments .....  some more logical than others.

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

abbey road d enfer

Re: Do you know any other way besides transformers or caps to block DC?
« Reply #12 on: June 16, 2019, 05:38:28 PM »
I read in a book that DC servos can be as bad as caps in low frequency phase shift, I believe Deane Jensen did some work on this.
What's wrong with LF phase shift? Particularly when there is a single dominant pole...
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Do you know any other way besides transformers or caps to block DC?
« Reply #13 on: June 16, 2019, 06:36:15 PM »
What's wrong with LF phase shift? Particularly when there is a single dominant pole...

More than whats wrong with LF phase shift, I think that the argument is that DC servos are not as transparent as one may think, since they provide a phase shift at low frequencies similar to capacitors.

JohnRoberts

Re: Do you know any other way besides transformers or caps to block DC?
« Reply #14 on: June 16, 2019, 07:31:11 PM »
More than whats wrong with LF phase shift, I think that the argument is that DC servos are not as transparent as one may think, since they provide a phase shift at low frequencies similar to capacitors.
Capacitors are generally very good, and servos are even better because they use better (film) capacitors.

The "myth" about servos is that they eliminate capacitors....  I wrote about that in the 80s for my column. Servos when properly executed are extremely transparent, even if they still use capacitors.  ::)

JR 

Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Re: Do you know any other way besides transformers or caps to block DC?
« Reply #15 on: June 16, 2019, 11:36:11 PM »
Capacitors are generally very good, and servos are even better because they use better (film) capacitors.

The "myth" about servos is that they eliminate capacitors....  I wrote about that in the 80s for my column. Servos when properly executed are extremely transparent, even if they still use capacitors.  ::)

JR

Do you have a link to your column?

abbey road d enfer

Re: Do you know any other way besides transformers or caps to block DC?
« Reply #16 on: June 17, 2019, 03:05:07 AM »
More than whats wrong with LF phase shift, I think that the argument is that DC servos are not as transparent as one may think, since they provide a phase shift at low frequencies similar to capacitors.
So, again, what is wrong with LF phase-shift, whether it comes from, capacitors, servos or transformers?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Do you know any other way besides transformers or caps to block DC?
« Reply #17 on: June 17, 2019, 06:28:55 AM »
I read in a book that DC servos can be as bad as caps in low frequency phase shift, I believe Deane Jensen did some work on this.
Beware when the term 'phase shift' is brought up as a source of bad sound. Most of the time, it means that the person blaming phase shift really doesn't know why things sound bad, but it's simple enough to blame 'phase shift' as a mysterious cause because it sounds somehow plausible and it's been used as a culprit many times before.

In a minimum phase filter (basically a filter with no extra delays added in), the phase shift and magnitude response are tied together inseparably - if you want a certain magnitude response (frequency response) then you have to have a certain phase shift, no more and no less.

So, if you obtain the same highpass from a servo that you obtain from a coupling cap, then their phase responses must be exactly the same. Something called the Hilbert transform (and a few assumptions about causality) will let you transform magnitude into phase and vice versa with mathematical exactness.

This doesn't mean that the two filters have to sound the same - the servo can have all manner of other flaws unrelated to magnitude or phase, and so can the coupling cap. But, beware when 'phase' is trotted out as a universal source of ephemeral bad sound. It rarely has much to do with anything related to 'bad sound', it's just the necessary result of a filter with a specific frequency response.

All that said, I find DC servos to be quite effective in terms of size, cost, and performance compared to coupling caps for low impedance audio circuits. They also have the benefit of correcting the offset of the amplifier that they're wrapped around, and not just highpassing the signal passing through, so you get a well defined DC offset from the entire system, and can thus use signal handling amplifiers that have slightly sloppy DC performance because they have great audio performance. These days, for about $5 in parts, you can make a balanced DC servo with a net residual offset of about 5-10┬ÁV that takes up a footprint of 10 mm x 12 mm and is no more than 1.75mm thick. Sure, you can screw them up if you're careless, but you can also get them right too, just like coupling caps.

Re: Do you know any other way besides transformers or caps to block DC?
« Reply #18 on: June 17, 2019, 08:07:12 AM »
So, again, what is wrong with LF phase-shift, whether it comes from, capacitors, servos or transformers?

Im doing achoustic instrumentation in which the phase between mics is critical, so yes theres something wrong with LF phase shift in some applications. Plus my point is that people believe servos are completely transparent when compared to capacitors, they are not

gyraf

Re: Do you know any other way besides transformers or caps to block DC?
« Reply #19 on: June 17, 2019, 09:36:56 AM »
For instrumentation, you shouldn't bother about the subjective "sound" of different capacitors - difference is barely measurable, if at all. 

Just scale up your capacitors to some low-low frequency (don't forget to scale up micamp input protection accordingly), and off you go.

If sub-audio phase correlation is important for your project (why?), then simply select lowcut capacitors to match in value - that'll give you matched phase as well..

Jakob E.
..note to self: don't let Harman run your company..


 

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