uncle

High pass filter on MicPre input
« on: January 04, 2009, 12:37:23 AM »
Does anybody haves schematic for simple high pass filter that can be put in front of mic preamplifier? I'd like to add that feature beside phase switch, pad and +48V

thanks


NewYorkDave

Re: High pass filter on MicPre input
« Reply #1 on: January 04, 2009, 12:44:10 AM »
If the input impedance of the preamp is mainly resistive, a couple of coupling capacitors will form a simple high-pass filter.

If you're building it into a preamp, it's usually better (IMO) to add it downstream, after the first stage of amplification.

gyraf

Re: High pass filter on MicPre input
« Reply #2 on: January 04, 2009, 03:41:58 AM »
Passive high-pass before amplification will probably result in rather large low-frequency noise (as a simple filter will make you loose input termination impedance when going towards zero Hz)

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

NewYorkDave

Re: High pass filter on MicPre input
« Reply #3 on: January 04, 2009, 02:05:09 PM »
That's an excellent point, I hadn't even thought of that.

clintrubber

Re: High pass filter on MicPre input
« Reply #4 on: January 04, 2009, 07:02:46 PM »
So you better don't, as pointed out by both gentlemen.

But if you for some reason still must, then have a look here:

http://www.ethanwiner.com/gadgets.html



Bye,

  Peter

clintrubber

Re: High pass filter on MicPre input
« Reply #5 on: January 04, 2009, 07:14:31 PM »
Passive high-pass before amplification will probably result in rather large low-frequency noise (as a simple filter will make you loose input termination impedance when going towards zero Hz)

Jakob E.

W.r.t. 'will probably', it's certainly the case.
If I recall it correctly though (after all these lovely non-tech days  ;) ) the reason for it is somewhat different
(or maybe it's another explanation simply resulting in the same):

the bass-cut is realized by inserting smaller valued caps i.s.o. larger ones (which are even totally skipped or at least effectively 'absent' in the transfer for the band of interest), which could be thought of as an increased source impedance for those lower frequencies, hence causing more noise in that region.

In other words: IIRC a calculation will show that the capacitor simply contributes a noise density component which is in accordance with it's resistivity at that frequency  (so 1/|w*C|)

Smaller cap --> more 'resistance' (which can be thought in series with the source impedance) --> more noise

Regards,

  Peter

JohnRoberts

Re: High pass filter on MicPre input
« Reply #6 on: January 05, 2009, 12:22:08 PM »
In transformerless mic preamps using low noise bipolar transistors or ICs, there can be a significant input noise current term. This input noise current is multiplied times the input source impedance (in parallel with input termination) to generate a noise voltage term that is combined with other input noise voltage sources. In principle you could tweak the input stage current density of a discrete preamp to be better optimized for the higher source impedance, but you are still dealing with more total noise.

In addition, increasing capacitor impedance in the mains hum frequency region could negatively impact common mode rejection if the capacitors aren't closely matched.

In general an active 2 pole HPF added after the mic pre stage is much less problematic. If you have the panel space available a pot could make this HPF stage adjustable.

JR
 
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

pstamler

Re: High pass filter on MicPre input
« Reply #7 on: January 05, 2009, 02:25:38 PM »
So I guess the upshot is that we need to know what kind of preamp you're talking about before giving an answer. Transformer? No transformer? Tube? Bipolar? FET?

Peace,
Paul

uncle

Re: High pass filter on MicPre input
« Reply #8 on: January 05, 2009, 10:34:35 PM »
Thanks for the answers..
it's transformerless 9k mic pre. I can add then a HPF after mic pre and before balancer board. How can I measure what cap value is for, for example 18db/oct @ 50Hz? How you calculate that?

 

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