aortizjr

EMI/RFI with Mic preamps (Inductors, Common Mode, etc.)
« on: October 05, 2005, 01:10:25 PM »
In this thread:

http://www.groupdiy.com/index.php?topic=10417

There is a lot of talk about building a mic pre-amp with good EMI protection. With some of the nicer ones, they use Common Mode chokes/inductors to suppress EMI noise.  Also with many of the designs available here, there are Ferrite beads and single inductors.

The problem is that theere are no values for these and I have had trouble sourcing parts. I lack the true understanding to really make decisions. And of course all companies have their different specs etc. etc.

So hopefully this will be a thread to clear this up for Mic pres.

The assumptions of the pre:

1. Already has some sort of Common  Mode Rejection (CMRR)
2. Balanced Input (otherwise Common Mode would not be applicable). But I think the idea of the single inductor could apply to non-balanced applications.
3. Capactor shunts in place to ground. These are to roll off the higher (out of audio band) frequency ranges.

So looking at this schematic:



You see the inductors/ferrites and common mode choke. What would be ideal/available for these?

For good engineering and DIY, let's focus on stuff that is small, readily available, easily purchased in single quantities, and inexpensive.

There is some nice stuff out there, but having to E-mail companies, distributers, and dealing with high minimum quantities and ordering directly can be cumbersome.  However, if we do go that route, perhaps provide information about purchasing these items.

In the next post I will post basically a summary of what I learned already with some additional questions.


alk509

EMI/RFI with Mic preamps (Inductors, Common Mode, etc.)
« Reply #1 on: October 05, 2005, 01:20:11 PM »
That "common mode choke" is actually a transformer...

Peace,
Al.

EDIT: Sorry, I just read the other thread and saw somebody already point that out (I thought you'd gotten this schematic from somewhere else).

Lest laziness get the best of you!

aortizjr

EMI/RFI with Mic preamps (Inductors, Common Mode, etc.)
« Reply #2 on: October 05, 2005, 01:20:56 PM »
So here is what I have learned so far:

From cuelist: 50mH is good for a common mode choke
From Samuel Groner: ~50uH is good for single inductors. 50mH is OK for a common mode choke.

Of course looking around in a catalog, there will be tons of common mode chokes, EMI filters, inductors, etc. Not all of them just have the Inductance (L) listed. Often they will just have frequency range, Ohms, etc. What is overkill and what is not? Is there a range? Minimum value for these?

Here is a good common mode one suggested by cuelist.

The distributor in the US is: http://allstarmagnetics.com

I have E-mailed tham and am waiting for a response on price and availability.

Here is one that I found at Digikey.

But it is pretty big and rated for 2A which is overkill. It is also $8 US which I consider to be expensive.

aortizjr

EMI/RFI with Mic preamps (Inductors, Common Mode, etc.)
« Reply #3 on: October 05, 2005, 01:22:40 PM »
Quote from: "alk509"
That "common mode choke" is actually a transformer...

Peace,
Al.


Yeah... I just threw that in there. I don't have a symbol for common mode choke. I guess I could just put a box there or draw one up...

I will change it...

alk509

EMI/RFI with Mic preamps (Inductors, Common Mode, etc.)
« Reply #4 on: October 05, 2005, 01:25:33 PM »
Just turn the transformer symbol 90 degrees and correct the connections.

Peace,
Al.

Lest laziness get the best of you!

aortizjr

EMI/RFI with Mic preamps (Inductors, Common Mode, etc.)
« Reply #5 on: October 05, 2005, 01:35:36 PM »
Quote from: "alk509"
Just turn the transformer symbol 90 degrees and correct the connections.

Peace,
Al.


Ok.. I think I got it right this time :)

Thank you for the info.

People are posting faster than I can update :)

soundguy

EMI/RFI with Mic preamps (Inductors, Common Mode, etc.)
« Reply #6 on: October 05, 2005, 03:23:49 PM »
not really a design thing but maybe helpful, in most of the cases where I have had bad RF problems, I installed a ferrrite bead on the + and - legs of the mic input to the circuit and that seems to work fairly consistently.  Jensen sells them on their website and they are pretty reasonably priced for bunches of them.

dave

chips are good with dip...

bcarso

EMI/RFI with Mic preamps (Inductors, Common Mode, etc.)
« Reply #7 on: October 05, 2005, 09:21:44 PM »
Quote from: "aortizjr"
Quote from: "alk509"
Just turn the transformer symbol 90 degrees and correct the connections.

Peace,
Al.


Ok.. I think I got it right this time :)

Thank you for the info.

People are posting faster than I can update :)


To make the common-mode choke function completely clear, put dots on the input side of each winding, or the output side of each.

NewYorkDave

EMI/RFI with Mic preamps (Inductors, Common Mode, etc.)
« Reply #8 on: October 05, 2005, 09:27:38 PM »
You can also buy ferrite beads from Mouser and the other usual suspects.

To increase their effectiveness substantially, turn the wire through them a couple of times if the inner diameter of the bead permits.

PRR

EMI/RFI with Mic preamps (Inductors, Common Mode, etc.)
« Reply #9 on: October 06, 2005, 01:47:12 AM »
> What would be ideal/available for these?

There is no universal answer. Anyway, we often don't need "ideal" performance (whatever that would be).

> ~50uH is good for single inductors. 50mH is OK for a common mode choke.

Note the huge difference in values. To make sense of this, you have to remember that a common-mode choke (which isn't really used as a transformer, though the construction and symbol are the same) has two values. And the second one is rarely stated.

For unbalanced current, or when driven the same on both sides, it has a high impedance, the H given in the catalog.

For current the same in both sides (and assuming the two sides are connected out of phase), the inductance cancels. In theory to zero, in practice to some small value.

If you just stick 50mH in front of a mike input, it will suck. 50mH*20KHz*6.28= 6K ohms at the top of the audio band. Since many mike inputs are less than 5K, often 2K, the top octave or two will droop bad. (Also the treble noise will rise in BJT inputs.)

50uH is 6 ohms at the top of the audio band, which does nothing to the sound. It is only 190Ω at 600KHz, the bottom of the AM broadcast band, and by itself (with a resistive amp input) will do little about radio signals well up into the short-wave band.

But it is never by itself, and amp inputs are rarely resistive up to MHz. If we throw in a capacitor, we can swamp some of the amp input impedance with a known stable impedance, and get a 2-pole filter that will cut hard above a certain frequency. But AT that frequency, the L and C resonate, which can make things worse. Can even suck enough power through the feedback loop to make the whole amp oscillate at MHz.

50uH and 470pFd will resonate at about 1MHz. Will it peak or slump? Well, 50uH is about 300Ω at 1MHz, and so is 470uFd, but the 1-side input of the sample amp is 1KΩ. The L-C network is very lightly loaded. It will be flat up to maybe 500KHz, rise to about a 1:3 or 10dB peak at 1MHz, then fall fast, ultmately asymtotic to -12dB around 1MHz.

If you work next to KRAP-AM on 990KHz, it will BOOST the station's signal. Yes, it will severely knock-down most police, taxi, CB, TV, and other sources (though in the upper TV band, waves are small enough to flow over the surface of these physically large MHz-band parts).

You really need to grok RF circuits well enough to rough-analyze tanks at a glance to get anywhere with such filters. And you should have RF-band test gear (and know how to use it) to verify that the actual parts and layout do what you think they do.

In most DIY, ultimate RF rejection is not needed. Simple tricks may suffice.

One thing is: don't be ashamed to load your input with carbon-film resistors. R9 2K7 above serves no audio purpose, actually degrades audio a fraction-dB, but gives RF something to overcome before it gets to the input devices. It also gives some loading on the L-C network. We'd like more loading to kill that 10dB 1MHz peak, but that would degrade audio performance.

Don't be too afraid to blindly throw caps on the XLR pins. Any good mike can drive 100 feet of cable, but rarely does. 100 feet of cable is 3,000pFd, so 470pFd or 1,000pFd is "nothing". The caps will drain-off considerable RF.

Don't be over-obsessed with low series resistance. Yes, 100Ω in series with the amp will degrade noise figure. When was the last time you really pushed your mike and room noise floor? If you use the big hot condensers, series resistance is almost a non-issue: their internal amp's self-noise is like the random noise of a 5KΩ resistor, so a 470Ω + 1,000pFd R-C low-pass in each input will not raise working noise floor.

We need to consider common-mode and differential-mode separately, both the filter and the amp input susceptability. Many LTP BPJ input can eat volts of common-mode even up at 1MHz, but are sensitive to small amounts of diff-mode MHz signal.

And while I hate the weight and cost, the "best" mike transformer (which comes closest to passing radio waves) is still a better radio-rejector than most things you can put in front of a transformerless input.

FETs tend to accept radio signals more gracefully. They also have less gain and higher noise, for some of the same reasons they shrug-off radio. The hottest FETs do approach BJT gain and noise. Even at the same gain, the curvature of a FET is more benign and won't convert radio to audio byproducts as well (bad) as a BJT.


Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
EMI/RFI with Mic preamps (Inductors, Common Mode, etc.)
« Reply #10 on: October 06, 2005, 04:50:05 AM »
Good that this topic came up - I spent a good deal thinking over it and I did not come up with a completely satisfying solution.

The best solution (judged from highest RFI suppression) is probably a common-mode transformer as used in the supergreen, but I suspect that it would eat up much time to implement such a solution properly (i.e. flat common mode and differential mode frequency response, low THD, very low resistance etc,).

At the moment, I'd probably settle for a 50 mH common mode chocke bypassed with 6k8 resistors (hopefully reducing peaking a bit) and followed by 1 nF C0G caps to chassis. A low input impedance (2k) would help as well.

Samuel

Carl_Huff

EMI/RFI with Mic preamps (Inductors, Common Mode, etc.)
« Reply #11 on: October 06, 2005, 11:59:30 AM »
Thank you PRR for that thoughtful and detailed analysis of the problem.  

I always make a point to read and study your posts on any topic.
Best Regards,
Carl Huff

chrissugar

EMI/RFI with Mic preamps (Inductors, Common Mode, etc.)
« Reply #12 on: October 09, 2005, 03:43:00 PM »
Thanks PRR, as always a pleasure to read.

chrissugar
Christian Mike Sugar
        CMS-LAB


 

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