Balanced Passive High Pass Filter

[craigmorris74]

Greetings,

Looking to build a passive, balanced high pass filter for line level.  Would this be feasible?  If so, would it have to look something like this:

Would it be difficult to match the capacitor values?  Ugh, super rookie question.  Just want something to stick after my mic pres, before the next step in the signal path.

Thanks,
Craig

 

[cuelist]

Possible, yes.

Keep in mind that the resistor you've drawn is effectively in a paralell with the input impedance (effectively resistance) of the next stage. So when you calculate the -3dB point of the filter you need to figure this in. Also, this will be 1st order filter with only -6dB per octave. High pass filters for this purpose are typically 2nd order or higher. To create a passive second order filter you'd need to cascade to 1st order filters with 1:10 impedance difference or use an LC filter (and inductor in place of the resistor in your schematic.

But why?  Is there no highpass filter in your mic pre?

 

[craigmorris74]

Sorry, there is supposed to be a resistor going to ground from both + and - lines.  Don't know what I was thinking.

I don't think I have a high pass in any of the pres I own-the majority are from kits SCA and JLM.  I think I would like it after the mic pre because I don't want to mess with considering phantom power.

Thanks,
Craig

 

[Kingston]

Read this: http://www.groupdiy.com/index.php?topic=34480.0

same topic, great info.

 

[tv]

Re: "inductor"

If you plan to install the high-pass in a  "semi-permanent" way, you could use your pre's input trafo winding (if yours has one) as a part of the filter.

 

[craigmorris74]

Good ideas and info, guys.  I'll research your leads and see what I can come up with.

Thanks,
Craig

 

[conleycd]

I might be mistaken but I seem to remember Joe Malone posting about a modification in his Go between - I think eliminating the pad but adding a resistor or a cap - to make a passive high pass filter.

If you are mostly concerned about roll off for microphones I don't think there is anything wrong with doing pre preamp.  You don't want the preamp typically to be amplifying all that rumble and noise.  It eats up headroom and you don't need (typically) below 80hz or even 100hz for most vocals.  I think Joe prefers a 6db roll off because it is slight, natural, and does the job.

CC

 

[PRR]

> supposed to be a resistor going to ground from both + and - lines.

Not at all necessary.

Unless you have huge common-mode rumble. Which is very unlikely in audio studios.

> I don't think there is anything wrong with doing pre preamp.

Not "wrong", but there are usually trade-offs.

Put a small (bass-cut) cap in front of a transistor amplifier, the random component of base bias current causes a random voltage which is amplified. Recall that transformerless transistor inputs run at high current with large base current. Instead of room rumble, you have base-capacitor rumble. This together with 1/f noise can completely confound a wideband noise measurement, and may be audible.

We often have a big C in the emitters as part of the gain control; same problem.

Put the C in front of the transformer into a tube, you will probably get a peak instead of a smooth roll-off, and you will find that small iron at low levels does not have a specific inductance, permiability depends on audio level.

I can't see a major problem with a C-R network between iron and tube, except the overall loading effect of the R. Where an R is needed to damp the iron, this may be quite valid.

But the usual technique is to put the bass-cut "between first and second stages". The first stage is low-noise, broad band, has enough gain to overwhelm 2nd stage noise, yet not enough gain to make rumble troublesome (IM or clipping). Often a gain of 10 or 20 is good. And also you can rig it so the bass-cut is unbalanced and higher impedance, saving a buck especially if you want switched frequencies. Your "2nd" stage gives the rest of the gain, any output frills needed, and might not be super low noise like the 1st stage must be.

Classically we'd do the subsonic filter after the mike preamp before the bus switch/drivers. If you have so much rumble that normal gain settings are clipping or making significant IM, you might want to look in the studio for rumble reduction possibilities.

Doing a lot of work in a VERY rumbly room, I mostly kept the rumble peaks 20dB down from zero VU. That sometimes meant the harpsichord never beat -20VU. But everything in the room was on tape. In post-production I could high-pass and boost to a good listening level.

Bass-cut probably "must" be switched. One day I have Children's Choir with essentially nothing below 280Hz, and can slope it at 200Hz; another week I have a pipe organ good to 32Hz and trucks on River Road going much deeper, I need a steep 25Hz filter.