Building a simple RC High Pass Filter?

Hi Guys,

I'm wondering if there might be an easy way to add a simple High Pass Filter,
say at 100Hz, wired to a switch, connected to a mic
input to have a bit of a Bass roll off?

Anyone done this or know if it can be done? A simple passive design, resisitor and cap?

Thanks in advance

Matt

Yes, provided you know the input impedance of the preamp, and provided you have a capacitance meter. Oh, it'll take two caps.

If the input impedance of the preamp is R (in ohms), calculate the capacitance by:

C = 159,000 / (R x F)

where C is the capacitance in microfarads, and F is the frequency in Hz. So if you want 100Hz, and the input impedance is 1500 ohms, then:

C = 159,000 / (100 x 1500) = 1.06uF.

But that's not the value of capacitor you want to run out and buy. What you really want is 2.12uF. Why? Because you're going to put one cap in series with each leg of the mic input, and these caps will be in series with one another, and two equal-value caps in series have a total equivalent capacitance of half the value of each cap. So you want 2.12uF; in practice, 2.2uF will do. So run out and buy a dozen 2.2uF caps, maybe more, and start sorting using your capacitance meter until you get a closely matched pair. (If you don't, your preamp's common-mode rejection will go to hell in a handbag.) Put those in series with the two legs of the inputs (pins 2 & 3 of the XLR). Then wire up a DPDT switch to bypass the two caps in one position of the switch, leaving the other position unconnected. Problem solved.

Only thing is, this is a trouble-prone way to solve the problem, given that the signal levels are very low. It's a good place to pick up extraneous interference, and unless the switch terminals are gold, you may have all sorts of problems when they get old. You also may be switching at a point where phantom voltage exists (tranformer preamps only), which is a good way to get BANGs in your output if you switch hot. (If you do this on a transformerless preamp, put the filter on the side of the blocking caps away from the XLR.)

That's how to do the job you outlined, but let me suggest two alternatives. The first is to buy an inline filter, which Shure sells and E-V used to. Plug it in when you need it, otherwise keep it in the BS bag. Or install an RC filter circuit somewhere else in the preamp; between the amplifying stage and a following level control is often a good spot.

Peace,
Paul

Paul,

Thanks for the assistance, as always.

I have access to a cap meter and can match as suggested, though I heed your
warning and the Shure unit might be just what I need at least I can use them
in other pres that DON?T? have a Hi-Pass.. Which is just about all of
them...

I?m planning on building some older revisions of Peter C?s Green Pre, one
that doesn?t have a Filter like the current one and though I might try a
retro fit, but this is looking like it isn?t worth it.

Cheers

Matt

Hi,
I'd like to make a simple high pass filter for a neve mic pre too, less expensive that the original.
What I know is that the high pass filter has to be between the two stages of the BA283 board at the same place of the 1073 eq.
What I don't know is : wich parameters I have to respect for a correct work of the high pass (Impedance...), do I have only to calculate the frequency of the filter and wiring it. These infos could be really good for any others preamps design, I think...


Thank you
:wink:

[quote author="matta"]a simple High Pass Filter,
say at 100Hz, wired to a switch, connected to a mic
input to have a bit of a Bass roll off?

Anyone done this or know if it can be done? A simple passive design, resisitor and cap?[/quote]
Question will already be answered for but it's at least a nice opportunity to mention Ethans site:

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



Bye,

Peter

hello,
thank you Clintrubber, I've looked at Ethans site and the infos are really interesting :thumb:

hello,
What I like to know is if it's possible to insert the RC filter directly without any other component at the input of my neve mic preamp....Or will I have to add one or more other(s) component(s)
Do I have to look at the impedance from the source (the mic) and of the preamp?
Must I put the RC filter before or after the input transformer?

Thank you by advance
:wink:

One could put it after the xfmr no ? or would that make the cap size
a problem ?

Well there is a two pole and or two stage type

the peavey tube preamp , i think , switched a simple r/c network in
two places at the same time for a lo cut , it felt pretty good .

so what was the problem ?

Any reason for reviving this thread after nearly 10 years?

Jakob E.