Adding 80Hz HP filter to a BA

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chrisp
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Adding 80Hz HP filter to a BA

Postby chrisp » Fri Nov 23, 2007 1:56 pm

What's good a good way to implement a 80Hz HPF on the BA? A simple LC at 0.039uf and 5.1K placed across the outputs? Switched in and out by a DPDT?
Chris P
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chrisp
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Postby chrisp » Wed Mar 19, 2008 8:51 am

Bump - any suggestions?
Chris P

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qu4ntum
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Postby qu4ntum » Thu Mar 20, 2008 10:23 am

wouldnt it be best to do the rolloff before anything else, ie at the input...?

chrisp
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Postby chrisp » Sat Mar 22, 2008 11:02 am

Yeah, I wondered ... but I don't want to mess up any impedance values.

The good thing about doing it up front is that the 10 pin connector is just sitting there unless you have a DI connected, so it can be used for this instead. Working with 16.5uf (two 33uf in series) and a 120R resistor to minimise ZΩ issues, the HPF DPDT switch might look something along the lines ... (Pic edited)

Image

Now the downside of this to me is that we've traded lowering the shunt value (to minimise impedance changes) for a higher uf value, but cap values have nowhere near the accuracy of resistor values, so we end up with a less precise filter - the -3db might be anywhere between 60-100Hz.

Am I overthinking this?
Chris P

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greenmanhumming
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Postby greenmanhumming » Sat May 24, 2008 6:20 pm

I think I'd want a steeper slope - any ideas?

i would have thought 12-24dB per 8ve was more appropriate for this type of filter

also, doesnt this put an extra cap in the signal chain? i would imagine that is best avoided if possible...

chrisp
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Postby chrisp » Mon May 26, 2008 9:26 am

Hi there Mr Humming

It all depends on what you're after - I was designing more for the sort of rumble filter you find on mics, and they tend to be simple first order LC filters like that shown. Adding caps to the signal path, yeah, but all designs are a compromise, and I've seen well regarded mics (who mentioned the C414?) with at least 2 caps in the rumble filter section - it's how you make a passive filter, after all. I'm not that concerned, but of course I could probably play around with the equation a bit more and get a single cap to work, rather than 2.

f=1/(2piRC), so for f=80Hz we get the relationship between the resistor and the cap as being C=1/(503*R). So 91Ω and 22uf as a single cap would do the trick, as would 200Ω and 10uf.

If you want more than this, I suspect you're getting more into EQ territory than I want to go, but even there, the low shelf on many highly regarded preamp EQ's (1073, anyone?) is a simple LC, again with two caps on some frequency selections.

Let us all know what you come up with.
Chris P

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I do lots of things. I believe eclectic skills are best.

Blake
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Postby Blake » Mon May 26, 2008 2:42 pm

Chris did you end up implementing a HPF on your animals?

chrisp
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Postby chrisp » Mon May 26, 2008 3:33 pm

Blake

Hand on heart, no, not yet. Its part of a general redesign on my BA builds that will add the HPF, a 10 led PPM meter, and a 10K impedance setting on my variable Z switch. Also looking to stack 8 of the above in a 2U rack.

The trouble with progressing this rebuild is that my BAs will be out of action, and at the moment they're just getting too much use, what with my own recording activities (I'm using them on a spoken word recording over the next month of weekends) and friends dropping by with the old line "you know those 4 funky little preamps you have in that box, well I really could use them for a little project I've got going ...." I tell you, build a BA and you'll never be short of friends.

Its gonna happen, but its not yet at the top of my DIY list, and probably will be Christmas project.
Chris P

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I do lots of things. I believe eclectic skills are best.

greenmanhumming
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Postby greenmanhumming » Mon May 26, 2008 8:01 pm

the c414 XLS bass cut is 12 dB/octave at 40 Hz and 80 Hz; 6 dB/octave at 160 Hz

i guess i'd better look at the schem to see about subverting the pad switch.

the cap accuracy is not a problem , i'll just select for the right value. So if i accept that there will be extra caps in the signal which ones do i buy? presumably the same type Joe uses that come with the kit?

Joe, can you sell me some extras for this purpose? in a range of suitable values?

i might want to use the hipass with the DI, so i'll probably try to connect the filter elsewhere...

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Postby Joe Malone » Thu May 29, 2008 10:43 am

I haven't read all of the above but to use the Pad switch as a HPF all you need is a Bipolar cap where the one of the two 470R resistors are on each side of the PAD switch and link over the other 470R and no Rpad needs to be fitted. This works if you have fixed impedance but if you have variable then the variable impedances need to be switched values and the HPF cap would need to be switched as well.

I always perfer 6dB slope as it is the best phase wise in the roll off area so the least audible. Also since the cap feeds the inductance of the input transformer the roll off slope changes to a higher one as DC is approached.
Joe :-)
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sb1
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Re: Adding 80Hz HP filter to a BA

Postby sb1 » Thu Apr 10, 2014 4:46 am

Hi Joe, How would you add a HPF switch separate from the BA. ie- without using the switches on the BA.
Thanks

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Re: Adding 80Hz HP filter to a BA

Postby Joe Malone » Tue Apr 15, 2014 7:56 pm

sb1 wrote:Hi Joe, How would you add a HPF switch separate from the BA. ie- without using the switches on the BA.
Thanks


If the transformer has bad transfer characteristics or not loaded well like API input is setup you can get a roll off followed by a bump. This can be fixed usually by making the cap smaller so the roll off is higher but need to graphed with something like RMAA to work out the best value.

Ideally cutting the track between transformer secondary to the +in of the opamp before any resistor from +in to 0v (or half rail in BA BAD) and adding capacitor across the cut track can work very well as a HPF.
Joe :-)

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mikeyb
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Re: Adding 80Hz HP filter to a BA

Postby mikeyb » Thu Jul 02, 2015 8:55 am

Hi Joe,
I'm hoping to implement maybe a 80/160Hz HP filter, cutting the track between tx secondary and + input on opamp.
What typical value of R would i be looking at?
Would i use Rl as 27k (oep lo ratio Cl cap) and use the formula 1/(2 x Pi x r x c) > C = 1/ (2 x pi x 27k x 80) = 73nF or 68nF nearest? For 160Hz would i halve the cap ie 33nF
But is this the turnover frequency, so to be an 6db down at 80 hz, i would have to turn over at 160Hz?
So for 6dB down at 80Hz > C=1/(2xPix27k x 160) = 37nF or nearest 33nF or 39nF
and for 6dB down at 160Hz C =1/(2 x Pi x 27k x 320) = 18nF or nearest 15nF or 22nF

I've just been looking at the HPF in the G9 schematic and if you put the cap values in the formula, the frequencies are doubled(presuming this is the turnover points)

Hope this is understandable!!

Cheers

Mike

mikeyb
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Re: Adding 80Hz HP filter to a BA

Postby mikeyb » Fri Oct 23, 2015 3:44 am

Hi Joe - would you implement the HPF like this?
What values for Rf and Cf would you use to implement a 80Hz HPF?

BA HPF.jpg


Cheers

MikeyB
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mikeyb
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Re: Adding 80Hz HP filter to a BA

Postby mikeyb » Wed Nov 11, 2015 12:30 am

Hi Joe, would you implement the filter like in my above jpeg?

Thanks

Mike

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