5v333

Low Budget Variable Passive HP filter
« on: July 08, 2019, 08:17:31 PM »
A friend of mine asked if it was possible to get a passive hp filter build with some frequncies from 100hz to 4k. with higher order slope than 6db/oct
he wanted to use it as an instrument when doing his dub music and he was not very interested in to investing alot of money in to it. so nothing to fancy.

i got a kick and decided to have a go at it to see if it was possible to earn a couple of coins and give myself a challange.

attached is a draft of what im thinking.

im thinking 12db/oct.
reactance of C and L should be 300R each to form a 600ohm divider at the choosen frequency so that we get -6db at f.
am i right about that?

lets start there. gotta go zzzzzzzzz.


Re: Low Budget Variable Passive HP filter
« Reply #1 on: July 08, 2019, 08:24:49 PM »
Why not us an RC filter if you're going low budget?

squarewave

Re: Low Budget Variable Passive HP filter
« Reply #2 on: July 09, 2019, 12:39:45 AM »
You need to factor in the characteristic impedance of the circuit. Right now your source impedance is 0 and your load impedance is 10K. That's wrong for two reasons 1) you must drive with characteristic Z for critical damping (otherwise you'll get peaking where the response has a boost at the corner frequency) and 2) 10K load is too high (your inductor would need to be huge). A good op amp can drive 600 ohms so I would design for a characteristic impedance of 600 ohms. Also there will be a 6dB loss that you need to "make up" in the output.

But as a general rule, if something has been done many times and it's what you want, you should probably just copy that. At the very least you might be able to find an off-the-shelf multi-tapped inductor. Otherwise are you going to make a custom inductor?

Traditionally the way this sort of circuit used to be done was with a CLC filter. Pultec, Cinema Engineering, Allison Labs and many others made all sorts of filters like this (I love low/high cut filters). So a CLC filter is series cap, shunt inductor and series cap (eg 6uF 970mH 6uF is 50Hz on the HLF3C). Drive that with 600 ohms and load it with 600 ohms. That makes damping just right.

However, this does require a 3 pole switch, it's 18dB / oct and it's actually not trivial to build because you're switching a lot of parts and using caps paralleled to get proper values, etc. If you're really trying to do cheap or even just simpler, then a basic 6db / oct RC filter in the feedback path of an op amp could be perfectly effective. You could then also use a conventional potentiometer and make it continuously adjustable instead of stepped. If by "dub music" you mean dubstep, then making it continuously adjustable might be more desirable anyway. Sometimes the simplest solutions are really the best.

mjrippe

Re: Low Budget Variable Passive HP filter
« Reply #3 on: July 09, 2019, 11:15:26 AM »
Well, since passive is the goal I would say CLC is your best bet.  If you terminate the output with a higher impedance than 600 ohms you will get ringing - which is a bit like a resonant filter on an analog synth.  I personally like that effect.  The Allison Labs is the only one I've seen that is continuously variable (over a set of ranges) and it uses a massive mechanical variable inductor.
« Last Edit: July 09, 2019, 10:37:52 PM by mjrippe »

L´Andratté

Re: Low Budget Variable Passive HP filter
« Reply #4 on: July 09, 2019, 11:21:08 AM »
My guess is your friend wants a passive filter, because this seems most simple (it is in case of RC), but things get complex quickly with inductors involved so most simple 2nd order would be active sallen key hp filter with battery power...?

Look up drawmer noise gate, the variable filter section hp+lp filters can be copied directly...
"Why not get an assistant to work the faders? Real vintage! And maybe cheaper... ;)"

5v333

Re: Low Budget Variable Passive HP filter
« Reply #5 on: July 09, 2019, 04:47:57 PM »
I appretiate the taking part in this!

For now passive is what is called for. I could dabate with him or i could make a small profit out of a simple/affordable design.
I guess it has a nice ring to it in his ears. Makes him inspired.

I can roll my own multi tapped inductor.

He is in to dub. Think scratch perry but with laptop and synthesisers.
 

The CLC might be better/affordable.
I need to read up on it.

But would the CL tank work with a 600R load instead?
I guess i have to breadboard some.

squarewave

Re: Low Budget Variable Passive HP filter
« Reply #6 on: July 09, 2019, 07:07:17 PM »
But would the CL tank work with a 600R load instead?
You would have to drive it with 600 ohms in series and load it with 600. But for HP the series resistance matters a lot whereas the load doesn't matter much at all. You could actually design for a 10K load (most line inputs are 10K) and get the benefit of almost no loss (meaning no makeup gain necessary). The response won't be like a perfectly damped filter but you can play with the cap size to get really close.

Apache5

Re: Low Budget Variable Passive HP filter
« Reply #7 on: July 10, 2019, 01:07:40 PM »
I think that the famous "big knob" King Tubby  has in his desk is a HPF CLC kind passive filter. Possibly an  Altec 9069-B.  An multiple tap inductor needed, a lot of condensers and a 3 deck switch.  The problem is to know the values of the original but maybe you can do your own. I dont thing it would be cheap... an active one is a lot cheaper for sure. There was a guy on facebook that has kits for a big knob with a custom inductor... "westfinga" I think

abbey road d enfer

Re: Low Budget Variable Passive HP filter
« Reply #8 on: July 10, 2019, 01:35:04 PM »
Why passive? Active is cheap and predictable, more flexible...
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

iampoor1

Re: Low Budget Variable Passive HP filter
« Reply #9 on: July 10, 2019, 01:50:02 PM »
Active might actually be simpler if you want a 12db rolloff


mikeyB

Re: Low Budget Variable Passive HP filter
« Reply #10 on: July 10, 2019, 03:28:27 PM »
Check filter on Drawmer noise gate DS201, very simple and low pass if you want it too! May need to tweak values - pots are rev log 100k duals, Colin at audiomaintenance should have them
making better music!

5v333

Re: Low Budget Variable Passive HP filter
« Reply #11 on: July 10, 2019, 06:54:27 PM »
Predictability and flexability is not what is askd for.
Its going to be more as an musical instrument than a tool for correction  in a mix.
He will prob be switching freq while recording/dubbing.
Prob feedbacking echoes through it etc.

Today i made a CL voltage divider with 10k load after it.
 Was aiming around 100hz (Z = 300Rc+300Rl)
One 4.7uf and five stacked 100mH.
12db/oct alright!
A broad peak of about 2dbs was present.
-3db @ about 100hz
-6db @ about 70hz

With a load of about 1.2k no peak and -6db @ 100hz.

I said to my friend that i could put a pot in there for making a broad  resonance boost around fc.
He liked that very much.

I soon as i can give him a price we will know if i have to make an active unit.
But right now i think it is selling it self.

5v333

Re: Low Budget Variable Passive HP filter
« Reply #12 on: July 10, 2019, 07:28:14 PM »
i just found some RM12 1000AL ferrite cores (without center holes...), two bobbins and some 0.25mm enameled wire at home.

it takes 685 turns to make 500mH inductor with AL1000.
an RM12 bobbin can take about 800turns of 0.25mm wire.

5v333

Re: Low Budget Variable Passive HP filter
« Reply #13 on: July 11, 2019, 07:22:20 AM »
sorry if im missbehaving here in anyway.

appretiate the all inputs and questioning.

you are all worth your weight in gold.

L´Andratté

Re: Low Budget Variable Passive HP filter
« Reply #14 on: July 11, 2019, 10:57:57 AM »
https://tinyurl.com/y23waesx

Although an active solution would be...  :-X

 ;D
"Why not get an assistant to work the faders? Real vintage! And maybe cheaper... ;)"

5v333

Re: Low Budget Variable Passive HP filter
« Reply #15 on: September 23, 2019, 06:10:08 PM »
i just managed to put this together and it became a small hit. one octave lower and one higher would of been smashing!

it can recieve balanced or unblanced sources. theres a switch to change the input circuit.

i inserted a variable series resistance from 0-300R that funktions as a resonance/curve  thingy. with 0R theres a resonant boost about 12db or something. with 300R the cutoff is wider and no res boost.

also inserted a DPDT switch that turns the CL upside sown, making it into a lowpass.

6frequencies

a bypass switch.

the output is unbalanced but turns into an assymetric balanced output if you put a TRS into it.


i put a cap to ground in the filter and filled the tank with 1/2 B+. this helped to reduce thumping/popping when using the switches.
theres still some random clicks happening but i dont think i can reduce that anymore. would of been great though.


i wonder if i done the output circuit correct. should i keep the cap at the noninverting input as a AC short to ground?
and should i pick other values for thoose resistors?


ruffrecords

Re: Low Budget Variable Passive HP filter
« Reply #16 on: September 23, 2019, 06:29:00 PM »
With the simple 2 x 20K pot divider across the supply that you are using to bias the op amp to half the rail you throw away the considerable power supply rejection ratio the op amps offer you. To regain some of it keep the pot divider but do not connect it straight to the op amp plus input. Instead decouple the bottom R with 47uF and feed from this point to the plus input via 100K resistor. Repeat for other op amp. Small improvement but worth doing.

Cheers

Ian
« Last Edit: September 24, 2019, 06:55:45 AM by ruffrecords »
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

5v333

Re: Low Budget Variable Passive HP filter
« Reply #17 on: September 24, 2019, 06:14:38 AM »
thanks Ian!


 

Related Topics

  Subject / Started by Replies Last post
14 Replies
4395 Views
Last post July 01, 2010, 10:19:06 AM
by flintan
8 Replies
3492 Views
Last post September 10, 2010, 02:16:03 PM
by moamps
12 Replies
6860 Views
Last post October 18, 2010, 09:54:48 AM
by johnR
12 Replies
4361 Views
Last post March 13, 2012, 12:18:23 PM
by buildafriend