DIY Harrison Filters?

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Dubka

Well-known member
Joined
May 1, 2010
Messages
109
Location
London, UK
For anyone who knows how or has schematics or whatever - how about a set of the famous Harrison HPF and LPF filters?

A 2u box of 8 would be amazing...  :)
 
this is a PCB that I did some time ago, I didn´t test this pcb but I built the filter together with the Nemek eq.

normal_harrisonfilterpcb~0.jpg


http://www.twin-x.com/groupdiy/albums/userpics/harrisonfiltersch~0.jpg


edit: build to built... verbal time...  ::)
 
Wow!  That's great! 
It looks a perfect size for mounting a number of them too..
Does anyone have the means to prototype this circuit?
If so, we could be in business  :)
 
I´m very interested to get the PCB layout or PCBs if possible. I want to upgrade my liveconsole with HP/LP filter and this seems to be exactly what I´ve been looking for.
 
I was thinking the pcb's might be best with the pots on the right hand side and HPF below LPF.
This would suspend the board vertically within the chassis so any number could be mounted side by side.
I'm picturing the front panel something like this...

X    LPF    LPF    LPF    LPF    LPF    LPF    LPF    LPF    X

X    HPF   HPF   HPF    HPF   HPF    HPF   HPF   HPF    X

(X = rackmount holes)
 
Raphel,

Did you end up preferring the linear pots or the rev log pots on this circuit?  Also did you end up scaling the resistors as mentioned in your nemek thread?  Or were you happy with the results as is?

-Casey
 
Sorry Casey, I didn`t try with the revlog, it work well for me with linear pots. my second nemek is still waiting some free time.

I use dual 50K linear pots and TL074 so I think I didn`t scale the resistors.

Rafael
 
I couldn't get a smooth enough range with linear pots with that circuit. I made a board with opa604's and rev logs worked better for me.

-Okko
 
Just thought I should post:

HP/LP section of the Harrison 32C console EQ.

In my console the op-amps were originally HA-4605-5 but I use LME49470 nowadays, practically same as in the Great River 32c IIRC.
 

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OK, so what's so special about these filters?
Well, you can see that the HP uses a follower with a gain of 2.
TYpically, a Butterworth filter based on a unity-gain follower (standard Sallen-Key) would need different values for the pot sections, the one going to ground being twice the one in the FB path.
The usual workaround is to use a follower with a gain of 1.414 (sqrt of 2) or 3dB with pots of equal value.
Apparently, they have gone a little further, which makes the HP filter a Chebychev with ca. 1.5dB ripple, that means that there is a 1.5dB hump just above the cut-off frequency, and the slope at cut-off is slightly higher than Butterworth. Cut the offending bass and boost just above, a nice trick.
The only drawback is that they have to deal with the additional gain, that they need to compensate somewhere (not a big deal anyway), as done by R1/R2 and IC1A.
For the LP, it is even simpler, the FB cap, which is usually twice the value of the one that goes to ground, has been increased by a factor of 2, giving the same C1.5 alignment. Cut the hiss and boost presence.
Nothing magic, but a proof that these guys at Harrisson know what they do, with real hands-on expertise.
 

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Hey Gabriel

I haven't gotten to prototype the current pcb design yet. Things have been crazy here with studio construction. However I'm getting back on it and should hopefully have a final design available in a few weeks. I think we may offer boards as well if there's any interest.

 
Hey Marc,

I just finished building 16 preamps and none of them have a simple roll off.

I'm looking for a simple design to make 8 channels of HPF in a rack format.

Please let me know if you have any suggestion!
Cheers
Gabriel
 
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