Calrec Eq - Frequency distribution versus scales

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Dr nEon

Well-known member
Joined
Jan 11, 2006
Messages
232
Location
Grate Britain
Hi everyone!
Newbie here with a calrec eq problem!

Firstly, a big thanks to Gyraf and all the regulars here...u guys are so inspiring!!

Ok , so I've built two channels of PQ1549 eq , from the rev4 pcb layout on jakob's site . The unit works first time :grin: ,has no noise or gain problems, and is nicely matched between the two channels.

However,on closer inspection.....

When I sweep tones , or check pink noise , I find that the frequency knobs are not corresponding exactly to what the original calrec scales suggest.

They are correct at the beginning and end of the pots travel ,but the 'curve' seems to differ..

For instance ;

hi mid 530 peaks at "530" mark
640 at 27deg above "640"mark
1K at 12 oclock position
2K at "3k6" mark
3k6 at about 10deg below "7k5"mark
7k5 at "7k5" mark

The lower side of the pot's range is expanded,while the upper side is compressed . All of the freq knobs are behaving exactly in this manner.

I started with omeg p20's , and then tried 16mm alpha , which also deviated from the panel dial markings.So now I have put in one of the lovely green eco range 16mm pots , ....but the curve of that is similar to the omeg p20. Back to square one! :roll:

Has anyone else experienced this ? ?

I am suspecting the pots , but could my problem actually be elsewhere in the circuit?

My psu is gyraf's mini-psu , using a 15 volt tx and 7815/7915.


Any help greatly appreciated!

nEon xx
 
It's probably the pots' resistance curves, as you say - Calrec used some - now obsolete - Ruwido types.

But ask yourself; is it important?

I'm not sure that the originals are on the mark regarding frequencies (like almost all parametric eq's) - you set them by ear anyway, and if you need to copy a setting, it'll still be the same (unless off course, you have versions with different pots)

Jakob E.
 
Hi Jakob!

Thanks for the quick reply :grin:

Ok , that's piece of mind for me..phew!

I understand your point about para eqs in general . GML8200 for instance was always beautiful, but pesky, to set up over a mix!

So, just for the record... the best option appears to be rebuilding an alpha 16mm pot ,as per your instructions . This has a much better sweep in the 2k5 to 7k5 region , and like wise in the other freq bands.

I'm hoping to build another 8 of these to mount vertically in a subrack,or put into the producer panel of my console. I just wanted to make sure I was on the right track before commencing!!

On that note , Jakob , could I power eight of those from the mini-psu design (with something like 50va tx) .. or is there another design that would be better suited?


Many thanks , Cheers!


nEon x
 
The minipsu - with sufficient (isolated!) heatsinking and a 30VA transformer - will deliver up to 1A worth of current - which should be able to drive 8 units. Maybe you should use 2200uF/35V electrolytics in stead of the 1000uF/35V ones then.

But you probably shouldn't push it beyond that..

Jakob E.
 
Ok , that's great to know...I'll do what u said with the caps , and I wont exceed 8 units.

Thanks again jakob , I'll post pics when I'm (eventually) done!

Cheers!

nEon x
 

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