Modding EQ for Variable Q shape

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porkyc

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Dec 30, 2008
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Part 2 from above.
Having now looked at the PCB, the pots are going to have to be dual-concentric with a pull, but trying to interlace the 2 mid pots (with said switches) and the existing HF/LF shelves will be difficult.
Probably best route is to strip the EQ out, as has been mentioned and replace with complete EQ daughter board including pass amps.
This is how the Roger Troisi EQs are done for the MCI JH600.
You might be able to use the existing pass amps, but the tuned circuits may well have to move to the daughter board.
The cost of the pots may well be prohibitive, so, cost effectiveness?..................
PC
 

porkyc

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Dec 30, 2008
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Part 3 from above.
Back on the don't bother path.
But, you could do a lot to improve the mids with just 3 capacitor changes.
The mids have no overlap so you cannot do anything at 3K and 10K at the same time.
Also, the Q on the mids is quite high, using caps at 3:1 ratio.
Changing 3 caps gives you a softer Q, (the same as the "Thriller" EQ) and overlapping ranges of 220-5k and 470-10K.
C20 2n2, C22 4n7, C21 10n.
These chosen because of stock values, the ranges can be tweaked but would require more esoteric values. You could lower the 4K7 Rs to widen the range of each band. Even just to 4K3 gives a slightly nicer range. With the 100K frequency pots you could have 2 very wide ranging mids. The MCI JH500 has 2 wide ranging mids offset by 1/2 octave. The Harrison MR2 just had 2 identical mids 400-8K, as did their MPC film board.

You can see why the Orion ranges are as they are, because of the splitting of the shelving/mids between the the chan/mon paths.
Never let sales people design consoles; all you get is "features" you never knew you needed. The rest of the world moves the whole EQ between ch/mon), thus giving you one good 4-band EQ as opposed to two not very good ones.

Improve what you've got and build a Net EQ or a Sontec clone. It'll be cheaper.
PC
 

Barrylime

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Joined
Sep 27, 2020
Messages
20
Mr. Grumpy here.
A. I can't help but feeling the rest of the world would make an outboard EQ if you are going to make anything. How many times at once do you need this capability?

B. You said, " I would love to know how realistic/feasible this mod is."

I would suggest that everyone here has answered your question as to the feasability of this.
ie not much/none!
You cannot easily modify a wien based bandpass filter to switchable/variable Q, the Q is a function of the ratio of the capacitors, as obviously is the frequency range. You also have to pad the input to the wien (with the higher Q) to maintain the +/- range, so the amount of switching that has to take place.................... No thanks!!

C. Having said "none" however, you could do daughter board(s) that replaces the wien with a SVF with switchable Q. The switching is simple (changeover/shorting out of a resistor). This would be a pull-pot on the level pot, (cheaper than on the dual RA frequency pot). 2 dual chips.
The point of the SVF is that the 3 parameters are independent (in theory).
This is how a Nashville based manufacturer did this. You either ordered stock EQ with wien circuits or enhanced EQ with SVF sitting in the same place in the circuit. The inverting pass amps were on the main board, the tuned circuits were on daughter boards.
My2p's worth.
Thanks for the reply! I have found someone who is designing the mod for me at the moment, I will update this thread in the months to come.
 

Barrylime

Member
Joined
Sep 27, 2020
Messages
20
Part 3 from above.
Back on the don't bother path.
But, you could do a lot to improve the mids with just 3 capacitor changes.
The mids have no overlap so you cannot do anything at 3K and 10K at the same time.
Also, the Q on the mids is quite high, using caps at 3:1 ratio.
Changing 3 caps gives you a softer Q, (the same as the "Thriller" EQ) and overlapping ranges of 220-5k and 470-10K.
C20 2n2, C22 4n7, C21 10n.
These chosen because of stock values, the ranges can be tweaked but would require more esoteric values. You could lower the 4K7 Rs to widen the range of each band. Even just to 4K3 gives a slightly nicer range. With the 100K frequency pots you could have 2 very wide ranging mids. The MCI JH500 has 2 wide ranging mids offset by 1/2 octave. The Harrison MR2 just had 2 identical mids 400-8K, as did their MPC film board.

You can see why the Orion ranges are as they are, because of the splitting of the shelving/mids between the the chan/mon paths.
Never let sales people design consoles; all you get is "features" you never knew you needed. The rest of the world moves the whole EQ between ch/mon), thus giving you one good 4-band EQ as opposed to two not very good ones.

Improve what you've got and build a Net EQ or a Sontec clone. It'll be cheaper.
PC
all very great info! I might consider some of the things you have said on the existing channels (seeing I have pcb replicas to make new ones and more drastically mod/change by removing the EQ).
 

cuelist

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Joined
Aug 21, 2004
Messages
248
Location
Sweden
Probably the lowest parts count mid-eq with variable Q is the one used in the Audient ASP. A David Dearden design. It does not have complete independence of three parameters (for that, look at Barry Porters design). But it sounds good and works well enough.

Similar topologies have been used by Allen & Heath in their ML5000 console as well as Crest Audio in their V12 console.

You conceivably "graft" this modification on to your D&R strip using a small mezzanine PCB and changing out some components on the existing PCB but as Abbey pointed out, you'd also need to find space of the Q-pots.

Anyway, this is how David described it to me a while back:

In my initial drawings for the ASP8024, I intended to only have a semi parametric EQ, ie without a Q control. It was then decided that it really should have parametric mids. The problem was space as the channel pitch was intended to be only 35 mm. I needed the most space efficient method which could fit into the available space and a standard state variable circuit needed 3 opamp stages.

The inspiration for the 2 opamp parametric came from something that BSS were using for configuring their electronic crossovers. They had a filter design breadboard and design charts for various frequencies to allow the construction of various bandpass filter responses. The basic circuit was an architecture that I hadn't seen before and I thought it had promise.

After some thought and experimentation I managed to make it work as a fully parametric block. I have never seen the circuit block used or suggested anywhere else. It probably came out of some obscure filter design book! The only limitation I found at the time is being able to get very low Q. As far as I remember (it was 1997!) It was good down to a Q of around 0.4 to 0.5 which was fine. There was very little interaction between Q and boost. Oh, and space! I had to increase the channel pitch to 37mm to get the last track in! I've never even looked at the circuit since so it seems to have stood the test of time, audibly and technically. There are more than 1000 of those consoles around the world and nobody has complained yet!

it is basically a WEIN bridge topology which was in common use in all Soundcraft consoles and also in by various other manufacturers in the 80's and 90's. However, they were all fixed Q with the Q and amount of boost/cut heavily dependent. The normal situation was to set the boost/cut to around +/-15db by adjusting the ratio of the capacitors (which I think was around a 2:1 ratio) and accept that the Q would be around 1.3.

Both mine and JP's version have equal value capacitors. I wonder whether it was JP or Chas Brooke at BSS came up with the elegant way of achieving independence between boost/cut and Q by simultaneously changing the gain of the boost/cut invertor opamp and the Wien opamp with a single resistor?

Sadly, we will not be able to ask JP, but if I see Chas I will ask him. You have now set me on a quest (no pun intended) to see if I can find any other use of the circuit. I certainly used a switchable Q Wein bridge at DDA in the 80's and I seem to remember that Amek did as well, but not in as elegant a way as this. It involved a brute force balance of positive and negative feedback to switch the Q.


JP = John Petrucelli In Memoriam: John Petrucelli, 1952-2017 - ProSoundWeb
 

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