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Author Topic: Series 600 EQ types  (Read 6319 times)
Berwyn636
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« on: April 13, 2016, 03:29:58 AM »

I posted this before we lost the archive here so I'm reposting as i can never find photos of the Troisi EQ. Here's all three types together for comparison.
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Harrison
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« Reply #1 on: May 04, 2016, 12:13:47 PM »

Whilst stating the obvious, these are not MCI EQS.
They are a custom 3 band fully parametric on all bands with pk/shelving on the hi and lo bands.
It looks like the lower dual concentric is the frequency and the upper is the Q.
They will have designed their own board to replace the MCI one and had the overlays made.

There are 2 ways of getting this type of EQ. State variable (or Bi-Quad; not the same thing) and the way ITI/Sontec do it, the name of which I can't remember.

If you've got some quad chips on the daughter board, it's probably state variable. It's not that difficult to do actually, once you've understood how the original MCI 600 EQ worked. Personally, I always thought the 600 console was really good until you put the EQ in, then the wheels came off.
The console is dead quiet, as clean as hell. 

You could take more pictures and let us all see, of course!
PC
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Berwyn636
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« Reply #2 on: May 07, 2016, 11:50:45 PM »

Done!

They seem to all be the dual op amp type 5532 so maybe they're in pairs to create the quad you were talking about?

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Harrison
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« Reply #3 on: May 13, 2016, 01:08:46 PM »

Thank you for the pictures.
I can see what he's done and it's not a bad solution and it meant that he could test the EQ independently of the module. (other than needing most of the MCI daughter board for the chips and connector).
PC
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Harrison
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« Reply #4 on: May 14, 2016, 04:17:38 PM »

The Troisi Eqs.
The Troisi daughter board replaces the MCI EQ completely as a stand alone. It does not appear to use the pass amps IC1000/1100.
The curves appear to be non-reciprocal, hence the ∞ mark at the most anti-clockwise level position. Most console Eqs are not like this. (You have non-recip Eqs as outboards normally; dBx 900 series, Valley People, and most notably the Orban 622(?) parametric).
The original ITI and Sontec outboards were not. The Orban made a big point of this notch capability, but it sounded like c**p.

All the pots have been replaced, but the original MCI boards appear to have been donors to the cause, hence the connector and the XR chips.  I would suggest that the original modules had the enhanced vari eq option in the first place. The only knob bits to be purchased therefore would be the lower of the dual concentrics.
You could do something like this to the standard module, but will need a jig and very accurate drilling. You could also do a dual concentric for the filters, thus both variable.
PC
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TonesOnTail
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« Reply #5 on: May 17, 2016, 03:34:49 PM »

This be very interesting....

It looks like the Trosi is very close to the Sontec 250, with some extra bits and bobs. If doing an EQ replacement, I think that this would probably be the best option. I bet you could cram 4 band, and sweepable HPF and LPF on there.... The only issue I see here is sourcing the CT 50k gain pots(as used in the TROISI as well),  and the dual concentric pots for Q and Freq. Schemo attached.

You could use 1 dual IC for each band, and one dual for the summing DC Servo. I would go around IC 1100, and use IC 1000 as usual for the PI and Filter section.
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Harrison
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« Reply #6 on: May 21, 2016, 07:41:27 PM »

Identify the problem, then fix problem. This also requires cost benefit analysis. ie Can I afford the 3 band fully parametric with variable Hi/Lo pass filters and a lot of metalwork needed? Do I need this on all channels?

The EQ sidechains and the Hi/Lo filters are the problem, so replace them with good ones.
The easiest fix on a stock EQ console is top and bottom shelf/peak, possibly with a mid having optional wide/narrow bell. Good (ie audible) Hi/Lo filters at say 50Hz and 12Khz (the API 550A is 50 and 15K on one switch).
If you want to get into metalwork, then variable Hi pass at least.
You now have 2 or 3 dual RA pots with pull switch and 2 or 1 without switch for mid and hi pass filter. Not wildly expensive. There are some IC1000/1100 pass amp mods, but not much.

The moment you decide on 3 band fully parametric, you are into dual concentrics having dual RA tracks for frequency and single audio taper for bandwidth, with pull switch for PK on a stock console on top and bottom. (And you’ve got to get the lower knobs for the dual concentrics; you may not be able to buy them in the quantities you want. Ritel knobs.). The cost is escalating.
Troisi started with the enhanced eqs so the metalwork was not an issue, just the new overlay.

So, dear Berwyn636, what do you want?!
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Harrison
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« Reply #7 on: May 22, 2016, 11:43:56 AM »

EQ Theory as applied to the JH600.

There is an excellent series of articles by Ben Duncan in old Studio Sound that traces the history of EQ in consoles, by the way, but this here will stick to the JH600 discussion.

The usual topology of a console EQ is:-
see 600series jpg

There is a series of inverter pass amps, a pot across each one so there is phase and anti-phase at either end of the ± pot, and you feed levels of either of these signals into a sidechain tunable circuit and feed the result back into that pass amp, thus adding or subtracting the relevant frequency (ies).
Most consoles have the pass amps in series, but sometimes, as on the 600, the first pass amp does both top and bottom with the second doing the mid. There is nothing wrong with this; the API 550A is the same, as is the JH400.

The JH600, however, has a different topology; called “feed forward”:-
see 600ff jpg

Here, the signal is fed at full level into a tunable circuit and the ± level pot steers that tuned signal into the next 2 pass amps. Notice how the level pot is not connected to any pass amp outputs; it only feeds inputs.
Now, given that surely you can’t mess up a series of unity gain inverting op amps, IC1000/1100 (although I would jump the 22µF coupling caps), the problem is plainly the tunable filters themselves, and it’s these that are on the daughter board and most easily replaced, this being done with any of the usual suspects, Wein, Twin-T or State Variable.


The Hi/Lo pass filters are just a mess and need to be fully replaced with correct Butterworth topology. It’s this that requires some track cuts and links on the main channel board to effect a filter insert point. (After the phase buffer).
With the stock EQ module, realistically, the only option is a better version of the 3 band EQ with decent switchable filters. Shelf/peak on top and bottom, possibly wide/narrow bell on mid. You could put fully parametric on all 3 with shelf/peak on top and bottom, but the cost is going to escalate. All of this is without any metalwork being done; the parametric will require an overlay.
If you have a vari-EQ module to start with, then 3 band fully parametric with adjustable Hi and Lo pass is possible, because the holes are already there. But this will get expensive requiring 4 dual concentric pots. 3 of one type (dual RA with single A and concentric), the filters being a double dual RA concentric.

The choice also becomes to continue to use the feed forward topology or to modify the ± level pots vs pass amps to be the more conventional series with sidechain. Any of this is possible.
PC
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Harrison
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« Reply #8 on: May 22, 2016, 11:46:48 AM »

Probably better if you could actually see the pictures!!
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Luddite
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« Reply #9 on: May 22, 2016, 12:13:57 PM »

This is an interesting discussion.  If you have a proposed schematic for an improved daughterboard, I'd take a crack at manufacturing a batch.
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Harrison
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« Reply #10 on: May 22, 2016, 01:52:00 PM »

As I have alluded to, it needs a definitive specification.
Yes, I have some circuits in mind based on what I would do if it were me.
Replacing the stock EQ with Wein based EQ from a "direct competitor" ie known good working circuits.
Shelf/peak top and bottom, wide/narrow mid, fixed switched Hi/Lo pass (50hz/12Khz).
Possibly variable Hi pass.

I'm at the stage of PCB layouts, but I need to go and measure accurately a module or two and adjust accordingly. I'm in London, (well just South West of) and my only known contact is in Glasgow. Ringo Starr's console used to be nearby, but I don't know what happened to that.
You could use the Ne***k El*** circuits for the same thing, they are SV filters.
If it were me, I would drop the feed forward stuff and go back to conventional series pass amps. This involves removing the 10K DIL resistor arrays and replacing where needed 1/8W resistors, and track cuts and links. The point being that since the 22­­µF caps are coming out, you don't need the 10K holding to ground resistor on the inverting leg of the pass amp, because you will return the sidechain into the non-inverting input.

I have to say that I obviously want to sell the mod, but not to live on so to speak. But most of this is the fun of doing it.
The other circuits that will do the job are the Q8 Westar consoles. This was the next console on for Tom Hay, who was instrumental on the JH600. But he got the circuits right by then!!

Replacing the enhanced EQ would be very similar to the Troisi EQ, but I would have reciprocal curves, not the asymmetric ones he's done. Definitely state variable, shelf/peak again top and bottom.
PC

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Harrison
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« Reply #11 on: May 22, 2016, 02:20:31 PM »

Some other thoughts.
I would not bother with the slide in connector that MCI use.It's too large and in the wrong place for the new daughter board. This would be removed from the main board.
I would connectorise the new board but the audio and relay supplies would be on a 5 way .156" molex and the audio on a 9-way D or 10-way ribbon connector coming from the "slide" connector points on the main channel board. The point being that you could fault find on the daughter board because it would still be connected but hanging loose. Nothing wrong with this....Neve consoles are full of ribbon connectors!
Relay supply because the pull switch on the frequency pot will activate the relay for the shelf/peak switching. ie the EQ would be normally shelf with peak the option, but it needs a 2-pole switch.
The vari-EQ replacement would not need pull switches and relays; the switch holes are already there.

No quad chips, you can't get the ones you want anymore. Dual chips only, MC33078, NE5532, LM 4562. The best ones, LME49720, have gone in DIP packets.
Do do the whole thing in SMD? or SMD chips, then you can go quad chips and use the LME49740. (if you are doing state variable)

Again, the easy way is to keep the feed forward and re do the sidechain board, but hardly anyone uses this feed forward configuration.
PC
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Luddite
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« Reply #12 on: May 23, 2016, 09:35:15 AM »

SMD would seem like the way to go for this.  I've come around to the point where I prefer designing that way.  And I'm sure Jeep would have used them.
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TonesOnTail
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« Reply #13 on: July 02, 2016, 03:17:29 PM »

OKAY! After a couple of months of planning, my first EQ card prototype is ready to install(Too bad I won't be able to test it until Monday  Undecided).

This is 3 bands Twin-T fully parametric filtering ala Sontec/GML with sweepable Harrison 32c hpf and lpf. It uses ordinary Dual DIP-8 op-amps, and I decided to roll with LME49720's for the test, but could just as easily be NE5532 or OPA2134 of OPA2604. I wanted to be the least intrusive to the actual channels strip as possible, so here are a few things to note:

This is designed for 4030 modules with Vari-Q metalwork. You could drill up your standard modules, but I would not suggest it.

The HPF and LPF get inserted in place of C1100. I left the MCI HPF and LPF alone, but in the "OFF" state. If you look at the 4030 schematic, you will see that the HPF is always ON, even when off, but at a non audible frequency(.3387hz). If this turns out to be a problem, I will jump in before it. After C1100, it is business as usual on the main board.

It uses the original header, so that once again, I cause the least damage to the main board.

On the prototype board, the HF and LF are wired for shelf, but I left a jumper to test the peaking function. I didn't want to invest a bunch of $$ into fancy pots for a test. The final version will have a peak/shelf switch, if it is deemed necessary.

The tantalums... The original 250A schematic calls for them, and I had them laying around, so I figured I would try them. They will most likely become Panasonic FC in the final version.

The WIMA's are all MKS2 Metalized Poly. I wanted to keep things are small, and tight as possible, to try and offset the heat and cramping of this area of the channel strip.

I will post an update in a couple of days....

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'75 MCI JH16-24 w/ JH114 xport ----- '84 Sony/MCI JH110C-HP 1/4" 2trk ----- '82 MCI JH636-VU-AF
Harrison
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« Reply #14 on: July 05, 2016, 11:49:58 AM »

Nice one.
Did you bother with shelf/peak. And using the MCI connector as a flying one is a good idea.
The early production modules do need the Hi/Lo filter rebuilt, though. They are inaudible.

But it looks good.
PC
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