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+  MCI Console and Tape Machine Forum
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| |-+  JH-600 Series Consoles (Moderator: Westrek)
| | |-+  Series 600 EQ types
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Author Topic: Series 600 EQ types  (Read 3767 times)
TonesOnTail
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« Reply #15 on: July 05, 2016, 08:11:13 PM »

Update:

THESE FILTERS ARE WONDERFUL, and smooth like butter! All of the adjectives that normally would be used to describe the 600 eq are no longer true, I.E. Grainy, Harsh, Phase-shift. They honestly now sound more like an LC based EQ, less dramatic, natural, etc.

A few things to note:

The Salen-Key HPF and LPF did not work as I intended. So as a test, I wired the Dual 50k Log pot for the HPF so that the first pot was being fed on one side from Pin 11 of the EQ header, the wiper feeding Pin 2, the other side to audio ground, and the second pot with one side fed from Pin 10, and the wiper and other side hitting audio ground(Look at the 4030 IO drawing). Low and behold, I had a sweepable HPF!. That said, I think a linear or rev log would be better suited for this purpose, as most of the mojo happened at the top end of the pot.
After the success of this test, I decided that for the purpose of the 4030, perhaps the Harrison-style HPF and LPF are unnecessary. That said, for the LPF I think I will implement an ON-OFF-ON DPDT toggle instead, keeping the 14 khz filter on the channel strip, and adding a pair of caps in series with pin 12 and 13 giving a second cut off frequency of maybe 10khz or 8khz. I find the LPF less necessary to be sweepable, anyway.

Mind you, I was not viewing the output on an FFT analyzer, so I have no clue what was actually happening with either the 3 band, or the modified HPF. That said, I can say this:
  • The Frequency range is much wider than the MCI for all 3 bands
  • The Q sweep is very wide, and an excellent tool on the High and Low bands. The final design will include a Peak/Shelf switch on the top and bottom bands. My tests with the jumpers proved that it would be a worthwhile feature.
  • It is far more transparent than the MCI EQ, but I feel like the max amount of Boost/Cut is less, probably more in the range of +/-8db, instead of +/-14db. This may require modification to the Gain pots, if this is deemed to not be enough.
  • I will preform some test and share the graphs.

BTW, I am far more interested in the idea of a group buy on PCBs and parts for anyone else interested in this, than turning a profit. If we bought enough parts, we could turn these out pretty cheap... I realize that this is not a 1 size fits all solution, but now that the ground work is laid, it wouldn't be all that difficult to roll different versions(0136 modules, standard EQ, etc). I just started with the Vari-Q 4030 version, because that is what I have in my console.

Also, I am fully open to suggestions for changes, and errata.
« Last Edit: July 05, 2016, 08:13:39 PM by TonesOnTail » Logged

'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 #16 on: July 06, 2016, 11:05:24 AM »

There are some redraws out there of the Ha**ison filters that are wrong by the way!!

Yes, you will have a much wider frequency range if you have just used the Sontec/GML values. Using dual 50K pots and tweaking the tuning caps will resolve this. Again, some experimentation.

Ideally, the Hipass variable should be 100K/50K dual rev audio to keep the Butterworth slope correct. The variable Lopass would still be dual 50K.

Also, I would jump C1003, C1102, 1103 on the main board. These are the 22µF caps in the pass amps.
AC coupling caps were put in with scant regard, just to be safe; they were not always needed.
You want (need?) the 2 on the input to the phase buffer. (First half of IC1000)

You can still get LM4562 chips, which are the same as LME49720.

If you find Barry Porter's "NetEQ", this is a good example of the feed forward design he likes.
I think the theory is it's quieter in that the tuned circuit is at full level and you are steering the amount you want into the pass amps, vs the conventional one that has the tuned opamp at full blast and you steer small or large levels into that. ie the tuned op amp is hanging on the pass input all the time waiting for an input (in the conventional design), whereas in the feed forward version you have control over the background noise being fed into the pass amps.

Either way, you have confirmed that it is the tuned filters themselves that are the culprit; they always were a bit, "my first EQ".
PC



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Rob
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« Reply #17 on: July 06, 2016, 12:08:56 PM »

Barry Porters Net eq sounds peachy !
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Harrison
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« Reply #18 on: July 06, 2016, 04:03:33 PM »

It's only when you start all this, you realise how poor the 600 EQ is. The hi and lo bands are very short of range.

Using the Sontec schematic, please see the attached table for values. I've used GML cap values.

f = 1/2πRC where C = √C27 x C28

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TonesOnTail
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« Reply #19 on: July 06, 2016, 04:57:35 PM »

Thanks for the chart. Those are about the values I used, except with R73 and R74 also being 1K6 Ohm. The pots definitely need to be REV LOG though. Very little resolution at the high range otherwise.

I did some more playing around this morning. I was able to increase the amount of Boost/Cut by lowering the value of what would be R61 on the Sontec schematic I will have to re-match R62 as well though, in order to make the Q peaks more even again. This brought me to around +-12db change in gain on average. Still sounds wonderful.

One problem I am encountering is self oscillation in the low band when I have gain pot turned full CCW. Bugs to work out.

Charts to come....
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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 #20 on: July 07, 2016, 08:15:39 AM »

R61 is the Sontec equivalent of the final feed resistor to the EQ in/out switch feeding the ±pots on the main board. (Rs 1, 4, 9 on the variQ board). Yes, you would have to tweak these to get the correct ±dB value.
Some people might suggest that ±15dB would be better, but each to his own.
The rev audio pots were a given. I think the Q pot is normal audio, BTW.

Yes, some designers have suggested that the twinT topology can hoot like a dog.

At the top end, the Sontec is 3K3 for R73/74, so the values are not cast in stone, and can be changed to suit the application. The use of 100K pots, for example, is that an outboard specialist EQ would need to be as flexible as possible; we are trying to make a good stock EQ (!?)
I did the calculation on 50K to slightly restrict the range of each band and also that the pots are the same, for a better price.
If you want the MCI ranges, then you are looking at 2 off 25K duals (600 uses 5K) and 1 off 50K or 100K dual, but part of the 600 problem is that the ranges are poor.
The Har**son 24 series was something like 25-800, 300-8K, 800-16K. The 32 series had an additional lower mid of 150-4K. AND the 24 series had a decent hi pass.
There is nothing wrong with the JH500A/B hi pass by the way, the C is inaudible; this was covered some time ago but now lost.

I think the Hi/Lo filters require a track cut on the main board to expedite an insert point; this is pretty much how everyone else does it. I'll detail this tonight. (I know this because I was working on an SVF version of the project and had some of the board designed already).
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Harrison
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« Reply #21 on: July 07, 2016, 09:49:35 AM »

"I think the Q pot is normal audio, BTW."

No it's not. It's linear.
PC
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Rob
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« Reply #22 on: July 10, 2016, 05:55:57 AM »

"Some people might suggest that ±15dB would be better, but each to his own."

The Trident 65 has +&-15dB, but I found the extreme boost available not very useable. The top end could kill an elephant at 100 paces.   When you sweep the eq with a lot of boost it sounds more like a wha wha pedal.....
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TonesOnTail
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« Reply #23 on: July 10, 2016, 07:08:50 PM »

The Trident 65 has +&-15dB, but I found the extreme boost available not very useable. The top end could kill an elephant at 100 paces.   When you sweep the eq with a lot of boost it sounds more like a wha wha pedal.....

I agree, and think that less is probably more in the case. Enough boost/cut to make the high Q mode surgical, but with the ability to have tighter resolution. I was thinking somewhere between +/-8db or +/-10, especially since you only get 5 steps in either direction...
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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 #24 on: July 13, 2016, 03:44:36 PM »

Rob,
You won't know this, but MCI had the bright idea for the later 600 series modules to have detented pots so it felt like switches.
But they weren't like the Soundcraft/DDA Sfernice 21 clicks, they were ± 5 clicks. Horrible.
Are the frequency pots detented as well anyone? I think they are, possibly totally defeating the purpose of pots.
Yes, pan and ±level should be centre-detented, after that, forget it.
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TonesOnTail
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« Reply #25 on: July 13, 2016, 09:11:49 PM »

Are the frequency pots detented as well anyone? I think they are, possibly totally defeating the purpose of pots.

They sure are! 11 positions just like the gain pots, in the entire EQ section.

On the other hand the 4030 modules also have center detented pan pots for two mix, send 3-4, send 5-6, and bus assignment.
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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 #26 on: July 14, 2016, 09:14:29 AM »

Yes.......................whoever made that decision?  So you already have a questionable EQ, which you then make PDC to a lot worse. Considering that the mid band has such a wide range 250Hz -10KHz, and now you put that into 11 position switch mode. Yuk! And the pots were more expensive!

The younger kids at MCI who did the MXP3000 put all of this right. There was choice of VCAs, choice of drop-in EQs. Admittedly, they used Alps little switches like Soundcraft/DDA which limited the lifespan of the console, but it did sound good. AND it had the best LM meter package going. (until the SSL Avant).
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MM1100
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« Reply #27 on: July 05, 2018, 11:11:25 PM »

Coming in two years later! Did we ever get to an updated EQ mod that can be replicated for DIY?
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TonesOnTail
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« Reply #28 on: July 06, 2018, 12:46:05 PM »

I made some prototype boards based on some fine and much appreciated correspondence with Harrison off forum. It was something like a twin-T Barry Porter hybrid with 3 bands of fully parametric EQ utilizing the feed forward summing amps and onboard gain pots from the stock 4030 revision channel strip. It worked well and sounded smooth, except at the extremes of the boost/cut ranges where they started to hoot and feedback pretty badly. I had worked on some design changes to tame the oscillations of the filter amps, but had not gotten to running a second prototype run of PCBs.

Harrison had suggested bypassing the entire EQ section if the main modules, and replacing the EQ summing amps and pots with a more common feedback topology. This would more than likely be the way to go, but would add further to the cost and modification per channel. Things like CT pots, multi deck pots with switches, etc, are mucho $$.  This quickly goes into the territory of exceeding the market value of 600 series channel strips. Even the first prototypes I made were around $100 usd per channel, using PET caps, sealed pots, etc.
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'75 MCI JH16-24 w/ JH114 xport ----- '84 Sony/MCI JH110C-HP 1/4" 2trk ----- '82 MCI JH636-VU-AF
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