200B Summing Mod

Hey friends!

Finally getting around to thinking about working on this 200b we picked up for cheap a while back. Thinking about rewiring it for a strictly summing box. Thinking of bypassing preamp and popping in a THAT1512 balanced receiver.  Would love to get some thoughts on this. I've attached a schematic for looking at. Seems pretty straightforward. Am I missing anything? Eventually I'd like to upgrade the summing section with some CAPI stuff, but that's a little more pricey. Ha ha.

I wouldn't mind looking into a transformer receiver instead. I'm a little less familiar with how to implement it, though.  I guess a transformer followed by a preamp to match impedance and maybe some makeup gain?

What's your reason for doing this?

I replaced the input stage in my 200b with a 4:1 line transformer and a 5532 based input stage. My reason was that I like the sound, CMMR and immunity to hum the transformer provides.

living sounds said:

What's your reason for doing this?

I replaced the input stage in my 200b with a 4:1 line transformer and a 5532 based input stage. My reason was that I like the sound, CMMR and immunity to hum the transformer provides.

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Well, I'm not really interested in redoing the pre-amps, because we have plans to get some other pres. But we don't have anything OTB, so being able to sum would be a great start to getting out. I've been summing through it already, and it sounds decent. Just looking to improve it.

Out of curiosity, where did you end up mounting your transformers?  And how would you describe the sound of your mod, because it does sound interesting!

I don't mean to sound picky but your schematic shows only replacing the mic pre with a THAT chip which make this a line level mixer rather than a summer. Summers don't usually have EQ. With your mod you are still going through four TL072 op amps the last one of which is severely overloaded by the combined load of the four 10K pots, the pan pot pull ups and the bus feed resistors.

Cheers

Ian

You could use a transformer, the secondary connected between ground and the non-inverting input of IC1a, and removing all the rest of the preamp.
You could also use a 1240, its output would be connected to your X point.
I don't think there's much to gain by replacing the input stage, though.
The weak point on the 200B is the bus structure, which causes longitudinal noise to develop; there's not much you can do, it's just the way ribbon cable acts. However, it's tolerable on smaller versions (up to 16 ch). If you have it part-filled, load the input modules close to the groups. A producer's desk in the middle is not a good idea.

abbey road d enfer said:

The weak point on the 200

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Can you explain that a little more?

Cheers

Ian

ruffrecords said:

Summers don't usually have EQ. With your mod you are still going through four TL072 op amps the last one of which is severely overloaded by the combined load of the four 10K pots, the pan pot pull ups and the bus feed resistors.

Cheers

Ian

Click to expand...

You're not being picky at all! This is a great point. And, come to think of it, I'm not particularly impressed with the EQ on the board anyway. Maybe I should bypass that as well, or put in a switch.  Or rebuild the EQ, but I don't think that's something I want to tackle right now.

abbey road d enfer said:

You could use a transformer, the secondary connected between ground and the non-inverting input of IC1a, and removing all the rest of the preamp.
You could also use a 1240, its output would be connected to your X point.
I don't think there's much to gain by replacing the input stage, though.

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Great input! Are you saying that I won't see much of a tonal improvement by replacing the stock ICs with the THATs or by introducing an input transformer?  The bus noise hasn't been an issue for me. It's a 24 channel.

ruffrecords said:

Can you explain that a little more?

Cheers

Ian

Click to expand...

IIRC, the 200B's reference bus is three connections of the ribbon cable, which, due to the modular structure is actually very long, about 3-4 times more than the actual console span. It results in a non negligeable resistance. Since there is no separate mix reference bus, all circulating currents are fed into the summing amps. Actually, many other mixers had the same flaw, though now the tendancy of going non-modular reduces this problem.

abbey road d enfer said:

IIRC, the 200B's reference bus is three connections of the ribbon cable, which, due to the modular structure is actually very long, about 3-4 times more than the actual console span. It results in a non negligeable resistance. Since there is no separate mix reference bus, all circulating currents are fed into the summing amps. Actually, many other mixers had the same flaw, though now the tendancy of going non-modular reduces this problem.

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OK, thanks. So to summarise, the use of ribbon cable plus the extended length of the buses plus no reference bus is the primary cause.

Cheers

Ian

ruffrecords said:

So to summarise, the use of ribbon cable plus the extended length of the buses plus no reference bus is the primary cause.

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Yes; the small frame (8ch) were pretty good though.

awesomealex9 said:

Well, I'm not really interested in redoing the pre-amps, because we have plans to get some other pres. But we don't have anything OTB, so being able to sum would be a great start to getting out. I've been summing through it already, and it sounds decent. Just looking to improve it.

Out of curiosity, where did you end up mounting your transformers?  And how would you describe the sound of your mod, because it does sound interesting!

Click to expand...

I have mounted the transformers on the inside of the backpanel PCB. This required unmountaing the backpanel and unsoldering lot's of resistors there.

It sounds like the tranformers sound. I used old Belclere line inputs from a BBC console. The transformer is followed by a passive low pass filter and then amplified via a 5532, gain can be set via the input gain pot. There is never any hum. I had a dislike for the way the consoles DC servo sounded.

You can replace the op amps with lower noise/distortion ones. Ian is correct about the TL072. I have replaced this with a 5532 as well. To make it work I would recommend the Soundcraft 6000's fader amp circuit (schematic attached). There's an added cap after the fader (you can solder it directly to the fader), and a change to the two resistors setting the gain of the op amp. And there's the 75 ohm resistor for added stability.

I don't know if it's all worth it. I made lot's of changes to my console, grounding, 2-ACA, decoupling, op amps, a discrete PSU and whatnot. I still have not arrived at a final perfect setting.

Maybe the ribbon cables are to blame? What kind of noise are we talking about? I was able to eliminate most of the hum via a star ground mod with thick speaker wire going to a bus bar from every modules output.

awesomealex9 said:

You're not being picky at all! This is a great point. And, come to think of it, I'm not particularly impressed with the EQ on the board anyway. Maybe I should bypass that as well, or put in a switch.  Or rebuild the EQ, but I don't think that's something I want to tackle right now.

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I think there is already a hard bypass switch so easy to leave it out. My main worry is the last TL072 driving the buses. it is overworked and will probably negate all the sonic loveliness introduce by the THAT chip.

Cheers

Ian

living sounds said:

I was able to eliminate most of the hum via a star ground mod with thick speaker wire going to a bus bar from every modules output.

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Yes, that's the brute force solution to this problem. A more refined solution would run a clean audio ground separate for the summing bus reference. Very extensive mod.