THAT 202XL recreation

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ilya
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THAT 202XL recreation

Post by ilya » Wed Nov 15, 2017 8:47 am

I'm thinking of building an equivalent ov 202XL VCA for my compressor. I've studied the documents here: viewtopic.php?f=6&t=34

While circuits look simple enough, there're a couple of things I'd like to clarify. All circuits use 3300ppm tempco resistor. All tempco resistors in the range of 2700-3600ppm were made by Panasonic and are phased out. There are some high values tempcos available (1k and 2k), but here the value is 50-100R. Is there any recommendation regarding replacing this resistor?

Also, in the 2002T circuit there's a note regarding R2/R3 selection procedure. What would that procedure be?

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mediatechnology
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Re: THAT 202XL recreation

Post by mediatechnology » Wed Nov 15, 2017 12:50 pm

I think in through-hole your choices are pretty limited.
You might follow this link: viewtopic.php?f=12&t=658

In surface mount you at least have more options.
The driving market (I'm guessing) for SMT tempco resistors might be LED drivers and Cellular base station RF power amplifiers.
In SMT KOA comes to mind: http://www.koaspeer.com/products/thermal-protection/

I used to stock the 1K PT-146 and they're big. http://www.precisionresistor.com/PT146- ... sator.html

What I might suggest is that you find a tempco you can readily source as close to 3300 ppm either negative or positive TC and then design the Ec driver circuit around the resistor.

I don't know the answer to your final question about the resistor selection but my hunch is that the input resistor is selected to match the 5 or 10% tolerance of the temperature sensitive shunt R.

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JR.
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Re: THAT 202XL recreation

Post by JR. » Wed Nov 15, 2017 4:37 pm

ilya wrote:
Wed Nov 15, 2017 8:47 am
I'm thinking of building an equivalent ov 202XL VCA for my compressor. I've studied the documents here: viewtopic.php?f=6&t=34

While circuits look simple enough, there're a couple of things I'd like to clarify. All circuits use 3300ppm tempco resistor. All tempco resistors in the range of 2700-3600ppm were made by Panasonic and are phased out. There are some high values tempcos available (1k and 2k), but here the value is 50-100R. Is there any recommendation regarding replacing this resistor?

Also, in the 2002T circuit there's a note regarding R2/R3 selection procedure. What would that procedure be?
I hate always being the wet blanket but WHY??????? (maybe I don't hate it, or I would stop doing it).

I actually have a patent for a VCA (US04818951 roberts 1989), and stopped using my own after we (Peavey) could buy better ones from dbx....

The last generation IC VCA from THAT corp is superior to all the former stuff...

If you just want some distortion there may be easier ways to make that.

JR

PS: Tempco of resistors is probably about VCA gain tracking over temperature. May not be a big deal for small gain change range... or simple dynamics processing.

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mediatechnology
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Re: THAT 202XL recreation

Post by mediatechnology » Wed Nov 15, 2017 5:24 pm

There is a replacement need for failing VCAs with the 202 footprint in legacy consoles.

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JR.
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Re: THAT 202XL recreation

Post by JR. » Wed Nov 15, 2017 5:58 pm

mediatechnology wrote:
Wed Nov 15, 2017 5:24 pm
There is a replacement need for failing VCAs with the 202 footprint in legacy consoles.
It would make the world a better*** place if the new VCA could be integrated into previous applications.

JR

*** lower noise, lower distortion

ilya
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Re: THAT 202XL recreation

Post by ilya » Thu Nov 16, 2017 4:12 am

JR. wrote:
Wed Nov 15, 2017 4:37 pm

I hate always being the wet blanket but WHY??????? (maybe I don't hate it, or I would stop doing it).

I actually have a patent for a VCA (US04818951 roberts 1989), and stopped using my own after we (Peavey) could buy better ones from dbx....

The last generation IC VCA from THAT corp is superior to all the former stuff...

If you just want some distortion there may be easier ways to make that.
But of course I'm not going to make a discrete VCA, this is just insane! What I'm talking about is an array of 218x chips (4 or 8 in 2002 and 202XL respectively) with a couple of op-amps. Tempcos are giving me a hard time though....

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JR.
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Re: THAT 202XL recreation

Post by JR. » Thu Nov 16, 2017 11:02 am

OK I still would like to see new silicon...(better than 2180s too)

3300ppm Rs available in SMD but they are noted as end of life... so won't be available forever.

You can probably tweak tempco by mixing and matching values in series /parallel but seems like a lot of effort.

You can hand solder SMD resistors with care (I have before in prototypes).

JR

ilya
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Re: THAT 202XL recreation

Post by ilya » Fri Nov 24, 2017 10:20 am

Sent a letter to THAT and it turned out I don't need tempcos for 2181 any more because it's already there on the chip. Now, if I understand correctly, I just remove the tempco (R8 in the 202XT circuit). Do I still need the R7 (6R8)? Can't figure what its function is.

Everything else seems to be pretty straighforward.

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JR.
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Re: THAT 202XL recreation

Post by JR. » Fri Nov 24, 2017 10:52 am

I believe THAT published a white paper about upgrading old VCAs with newer ones... maybe check their website.

I would guess you still need the resistor, just doesn't need a special tempco.

JR

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mediatechnology
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Re: THAT 202XL recreation

Post by mediatechnology » Fri Nov 24, 2017 11:21 am

ilya wrote:
Fri Nov 24, 2017 10:20 am
Sent a letter to THAT and it turned out I don't need tempcos for 2181 any more because it's already there on the chip.
I don't think that's correct.
If you look at the 2181 datasheet page 2 table 3 it shows a +0.33% gain control tempco.
You still need -0.33% to correct it.

Also see equation 1 on page 5 of the 2181 datasheet for the delta-T term in the denominator: http://www.thatcorp.com/datashts/THAT_2 ... asheet.pdf

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