Api opamp based VCA

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mono-poly
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Api opamp based VCA

Post by mono-poly » Thu Feb 25, 2010 12:07 pm

Would that be possible to make and would it be interesting soundwise?
To have a "high-end" vca like that with an api compatible opamp submodule users could choose to have placed on them?
Or am i just dreaming about some crap?

Example of sub modules

http://www.thelisteningsessions.com/ima ... top-lg.jpg
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Post by parasitk » Thu Feb 25, 2010 12:41 pm

:hyper: :hyper: :hyper:

I'm definitely into this idea.

On that note, I just received my SL-2520 "red dot" op amps today!

Image
:love:

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meridic
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Post by meridic » Thu Feb 25, 2010 12:56 pm

I have thought about such things over the years as well.

The thing is the vintage discrete opamps while good sound are so complete overkill for the situation, they are ment to drive low impedances and transformers. They are also bipolar on the input, so they have low input impedance as well, They are just not suited for modular synths.

But that does not mean we should not have discrete opamps and OTAs to play with, we just need to look at the early days of synths and not recordings for our inspiration, back when synths were made with transistors. The old synths often have plenty of examples of discrete OTAs and opamps hard wired into their circuits, they just need to be extracted and tweaked to work happily outside their intended environment.

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Post by mono-poly » Thu Feb 25, 2010 1:02 pm

Yeah i've been looking at the SL-2520 and the Forsell 992.
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Post by meridic » Thu Feb 25, 2010 1:03 pm

Whats the "SL Red dot" stand for?

I have been out of that community for awhile.

Now that I think of it there was a mono/poly over at prodigy pro forums land.

But that was long ago.

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Post by parasitk » Thu Feb 25, 2010 1:05 pm

meridic wrote:Whats the "SL Red dot" stand for?

I have been out of that community for awhile.
It's for an API 312 preamp "clone" I'm working on...

http://scottliebers.com/op_amps/op_amps_2520.htm
SL-2520 - Two versions of the SL-2520 were designed as replacement opamps for vintage console restoration. The vintage or “red dot” version is built to match the Huntington and Melville NY opamps from the 70’s.

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Post by meridic » Thu Feb 25, 2010 1:16 pm

Huntington and Melville NY opamps from the 70’s.
"new york opamps"? You say more colored as well, color + 2520 = melcor 1731. Is that what you are getting at here? I have never been a fan of the 2520 but I do love the melcor quite abit and have thought about the silly idea of putting some of those in a synth over the years.

I really do not know my discrete opamp triva though, I just know schematics and sounds.

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Post by paults » Thu Feb 25, 2010 1:20 pm

$4 in parts = $70 sell price. Nice work if you can get it :tu:

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Post by parasitk » Thu Feb 25, 2010 1:28 pm

meridic wrote:"new york opamps"? You say more colored as well, color + 2520 = melcor 1731. Is that what you are getting at here? I have never been a fan of the 2520 but I do love the melcor quite abit and have thought about the silly idea of putting some of those in a synth over the years.

I really do not know my discrete opamp triva though, I just know schematics and sounds.
I'm just a fan of the old API sound (and I do happen to like the 2520s), which is why I'm building the pre amps. I couldn't engage in a detailed conversation on the whys or hows. My ears like it - that's all the reason I need.

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Post by meridic » Thu Feb 25, 2010 1:42 pm

I sort of went on a tangent there.

Yes it could be done. but you would need to stick an OTA or other such gain control element in there if you want to use proper 2520 module. A diode ring between a pair would work, or a vactrol.

EDIT: Both the diode ring and the vactrol would want a higher impedance to follow them then the 2520 offers, so a differential FET pair would be good to keep things discrete and deal with the impedance. But you could slap a TL074 to do input summing buffer the gain control and CV summing if you want, the 2520 can certainly handle driving the output on its own.

But if we just took the 2520 circuit and tweaked it we could get a very nice circuit for the modular world. We do not need the big output transistors, modify the driver stage to be the new output stage instead. Throw a differential pair of FETs on the input and replace the resistor current source on the old input bipolar pair with a transistor so we can control the current and we have VC.
Last edited by meridic on Thu Feb 25, 2010 1:48 pm, edited 1 time in total.

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Post by meridic » Thu Feb 25, 2010 1:45 pm

I'm just a fan of the old API sound (and I do happen to like the 2520s), which is why I'm building the pre amps. I couldn't engage in a detailed conversation on the whys or hows. My ears like it - that's all the reason I need.
The melcor is the opamp that birthed the 2520. They are a great deal alike from the circuit standpoint, but the melcor has more color, imparts quite abit more of itself on the signal.

EDIT: I guess I do like the 2520, I just think that the melcor does everything that the 2520 does and does it better.

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Post by Peake » Thu Feb 25, 2010 1:57 pm

Oh dear, wasn't I just talking about this?
This is not the place I'd imagined it to be.

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Post by mono-poly » Thu Feb 25, 2010 2:02 pm

Mike i hope not about the $4 part thing.
Wonder what you think about this api/descrete thing!
The farad is the captain of my print.

http://www.mono-poly.nl
http://www.noodlebar.org

WTB blue lantern purple vco

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Post by meridic » Thu Feb 25, 2010 2:54 pm

To keep things 100% 2520 for the signal path you could use PWM control. A resistor shorted to ground through a VC switch. Pulse the switch at a very high rate, the slower the rate the higher the resistance becomes. But that does require a high speed pulse VCO.

Edit; It would not surprise me if the APIs took issue with the high speed pulsing though.

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Post by bananeurysm » Thu Feb 25, 2010 7:29 pm

hmm.. i also have some discrete 2520 pcbs i've stuffed for apis... you're giving me ideas now.

there are also a TON of drop in replacements out there.. as i'm sure yall are aware. (john hardy, JLM audio, etc)

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Post by Peake » Thu Feb 25, 2010 9:23 pm

Yes, I was mentioning it in the "IC Upgrades...?" thread, or at least in the locked version.
This is not the place I'd imagined it to be.

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Post by Peake » Fri Feb 26, 2010 5:23 pm

I've also been talking about using audio transformers in synths, so when the first modules show up using these or some combination of these... :yay:
This is not the place I'd imagined it to be.

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