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Api opamp based VCA
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Api opamp based VCA
mono-poly
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/images/AP-1top-lg.jpg
parasitk
hyper hyper hyper

I'm definitely into this idea.

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


love
meridic
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.
mono-poly
Yeah i've been looking at the SL-2520 and the Forsell 992.
meridic
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.
parasitk
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

Quote:
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.
meridic
Quote:
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.
paults
$4 in parts = $70 sell price. Nice work if you can get it thumbs up
parasitk
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.
meridic
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.
meridic
Quote:
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.
Peake
Oh dear, wasn't I just talking about this?
mono-poly
Mike i hope not about the $4 part thing.
Wonder what you think about this api/descrete thing!
meridic
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.
bananeurysm
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)
Peake
Yes, I was mentioning it in the "IC Upgrades...?" thread, or at least in the locked version.
Peake
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... applause
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