[sdiy] what opamp for "warmth"? (I know, I know...)

Richie Burnett rburnett at richieburnett.co.uk
Sun Apr 9 01:03:41 CEST 2023


Strangely Roland chose to use a different transistor for the VCAs processing the white noise part of the snare sound compared to the small-signal NPNs used everywhere else in the TR-909 for the other drum and cymbal VCAs. I'd love to understand why they did this!?

The part they chose for the snare VCAs is the 2sd1469 which is a more beefy NPN switching transistor capable of handling up to an amp of collector current. Looking at the datasheet it looks like a strange choice! There must be something unique about it to warrant introducing another part into the BOM.

-Richie.

---- René Schmitz wrote ----

>On 07.04.2023 23:06, Tom Wiltshire wrote:
>> For synth pads, the envelope would be much slower, so the control feedthrough wouldn't be so obvious. I'm not saying it wouldn't *exist* but a slow rise of 100msecs+ in DC level is a lot easier to get away with than a sharp hit!
>
>Indeed.
>
>If you look at the JX3P schematic you can see that the control voltage 
>is filtered, with 2* 100k / 22n. This reduces the speed of the 
>envelopes. Similarly done in the 909.
>
>> Tom
>>
>>> On 7 Apr 2023, at 19:24, brianw <brianw at audiobanshee.com> wrote:
>>>
>>> Speaking of control feedthrough, there was a paper written about modeling the TR-808 where it was mentioned that the "pop" or "click" from the CV was beneficial for percussive sounds. That wouldn't be so good for synth pads!
>>>
>>> Brian
>>>
>>>
>>> On Apr 7, 2023, at 4:51 AM, René Schmitz <synth at schmitzbits.de> wrote:
>>>> You'll find the same VCA in the 909 in a few places. (Bassdrum , Toms, Clap..)
>>>>
>>>> Note how far the signal is divided down.
>>>>
>>>> It's noisy, and nonlinear, and has some control feedthrough. Show what you can get away with for a single monophonic signal.
>>>>
>>>> The MS10 had the single transistor, the early "Korg35" MS20 had the same.
>>>> The later MS20 had a second transistor, which makes things more linear/symmetrical and you can run larger amplitudes -> less noise.
>>>>
>>>> According to the schematics the transistors shall be selected. jx3p also says "SEL".
>>>>
>>>> Anyone would know what for? Equal gain?
>>>>
>>>>
>>>> Best,
>>>> René
>>>>
>>>> On 07.04.2023 11:04, Adam (synthDIY) wrote:
>>>>> David G Dixon via Synth-diy pisze:
>>>>>>>>> Whatever the VCA is in the Korg MS-20 really warms up the sound of that synthesizer.  You can find the schematic on the internet.  I have something redrawn by a fellow named Marjan Urekar that I looked at years ago.  It involves two transistors.  It is evidently also the VCA used in the Roland JX-3P.
>>>>>
>>>>> Interesting.... here's the JX3P version
>>>>>
>>>>> <grab 2023-04-07 at 6.57.54 pm.png>
>>>
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>
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