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

brianw brianw at audiobanshee.com
Sun Apr 9 01:24:01 CEST 2023


At first, I thought you were talking about the 2SC828RNZ, which is the noise source in the TR-808.

A very quick glance at the TR-909 hints that the noise source is a LFSR "digital" noise, and perhaps that digital output is too much for "small signal" voltages?

I couldn't find the 2SD1469 ... what's the part designation on the schematic?

Brian

On Apr 8, 2023, at 4:03 PM, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
> 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 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 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




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