K5synth

nelsonj_sce a \ufffdcrit :
> Wow- how did you confirm the VCA's are the noise source? 
>   
By removing the opamps situated before the VCAs. They had no input, and 
still lots of noise.

Removing the opamps after the VCAs is useless since there is still a 
sound output without them, because there are bypass resistors still on 
the PCB.

I saw this when I (finally) replaced the opamps with OPA4228s.
> Did you replace them with anything else? What VCA's did you use as 
> replacements?
>   
Once I'll learn enough C or assembler to drive the VCAs I plan to use, 
I'll be able to replace the VCAs with what I want. I'm planning to use 
PGA2320s (http://focus.ti.com/docs/prod/folders/print/pga2320.html) 
which produce about no noise. The problem is that it needs a serial 
signal to adjust the volume, so I'd take a PIC12 as an ADC to send the 
slider's position to the VCAs.
> I was unable to get the spec sheet for the JRC chips, did you ever 
> find it?
>   
A while ago in fact. I think it's easily available now that placeslike 
datasheetarchive exist. The unit might be a JRC clone in the K5, I'm not 
sure, but the real thing is a Mitsubishi M5222L, in a SIP package.

Here's the datasheet: 
http://www.datasheetcatalog.com/datasheets_pdf/M/5/2/2/M5222.shtml
> I ordered a handful of discountinued VCAs for my OB-X (M3080???) and 
> you can still get them.  If you can figure out the pin configuration 
> of the current JCR in the K5 (8 pins linear as I recall)I will see if 
> they can be mapped to these other M3080 VCAs (that are 4 pins on each 
> side so it will require some wiring as the VCAs won't fit correctly).
>   
Or use a small PCB and single-row right-angle pin blocks.

I'm not sure if it will be easy to connect. IIRC, the volume control 
goes between -6v and 0v(But then, it's just a matter of connecting the 
volume control on the VCA's PCB instead of the synth board).
> --- In k5synth@yahoogroups.com, Antoine Desch\ufffdnes <antdes45@...> 
> wrote:
>   
>> Hi,
>>
>> I finally started to try a few mods to reduce the noise that the K5 
>> produces(that the 56k resistor mod doesn't fix) I can now confirm 
>>     
> that 
>   
>> the VCAs(Voltage Controlled Amplifiers, they get the position from 
>>     
> the 
>   
>> volume slider and amplify/reduce the sound accordingly) are the 
>>     
> main 
>   
>> source of noise. I should start working on an alternative sometime.
>>
>> It looks like I'm Burr-Brownizing the K5.
>>
>> Cheers,
>> Antoine
>>