[sdiy] Korg Monotron... Korg 35 chip?

[Richard Atkinson]

Hajimemashite Sam-san,

Maybe a better thing to do is to add in an effect loop after the low pass 
filter and construct a separate Korg35 style high pass filter using a few 
transistors and a few resistors. That would give a LPF into HPF style 
architecture rather than the HPF into LPF architecture of the MS20, but 
there may be some advantages to this approach.

The Yamaha SY-1, SY-2, CSY-1, CSY-2A and the solo manual of the D85 organ 
all use LPF into HPF, whereas the CS-50 / 60 / 80 and Korg 700S, MS20 use 
HPF into LPF. Non-linear filters such as the Korg35 and diode bridge 
designs are not commutative; i.e. they give a different result depending on 
which way round they are connected.

Richard ATKINSON
Cambridge, UK

On Jul 4 2010, HOSHUYAMA wrote:

>Hello list,
>
>> > Would that also mean that I could use one Monotron as 
>> > highpass, and one as lowpass?
>> 
>> Good question - so far I have not seen a mod to reconfigure a Monotron 
>> for the highpass filter (which may mean it is difficult). Can't wait to 
>> get my hands on one to see how do-able that is! But note that the 
>> Monotron isn't using the Korg35 per se, just the same arrangement of 
>> transistors that are in the Korg35 (as far as I can tell from the 
>> various photos of it around the web).
>
>I already succeeded at adding a high pass input to the monotron filter
>in an experiment.  But, it is difficult to change the signal pass on 
>the PCB with tiny SMDs.  Actually, I broke a PCB and had tough time 
>to recover.  So I did not use high pass configuration in my modification.
>
>Additional info for those who are thinking of modifying a monotron.
>Correcting my English is welcome.
>
> - DC-DC converter is used. Stable 5V is generated from 3V or less. - A 
> bias voltage 1.35V generated by an opamp from 5V is used for virtual 
> grounding. - 74HC14 is used for VCO and VCLFO. - The left LM324 is for 
> gate generator, VCO CV mixer, and exponential converter. - The right 
> LM324 is for VCF CV mixer, VCF signal mixer/attenuator, and VCF buffer. - 
> 6-pin chip beside the left 324 is a pair transistor for VCO. - Large 
> 3-pin SMDs are N-channel JFETs. - Small 3-pin SMDs are bipolar 
> transistors. - SMD capacitor values can not be seen. (tiny brown SMDs) - 
> Resistor values are easy to read. (tiny black SMDs) - A 3.5mm jack has 4 
> terminals: chip, 1st ring, 2nd ring, and 3rd ring (ground).
>   In the external input jack, 2nd ring is used to turn off the gate 
> signal to VCF.
>   In the headphone output jack, 2nd ring is used to control the power 
> amp IC to
>   mute the loudspeaker.
>
>Best,
>
>Sam HOSHUYAMA
>Saitama, JAPAN
>
>"Tim Stinchcombe" wrote:
>> >  Hey all, has anyone else noticed the Korg Monotron?
>> 
>> Reckon so...
>> 
>> A couple of recent threads at Muffwiggler:
>> 
>>  
>>  
>> <http://www.muffwiggler.com/forum/viewtopic.php?t=14577&postdays=0&postorder 
>> =asc&start=0>
>> 
>>  
>>  
>> <http://www.muffwiggler.com/forum/viewtopic.php?t=18358&start=0&postdays=0&p 
>> ostorder=asc&highlight=>
>> 
>> And there are many Japanese blogs/sites with some great-looking
>> modifications:
>> 
>> 
>> http://www.korg.co.jp/Product/Dance/monotron/welove/
>> 
>> 
>> http://beatnic.jp/monotron/
>> 
>> Google translate:
>> 
>>  
>>  
>> <http://translate.google.co.uk/translate?u=http%3A%2F%2Fbeatnic.jp%2Fmonotro 
>> n%2F&sl=ja&tl=en&hl=&ie=UTF-8>
>> 
>> 
>> http://masa921.hp.infoseek.co.jp/monotron_mod.htm
>> 
>> Google translate:
>> 
>>  
>>  
>> <http://translate.google.co.uk/translate?u=http%3A%2F%2Fmasa921.hp.infoseek. 
>> co.jp%2Fmonotron_mod.htm&sl=ja&tl=en&hl=&ie=UTF-8>
>> 
>> > In the 
>> > adverts it states that it uses the same filter as the MS-10 
>> > and the early MS-20. My question is this: The MS-20 has 
>> > lowpass and highpass filters. The MS-10 only low pass. Would 
>> > this mean that the korg 35 chip is configurable as both?
>> 
>> The original was, yes, just hook it up slightly differently for 
>> highpass...
>>  
>> > Would that also mean that I could use one Monotron as 
>> > highpass, and one as lowpass?
>> 
>> Good question - so far I have not seen a mod to reconfigure a Monotron 
>> for the highpass filter (which may mean it is difficult). Can't wait to 
>> get my hands on one to see how do-able that is! But note that the 
>> Monotron isn't using the Korg35 per se, just the same arrangement of 
>> transistors that are in the Korg35 (as far as I can tell from the 
>> various photos of it around the web).
>> 
>> Tim (still pissed at Korg for not releasing it worldwide in one go)
>> __________________________________________________________
>> Tim Stinchcombe 
>> 
>> Cheltenham, Glos, UK
>> email: tim102 at tstinchcombe.freeserve.co.uk
>> www.timstinchcombe.co.uk
>> 
>> 
>> 
>> 
>> 
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>> 
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>> 
>>  
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