[sdiy] Korg KR-55 drum machine query

Adam (synthDIY) synthdiy at adambaby.com
Tue Nov 14 04:12:17 CET 2023



> On 14 Nov 2023, at 9:49 am, Tom Wiltshire <tom at electricdruid.net> wrote:
> 
> Yeah, I did a deep dive on the DR-110 a while back:
> 
> https://electricdruid.net/analogue-drums-boss-dr-110-drum-machine/
> 
> Despite what Boss/Roland say about "Interrelations between frequencies are so critical that slight deviation of one frequency can cause beat sound or distortion" in the service manual, I don't believe the frequencies are *that* critical. Shortly after, they mention that the caps need to be 5% tolerance (so *much* tighter than the usual 20%!) but that would give +/-15Hz roughly for even the lowest frequency they list. That's a pretty hefty beat frequency, even before the oscillator you're beating against drifts out by even more in the other direction.
> 
> What I do find interesting is the different ways this "metallic noise" thing has been done. DR-110 uses *four* mixed squarewave sources, bandpass filtered. TR-606 uses *six* squarewave sources, mixed.

Yes, the CR-8000 is similar, and in addition you get a killer "Rimshot" sound from the lowest 2 oscillators!
Mentioned here
https://mezzoauto.blogspot.com/2017/06/roland-cr-8000-compurhythm.html
Roland CR-8000 CompuRhythm
mezzoauto.blogspot.com


Economical Roland engineering at work again!


A





























> KR-55 uses ring-modulated pairs of six squarewave sources, added together (extra complex!). 
> 
> Tom
> 
> 
>> On 13 Nov 2023, at 23:08, Adam (synthDIY) <synthdiy at adambaby.com <mailto:synthdiy at adambaby.com>> wrote:
>> 
>> Ha, Love it!
>> S'funny, you'd think these frequencies would not be all that critical for what basically is metallic noise, but some schematics suggest otherwise (808, 606, Dr-110)
>> This from the Dr-110...
>> 
>> <grab 2023-11-14 at 9.03.25 am.png>
>> 
>> 
>> A
>> 
>>> On 13 Nov 2023, at 11:44 pm, Tom Wiltshire <tom at electricdruid.net <mailto:tom at electricdruid.net>> wrote:
>>> 
>>> Hi All,
>>> 
>>> I've been studying old drum machines again. This time, I've been looking at the Korg KR-55.
>>> 
>>> I've come up with one quirk or schematic error, and I'd like you to take a look and see if you agree with me.
>>> 
>>> The machine uses eight simple squarewave oscillators to generate two "metallic noise" signals for cymbal synthesis. These are called Ring1 and Ring2, because they're generated using pairs of oscillators "ring modulated" together by 4070 XOR gates. The schematic looks like this:
>>> 
>>> https://electricdruid.net/wp-content/uploads/2023/11/RingGenerator.jpg
>>> 
>>> The whole thing is here:
>>> 
>>> https://www.synthxl.com/wp-content/uploads/2021/10/korg-kr-55-Service-Schematic.pdf
>>> 
>>> The schematic gives us frequencies for the eight squarewaves. We also know that the general pattern for such oscillators is Freq = k/RC, so it's just a question of working out what k is in this case (using the LM339 on a 5V supply).
>>> It occurred to me that we can assume the designers wanted the tuning pots roughly in the centre, so if we feed in the required Frequency, with the midpoint R value and the given capacitor, we can work out k. That gives us this:
>>> 
>>> Required Freq	R	C	K
>>> 272 Hz	125 KΩ	22 nF	748000
>>> 8529 Hz	49 KΩ	8.1 nF	3385160.1
>>> 633 Hz	125 KΩ	9.1 nF	720037.5
>>> 9073 Hz	49 KΩ	1.5 nF	666865.5
>>> 1458 Hz	125 KΩ	3.3 nF	601425
>>> 13500 Hz	49 KΩ	1 nF	661500
>>> 731 Hz	125 KΩ	7.5 nF	685312.5
>>> 3946 Hz	125 KΩ	1.5 nF	739875
>>> 
>>> This is all pretty reasonable, with values between 600,000 and 750,000, which is not much of a spread. However, that second row is an exception - it's way out. Working out the average value, excluding 8529Hz, gives us k = 689,002. We can then put that k value back in and see where the tuning pots need to be in theory:
>>> 
>>> Required Freq	Required R	Midpoint R
>>> 272 Hz	115.1K	125 K
>>> 8529 Hz	10.0K	49 K
>>> 633 Hz	119.6K	125 K
>>> 9073 Hz	50.6K	49 K
>>> 1458 Hz	143.2K	125 K
>>> 13500 Hz	51.0K	49 K
>>> 731 Hz	125.7K	125 K
>>> 3946 Hz	116.4K	125 K
>>> 
>>> As you can see, all the "Required R" values are pretty close to the midpoint R values. The fifth line, 1458Hz, is pushing it a bit - we have 150K as the maximum value from the resistor+trimmer, so that's a bit close to the end, but it's still within, just about.
>>> 
>>> The BIG EXCEPTION is that second line, which needs an R value five times smaller than the available 39K+20K trimmer. What's going on? Is this just a mistake on the schematic? Is the frequency wrong, or is it the cap value that is wrong?
>>> 
>>> Any thoughts much appreciated.
>>> 
>>> Thanks,
>>> Tom
>>> 
>>> ==================
>>>        Electric Druid
>>> Synth & Stompbox DIY
>>> ==================
>>> 
>>> 
>>> 
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>> 
> 

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