[sdiy] Korg KR-55 drum machine query

[Richie Burnett]

Hi Tom,

I remember doing a similar exercise when I modelled the TR-808 cymbal 
oscillators, but I had the actual machine and also a bunch of samples to 
refer too.

Do you have access to either a KR-55 or at least some decent samples that 
you could analyse?  The frequencies of the six free-running oscillators 
stand out as fairly prominent spectral lines in the TR-808 cymbal and hi-hat 
sounds, (despite all the clipping and filtering that follows the 
oscillators!)

-Richie,



-----Original Message----- 
From: Tom Wiltshire
Sent: Monday, November 13, 2023 1:44 PM
To: Paul Perry via Synth-diy
Subject: [sdiy] Korg KR-55 drum machine query


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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