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 Forum index » DIY Hardware and Software » Circuit Bending
Replacing MG-1 5837 Noise Generator With Analog Circuit
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facon



Joined: Jan 14, 2015
Posts: 16
Location: Michigan, USA

PostPosted: Wed Jan 21, 2015 9:35 am    Post subject: Replacing MG-1 5837 Noise Generator With Analog Circuit Reply with quote  Mark this post and the followings unread

I'd like to change out the digital noise generator chip in my MG-1 for an analog white noise circuit. I've found a ton of different schematics for noise generators and went with one that happened to have 12V supply. I'm wondering if it will be fine to just stick this circuit in place of the 5837.

I believe the section after the noise generator is a pink noise filter with a gain stage. Is this accurate?

Will I need to add any additional gain to get it to the same level as the 5837? Or is this what the second 2N3904 is responsible for? If so, should I replace the 1K resistor with a trim pot to match the output of the 5837?

Should I add some extra power supply filtering from the +12V connection? Will adding a 100uF cap from +12V to ground have any negative affects on the noise?

5837 Datasheet - http://www.dutchsynth.nl/images/mm5837.pdf

MG-1 Schematic - http://jdilisio.com/synth/MG-1Schematics.jpg


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facon



Joined: Jan 14, 2015
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Location: Michigan, USA

PostPosted: Fri Jan 23, 2015 7:16 am    Post subject: Reply with quote  Mark this post and the followings unread

Well, I'm thinking this won't work as it is. It looks like the gain of the Q2 is only going to get me around 2V peak to peak. According to the MG-1 schematic, the 5837 is giving around 10V peak to peak from +2V to +12V.
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richardc64



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PostPosted: Fri Jan 23, 2015 9:45 am    Post subject: Reply with quote  Mark this post and the followings unread

You might get a higher noise level by using the values shown below.

Alternatively you could try reducing R7 to increase the level going into IC2B, and/or decreasing R11 so the opamp will have more gain.

A daring step would be to connect R15 and C7 to -15 instead of Gnd.


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facon



Joined: Jan 14, 2015
Posts: 16
Location: Michigan, USA

PostPosted: Fri Jan 23, 2015 12:47 pm    Post subject: Reply with quote  Mark this post and the followings unread

Great. I think at first I'll try adjusting R7 to see if I can get it close.
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varice



Joined: Dec 29, 2004
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PostPosted: Fri Jan 23, 2015 1:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

facon wrote:
Great. I think at first I'll try adjusting R7 to see if I can get it close.

I wouldn’t do that as it will probably affect the spectrum of pink noise filter circuit.

It would be best to increase the gain of the op-amp instead.

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facon



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PostPosted: Fri Jan 23, 2015 8:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

I tried the circuit that I had posted earlier. I hooked up a pot to R160 and found that around 2.7K is what was needed to get to the same volume and to trigger the sample and hold correctly.

I did change R13 to 470K because that's the closest value I had on hand.

I'm just now noticing that when I have noise up, I hear a click along with the LFO. I'm not sure if it did that before, but overall I'm happier with it. I may need to tweak a little more over time.

Thanks for the help!
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varice



Joined: Dec 29, 2004
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PostPosted: Sat Jan 24, 2015 12:34 am    Post subject: Reply with quote  Mark this post and the followings unread

facon wrote:
...I'm just now noticing that when I have noise up, I hear a click along with the LFO. I'm not sure if it did that before...

AAARGH Exclamation To go to the trouble to get rid of the annoying repetitive cycling of the 5837 only to now pick up interference from the LFO. Evil or Very Mad Not an uncommon problem though. If you are now hearing clicks from the LFO only when you have the noise source in the mix (I don’t hear any LFO clicks with my MG-1), then I would suggest a couple of things to try. First, I would make sure that the common (ground or 0V reference) of your added noise source circuit is connected as close as possible to the point where R160 is connected to common. I would also connect the added noise circuit positive voltage supply directly to the MG-1 positive power supply output.

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facon



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PostPosted: Sat Jan 24, 2015 8:50 am    Post subject: Reply with quote  Mark this post and the followings unread

I removed 5837 and R153. The positive connects to where R153 was connected, ground goes to pin 1 and the output goes to pin 3. I didn't remove C29. Since my re trigger circuit takes up the space above that section of the pcb, I had to place the new noise generator pcb on the other side of the LfO slide pot. The wires do touch the pot. I think I'm going to try installing it on the other side of the PCB with shorter wires.
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facon



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PostPosted: Sat Jan 24, 2015 11:21 am    Post subject: Reply with quote  Mark this post and the followings unread

I tried everything and connecting it directly to 12V at C14 on the bottom board did the trick!

Thank you so much. Now if I can figure out why I'm still hearing my oscillators when the mixer controls are fully down, ai'll be very happy with this synth.
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