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r/synthdiy•
ppprograming

Noob Help; why no noise? (DIY S&H/Noise debug)

As some of you might remember I am working on debugging the Erica Synths S&H+Noise module. (thread here: https://www.reddit.com/r/synthdiy/comments/y6mskv/help_with_debugging_mkixerica_shnoise/ )

In order to investigate the basics / out of desperation I build the basic noise circuit on a breadboard using a 547C (I bought a whole bunch of 547 and 548 to help try things). I build the following diagram on a breadboard: (tried several transistor samples

https://imgur.com/1OKqwX1

However when I measure with the oscilloscope at the emitter I am not seeing any noise. Also when I try this with several different transistors of type 547 or 548.

I have:

  • Confirmed (using the datasheet) that the transistor has the right orientation.
  • Confirmed voltage across the circuit (as ~18.5V)
  • Confirmed the voltage between emitter and base (as ~12V)

What the hell am I doing wrong? This is like a minimal thing?

Update: https://imgur.com/a/a9b16xD images of breadboard and measurement. Measurement taken from the emitter.

As an aside, I also looked at https://synthnerd.wordpress.com/2020/03/09/synth-diy-a-white-noise-generator/ for help (outside the MK stuff) and it seems helpful. Except it isn't working for me for some reason.

update2: as per request I have extended the breadboard circuit with a tl072 opamp. Still nothing.

Posted Nov 5, 2022, 7:34 PM UTC · edited

323
paul6524•

I think I know what you're seeing / not seeing. You've probably already watched it (looks like a screen grab in your link), but check around the 6:50 mark in the Moritz Klein video - https://www.youtube.com/watch?v=0yB_h_wFkh4

The voltage between emitter and ground will look like 12V DC on a scope. Center the signal and zoom in and then you will see your noise. The opamp section just beyond the initial transistor / resistor pair will bias and amplify the noise signal into a useful audio signal.

Regarding your Erica build, I'm guessing it's a bad ground somewhere? maybe at the base of the transistor. Find a ground on the board and check for continuity at the base of the transistor. Which leg can you touch to make it work? Usually when things act like this I just opt to reflow the joints in that area. A lot of times, I don't really know exactly what was wrong, but thats fine if it ends up working.

FWIW - good call on breaking it down on a breadboard to better understand what's going on. Can be slow, but its the best way to understand what's going on well enough to troubleshoot.

Good luck!

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ppprogramingOP•

unfortunately that is not the case.

I expected 200-400 mV peak-to-peak in terms of noise. Instead I am seeing practically a flat line.

I took a picture of the measurement and my breadboard: https://imgur.com/a/a9b16xD

Really not sure why this isn't generating noise, but that would explain why the module is not working. Or at least this behaviour is consistent and not what I would hope, expect.

I did try several transistors and I am seeing the same on all of them. Super weird.

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paul6524•

Just for fun, try bending the collector lead so that it hangs out in free air, instead of in the board.

If you haven't already, I would check continuity and expected resistance across every part of that circuit. Breadboards can be finicky. Those resistor leads also look really thin and prone to breakage - they *should* work, but I'm just looking at eliminating any issues. Like you've said, this really should be working. Check that the 100k resistor is in fact 100k, try a 10k and see if there is any change in the output. Are your transistors from a trusted source and DEFINITELY NPN? A bad or incorrectly labelled set of transistors would explain all of this away. ebay and amazon have both screwed many of us in the past. Not all the time, but its always a possibility.

You can check the transistor with the following methods if you haven't already done so - https://www.electricaltechnology.org/2013/03/how-to-remember-direction-of-pnp-and.html

For my own sanity, I would also be careful about keeping the power rails red / pos and blue / negative / ground. Looks live you've got it wired correctly, but that's the kind of stuff that tricks my brain when I'm tired and have been staring at things too long.

I'd try rebuilding the exact same thing with new components just to rule things out. Keep the leads from kinking, flip the breadboard around just for fun, and see what happens.

Also agreed with the other user who mentioned building the amplification / offset circuit. Will definitely help with visualizing things. You just might test the amplification portion by feeding it a simple DC voltage and ensure that it biases and amplifies as you'd expect.

Does your breadboarded version change when touching the leads like your PCB version did?

And as simple as this should be, this is the kind of stuff that shows up periodically and drives us all insane. You'll also end up building things that just work perfectly without any fanfare. Synth DIY is a fickle beast some times.

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ppprogramingOP•

Thanks a lot Paul.

To respond to a few of your comments:

  • Thanks a lot for the link to the transistor checking. These checks out the transistors.
  • I am aware of the confusing power-rails, but I am used to this due to building all the MK stuff.
  • The circuit is OK in terms of continuity and resistance values & connections. In any case it was build with all new components.
  • My breadboarded version does not respond to touching the live circuit unfortunately.
  • I think I got my 557s and 558s from Reichelt, which is a good supplyer.

However, I found another (even simpler) circuit online which suggested that the BC337 has great noise quality at a low voltage. And I had a bunch of those lying around. (I have been buying some extra seemingly useful components every time I needed some, so I had some transistors to dig through.

This even simpler circuit (ram +voltage in emitter, connect base to ground through big resistor) worked. Very well with the 337, but also with the previously problematic 557. Which makes me think it isn't the transistors fault.

You can see some measurements with my oscilloscope of the output. The bigger one is of the 337, the smaller one of the 557: https://imgur.com/a/g2oHLqN

Now, this doesn't look like typical noise spikes to me so I think my oscilloscope is doing some fuckery. Also I don't trust any transistors I have anymore ;)

So ...

  • I am going to try to make a small perfboard noice circuit (transistor (the 337), big resistor, 100nf cap) and bung it on the power-pins of the module and plug the output into the opamp (which i know works). Hopefully resulting in a working module (even if it is a bit cludgy).
  • I want a component tester for quickly testing transistors.
  • I want a different (real) oscilloscope, as the breakdown measurements look WAY too regular.

Should any of this not make sense please let me know :)

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paul6524•

Kludgy is good - call it a "daughterboard" and no one has to know...

I've thought off and on about getting a transistor tester and just never gotten around to it. While they might be useful in this situation, I've never really had it come up otherwise. I just pull from the same tape for "matching", or design in some matched pair IC. If you get one and find it useful, post about it for sure.

Yeah I've got the same DS150 scope... It has its uses, but it also sucks. Something more real is on my "someday" list.

I just built a Noise Plethora - I'll see if I can get a shot of its white noise output on the DS150 just for kicks.

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littlegreenalienSkullAndCircuits•

Amplify and get a worse transistor.

Look at the noise generator in my metal-o-tron ( https://www.skullandcircuits.com/metal-o-tron-ii/ ) bottom left of the schematics.

There are 2 amplification stages which provide a ton of gain (about 100 times) as the generated noise is very quiet. I tried several different transistors and found most of the common ones to be unusable because they were too good. I settled for a BC550 since they're still available and do the job (although noise levels can vary widely between batches ). I probably tries BC547 as well as 2N3904, both being too clean. If it's a one off, just harvest something out of old electronics and give it a try.

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ppprogramingOP•

Thanks man. I like the tip about finding a worse transistor. Might give that a try if I can find some 550's. By the way is there an overview/comparison of transistors somewhere that would be useful? Or is this all just absorbed knowledge.

It's to get my other thing to work. I might perfboard it and connect it up to the input pin of the opamp in the S&H module. (and steel power from the module as well off course.

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abelovesfunI run AISynthesis.com•

Have you contacted the maker? I would start there.

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ppprogramingOP•

Huh? The question is what could be wrong with the tiny circuit on the breadboard. E.g. am I measuring wrong or something else. This is not necessarily related to the module that I am trying to build.

The schematic of the noise source is similar/identical to others I've found.

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abelovesfunI run AISynthesis.com•

My bad. I missed the breadboard part. I forgot that they don't support the edu modules.

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thinandcurious•

I don't think you're going to be able to measure the noise from the transistor directly, the signal is very quiet and requires a fair amount of amplification. On the breadboard I would suggest to add the amplification circuit from the S&H module as well as the 47uF and 4.7k power rail filter. This is the part of the schematics I'm talking about: https://imgur.com/a/O6EDT3b

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ppprogramingOP•

Really? Most places I've seen, including the MK video, show measurements directly from the emitter. Why wouldn't my crappy oscilloscope be able to show that?

I am ofcourse happy to throw in an 741 later and see if that helps. I just don't think it will.

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ppprogramingOP•

Update: set up the complete noise source circuit on the breadboard, using a tl072 from the parts bin. Still seeing nothing on the output :(

So I am thinking that the transistor is not generating noise for some reason, even though there should be plenty of voltage.

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hafilax•

Your orange jumper doesn't connect to the resistor on the breadboard.

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ppprogramingOP•

It does. Maybe a bit hard to see from this angle but it does.

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hafilax•

No it doesn't.

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ppprogramingOP•

It does, you cannot see it very clearly but the lead bends to the left (in the picture) and is connected to the hole directly to the left of the resistor.

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hafilax•

The hole to the left of the resistor doesn't connect to the resistor. Connections are vertical in that section of the breadboard.

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