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 Forum index » DIY Hardware and Software
Simmons SDS3 noise circuit
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fonik



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PostPosted: Thu Oct 15, 2009 11:07 am    Post subject: Simmons SDS3 noise circuit
Subject description: how does it work?
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hi all,

below you see the noise circuit of the SDS3.

the balance and the compensation pins of the LM301 are connected to those "twisted wires". i wonder how to implement this correctly. just use some wire without insulation and twist it? how long? how many twists - and why?
how does it work?


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sds3_noise.gif



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richardc64



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PostPosted: Thu Oct 15, 2009 11:21 am    Post subject: Reply with quote  Mark this post and the followings unread

It's called a 'gimmick' capacitor,
http://en.wikipedia.org/wiki/Capacitive_coupling#Gimmick
a way to get a very small values in the range of 3pf-15pf. The wires should definitely be insulated, and solid, so they stay twisted together.

Frankly, I don't see the need to exactly duplicate this circuit, since there's so many transistor noise circuit designs available.

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goodrevdoc



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PostPosted: Thu Oct 15, 2009 12:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

I believe the gimmick capacitor is in place of a proper compensation cap on the op amp. It seems to let one be able to 'tune' the noise, at least the high end of it. As for the rest of the noise source, I don't remember seeing a transistor with the base not connected. It is generally the collector, yes?
-justin
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goodrevdoc



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PostPosted: Thu Oct 15, 2009 12:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

Oh I forgot to ask, have you breadboarded it? Does it work as drawn? Sorry, I just can't get that picture out of my head. I'll have to play around with this when I get home.
-justin
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Blue Hell
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PostPosted: Thu Oct 15, 2009 12:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

goodrevdoc wrote:
I don't remember seeing a transistor with the base not connected.


Hmm .. odd ... indeed, the noise usually is generated by the reverse breakdown of the BE junction (zener effect) ... can't think of any mechanism here.

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goodrevdoc



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PostPosted: Thu Oct 15, 2009 12:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

Could it be an error in the schematic? Because if it were the base connected to ground and the collector left unconnected, then the circuit exhibits all of the normal characteristics of a run of the mill white noise generator. Any body have gut shots?
-justin
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ickystay



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PostPosted: Thu Oct 15, 2009 12:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

You can see the twisted pair on the little two channel SDS:

http://www.simmons.synth.net/sds34/pics/sds_4_open.jpg

(bottom left plastic opamp)

Photo linked from:

http://www.simmons.synth.net/sds34/
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fonik



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PostPosted: Thu Oct 15, 2009 1:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

goodrevdoc wrote:
Could it be an error in the schematic? Because if it were the base connected to ground and the collector left unconnected, then the circuit exhibits all of the normal characteristics of a run of the mill white noise generator. Any body have gut shots?
-justin

or base even connected to -VEE. saw it before.

anyways, i did not put it on the breadboard yet. but from what you say and from what i've seen elswhere, the collector should be left off and base should be connected.

i agree to use another noise circuit, or use a common OpAmp and leaving the twisted wires off. i would be fine with that. however some people might want it on a possible PCB just as it has been on the original SDS3...

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goodrevdoc



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PostPosted: Thu Oct 15, 2009 2:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
or base even connected to -VEE. saw it before.

Yes, I'm in the middle of converting My SLMS to SLMS+ and this is one of the kludges. Moving the base from ground to -V. The reasons why are explained in another thread here:
link
As for the op amp, a 301 is a single so it should be no problem to replace with a TL071, 741, etc. while still having pads at holes 1 and 8 for those who would prefer to get 'twisted' Wink
-justin
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fonik



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PostPosted: Sun Oct 18, 2009 10:49 am    Post subject: Reply with quote  Mark this post and the followings unread

i breadboarded it today, and here is a follow up:

it is working nice with the transistors base tied to GND and the collector left off - as expected.

additional observations:
the 2n5172 i more "noisy" than any of my 2n3904.
leaving off the twisted wires is fine, the wires would have some kind of tuning or filtering effect, as mentioned before.
a TL071 does not work that well. the outcome is a much lower noise level and i needed to add a 10pF capacitor in the OpAmps feedback path to prevent oscillating.

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jumunius



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PostPosted: Fri Jun 04, 2010 1:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

Curiously, if these twisted wires have a tuning/filtering effect, how does one tweak the tuning/filtering to their liking? More twists? More wire? Or do they just have a fixed effect? I don't see any pots in the schematic.
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vtl5c3



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PostPosted: Tue Jun 08, 2010 7:03 pm    Post subject:   Reply with quote  Mark this post and the followings unread

Glad to see some work being done on the SDS3. You da man, Fonik! thumright
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fonik



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PostPosted: Tue Jun 08, 2010 11:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

i fear you got it wrong Laughing

nothing done since 10/2009. and i passed the proto PCB to someone else...

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