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fonik
Joined: Jun 07, 2006 Posts: 3950 Location: Germany
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richardc64
Joined: Jun 01, 2006 Posts: 679 Location: NYC
Audio files: 26
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Posted: Thu Oct 15, 2009 11:21 am Post subject:
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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. _________________ Revenge is a dish best served with a fork... to the eye |
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goodrevdoc
Joined: Sep 11, 2006 Posts: 288 Location: Philadelphia, PA
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Posted: Thu Oct 15, 2009 12:14 pm Post subject:
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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
Joined: Sep 11, 2006 Posts: 288 Location: Philadelphia, PA
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Posted: Thu Oct 15, 2009 12:21 pm Post subject:
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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
Site Admin
Joined: Apr 03, 2004 Posts: 24075 Location: The Netherlands, Enschede
Audio files: 277
G2 patch files: 320
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Posted: Thu Oct 15, 2009 12:28 pm Post subject:
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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. _________________ Jan
also .. could someone please turn down the thermostat a bit.
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goodrevdoc
Joined: Sep 11, 2006 Posts: 288 Location: Philadelphia, PA
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Posted: Thu Oct 15, 2009 12:49 pm Post subject:
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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
Joined: Nov 15, 2006 Posts: 142 Location: Oregon
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fonik
Joined: Jun 07, 2006 Posts: 3950 Location: Germany
Audio files: 23
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Posted: Thu Oct 15, 2009 1:35 pm Post subject:
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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... _________________
cheers,
matthias
____________
Big Boss at fonitronik
Tech Buddy at Random*Source |
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goodrevdoc
Joined: Sep 11, 2006 Posts: 288 Location: Philadelphia, PA
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Posted: Thu Oct 15, 2009 2:06 pm Post subject:
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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'
-justin |
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fonik
Joined: Jun 07, 2006 Posts: 3950 Location: Germany
Audio files: 23
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Posted: Sun Oct 18, 2009 10:49 am Post subject:
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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. _________________
cheers,
matthias
____________
Big Boss at fonitronik
Tech Buddy at Random*Source |
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jumunius
Joined: Apr 19, 2010 Posts: 346 Location: San Francisco, CA
Audio files: 13
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Posted: Fri Jun 04, 2010 1:26 pm Post subject:
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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
Joined: Sep 08, 2006 Posts: 425 Location: PDX
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Posted: Tue Jun 08, 2010 7:03 pm Post subject:
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Glad to see some work being done on the SDS3. You da man, Fonik! |
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fonik
Joined: Jun 07, 2006 Posts: 3950 Location: Germany
Audio files: 23
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Posted: Tue Jun 08, 2010 11:23 pm Post subject:
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i fear you got it wrong
nothing done since 10/2009. and i passed the proto PCB to someone else... _________________
cheers,
matthias
____________
Big Boss at fonitronik
Tech Buddy at Random*Source |
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