Negistrem/Negistron build report

[earthtonesaudio]

I built this recently and was asked to do a build report, so here 'tis.

Schem:


People had problems getting it to work at all, but I didn't.  Here are my observations:

1. the BJT is connected correctly.  Google "negistor," or "negatron" for more info.  Basically it's avalanche current charging up the cap that gives you the LFO.

2. I've never seen a JFET wired up like that, but it works.  The thing is, you have to make sure the JFET you use is meant for use as a variable resistor.  J201's will work, but not well, and not all of them.  I used a J113 and it worked very well (only 1 out of 5 didn't work well).  I don't recall the specific spec, but I think it's good for the JFET to have a low Vgs (off) spec.

3. The resistor/cap to ground is a lowpass filter which takes the ticking/popping out of the LFO.  Too small a cap and you can hear the pops.  Too big, and you lower the maximum LFO rate, since it adds in parallel with the first cap from emitter to collector/ground.

4. I got it to "kind of" work with a BJT in place of the JFET.  "Kind of" meaning it distorted in a misbiased, farty way.

5. The LFO waveform is sort of like a sawtooth wave.  NOT a sine wave tremolo.  Think a sawtooth with rounded top/bottom, but slightly asymmetrical.  The lowpass cap is what does the rounding off of the edges.

6. Output is unity or below.  The JFET basically acts like a volume pot.  Does not amplify.  I don't know if it inverts the signal or not... I'm guessing not.

This is a cool circuit.  So few parts (and the only potentiometer you really need is the "rate" control, others can be fixed resistors) and a pretty good tremolo... definitely unique sounding. 

I plan on building a tremolo at some point which incorporates at least one Negistrem.  I think it would be really cool to have multiple-rate tremolos running at the same time...

[ambulancevoice]

has anyone built this pedal successfully? cause i think it was just a happy accident maybe

sure sounds great and fun though

[nelson]

I plan on building a tremolo at some point which incorporates at least one Negistrem.  I think it would be really cool to have multiple-rate tremolos running at the same time...

http://www.diystompboxes.com/smfforum/index.php?topic=66666.0

[earthtonesaudio]

has anyone built this pedal successfully? cause i think it was just a happy accident maybe

sure sounds great and fun though

Yep.  Successful on 1st try.  People were using j201's for the JFET and something other than 2n2222a's for the BJT (metal can version).  You have to use a JFET that is meant for either switching or voltage-controlled-resistor operation in order for it to work well.  J201's are meant to be used as linear amplifiers, kinda the opposite intended use.  I'm not sure what it is about the metal can 2n2222a that makes it work well here... probably low reverse breakdown current/voltage or something.  Higher supply voltage (12 or more volts) in theory would allow you to use a greater variety of transistors for the BJT.  I was able to get the effect using other transistors, but some worked better than others, and a few didn't cut it at all.

So... it's picky about the device used.  Can't just be any JFET or any BJT.  But, once you find the right type, most of them work without any fiddling.  Meaning, I tried all the J113 JFET's I had, and all the 2n2222a's I had... and all of them worked.  About 1 in 5 of the JFET's was noticeably poorer sounding.  And I've heard that about 1 in 20 2n2222's won't work from others.  I don't know how reliable these statistics are, but you can imagine why you don't see any commercial pedal built with these circuits.  Each one would have to be tested or it would be a dud!  However, I think that makes it just about perfect for the DIY/boutique communities. 

I plan on building a tremolo at some point which incorporates at least one Negistrem.  I think it would be really cool to have multiple-rate tremolos running at the same time...

http://www.diystompboxes.com/smfforum/index.php?topic=66666.0

Wow... that's pretty intimidating.  I was thinking I would just tack on a Negistrem as a bonus feature on an EA Tremolo or something like that.  But 4 Negistrems... that would be something.


...Overall, I like the Negistrem because it has a unique sound and is a really cool circuit.  But it's that unique sound that also makes me not like it so much for something like my "only tremolo" sound.  Maybe if there was an easy way to change the waveform from sawtooth to sine... but that would be a much more difficult project.