What are the trimpots of Maestro FSH for and what means "selected for noise"?

[gigimarga]

Hello,

After i've spent a lot of time trying to debug Maestro FSH-1 (http://www.tonepad.com/getFile.asp?id=96), thx to gijimmbo (you're great man!!!) who started the post http://www.diystompboxes.com/smfforum/index.php?topic=69381.0,  i've bring it to life only flipping the 3 FETs!!!

Trying to tweak the S/H section i observed that the 10K trimpot is more effective than the 100K trimpot, so what they are for?
So, the first question is: "It is a method to tweak 100K independently from the 10K trimpot (using some predefined voltages...)"?

After i'd tested 4-5 transistors (2N3904, BC337, 2N2222,...) for Q1 i obtained a lot of different sounds, even for the same type of transistor (mainly on 2N3904 i observed that).

So, how can i test if a transistor is a good "selected for noise one" (i don't want to spent my life selecting them...)?
May i try some germanium npn for Q1?

Thx a lot all (with a bonus for gijimmbo)!

[frequencycentral]

So, how can i test if a transistor is a good "selected for noise one" (i don't want to spent my life selecting them...)?

You need to use a socket for Q1 and try a few different trannies. Insert it, play with the trimpot, listen. Some trannies are 'noisier' than others, some produce different types of noise. Your'e probably looking ideally for white noise as opposed to pink, red etc. The S/H works by 'sampling' the random noisy output of Q1 and 'holding' that voltage with the 0.5 cap between Q5 and Q6. You could also try changing the 100K at the emitter of Q1 for a lower value for more noisyness.

[gigimarga]

Thx a lot frequencycentral...it seems that the only chance is to test a lot of trannies...

[nordine]

while toying with S&Hs, i've found that low gain npn trannies give me the best noise, say 2n2222/pn2222/2n3904... i've had bad luck with bc549 and higher gain devices.. good luck

[gigimarga]

Thx nordine...my observations are the same as yours...i will try today some old silicon trannies...i think they are very noisy

[gijimmbo]

gigimarga,
can you post your voltages for IC1?  mine seem funny now.  the filter is still sounding great, but absolutly nothing still on the S/H.   i redid the wiring on the switches (DPDT on-on toggles) and the pots thinking maybe i had a loose wire or something.  same problem.  went through and looked for more solder bridges, bad joints, etc. etc.  nothing seems to help.  i finally went back and measured the voltages again and found that they were the same except for IC1.  can you confirm please?
thanks man

[Mark Hammer]

There is no particular goal when selecting transistors for noise, since noise is random, correct?  It is a common recommendation, however, for synthesizer builders and designers to try out several transistors and pick the one that seems to give the most/best noise.