TWO STICKS of DERM "squeal"

jspake

Member
hi, i commented on another thread, but it doesn't seem to be showing up on the "what's new", so maybe nobody has seen the question. hope this is not an anoying redundancy if you've scrolled past this question already.

i've just tested my two sticks of derm board and the effect is quite nice, but there is a very high pitched squeal/howl type of noise accompanying the distortion. i have seen a suggestion to go for another NPN transistor in the middle slot, that has an hfe of around 200. i don't currently have a way to test that (on the way) but the handful of the 2n3904 units that i tried sounded exactly the same. is there a nearby component value that could be adjusted? is there another common NPN that i might try, or should i just try all of my 30 or so 3904s and hope one works out? thanks for any help!
 
Do you have any BC546b, BC547b, 2N4401
Do you realize that you could be getting unwanted noise if...ie
Has it been mounted in the Enclosure with Input & Output jacks properly grounded to Case?
 
i may have a couple 2n4401s, but not the others. when i tested the circuit i was using a test rig that i’m building, which is not mounted to an enclosure yet. however, i’m working on enclosures for both the tester and the derm today, so hopefully by tomorrow i’ll have a housing for both. would that make a huge difference? i thought i was supposed to rock it before i box it? ?
 
rocking before boxing is good. but realized that a the unboxed pedal won't be shielded the same way it would be inside a metal enclosure that is grounded to the PCB. so you can tell if a pedal is working before you box it, but a working pedal outside the box may still be a bit noisier that a working pedal inside the enclosure.

but a squeal is more likely to be something other than just an unboxed but otherwise working effect.
 
i see. i did expect it to be somewhat more noisy unboxed, but also didn't think the squeal would be coming from a lack of shielding, etc... i'll be ordering some of those alt transistors and see what i come up with.
 
i may have a couple 2n4401s, but not the others. when i tested the circuit i was using a test rig that i’m building, which is not mounted to an enclosure yet. however, i’m working on enclosures for both the tester and the derm today, so hopefully by tomorrow i’ll have a housing for both. would that make a huge difference? i thought i was supposed to rock it before i box it? ?
2N4401's are Good, Lower noise floor than 2N3904.
 
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