Em-Drive Sustain

[inefficiency]

Hey guys, I built another Em-Drive the other day for a friend and I'm noticing it seems to have pretty bad sustain compared to the last one I built.

It almost seems like the note is getting gated slightly.

I had an issue with a fuzz that did this in the past and I think it was due to the transistor being biased incorrectly.

Is this a feasible reason for this happening? If so, I'll just stick a trimmer in or something. Would it be the 2M resistor that the trimmer replaces?

[rocket88]

I definitely think it's a bias issue. When I built mine I did the layout johnk posted under the comments section, because it has a trimmer. I forget the value though. The nice thing is if you put a trimmer for biasing you can try different transistors and see what you like, not just seeing if they bias correctly with the spivey value.

[inefficiency]

Looks like I will have to do some tweaking to my modded layout.

I used Mark's layout to start with, but added pulldown resistors for the input and output caps because I was getting popping. I also added a switch to add a couple of clipping diodes.

It could be something to do with my mods, but with so few components with or without, it just seems to me like the only issue will be the transistor.

Unfortunately, I didn't pick up on it when I was making sure it worked, so the thing is completely boxed up and looking nice. I might just have to try a few transistors of that type until I get one that has gain that suits or whatever.

This is the layout I used. Probably could be more compact, but it works. Let me know if you spot anything wrong.

EDIT: The transistor says ME4003, because I thought I'd try one of those out, but it wasn't high enough gain so I ended up using the standard MPSA18. I might try the ME4003 again if I can find one with higher gain.

[rocket88]

it looks ok to me. i would personally put the diodes to ground, but other than that it should be ok. i don't know about the amount of gain needed in the transistor for this one, but i think i used 2N3904, which i don't think are anywhere near the amount of gain as an MPSA18. but i know when i was trying different transistors now all biased right and it got gated and farty, which is why i made one using a trimmer. when i tried each transistor again, i had to adjust each transistor to get the right sustain and sound.

[inefficiency]

The diodes are going to ground, just via a resistor. I socket it so I can try different values.
Someone here explained to me why you shouldn't just put them straight to ground. I can't remember who it was, but whoever you are, thanks!

Essentially, when I take out the resistor and put a jumper I get loads more clipping, but it's really high pitched, harsh and I lose all "fullness" - When I put a 4.7k it retains the fullness, but the gain is barely noticeable - When I put a 2k resistor, I get a bit of both.

It seems to react really differently to different amps though - that might be a spot for a trimmer also. I thought about putting it on an external pot, but I'll see how useful the trimmer is first and go from there.

[Silver Blues]

I think it was Induction that said it. That was an eye-opener for me too. Good tip.

[rocket88]

i forgot about that. i think silver's right, it was indcution. it makes sense and a neat idea, because if i understand it right the resistor makes the diodes soft clip, rather then hard clip. hence the lack of harshness. should put that in my mental toolbox, would be cool to experiment with.

[inefficiency]

Yeah, induction.

It's kinda irritating because I'm struggling to find the right blend of bass and depth vs clipping - Just can't seem to nail it spot on.

Looks like maybe this pedal needs trimmers all round.

[inefficiency]

So I tried building this again and using John's layout with the trimmer. All I added was a couple of pulldown resistors, because I've had trouble with popping in the past.

He specifies to use a 10k trimmer, but all I had were 5k trimmers. I figured seeing as it is replacing a 4k7 resistor, it wouldn't be an issue, just I would get much play with it.

When I turn the trimmer, nothing happens til it gets to near the top of it's travel (I guess where 4k7 would be) and it kinda crackles a bit, like the transistor is biasing, but there is no sound from the guitar at all.

Could the wrong trimmer be affecting this? I'm thinking probably not. If this is not the case, then I believe it is the gain pot - which is a sketchy, cheapo one. Just thought I'd check here first.

[rocket88]

i think you might need a larger trimmer. i checked mine, and it really doesn't do much till i hit the mid way point.

[inefficiency]

Cool. Guess I know my next purchase. Thanks man.

[Jopn]

Or socket different resistors into there.

[inefficiency]

Yeah, I realise I can do that, but I think a trimmer is going to be a lot more convenient for trying different transistors and such.