Neovibe R35 and R36 trimpot

[rhdwave]

Okay, i know i've been flooding the board with Neovibe questions lately...but i've become obsessed i guess.  Anyway, here goes: So to get the right mix between the dry and wet signals, it's been suggested to use a 200K trimpot to replace the 2 100K fixed resistors which would normally go in the R35 and R36 spots.

It has also been suggested to use 2 82K resistors in series with the outer lugs of a 50K trimpot to get a better result.  I know many people have utilized the 200K trimpot method.  I've figured the outer lugs must be soldered to the pads which were originally designated for the left side of R35 and R36 (that is the side in which those two resistors would have been isolated from each other) and the wiper must be connected to the N pad. 

However on my pcb this is impossible to do given the distance apart that the three pads sit.  It is possible to get the trimpot in the two resistor pads, but not at the same time as the N pad. 

Should i drill  an additional hole next to the these two pads and then use a jumper to connect the wiper of the trimpot to pad N?   I guess this would be a solution.  Or, would it be possible to simply solder this to the two pads right next to the N pad? I was thinking this would mess things up as it would connect the outside the outer lugs of the trimpot. 

Also, just wondering if anyone has tried the 82K resistors in series with either a 50k pot or trimpot?

Again, sorry for all the trivial Neovibe posts...i just want to make sure i get all this right.  It's been a challenging build for me, but one i've really been enjoying. 

[John Lyons]

If you use metal film resistors you should be close to the right resistance in the first place.
I tried a 50K trimmer as described and found that the sweet spot is very small and it's not really something to tweak unless you aren't getting much effect from the neovibe.
If you have already finished everything and it sounds pretty good and you want to tweak it to see if you can get it any better then just use the resistors and 50K (linear) pot soldered to wire jumpers at the board. The board is very tight so you can't physically mount a trimmer there.
The trimmer may give you the last 10% out of the neovibe...but maybe not.

John

[rhdwave]

Thanks John! Yeah, i was going a little crazy trying to figure out that trimpot that just didn't seem to fit.

[R.G.]

Nota Bene:
The original Univibe did not have provisions for a trimpot either, and used 5% resistors. So the "vibing" was highly variable.

The size of the wet and dry signals varies because of the tolerances on the preamp gain setting resistors and the fact that the discrete darlington phase stages have some signal loss. So the cancellation for notches was hit and miss.

The trimpot is not something that ever needs messed with once you have it set correctly. What I do is to use the 82K/50K trimpot method. This makes the trimpot's sweet spot much bigger. The smaller the trimpot in relation to a total of 200K, the smaller the range of adjustment. You only need about +/- 10% to cover all the possibilities, so a 200K trimpot would have an unnecessarily wide range and need very picky adjustment.

The idea is that you set the trimpot, then measure the trimpot plus the end resistor and sub in fixed 1% resistors when you get them measured. It should never need tweaked again.

[rhdwave]

Okay, that sounds good.  So I should solder in the 82K resistors and then jumper from the right side of the resistors to the outer lugs of the trimpot? Then check values once i hit the sweet spot, desolder and add 1% resistors? In regards to the total resistance, it will obviously be greater than the 2 100k resistors since the 50k trimpot plus 164K will equal 214K...but i guess it's not a series connection and as you said this will cover all the range of possibilities?

Thanks again!!!

[R.G.]

Which version ... exactly... of the board do you have?

[rhdwave]

I'm not sure exactly as i'm not at home right now...but i etched it myself from the general guitar gadgets site.  I did the etching like a week or a week and a half ago.  On the ggg site i noticed they have new pcb's for sale that have all of the artwork with the resistor and capacitor names (eg. r49 c20) filled in on the non copper side.  I don't think they had this when i printed the page out...i'm just saying this for general reference on time frame.

[R.G.]

Yeah, I think JD's gone full commercial fabrication on PCBs to get silk screen and solder mask.

The version number of the PCB will be etched in the copper if you etched from an on-line layout.

[rhdwave]

Okay, just got home...it's 4/03

[fpaul]

I'm trying to find the easiest way to do this so I'm trying to resurrect this thread .  I have the ggg board- version J 02/07.  It looks like it has a place for a trimpot but I am guessing you would have to use a 200K, then remove it and put in the correct resistors.  If you put in both the resistors and a trimmer it would be in parallel instead of series, is this correct?  I have the board ready to go except for the LDRs, bulb, two huge electros and these two resistors, so hopefully someone can confirm this soon.  If not I'll probalbly just stick in 2 100Ks and hope for the best.

This is my second attempt at a Neovibe; first tried making my own board.  I burnt off the little bit of copper left on some of the transistor pads.  My biggest mistake was putting the transistors on last.  Should have been first.  Also should have used a much smaller drill bit.  This "store bought" board  is giving me new respect for my soldering skills though; it's so easy it feels like i'm cheating.  Oh yeah, I also forked out 50 bucks for a Radio Shack digital soldering station which is a tremendous improvement also. 

Cheers

[R.G.]

Yes, the spot for the trimpot is for a 200K to 250K trimpot. You can do the measurement, or you can simply leave the trimpot soldered in.

Simply putting in resistors, even high precision ones for the mixing resistor is not enough. The best effect comes from exact cancellaion; and the two paths through the circuit are imperfect and may have different gains, so a "perfect" 1+1 adder may not result in the best sound. You might need a 1.05 +0.97 adder. The trimpot lets you do that.