MXR MicroAmp - Treble Mod Poping

[LaloFP]

Hi!

  Im tryin to Mod an MXR MicroAmp to be Unity Gain when turned Off, not TrueBypass.

  For doing that I cut the C3 contact to ground, and seems Ok to me.

  But I wanted to add a TrebleMod Switch (In green). The values are 0.005uf and 68K. I took those from the Dallas RangeMaster and sounds Ok to me.

  The problem is that if I want to turn off the Treble Mod (using a SPDT between the resistor I added and ground), I hear a Pop. And I want to turn off the Mod when I turn off the pedal, so I want to eliminate that Pop!

  I tried several things with large resistors and caps, like usual poping pedal problems, but I still have the Pop.

  What do you recomend?

[Mark Hammer]

I don't know what the value of the green resistor to ground is, but simply bridging the green cap so that C1 and R1 are effectively connected ught to do the trick without popping.

What sort of treble mod are you aiming for?  You might actually do better by sticking another R-C pair in parallel with R3/C3.  For example 100nf/1k will provide some boost starting around 1.6khz, albeit not until you start to reach higher gains.

[MrStab]

shouldn't the green resistor be tied to +4.5V, not ground? just checking!

[LaloFP]

I don't know what the value of the green resistor to ground is, but simply bridging the green cap so that C1 and R1 are effectively connected ught to do the trick without popping.

Im using 0.005uF for the cap and 68K for the resistor. I took those values from the Dallas RangeMaster.

I tried bypassing the green Cap before, and it worked for the Poping, but the overall volume was always dropped down, and I want to be able to bypass the TrebleMod and letting the original gain and volumen intact.

Maybe other values for cap and resistor?

What sort of treble mod are you aiming for?  You might actually do better by sticking another R-C pair in parallel with R3/C3.  For example 100nf/1k will provide some boost starting around 1.6khz, albeit not until you start to reach higher gains.

Thats was what I first tried, because it seems to be the natural thing with this design, but I didnt like the behavior: like you mension, I need to go to the higher gains to get the Treble, and when Im on the top of the gain, its to much bright and harsh.

What I want is a stable treble booster in all of the Gain range.

I like the sound and behaviour with that green RC. The problem is the Poping when I turn off the Mod.
I dont care if it happends when I do that with the SPDT, like a traditional Mod.
I care when it happends when I turn off the pedal in non-truebypass mode. In that case, the poping is unaceptable 

shouldn't the green resistor be tied to +4.5V, not ground? just checking!

I tested it right now and the Pop is still present and louder!

[Kipper4]

I'm gonna stick my neb in and ask would this work? Or will it still pop?

[Cozybuilder]

Maybe something like this?

[MrStab]

Im using 0.005uF for the cap and 68K for the resistor. I took those values from the Dallas RangeMaster.

The problem could be because you're making such a large jump, from 10M to 68k. Even if this isn't causing the pop, it will screw with your impedance badly. I was going to suggest a bunch of solutions, but the best one is probably to put the filter AFTER the opamp. You could replace C5 with 0.005u, with the 68k as your pulldown resistor.

You could maybe keep the green stuff where it is but make it 500p (0.0005u) & 680k, then reduce R1 and R2 to 2M2 each, to make the difference smaller. 10M is unnecessarily-high anyway.

Are you using a buffered pedal or active pickups before the MicroAmp?

I tested it right now and the Pop is still present and louder!

my suggestion wasn't to do with the pop, but that the green resistor should now act as the bias for the opamp, as well as being part of your filter (biasing was previously provided by R2). if it still worked regardless then maybe i'm talking crap!

[LaloFP]

I'm gonna stick my neb in and ask would this work? Or will it still pop?

Maybe something like this?

Those Cap changes didnt make any audible difference

Im using 0.005uF for the cap and 68K for the resistor. I took those values from the Dallas RangeMaster.

The problem could be because you're making such a large jump, from 10M to 68k. Even if this isn't causing the pop, it will screw with your impedance badly. I was going to suggest a bunch of solutions, but the best one is probably to put the filter AFTER the opamp. You could replace C5 with 0.005u, with the 68k as your pulldown resistor.

I tested that and the sound is different and I dont like it. Just cutting some bass gives less of that "treble booster"-sound like

You could maybe keep the green stuff where it is but make it 500p (0.0005u) & 680k, then reduce R1 and R2 to 2M2 each, to make the difference smaller. 10M is unnecessarily-high anyway.

Are you using a buffered pedal or active pickups before the MicroAmp?

If I change the cap and resistor values like you mension I got a background noise constantly. I dont know why.

And Im not using buffered pedals or active pickups before the pedal.

And Im sorry... I made a mistake in the schmatic I shared here. Im not using the highpass between C1 and R3, Im using it in first place, before the R1.

I tried to place it between every posible spot in the first part of the schem, but it only works if I use it before everything

Any ideas? Is the only thing I need to finish the mod! :/

[MrStab]

okay, try this:

remove the filter. change R1 to 1M, and make C1 330pF for effect ON, and keep the 100nF in parallel for bypass mode. R1 stays the same in both modes.

your original 68k/5nF (0.005u) has a rolloff of 468Hz - this new one would be close, 482Hz, and you get to keep a high impedance. but you'd also have to use a ceramic cap (afaik), which can introduce noise.

[LaloFP]

okay, try this:

remove the filter. change R1 to 1M, and make C1 330pF for effect ON, and keep the 100nF in parallel for bypass mode. R1 stays the same in both modes.

your original 68k/5nF (0.005u) has a rolloff of 468Hz - this new one would be close, 482Hz, and you get to keep a high impedance. but you'd also have to use a ceramic cap (afaik), which can introduce noise.

Stab! I tested that now and the R1 change doesnt affect negatively, but when I make C1 330p (those are in film too) there is a lot of background noise like ground and the signal is easily distorted, so I thing that opamp biasing is wrong

Equally, the treble thing is very small and the Poping is still there.

Im thinking of using my original idea (not between C1 and R3. In first place, before R1). Is that so bad? In sound I found it right and I really like it. The problem is the Poping and the impedance thing you say (that I dont really understand)

[MrStab]

Stab! I tested that now and the R1 change doesnt affect negatively, but when I make C1 330p (those are in film too) there is a lot of background noise like ground and the signal is easily distorted, so I thing that opamp biasing is wrong

Equally, the treble thing is very small and the Poping is still there.

DAMN!!! i'm really sorry, i screwed up there! i meant R2, NOT R1!

sorry to have wasted your time, i should have checked more closely. changing R1 instead of R2 could cause the kind of problems you describe, depending on what's paralleled to it. also, it won't affect the filter in the way we want it to.

[LaloFP]

Stab! I tested that now and the R1 change doesnt affect negatively, but when I make C1 330p (those are in film too) there is a lot of background noise like ground and the signal is easily distorted, so I thing that opamp biasing is wrong

Equally, the treble thing is very small and the Poping is still there.

DAMN!!! i'm really sorry, i screwed up there! i meant R2, NOT R1!

sorry to have wasted your time, i should have checked more closely. changing R1 instead of R2 could cause the kind of problems you describe, depending on what's paralleled to it. also, it won't affect the filter in the way we want it to.

Ahhh! haha I thought that the R1 and C1 were not a filter.

I tested the R2 and C1 thing and the highpass is there, but I get a big Hum noise  and still pops (I suppose it can be because the Hum)

How is that the original filter I proposed can screw it with the impedance if I put it before R1? Because that sounds great even it Pops.. 

[duck_arse]

lalo - what value cap is it you are adding in the green mod, first post? you should be able to switch (on a switch) the resistor to 4V5 to a much lower value than 10M, so that the new-formed filter rolls over at your wanted frequency.

try 33k~39k across the 10M, see what is.

[dschwartz]

Leave the input as original..just put a 47n in parallel with C3, and switch C3 in or out . you dont mess with input/output impedance, save a resistor, and no pops..

[LaloFP]

Leave the input as original..just put a 47n in parallel with C3, and switch C3 in or out . you dont mess with input/output impedance, save a resistor, and no pops..

The problem with that is that the EQ depends on the gain, and I want it to be independent on all the Gain range.

lalo - what value cap is it you are adding in the green mod, first post? you should be able to switch (on a switch) the resistor to 4V5 to a much lower value than 10M, so that the new-formed filter rolls over at your wanted frequency.

try 33k~39k across the 10M, see what is.

Yes, that works, but the Popping is still there :/

[LaloFP]

So guys, the mod to wich I want to remove the Pop:


Do you think is there a way?

[Mattnezz]

If the popping is still there with the schematic above, I think it comes from bouncing switchcontacts (at the sensitive opamp input) rather than from a DC potential difference.
I also notice the input impedance of the circuit drops dramatically this way, a 18k resistor is a heavy load to drive for most (passive) guitars, and may cause a drop in overall level and high frequencies.
Maybe go for a really small C1 and a 2M2 for R2, to make te difference in impedance less of a factor.
You might want to consider to switch the resistor with a fet (j113?) to eliminate mechanical switch noise.

[MrStab]

Do you think is there a way?

no, because you're dropping the impedance of a bare guitar input so much.

i didn't consider this before: i think some (or all?) of the popping may be because there's also a sudden (but slight) change in the voltage between Vref and the signal path when you switch from the big resistor to the small one, and if there's one thing i've learned from this forum, it's that sudden voltage change = pop.

if you've tried everything you can think of, i think you need to change your method. if you really want to keep the sound you have, maybe you could just switch between a small buffer PCB and the MicroAmp?

overkill suggestion: measure DC voltage on the signal path with the 10M resistor, and make a separate Vref with a slightly lower voltage to keep it the same for the smaller resistor! (don't really do that! lol)

[MrStab]

from a DC potential difference

beat me there!

[dschwartz]

10 meg vs 100k is asking for pop..dont switch the resistor to vref..

Lower the 10meg to 1meg and just switch the input cap