MXR Micro Amp - Unwanted bass boost

[stonerbox]

Hi guys!

I built the Micro Amp (yep, I like it clean). It works great and all but it seems it does not only boost the volume somehow it also boosts the lower frequencies, like the way low.. which is a bit weird..

What could be causing this and what could be done different?

Substituted:
R1. 22M > 1M
R2. 10M > 1M
C5. 15Uf > 22Uf
D1. 1N4001 > PR1407

[stonerbox]

My C1 is a 0.01Uf! *facepalm*

EDIT: Maybe this is not the problem here, since that cap should decrease the bass?

[chptunes]

C1 and C5 should have a lot of influence on your low frequency processing.  If your C1 is already dropped down to 0.01µf, try dropping C5 down to 10µf or 1µf.. or even 0.1µf, if you are not planning to plug in a Bass guitar.

[stonerbox]

C1 and C5 should have a lot of influence on your low frequency processing.  If your C1 is already dropped down to 0.01µf, try dropping C5 down to 10µf or 1µf.. or even 0.1µf, if you are not planning to plug in a Bass guitar.

Just changed the C5 to 4.5Uf made no difference at all. hm... Lower?

[slacker]

If you want C5 to start having any effect you need to change it to values smaller than about 0.2uF. I would leave it how it is though, making it smaller is cutting the bass after it's already being boosted, it would be better not to boost it in the first place, you can do that by making C1 smaller. With your current values anything above about 15Hz is boosted, if you make it 0.0022uF or smaller that will start cutting bass from a guitar signal.

[stonerbox]

If you want C5 to start having any effect you need to change it to values smaller than about 0.2uF. I would leave it how it is though, making it smaller is cutting the bass after it's already being boosted, it would be better not to boost it in the first place, you can do that by making C1 smaller. With your current values anything above about 15Hz is boosted, if you make it 0.0022uF or smaller that will start cutting bass from a guitar signal.

Thanks man, I'll do that!

I just don't understand why it came out this way? It's SUPER-low boosting right now.. like 15-120hz is extremly attenuated. When listening to other clones and the original MXR it's nothing like that. It's just plain weird...

[slacker]

Sounds like there might be something wrong with your build, it should be a pretty flat boost over audio frequencies. Are you sure C2 is 47pF if you accidentally used 47nF or some other larger value that would reduce high frequencies a lot.

[stonerbox]

Sounds like there might be something wrong with your build, it should be a pretty flat boost over audio frequencies. Are you sure C2 is 47pF if you accidentally used 47nF or some other larger value that would reduce high frequencies a lot.

Nailed it! Got a 47nf here! 

[PRR]

> Got a 47nf here!

Rolls-off everything above 56Hz. So your low-note is slightly down, your mids are way-down, and your highs are nowhere.

[stonerbox]

Only had one 10pF and some of 220pF caps to choose from. Tried both and none of them seemed to affect the signal in a audible way.

Think I will settle for the 220pF, unless it's too high? What does the C2 cap and R4 resistor do?

[PRR]

> What does the C2 cap and R4 resistor do?

Rolls-off supersonics so they don't make mysterious trouble.

If 47nF rolls-off at 56Hz, then 47pF rolls-off at 56KHz. Far above the guitar band.

Similarly, 220 is about four times bigger than 47, so 47pFd rolls-off around 56KHz/4 or about 14KHz, still well above the guitar band. I think that's fine.