BSIAB II built but which components effect bass and low mids? Help Please

[B_of_H]

http://www.generalguitargadgets.com/pdf/ggg_bsiab2_sc.pdf


Hello,

I built this on perfboard today.  I already own a bsiab kit from GGG and I wanted to try to make another.  It works fine but is very compressed with no Bass and low mids.  Essentially it sounds like it's through a wah with the toe down. 

I didn't have every component I needed so I made some subs.

I used:

C1 - 180p in place of 250p ( I have a 300p)
C2 -  .1u in place of the .068u
C5 - 390pf in place of 470p (I have a 500p)
C6 - 120p in place of 150p (I have a 180p)

Q5 - mpf102 in place of 2n5457 (I have plenty of 201,102 and 5457's)

R3 - 470r in place of 680
R7 - 51k in place of 47k



Also I put a few resistors in series to get close to the values on the schem (can I do this?):

R2 (180r>180r)
R15 (2.7r>2.7r)


Any ideas on where to start?  Thank you in advance for any suggestions. 

[B_of_H]

Here are some pics.  I put a 500p at C5 and it didn't help.
It has more gain than the kit I built from GGG but very fuzzy and little bass and low mids still.

[newbie builder]

If you increased C12 and C4 (both .022uf caps) in value (something like .1uf) that'd be a quick way to let more bass through- I'd try one at a time though because it might get mushy.

[Mark Hammer]

R7 is probably the culprit, or at least one of the major offenders.

Note that the tone control circuit is a combination simple lowpass and highpass filter.  The tone pot pans between the two filter outputs.  As shown, the lowpass filter, consisting of R7 and C10 ( http://www.generalguitargadgets.com/pdf/ggg_bsiab2_sc.pdf ) provides a rolloff starting around 154hz.  With 51k, that moves down to 142hz.  That's not a dramatic change in rolloff point but it is also accompanied by a reduction in the amplitude of the bass signal that does manage to get through.  The upshot is that it results in a shift in the balance of lows to mids.  Not huge, but noticeable.

Your substitutions of two-in-series for the values you didn't have were absolutely the right thing to do.  Just keep in mind thatthey don't have to be identical value; they only have to add up to the right amount.

R3 might be a little low-ish, though I suppose that would depend on the FETs in that position.  You might want to consider tinkering with the bias to address the over-compressed sound.

[B_of_H]

Thanks,

It's so cool that a complete idiot like myself can get help from people who know what they are talking about.  God Bless the Interweb.

I got my first kit from BYOC in January and just started putting together stuff on perfboard a month ago.  I wouldn't have got very far without this place.   This hobbie has killed my golf game but I'm really having fun (and saving money on greens fees).  I'm not much of a guitarist but I like to make noise and experiment.     

Mark and Newbie Builder you guys rock!

I'll update the results soon.

[B_of_H]

I changed R7 to 43k and it did increase the volume of the bass but the freq. is still very nasal but more like a bad fuzz sound.   (Like the guitar part on communication breakdown through a cheap stereo "duh-duh-duh-duh-da-duh-da")

There is way more high end than my GGG kit and I took C14 out of that one.


Should I change C10?

[shredgd]

Hi,

C1 is there to avoid radio frequencies to get into the circuit, so your slightly different value has not to do with mids and bass (though a bigger cap there - in the range of tens of nF - would cut progressively more presence and highs).

Instead I remember I read that C2 is inversely proportional to the bass content, so I would correct that value.

C5, together with R6, forms a filter which I never understood well (I'm not an electronic engineer) but, if this helps, I have 560p in my pedal, because "scottosan" once posted some wonderful sounding clips and he sayed he had that value in his pedal. I believe a bigger cap here will let more mids pass.

C6 role is to keep the high content (presence, mostly) of your sound when using the drive pot in its lower positions (the same as the hi-pass cap on a guitar's volume pot does). You might keep the value you used if you're not hearing an excessive high loss when using a low gain setting compared to the high gain settings (this might also be a desirable effect by someone).

Regarding the mpf102, I've never used that tranny, so I can't be of help.

The series resistors to get the right value are ok.

That lower value R3 should give you more gain from the first half of the circuit, so a more compressed sound (like Mark Hammer already said). The end result is subjective, though.

In conclusion I suggest you to fix C2 and C5 (and, only in a second time, R3 and Q5).
In addition you will probably like a simple mod to increase the bass in this pedal, which consists in raising C8 to 1u (you can even simply parallel a 1u cap to the already existent 0.1u). I love this mod!

Giulio

[Alverman12]

Hi!
I built the Bsiab2 but I found it a bit "closed" I know that I shouldn't post here ......but how can I open it up ?
I'd like also to add some treble.......however this stomp has a fighting spirit, it's awesome  The one I built has all J201s and it's nicely compressed (it seems there's a compressor inside)

Thanks to everybody

PS: Please forgive my poor english knowledge

[B_of_H]

The reason C2 is larger is because I thought it would increase bass. 

Can anyone confirm that it is inverse?  (shredgd - i'm not trying to be a pita or doubting your knowledge I just want some comfirmation before taking action)



Alverman12 -

On the kit version I took out C14, changed C6 to 22pf, changed C2 to .1u and I think it sounds more open now. 

I also tried just taking C13 and C14 out of the stock schem which let more high end through (it sounded great IMO) and took only a minute to do.

[shredgd]

The reason C2 is larger is because I thought it would increase bass. 

Can anyone confirm that it is inverse?  (shredgd - i'm not trying to be a pita or doubting your knowledge I just want some comfirmation before taking action)

You're doing right looking for a confirm, absolutely no pita 
After a hard search, I finally managed to find that thread...:

http://www.diystompboxes.com/smfforum/index.php?topic=41148.0

Look at point 2) of the first post; he says the bigger the cap, the more high mids will pass.

Hope this helps,

Giulio

[B_of_H]

Thanks, I have a couple of people that needed to see that thread.

Can I paypal you a beer?   

[snoof]

Instead I remember I read that C2 is inversely proportional to the bass content, so I would correct that value.

I've read posts on both sides of this statement.  If C2 is acting as negative feedback, then your statement would seem to be true.  it's just a guess on my part if it is indeed acting as a feedback crkt.  can anyone confirm or deny??  Mr. Hammer??

[B_of_H]

I think I'm going to just try a smaller cap and see what it does.

[snoof]

bumpity