Colorsound Power boost bass knob

[VoodooChili]

Hey everyone, first post here as i thought you might be able to give me some insight.

I recently built a BYOC power boost and thought I would ask the experts here if it's behaving normally or not. The bass control seems like it has very little effect until you get past halfway and start adding bass. It doesn't seem like it cuts much, if any, in the first half of the knob. The treble control functions as I would expect it. Just wondering if that's normal for the bass control as I've never played another power booster. Thanks.

Pic of the build:

[antonis]

Hi & Welcome..

It's always good to have a schematic to refer on:



Q2 & Q3 Collector values are small enough to mess with Bax tone control values..

Maybe you should play with R9 value in conjunction with R11/R12 ones..
(i.e. lowering R9 while raising R11 & R12..)

P.S.
By lowering R9 you might lower Treble gain..

[VoodooChili]

Hi & Welcome..

It's always good to have a schematic to refer on:



Q2 & Q3 Collector values are small enough to mess with Bax tone control values..

Maybe you should play with R9 value in conjunction with R11/R12 ones..
(i.e. lowering R9 while raising R11 & R12..)

P.S.
By lowering R9 you might lower Treble gain..

Thank you. So raising R11 and R12 would help cut more bass as the control is lowered? What would be good values to start with for these? Sorry if this is a newb question, I'm very familiar with guitar electronics but this is my first pedal build.

[antonis]

So raising R11 and R12 would help cut more bass as the control is lowered?

Actually, the problem is located to R9 and R13 equal values..

Without analysis of active tone control and BJT bias configurations, just try to make R9 = 15k to see if Bass pot responce is improved..

[VoodooChili]

try to make R9 = 15k

Awesome. I'll give that a try and see what happens

[Rob Strand]

Awesome. I'll give that a try and see what happens

If you change R9 it will change the treble control.  It raises the frequency and reduces the treble boost by quite a bit.

Unfortunately the treble frequency on that unit is very low and it interacts with the bass control.   Kind of forces the bass frequencies low.   If you reduce R9 it will only help a bit.   You could try changing the bass cap to 47n, limited results as it's working against the treble control.

The best way to fix the bass control would be to reduce the treble caps.    You need to nudge up the treble frequency to a point where the bass control will works more effectively.   If you change the bass cap to 47n or 68n and the treble caps to 6n8 or 4n7 it might be enough.   If you don't like the sound of the treble it's going to be hard to work around the problem.   Using 6n8 treble cap allows the bass control to work and then the 68n cap helps the bass.

[VoodooChili]

Awesome. I'll give that a try and see what happens

If you change R9 it will change the treble control.  It raises the frequency and reduces the treble boost by quite a bit.

Unfortunately the treble frequency on that unit is very low and it interacts with the bass control.   Kind of forces the bass frequencies low.   If you reduce R9 it will only help a bit.   You could try changing the bass cap to 47n, limited results as it's working against the treble control.

The best way to fix the bass control would be to reduce the treble caps.    You need to nudge up the treble frequency to a point where the bass control will works more effectively.   If you change the bass cap to 47n or 68n and the treble caps to 6n8 or 4n7 it might be enough.   If you don't like the sound of the treble it's going to be hard to work around the problem.   Using 6n8 treble cap allows the bass control to work and then the 68n cap helps the bass.

Thank you. Do you know if the regular power boosters work this way as well? It seems like in the videos I've seen the bass knob is a lot more effective. Just wondering if this is normal behavior or not.

[Rob Strand]

Thank you. Do you know if the regular power boosters work this way as well? It seems like in the videos I've seen the bass knob is a lot more effective. Just wondering if this is normal behavior or not.

It's hard to second guess what your unit is doing or what you expected.   All I can say is that tone control has low bass and treble frequencies.   I can see why someone could perceive the bass control as ineffective.   The fact you are getting the problem in boost and cut positions kind of confirms that.

There's always the possibility of a something wrong with the build.  For example if R7 and R8 were 47k and not 4k7 the bass control won't do much.   Wrong cap values.  Plenty of possibilities.

If you had a sine generator and a multimeter (or oscilloscope) you can measure the bass boost/cut at a couple of frequency points and confirm the bass control is working as expected.   You need a low enough signal level that there's no clipping.

[VoodooChili]

Ok, thanks for the insights. Food for thought anyway   

[antonis]

Food for thought anyway 

And further reading (if you wish for..)

pages 2-46 to 2-55

[VoodooChili]

And further reading (if you wish for..)

pages 2-46 to 2-55
[/quote]

Right on, i'll check that out.

[Rob Strand]

And further reading (if you wish for..)

What you won't find is the effect of the 39k bass resistor position.

Here's a comparison of four circuits:
- ck1: the original
- ck2: the circuit with tweaked caps to allow the bass frequency to increase
  keeping the resistor values the same.
- ck3: a circuit which tweaks the response by changing the 39k bass resistor
  just enough to match ckt2 without changing anything else.
  The idea is not to change the caps but I've made a slight tweak to the caps
  so the comparisons look more equal.
- ckt4: a circuit which sets the bass resistor to 4k7 (a common practice
  of matching this resistor to the other bass resistors)
  The caps have minor tweaks to match ck2.


1) Schematic:



2) Response comparison of when individual bands boosted:

The main observation here are:  The bass frequency can be increased.
The modified circuits can be made to match up.  As expected the maximum
boost is affected a bit in ckt's 3 and 4.




3) Response comparison when bass and treble simultaneously boosted:

Here's where things get different.  When you make the 39k bass resistor smaller,
the treble response is greatly affected by position of the bass control.
It's a very strong effect and it gets worse when you follow the common practice
of making the 39k base resistor equal to the other two bass resistors (as per ckt 4).

Clearly things are not equal.



For clarity I've only covered the main points of the tone circuit.  No transistors.  No extra caps.

EDIT: images regenerated.

[VoodooChili]

And further reading (if you wish for..)

What you won't find is the effect of the 39k bass resistor position.

Here's a comparison of four circuits:
- ck1: the original
- ck2: the circuit with tweaked caps to allow the bass frequency to increase
  keeping the resistor values the same.
- ck3: a circuit which tweaks the response by changing the 39k bass resistor
  just enough to match ckt2 without changing anything else.
  The idea is not to change the caps but I've made a slight tweak to the caps
  so the comparisons look more equal.
- ckt4: a circuit which sets the bass resistor to 4k7 (a common practice
  of matching this resistor to the other bass resistors)
  The caps have minor tweaks to match ck2.

Thanks for the info. I'm not trying to increase bass but rather decrease it, or at least make the minimum bass setting less bassy.

[antonis]

Minimum Bass frequency is set by 0.159/(C8 * R7) formula..
(assuming R7 = R8 )

P.S.
Max bass frequency is set by 0.159/(C8 * BASS)..
Max bass boost is set by (BASS + R7) / R7..
MAX bass cut is set by R7 / (BASS + R7)..

[VoodooChili]

P.S.
Max bass frequency is set by 0.159/(C8 * BASS)..
Max bass boost is set by (BASS + R7) / R7..
MAX bass cut is set by R7 / (BASS + R7)..

So if i wanted more cut from the control i would change R7/R8 (4k7) to something else?

[Rob Strand]

Thanks for the info. I'm not trying to increase bass but rather decrease it, or at least make the minimum bass setting less bassy.

There's two parameters: Frequency and boost.    If the bass is at the wrong frequency it is ineffective.   If the boost is too much it's annoying to use.

Those tone controls generally have equal boost and cut.   If you try to make them asymmetrical the center position is not longer flat.    In small doses you can get away with some asymmetry.  So for more cut than boost you would make R8 smaller than R7.

The amount of boost at low frequencies  is set by the resistor R7/R8.   If you increase R7/R8 it will also decrease the frequency where the the bass frequency starts.  So you might need to decrease the bass cap by the same factor you increased R7/R8.

When you play around with these things is pays to check the response using something like LTSpice.