amz miniboster finished. treble too "tinny"...?

[nag hammadi]

hey guys.  i finished an amz minibooster  :

http://generalguitargadgets.com/diagrams/joam_lo_sm_amz.gif

i like it okay.  i used the j201's. 

my only complaint is that the treble is too "tinny" sounding.  the bass output is okay, so i don't want to increase bass - i just want to thicken the highs.

any suggestions from you mad scientist types? 

[brett]

Hi.
try a 0.001uF cap between the input and ground.
In the old days quite a few pedals used this trick.
Thatt'll give you a -3dB point around 10kHz.
If that's not enough, try 0.0015uF or 0.0022uF.
cheers

[Skreddy]

IMO, the .001uf input cap as specified by AMZ seems pretty damn small.  I'd expect that circuit to sound tinny.  I suggest something like .047uf (still small enough to avoid woofiness) or .068 (for a little more lower-midrange beef) instead.  If that makes it too bassy, try a smaller output cap instead of .1uf.

[brett]

Hi.
MOSFETs have very high input impedance.  In fact, the gate is *insulated* from the rest of the device.  Because of this, the input cap can be almost as low as you like, so long as the resistance of the biasing system and other circuitry connected to the gate is high.  In the case of the AMZ mini-booster, these other components have almost 10M of impedance.  An input cap of 0.001uF in this circuit gives a cutoff frequency of about 20 Hz.  In other words, ample bass for a bass guitar.

What I was suggesting above was a capacitor between the INPUT and GROUND.  This forms a single-pole filter that passes low frequencies and carries high frequencies to ground.  Assuming the pickup impedance is 15 kohms, and a capacitor of 0.0015uF is used, the rolloff frequency would be 7 kHz, with 6dB per octave roll-off above that.  Just about right.

Alternatively, the output impedance is about 3k.  This can also be used to make a low-pass filter.  A 0.0068 to 0.015uF cap to ground (just before the 0.1uF output cap) will take out some treble.  The bigger the cap, the more treble is taken out.

good luck

[nag hammadi]

i am not sure i am trying to take out the treble or add more bass.  i am just trying to "warm" the treble.  maybe my vocabulary is failing me...

[petemoore]

  Perhaps [adjust values] use a resistor and a cap parallel, in series with the signal path? BSIAB has an example of this between it's Mu Stages. [470k with a 470p across it IIRC<]. this should shift the frequencies boosted toward the treble.
  You said 'tinny', so this would just probably add to the tin, however you said 'thicken the highs'...perhaps a combination of shifting freq's boosted up, then attenuating just the 'high highs' with a multi-pole LP Filter would thicken them up.

[JHS]

Use KP and Silva Mica caps and plain foil or multilayer tantals for the electrolytics, most wimpy treble come from cheap caps and their losses. Switch the FETs and select by ear, they stray a lot and can produce thin treble too if they are weak ones.

JHS