MOSFET Boosters and EMG's

[YouAre]

I've got a buddy with a guitar equipped with EMG's that I'm building a MOSFET boost for (Orman's circuit). I'm worried that the high impedance of the booster will do some funny business with the low impedance output of the EMG's. Do you guys think I've got anything to worry about?

[5thumbs]

In theory, low output impedance driving a high input impedance is optimal.

From Jack Orman's Muzique.com article, "Pedal Impedance: The In and Out of Impedance":

In general, the ideal situation is to have a low output impedance connected to a high input impedance in order to pass the best fidelity signal across the audio frequency band. A unity gain buffer, which has a high input and low output Z, will sometimes be used as a line driver to preserve the full frequency response of the signal.

Since the EMG active pickups are essentially magnetic pickups running into an active, high-impedance buffer with low output impedance, it's kind of like plugging your Strat into a MOSFET Booster, then plugging that MOSFET Booster into another MOSFET Booster.  I don't anticipate that configuration would have any impedance-matching problems, so my gut on the EMG active pickup->MOSFET Booster combo is the same...no worries.

Now I don't play EMGs, so I have no practical experience to share.  Perhaps others can offer specific experiential data to supplement the theoretical.

[shredgd]

The mosfet boost is quite bright, or at least that was the general impression I had (along with excessive bass) after gigging extensively with it (used as a volume boost). Probably because it is so hi-fi, it gives the same boost for all frequencies... but our ears hear more treble and bass the more you raise the volume... That's why I personally prefer boosts which give a "mid bump". So if your friend complains about harshness in his sound, I would suggest something different (I reached peace with the very simple but quite perfect EH LPB..).
Just my opinion/experience.

Giulio

[5thumbs]

The mosfet boost is quite bright, or at least that was the general impression I had (along with excessive bass) after gigging extensively with it (used as a volume boost). Probably because it is so hi-fi, it gives the same boost for all frequencies... but our ears hear more treble and bass the more you raise the volume... That's why I personally prefer boosts which give a "mid bump". So if your friend complains about harshness in his sound, I would suggest something different (I reached peace with the very simple but quite perfect EH LPB..).
Just my opinion/experience.

Giulio

The "hi-fi" comes from the very high input impedance of Jack's MOSFET Booster.  Couple that with the clean "clean boost" from the MOSFET and you've got a pretty darn transparent boost.  The increased treble you are perceiving is what is actually being lost to loading from lower-input-impedance amps/devices (when you're not using the MOSFET Booster as the first-in-line device.)

You could lower the input impedance if you wanted to get less treble "sparkle" from the MOSFET Booster.  I believe you can just start progressively reducing the size of that 10M resistor before the MOSFET gate until you find the input impedance that gives you the treble "sparkle" you're looking for.  Try anything between 10M and 100K and see how you like the changes in the sound.

Good luck!

[aziltz]

you could just add a trimmer to lower the input impedance and tweak to his liking.

[YouAre]

Stupid notification system didn't send me an email....

I guess I'll look into subtle high and low pass filters too. Take off some extreme highs and extreme lows to make it potentially sound less hi-fi and harsh if that's an issue.

Has anyone tried filters on the mosfet boost? Should I put them at the front end or tail end of the mosfet?

Thanks!

[Ben N]

The increase in brightness that people perceive with the Mosfet Boost probably wont be there with EMGs, because the buffering effect is less significant when fed by a lo-z output.

[5thumbs]

Stupid notification system didn't send me an email....

I guess I'll look into subtle high and low pass filters too. Take off some extreme highs and extreme lows to make it potentially sound less hi-fi and harsh if that's an issue.

Has anyone tried filters on the mosfet boost? Should I put them at the front end or tail end of the mosfet?

Thanks!

Ben's got a good point about the input impedance not have the same effect on EMG actives as they will passives.

That aside, you could start darkening the tone with a capacitor shunt.  Try different values (0.01μF/0.022μF/0.033μF/0.047μF are some common cap shunt values for darkening boosters/distortions.  On the MOSFET Booster, you could try placing the cap shunt between signal and ground between the 0.1μF cap after the MOSFET and before the 1M output pull-down resistor (if implemented).

If you want more control over the corner frequency of your LP filter, you could then put a pot in series with the signal as a variable resistor with a capacitor to ground right after the pot.  This will allow you to make an LP filter with a variable corner frequency.  Jack Orman's Simple R-C Filter Cutoff Calculator can help you play around with the pot and cap values until you find the corner frequency you're looking for.  FWIW, I'd recommend setting the corner frequency of your variable LP filter around 7.5Khz as the highest value, and varies the corner frequency lower as you change the resistance of the pot.  Setting the corner at 7.5Khz will still allow a lot of treble frequencies to pass, but without things getting muddy.

Also, I would be remiss if I didn't point out that the guitar your friend/customer is playing has a lovely LP filter built into it called a Tone Control.  Sometimes, it's best to just roll off the guitar tone a bit and not mod an otherwise acceptable pedal design.  (I realize that's not always an option, but it is worth pointing out.)

[Ben N]

You also don't have to use keep the input impedance of the Mosfet boost super high. If you don't need the high-Z, I think you can lower the value of R3 to 1M or 500k. Probably get lower noise that way, too.