Mosfet boost question for Jack or anyone..

[Davefx]

Getting ready to put together the Mosfet boost and wanted know if the input cap value of .001 was a typo, because it appears like it would be a treble booster, or is it supposed to be that.  If it was covered on a previous thread, I apologise.

Also if I wanted to run it on 18v, could I and what would I need to change?

Thanks in advance..

[bwanasonic]

Well I used a .001 input cap in mine and it has a very neutral, balanced tone. As for running it on 18v, you might want to try it with 9v before you go down that route. I have not found headroom to be a problem with just a 9v battery, although I don't use really high-output pickups. See if you have a need to run it at 18v first.  This is probably my *desert island* stompbox, and while I am sure there are tweaks that might suit some users, I would advise building it exactly as per JO's schematic as a starting point. It's pretty near perfection AFAIC.

Kerry M

[Davefx]

The friend I'm building it for insists on 18v...  But I'll try it 9 first, I'm sure it is great, but some folks are fickle, ya know.  I imagine I should adjust the voltage divider so that the drain gets 9-9.5v instead of the 4.5 to 5, running 18 that is.

Hmm..

[bwanasonic]

The friend I'm building it for insists on 18v...  But I'll try it 9 first, I'm sure it is great, but some folks are fickle, ya know.  I imagine I should adjust the voltage divider so that the drain gets 9-9.5v instead of the 4.5 to 5, running 18 that is.

Hmm..

It sounds like "A little knowledge is dangerous" phenomenon.  Why would someone insist on a specific operating voltage for a circuit they know nothing about? Do they have the idea 18v is *better* than 9v? Maybe they should try this logic with their amp and see how well it works ( gee, I figured if 110v sounded good, 220v would sound even better! Who knew it would burn my house down!).  It's downright scary how these "urban legend" type concepts spread.

Kerry M

[javacody]

Yeah, tell the guy he's wrong and that he really wants 36 volts.   :lol:  Tell him that 18 volts is old voodoo and all the new mojo pedals use 36 volts!  :twisted:

Just kidding, it's easy enough to add another battery clip in. There is something in the FAQ about doing this.

[Davefx]

Regardless, I need to know if I need to change the circuit.  I know how to GET 18v.  All he has left is 18v outputs on the PS he has.  He wants no batteries.  So whether it sounds better or not, that's what he NEEDS, so I will try to oblige any way I can.

Thanks for your help.

[Jered]

Dave, try a 200k trimmer for R2 to get 1/2 supply voltage.
 Jered

[bwanasonic]

You might also need to change the zener diode in the circuit. I don't know how happy the mosfet will be at 18v, but that should be easy enough to figure out (I really like the BS170 in there). If you do build it for 9v and then 18v, please post on any difference in sound.


Kerry M

[amz-fx]

The input capacitor value is correct.

18v will give you more clean headroom. To run it on 18v, make sure that all of your capacitors are rated for 25v minimum. Make sure that the external power supply is actually supplying 18v and not 25 or more, which is possible if it is an unregulated wall-wart.

If everything checks out, merely apply the 18v power to the circuit. No bias adjustment should  be necessary.

regards, Jack

[Davefx]

Thanks Jack and everyone..  Jack, just because I'm a theory dummy, is it because of the ultra high input impedance of the circuit, the .001 doesn't act trebly?  Usually you see .22 and higher for full range boosts.

Just curious..

Thanks.

[Davefx]

Also.. Do I have to change the voltage of the zener to 18v?  Could I use any voltage or does it have to be at least 9v.  

Thanks

[amz-fx]

The zener can be anywhere from 7v to 12v.  You would not want to go more than 12v.   You don't have to change it if you have a 9v in there now.

-Jack