quick question about the amz mosfet boost?

dmk · May 27, 2007, 11:25:47 AM

greetings all.
i built me a mosfet boost the other day with a master volume at the output. reason been that i really like the way it clips with the 5k gain control on full, nice and crunchy. is there anyway to get it to crunch up even more? i was thinking of maybe putting another booster cascading into it ala super duper.
cheers

mark

markm · May 27, 2007, 11:29:36 AM

You could try the mods shown in this Layout;
http://aronnelson.com/gallery/album76/Modified_MosFET_Booster_LAYOUT
I agree, it sounds good Cranked up!

dmk · May 27, 2007, 11:48:36 AM

what did you change from the original?
excuse the ignorance. all i can see is you got rid of the 5k gain pot.

markm · May 27, 2007, 11:55:19 AM

Essentially.......you're right.

Now, what you could try doing is decreasing the value of the source resistor (R5) to like say 1K.
That'll bump up the Gain.

dmk · May 27, 2007, 12:57:17 PM

that sounds more like it.
makes sense now looking at the schematic

my brain hurts some days

cheers

dmk · May 28, 2007, 11:05:10 AM

umkay. i tried changing R5 to a 1k, sounded spluttery and gated. i used a 2n7000 instead of a bs170 if that makes any difference.
bit stumped really. i'll give cascading 2 boosters a try next i think.
cheers for the help

markm · May 28, 2007, 11:57:19 AM

Quote from: dmk on May 28, 2007, 11:05:10 AM
umkay. i tried changing R5 to a 1k, sounded spluttery and gated. i used a 2n7000 instead of a bs170 if that makes any difference.

Well, the pinout is different.

dmk · May 28, 2007, 12:04:52 PM

Quote from: markm on May 28, 2007, 11:57:19 AM
Quote from: dmk on May 28, 2007, 11:05:10 AM
umkay. i tried changing R5 to a 1k, sounded spluttery and gated. i used a 2n7000 instead of a bs170 if that makes any difference.

Well, the pinout is different.

just reversed i think isnt it?

dmk · May 28, 2007, 12:19:47 PM

i started to really doubt myself so i went and tried flipping the 2n7000 around. i definatly have it in the right way

grapefruit · May 28, 2007, 10:37:59 PM

If you reduce the SOURCE resistor you'll have to adjust the GATE bias resistors to get the DRAIN at the correct voltage.

Cheers,
Stew.

markm · May 28, 2007, 11:40:06 PM

Quote from: grapefruit on May 28, 2007, 10:37:59 PM
If you reduce the SOURCE resistor you'll have to adjust the GATE bias resistors to get the DRAIN at the correct voltage.

Cheers,
Stew.

I would say this is correct in some cases however, both the MosFet Boost and the SHO use a VR tied to the source of the MosFet and their gate bias is fine.
Dropping the source resistor down to 1K would be equal to the boost control on the SHO just about wide open.
Now that I think about it, perhaps your point would apply more to the MosFet boost as the bias configuration is indeed different?

grapefruit · May 29, 2007, 03:07:35 PM

I haven't seen the SHO circuit, but with the mosfet booster I doubt that you'd get much, if any, more gain by reducing R5, and it will alter the bias. The ac gain is controlled by R6.

Stew.

markm · May 29, 2007, 04:18:02 PM

Quote from: grapefruit on May 29, 2007, 03:07:35 PM
I haven't seen the SHO circuit, but with the mosfet booster I doubt that you'd get much, if any, more gain by reducing R5, and it will alter the bias. The ac gain is controlled by R6.

Stew.

Yes.
The least resistance is set by the Source resistor plus whatever tolerance is figured into the equation by the 5K pot.

dmk · May 29, 2007, 04:56:20 PM

think i'm probably just going to build a 2nd booster and cascade the two.
i'll let you know how it goes

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quick question about the amz mosfet boost?