ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Re: mosfet clippers ?

:

12/17/1999 7:51 AM

Re: mosfet clippers ?

thanks for more great info Aron and ED...
I have ran into diodes in the feedback loop
causing that spattring sound on the decay
with a few diode set ups,pretty ugly for shure.
useing a 12 way rotary switch to test diode set ups and wanted to add the MOFET ideas. now it looks like a to-92 socket is in order here....thanks Aron I will try hacking
in diodes to ground just to see what happens.. trying to nail that SRV tone in messing with the ts.
I am a metalhead at heart but have started learning some
SRV tunes and the TS just seems to get me pretty close. I get to crank my amp up pretty good out here...and mixing the gain on my voodoo and the gain on the ts ..it stays pretty clean/yet heavy for metal stuff as well...unlike all my distortion boxes. I have been druling over the shaka..but want to finish this project first...
I am very happy with what I have so far but want to leave no stones left unturned.
then I will move on to a diodes to ground layout,but sounds like you got that one nailed Aron.
thanks again everyone...

12/17/1999 3:01 PM

Something you have to watch with MOSFETs is that in one direction they act like a soft-conduction silicon diode, the other they act like a MOSFET, depending on what you do with the gate. If you tie the gate to the source, it deactivates the MOSFET, so you get just the diode. If you tie the gate to the drain, you get the diode-connected MOSFET action in that direction.

I'm wondering if the splatting is from the "off" diode/MOSFET being turned on at the wrong time. If that happens, it would certainly sound ugly.

Just a guess.

12/17/1999 8:35 PM

thanks RG I am waiting for more parts to arrive
before I check out MOSFET clippers.this will help a great deal.I think you are right
I have found if the voltage drop is diffrent by pretty good
amount..I get spatting as notes decay..sounds good at full drive settings but not at SRV type settings I.E. "lower gain"
sounds like real bad crossover distortion or something..
some of the diode set ups I used on my tester deal are pretty badly mismatched ...I just started to run out of ideas..
onward through the fog..
hys

12/18/1999 12:25 AM

quote:
"I think John G. knows, maybe He'll drop a little hint?"

When I use the MOSFET clipper in the feedback I connect the gate to the drain and put a germanium diode in series with the source, anode of the diode connected to the source of the MOSFET. That's it.

I think the real key is to get a MOSFET with as low of a Vgs(on) as possible. When the battery starts getting low on mine it starts sounding extremely ratty. Not very musical at all. So it's fresh alkaline batteries only for this one.

Is that a big enough hint?

--johng

12/18/1999 1:17 AM
hys chip	thank you john ..even I understand that hint.. the germanuim diode will be on oppisit ends of the MOSFET's if I use 2 of the same set ups for - and + blew yet another new speaker today looks like a pair of webbers high powers are next up to bat... its allways somthing ..ugg

12/19/1999 1:25 AM
Ed Rembold	Thanks John, That clears it up for me, I just can't have even the possibility of that "splatting". And yes, Aron I did know what we were talking about, At least this time. Ed R.

12/19/1999 2:31 PM

Mosfet Splatting

The integral reverse diode (anode=source) is the soft knee diode (I'll call the other one the normal diode). The reverse diode has a low Vf, much lower than the normal diode. As long as the gate is floating or connected to the source, the reverse diode is the one that can conduct.

Diode clippers are almost always used in pairs, and as long as the opposing diode in the clipper pair has a lower Vf than the normal mosfet channel, the opposing diode will conduct and offer a low Z path for the signal before the splatty mosfet junction can conduct. The only way this will not happen is if the Vf of the opposing diode is higher than the Vf of the normal mosfet diode - and it's not for a Ge (or even 3 in series), Si or the reverse mosfet.

What this boils down to is that as long as the gate of the mosfet is not connected to its drain, it is very unlikely that the normal mosfet diode will conduct and make a splatty sound.

best regards, Jack

Page 2 of 5