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Old 2nd February 2012, 02:58 PM   #41
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your right, i didnt think of the internal diodes, surely there are a few similar types that dont have them?
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Old 2nd February 2012, 05:01 PM   #42
shanx is offline shanx  Canada
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Default Mosfet internal diode

Hi Razorrick, I believe that for mosfets the internal diode is intrinsic to the construction of that type of device, so for mosfets at least I don't think you could get one without it. That was the funny thing about some of the commercial products touting that they are using mosfets for clipping, in reality most of them were not even using the G-D connected diode connection for clipping(!) the signal routes through the SI internal diode as it has the lower threshold than the GD diode..but saying that it sounds better than a standard Si diode may be true, the internal diodes are a little slower (spongier?) to clip..
There are, I think some workarounds to that issue with mosfets that I will try out.
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Old 24th February 2012, 03:32 PM   #43
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Quote:
Originally Posted by shanx View Post
DPDT footswitch is installed and wired per previous diagram with in/out 1/4'' jacks. 9VDC adaptor power jack mounted and wired, not using battery connector on this build. FYI on most commercial 2.1 mm jacks for powering pedals, the polarity is negative on pin and positive on barrel. Some advice: use the insulated type jack that won't short to the case, as the case is referenced to negative. Double check to make sure your adaptor is polarized correctly before power up.
hi shanx, newb here just blowing thru some threads to get a feel for the forum and this was the first i landed on...lucky you!

if you're gonna run this with other pedals in the future, particularly on stage, you may run into some problems with your mains wiring...get a lot of noise and garbage...particularly as dirt pedals are usually high gain and first in line.

my humble suggestion would be to add a simple network to your power supply...you can wire it right to the 2.1mm power jack if need be...

use an 1n400X diode, with the band pointing to B+ in parallel with a 100-470u cap wired the same way...you can have the band on the cap denoting the negative side go directly to the negative terminal, and the positive to the positive terminal of the jack if need be. this will help filter mains noise and hum out...if need be, a small resistor in series (100-330r) with the b+ can also help if needed.

i find without the extra stage of filtering on power supplies in guitar pedals, particularly dirt pedals, that the pedal can be subject to a lot of unwanted and unnecessary noise. this can help.

that said, looks like a cool design, well done mate!
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Old 24th February 2012, 03:34 PM   #44
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Quote:
Originally Posted by shanx View Post
Been a while since I posted. I was looking into tying G and D together on the mosfets I have using them as 'diode' configuration, and this has been done in some commercial design stuff. There are some issues, because a mosfet example 2N7000 like others has an internal connected body diode. In some , but not all, commercial mosfet clipping designs the clipping circuit (with paralleled diode connected mosfets) the signal ends up only being clipped by the internal Si body diodes (!) and not even using the Mosfet 'diode' circuit. They even add Ge diodes in series to soften out the clip. Well that is something to consider.
I am going to pursue some of my own ideas on this now.
i believe the entire reason for using mosfets as diode clippers is precisely to take advantage of the internal diode's clipping mate. again, i'm a newb, so pardon if i'm off base here.
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Old 19th March 2012, 07:56 PM   #45
shanx is offline shanx  Canada
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Hey all,
Sorry been a while since posting..as life tends to send lots of other tasks our way.
Pinkjimiphoton, you are right that internal body diodes in the mosfet based distortions are used in fact as diode clippers. For the most part, the commercial designs tend to state that they are using diode connected (G and D tied together) mosfets to get the ''soft distortion'' from the mosfets. Their claims are not exactly true(!) But hey maybe it is the creative marketing departments..I was just pointing out that fact for people to look closer at. Thanks for bringing it up and confirming it. Your filtering comments for the Power supply are very welcome too. I am using a well regulated DC supply powering 5 or 6 pedals, and so far no big problems. Those small plug-type AC adaptors are indeed very noisy, with no DC regulation.
In the meantime on the build I am doing for a friend I started working on some minor improvements:
2 footswitches, one is bypass for the whole pedal and the other one to select between the soft clip in the NFB loop, or the diode clipper circuit at the output.
I installed a DIP switch on the board to be able to select various combinations of clipping elements. Stock Ge, LED, and Mosfet.
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Old 19th March 2012, 09:25 PM   #46
shanx is offline shanx  Canada
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Red face 2nd build pics

Here is the 2nd build pedal, with the bigger 1590DD case, 2 (3PDT) switches, LEDs and other hardware installed. Board needs to be finished/wired in. Gettin there
Attached Images
File Type: jpg 2nd build 001.jpg (8.5 KB, 78 views)
File Type: jpg 2nd build 002.jpg (10.0 KB, 80 views)
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Old 19th March 2012, 09:40 PM   #47
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hi shanx, looks good! please post clips when done!! peace pink
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Old 20th March 2012, 01:26 PM   #48
shanx is offline shanx  Canada
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Thanks! I will get some clips posted soon, and decent pics once the paint is on.
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Old 24th March 2012, 12:04 AM   #49
Simon B is offline Simon B  United Kingdom
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Default Reverse polarity protection

I pretty much always put this in, usually as a schottky type diode in series in the power feed, so as to protect against wrong-way-round wall warts and battery connection.

I've even fitted it to a cheap chinese 5 band graphic eq pedal. The pcb had a position for it, nicely silk screened, with a wire link inserted instead - presumably to reduce production costs.

You can do it with a mosfet too, for just about zero volts drop, see RG Keen's site:

Advanced Power Switching and Polarity Protection for Effects

Oh no, I'm now back in the infinitely expandable world of distortion effect design.........
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Old 25th March 2012, 01:17 PM   #50
shanx is offline shanx  Canada
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Thanks for the post and link, Simon. I should put that protection in.
Infinite possibilities for distortion, yes! I tried to get a small range that would be workable by putting in the DIP switches on the post clip circuit..so it gives a little bit of flexibility.
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